An innovative fracture plugging evaluation method for drill-in fluid loss control and formation damage prevention in deep fractured tight reservoirs

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2-15-2024

An innovative fracture plugging evaluation method for drill-in fluid An innovative fracture plugging evaluation method for drill-in fluid loss control and formation damage prevention in deep fractured loss control and formation damage prevention in deep fractured tight reservoirs

tight reservoirs

Chengyuan Xu Lei Liu

Yang Yang Yili Kang Zhenjiang You Edith Cowan University

Follow this and additional works at: https://ro.ecu.edu.au/ecuworks2022-2026 Part of the Civil and Environmental Engineering Commons

10.1016/j.fuel.2023.130123

Xu, C., Liu, L., Yang, Y., Kang, Y., & You, Z. (2024). An innovative fracture plugging evaluation method for drill-in fluid loss control and formation damage prevention in deep fractured tight reservoirs. Fuel, 358(Part A), article 130123.

https://doi.org/10.1016/j.fuel.2023.130123 This Journal Article is posted at Research Online.

https://ro.ecu.edu.au/ecuworks2022-2026/3488

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Fuel 358 (2024) 130123

Available online 25 October 2023

Full Length Article

An innovative fracture plugging evaluation method for drill-in fluid loss control and formation damage prevention in deep fractured tight reservoirs

Chengyuan Xu

^a^,^*

, Lei Liu

^a

, Yang Yang

^a

, Yili Kang

^a

, Zhenjiang You

^b^,^c^,^d^,^*

aState Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu, China

bCenter for Sustainable Energy and Resources, Edith Cowan University, Joondalup, WA 6027, Australia

cSchool of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072, Australia

dCentre for Natural Gas, The University of Queensland, Brisbane, QLD 4072, Australia

A R T I C L E I N F O Keywords:

Deep fractured reservoir Lost circulation Formation damage Evaluation method Fracture plugging Evaluation index

A B S T R A C T

Lost circulation, resulting from the undesired loss of drilling fluid into formation fractures, stands as a significant technical obstacle in the exploration and production of oil, gas, and geothermal reservoirs. Effective mitigation of this challenge requires the development and application of robust experimental evaluation methods to assess the effectiveness of fracture plugging. The traditional approach to fracture plugging evaluation relies on a uniform evaluation index and experimental parameters for various lost circulation types. Unfortunately, this practice frequently results in inconsistent performance of loss control formulas during laboratory experiments and field tests. To address this issue, this paper introduces an innovative evaluation method that accounts for the specific characteristics of the three major lost circulation types. By adopting this approach, a more scientifically rigorous design and optimization of loss control formulas can be achieved, ensuring their effectiveness in managing lost circulation challenges. The development of the new method involves a systematic five-step establishment process: lost circulation type determination, evaluation index weight calculation, fitness degree analysis between laboratory experiment and field test, experimental parameters optimization, and quantitative scoring of loss control formula. Analytic hierarchy process is adopted to calculate the evaluation index weight. Quantitative scoring model is proposed to finally determine the integrated formula score for the quantitative evaluation and scientific optimization of loss control formula. To bridge the gap between laboratory and field applications, laboratory evaluation tests are developed to address different types of lost circulation scenarios. The experimental results demonstrate significant improvements achieved through the optimized formula. Specifically, the maximum plugging pressure increased from 5 MPa to 20.8 MPa, while the initial and cumulative loss volumes witnessed reductions of 30.3 ml and 121.2 ml, respectively. Moreover, the evaluation method proposed in this paper exhibits a fitting degree of over 90 % when compared to the actual control effect on drilling fluid loss.

These findings substantiate the successful establishment of a connection between laboratory evaluations and field performance, providing valuable insights for future applications. Finally, a novel evaluation method for assessing the fracture plugging effect is established, accounting for various lost circulation types in deep fractured tight reservoirs. The reliability of this proposed evaluation method is validated by field test. Building upon this method, a high-score formula is designed and effectively deployed in a deep fractured tight reservoir in Tarim Basin, China. The successful application highlights the practical value and robustness of the developed evaluation method, offering promising prospects for future operations in similar reservoir settings.

1. Introduction

Lost circulation resulting from the undesired loss of working fluid into formation fractures remains the foremost technical challenge encountered during the exploitation of oil, gas, and geothermal

resources. This pervasive issue has significant implications for safety, the environment, and the economy, and has persistently plagued the industry for an extended period [12]. The working fluid commonly employed, such as drilling fluid, completion fluid, workover fluid, and others, plays a crucial role in oil, gas, and geothermal operations.

* Corresponding authors at: Center for Sustainable Energy and Resources, Edith Cowan University, Joondalup, WA 6027, Australia (Z. You).

E-mail addresses: [email protected] (C. Xu), [email protected] (Z. You).

Contents lists available at ScienceDirect

Fuel

journal homepage: www.elsevier.com/locate/fuel

https://doi.org/10.1016/j.fuel.2023.130123

Received 22 April 2023; Received in revised form 25 August 2023; Accepted 14 October 2023

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However, when working fluid loss occurs, it can trigger profound effects, including deep penetration of solid and liquid phases. This invasion often results in undesirable consequences, such as the swelling of clay minerals, migration and retention of formation fines, reservoir pore blockage, reduction in phase permeability, and imbalances in pressure between the wellbore and formation [321] (Fig. 1). These effects collectively contribute to a range of challenges, impacting operational efficiency, reservoir performance, and overall wellbore integrity.

Consequently, working fluid loss not only directly causes fluid loss itself but also gives rise to detrimental consequences such as formation permeability damage and a host of complex downhole issues [4]. As a significant contributor to non-productive time during the drilling process and diminished productivity during the production phase, working fluid loss exerts a substantial impact on oil, gas, and geothermal reservoir operations [5]. Addressing this challenge is imperative to minimize downtime, enhance productivity, and optimize overall reservoir performance. In recent years, naturally fractured reservoirs have emerged as a focal point within the energy industry. With an increasing number of companies venturing into the exploitation of progressively chal- lenging reservoirs, often found in deeper and more complex conditions, the significance of naturally fractured reservoirs has gained prominence [6]. This shift reflects the industry’s pursuit of untapped energy resources and underscores the need to navigate the intricacies of these reservoirs to maximize production and optimize exploration and production strategies. Naturally fractured reservoirs are characterized by their distinct features, namely developed natural fractures and ultra-low matrix permeability [722]. The presence of developed fractures presents advantages for the economical and efficient development of oil, gas, and geothermal reservoirs [8]. However, these fractures also intensify the challenges associated with working fluid loss, subsequently leading to a decline in permeability and productivity (Fig. 1). While the developed fractures offer opportunities, their presence necessitates careful consideration and effective management strategies to mitigate the negative impacts on reservoir performance and optimize productivity in such complex reservoir systems.

At present, the evaluation method of drilling fluid loss control is mainly based on the loss volume, maximum plugging pressure and plugging time to evaluate the plugging effect of plugging formula [111223]. The less the loss volume, the greater the maximum plugging pressure, the shorter the plugging time, and the better the plugging effect. When conducting laboratory measurements of these three indicators, it is typically necessary to replicate a portion of the field environment. For example, the formation temperature is simulated by heating the instrument [13], the fracture width is changed by confining pressure on the plunger [14], and the sand bed is used instead of the

plunger to simulate the pore loss of the formation [15]. The plugging effect of the formula can also be evaluated by drilling real cores [1617].

The disadvantage is that comparative tests cannot be carried out.

Nevertheless, regardless of the adjustments made to the laboratory environment, it remains impossible to completely replicate the exact conditions of the formation. The inherent disparities between the laboratory and field environments significantly impede the accurate evaluation of the plugging formula’s effectiveness. Consequently, the absence of experimental data support hinders the optimization of the formula. The limited correlation between laboratory experiments and field conditions underscores the need for innovative approaches to bridge this gap and enable more reliable and data-driven formula optimization.

In addition to observing the object of action, the evaluation of the plugging effect of the formula can also be evaluated by its own material performance parameters. Lost circulation characteristics determine the type of key performance parameters of plugging materials, and the applicability of plugging materials to drilling fluid loss conditions and types determines their effect [1819]. The performance parameters of plugging materials were evaluated, including particle size distribution D90, fiber aspect ratio, friction coefficient, compression resistance, acid soluble rate, high temperature resistance and so on [20]. Although quantitative evaluation methods for the key performance parameters of plugging materials are available, there is no relevant research on the evaluation of plugging effect based on loss control factors of different loss types. Our previous work developed a material evaluation method for the optimized selection of plugging material, accounting for the geometric and mechanical material parameters [32]. However, it cannot be used to evaluate the fracture plugging effect and guide the optimized design of fracture plugging formula.

A significant limitation in most studies examining the evaluation of drilling fluid loss control effect is the lack of consideration for the specific type of formation loss. This oversight leads to evaluation methods that lack pertinence and consequently yield substantial disparities between the evaluation results obtained in the laboratory and those observed on-site during loss control operations. As a consequence, the guidance provided by these methods proves insufficiently instructive. To address this gap, there is a critical need for evaluation methods that accurately account for the type of formation loss, leading to more consistent and informative results that can better guide loss control practices.

In this paper, an innovative evaluation method for assessing fracture plugging effect is introduced, incorporating several key components.

These include lost circulation type determination, evaluation index weight calculation, fitness degree analysis between laboratory

Fig. 1. Schematic diagram of working fluid loss and induced formation damage and production decline [910].

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experiments and field tests, optimization of experimental parameters, and quantitative scoring of the loss control formula. Building upon the classification of drilling fluid loss causes, this study establishes a direct relationship between different loss types and the primary control factors involved. This correlation allows for the adoption of distinct laboratory evaluation methods tailored to specific loss types, resulting in an approach that better aligns with real-world conditions. By considering the unique characteristics of each loss type and employing corresponding evaluation methods, this study ensures a more accurate and practical assessment of drilling fluid loss. Such an approach improves the overall relevance and applicability of the evaluation process, providing valuable insights that closely mirror the actual operational scenarios encountered in the field. The adopted approach in this study incorporates the analytic hierarchy process to calculate the evaluation index weight. By utilizing the fitness degree analysis, it effectively minimizes the discrepancy between laboratory loss control experiments and real field conditions.

Furthermore, a quantitative scoring model is proposed to determine the integrated formula score, considering three evaluation indexes. This model enables a comprehensive and quantitative evaluation of loss control formulas, facilitating scientific optimization. Importantly, this approach offers a generally applicable method to assess loss control effectiveness using multiple indexes, contributing to a more robust and holistic evaluation process. Finally, a novel evaluation method for assessing the fracture plugging effect is established, accounting for the various lost circulation types in deep fractured tight reservoirs. The reliability of this proposed evaluation method is validated by field test. A high-score formula is designed based on the evaluation method and effectively implemented in a deep fractured tight reservoir in Tarim Basin, China. The successful application of the formulated approach not only underscores its practical value and robustness, but also paves the way for promising prospects in future operations within similar reservoir settings.

2. Lost circulation types and controlling factors

Through different methods, working fluid loss can be classified differently. At present, loss is mostly classified according to the causes of loss [2425]. Referring to the research on the deterministic model of the cause of loss, the loss can be divided into the following three categories from this perspective: ①Induced fracture loss; ②Extension fracture loss;

③Natural fracture loss [26]. The main controlling factors of lost circulation control effect are different for different loss types, and the effects of plugging strength, plugging efficiency, and plugging compactness on lost circulation control effect are different. Therefore, the weight proportion of maximum plugging pressure, initial and cumulative loss volume of plugging zone in the comprehensive evaluation of lost circulation control effect are different.

2.1. Lost circulation with induced fractures

In the induced fracture loss, the undisturbed rock is a complete rock mass in situ near the wellbore (Without tension fractures). If loss occurs, the formation must be fractured and extended. The reason for the loss is that the fracture pressure of the formation is less than the effective pressure of the drilling fluid column, and the fracture is generated and reaches the pressure required for fracture propagation [26] (Fig. 2).

Induced fracture loss is easy to occur when drilling high pressure oil and gas reservoirs or killing wells, because the drilling fluid density is too high to fracture the low-pressure formation. When the running speed of the current drilling and casing is too high or the drill bit and the centralizer are wrapped in mud, the drilling tool is lifted sharply, which causes pressure excitation. The formation is fractured and finally loss occurs (Fig. 3).

The main controlling factor of lost circulation control effect is plugging efficiency. The longer the fracture extends, the more difficult it is to plug. Taking prevention as the main means, prevention and governance are combined. Loss control needs to be fast and efficient to avoid formation fracture and further fracture extension. The plugging effect depends on the fracture restart pressure and extension pressure after the plugging material is applied. Therefore, in view of the induced fracture loss, timely plugging of the loss channel should be considered to improve the plugging efficiency and enhance the drilling fluid loss control effect.

2.2. Lost circulation with extension fractures

In the extension fracture loss, the open loss channel exists around the wellbore under in-situ conditions, but the solid phase in drilling fluid cannot enter the lost-circulating zone freely within a certain pressure difference. When the drilling fluid column pressure is transferred to the fracture surface, due to the combined effects of positive pressure difference, temperature and seepage, the fracture is expanded (the fracture width and length both increase), and finally the solid phase and liquid phase in drilling fluid enter the formation [26] (Fig. 4).

The main causes of lost circulation are the existence of micro- fractures in the formation, the sensitivity of fracture width to wellbore pressure, and the fact that the fracture width exceeds the critical width that drilling fluid can effectively plug (Fig. 5).

The main controlling factors of lost circulation control effect are plugging efficiency and plugging strength. The fracture width extends from the original width to the resulting loss width, and the fracture extends to form a fracture network. The plugging effect depends on the fracture extension pressure and the strength of the plugging zone. For the extension fracture loss, the plugging efficiency and plugging strength are very important when plugging the loss channel. Timely and high strength plugging is a powerful guarantee to improve the control effect of lost circulation.

Fig. 2. Cause deterministic model of induced fracture loss.

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2.3. Lost circulation with natural fractures

In the natural fracture loss, the fracture exists in the formation, and is connected with the wellbore. The drilling fluid enters the formation freely under the positive pressure difference. Due to the existence of loss channel, drilling fluid cannot effectively plug the channel, which leads to loss. When the drilling fluid column pressure is greater than the

formation pressure, loss will occur. This type of loss can be divided into controllable and uncontrollable loss (Fig. 6).

Natural fracture loss refers to the type of loss that can be successfully plugged by conventional plugging technology, but it is often accompa- nied by fracture propagation and extension, which makes conventional plugging methods difficult to work (Fig. 7).

The main controlling factors of lost circulation control effect are Fig. 3. (a) Schematic diagram of lost circulation [27] (b) Flow diagram of lost circulation. Pb—Fracture open pressure；Pfp—Fracture extension pressure； Pz—Plugging zone strength；Pp+f—Pore pressure and hydraulic friction.

Fig. 4.Cause deterministic model of fracture extension loss.

Fig. 5. (a) Schematic diagram of lost circulation [27] (b) Flow diagram of lost circulation. Pb—Fracture open pressure；Pfp—Fracture extension pressure； Pz—Plugging zone strength；Pp+f—Pore pressure and hydraulic friction.

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plugging strength and plugging compactness. Drilling encounters fractures that cause lost circulation, and the fractures are connected into a network. The plugging range is wide, and there are many weak points.

Natural fracture loss has no high requirement for plugging efficiency. As long as the loss channel can be plugged, the fracture plugging zone can have a certain strength. The control effect of lost circulation is determined by plugging status and plugging strength.

3. Methodology

The process of fracture plugging effect evaluation method is shown in Fig. 8. Firstly, according to the loss characteristics and loss data sta- tistics of the work area, the loss type and its proportion are determined.

Secondly, based on the analytic hierarchy process (AHP), the weight of the evaluation index is determined, including analytic hierarchy process and establishment, judgment matrix establishment, consistency check and relative weight calculation [29]. The importance and scores of the indexes are determined based on field data and expert scoring. Then, the fitness degree of the experimental evaluation method is calculated based on the proposed analysis method of the fitness degree of drilling fluid loss control effect. According to the calculation results, the parameters of the evaluation experiment are optimized, including the parameters of the fracture module and the experimental steps. Determine the evaluation experiment method for this loss type. According to the evaluation results, the loss control effect of the formula is scored and rated in combination with the weight of the index. Finally, the formula is optimized based on the final score, and the formula with the best loss control effect for this loss type is obtained.

3.1. Determination of lost circulation type

It can be seen from Sec. 2 that the main control factors of different loss behaviors are different, and the evaluation index of control effect

will also change, so it is necessary to judge the type of loss in advance. By collecting the field engineering geological characteristics data of fractured formations and referring to the dynamic model of drilling fluid loss, including three loss models of induced fracture type, extension fracture and natural fracture type, the drilling fluid loss rate-time characteristic curve of the loss model is generated as the characteristic map. The data of drilling fluid loss rate at the initial stage of lost circulation in the well to be determined were recorded, and the curve of drilling fluid loss rate-time relationship was drawn. The field drilling fluid loss rate-time relationship curve was compared with the characteristic plates of different loss types to determine the type of lost circulation in fractured formations. The method is as follows:

(1) The field engineering geological characteristics data of fractured formation are collected, and the corresponding drilling fluid loss dy- namics model is established. The dynamic model of drilling fluid loss includes induced fracture type, extension fracture type and natural fracture type.

(2) Solve the above dynamic model of drilling fluid loss, and obtain the drilling fluid loss rate of the three loss models at different times. The drilling fluid loss rate-time characteristic curves of different loss models are made as the characteristic plate (Fig. 9).

(3) The drilling fluid loss rate data in the early stage of lost circulation in the well to be determined are recorded, and the drilling fluid loss rate-time relationship curve is drawn.

(4) By comparing the drilling fluid loss rate-time relationship curve of the field well with the characteristic plate of different loss types, the drilling fluid loss type of the target well is diagnosed.

3.2. Evaluation indexes of loss control effect

The control effect of drilling fluid loss is a comprehensive reflection of the strength, efficiency and compactness of the fracture plugging zone. The control effect of drilling fluid loss is characterized by the Fig. 6. Cause deterministic model of natural fracture loss.

Fig. 7. (a) Schematic diagram of lost circulation [27] (b) Flow diagram of lost circulation. Pz—Plugging zone strength；Pp+f—Pore pressure and hydraulic friction.

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maximum plugging pressure, initial and cumulative loss volumes in laboratory experiments (Fig. 10).

(1) Maximum plugging pressure.

The strength of the maximum plugging pressure is a comprehensive reflection of the strength and structural stability of the fracture plugging zone. The strength of the fracture plugging zone is characterized by measuring the maximum plugging pressure. The maximum plugging pressure refers to the difference between the wellbore liquid column pressure and the formation pressure when the fracture plugging zone is destroyed. The greater the maximum plugging pressure, the stronger the ability of the fracture plugging zone to resist external forces, thus the more stable the structure.

(2) Initial loss volume.

The volume of the initial loss reflects the speed of the formation of the fracture plugging zone, i.e., the plugging effect. The formation speed

of fracture plugging zone is characterized by measuring the initial loss of plugging formula. The initial loss refers to the loss of drilling fluid after the plugging material enters the fracture before the formation of fracture plugging zone, which is characterized by the loss within 1 min before the formation of the plugging zone. The less the initial loss, the shorter the time for the plugging material to bridge and form a fracture plugging zone.

(3) Cumulative loss volume.

The cumulative loss volume is a comprehensive reflection of the structural compactness of the fracture plugging zone. The denser the structure of fracture plugging zone, the less the loss of drilling fluid. The compactness of the fracture plugging zone structure is characterized by the cumulative loss volume of the plugging formula. Cumulative loss volume refers to the volume of drilling fluid loss from the plugging material entering the fracture to the failure of the fracture plugging zone. The less the volume, the denser the fracture plugging zone structure.

3.3. Weight calculation of evaluation index 3.3.1. Hierarchy establishment

Combined with the characterization system of drilling fluid loss control effect in Fig. 10, the hierarchical structure of loss control effect is established as follows (Fig. 11):

3.3.2. Judgment matrix establishment

Based on the laboratory experimental results, field tests data and the experience from research experts and engineers, the relative importance of the evaluation indexes is determined according to the scales 1 ~ 9 shown in Table 1. The judgment matrixes of relative importance with respect to the C layer is constructed as shown in Tables 2 ~ 4 [30].

3.3.3. Consistency check

In order to ensure the validity of the judgment matrix, it is necessary to test the consistency of the evaluation results of the judgment matrix.

Fig. 8. Flow diagram of the proposed fracture plugging effect evaluation method.

0 5 10 15

0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014

Natural fracture loss

Loss rate(m3/min)

Time(min)

Extension fracture loss Induced fracture loss

0 5 10

0.0000 0.0005 0.0010

Fig. 9. Characteristic plate of loss rate-time curve.

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The consistency of the judgment matrix is tested by the ratio CR of random consistency. When the test result of the consistency of the judgment matrix is CR < 0.1, the constructed judgment matrix is considered to be effective. Otherwise, the judgment value of the relative importance of the two elements should be adjusted until the consistency condition is satisfied. The calculation formula of CR is given by CR=CI

RI (1)

CI=λmax− n

n− 1 (2)

where RI value is determined according to Table 5, CI is coefficient of concordance which is related to the order n and the maximum eigenvalue of the matrix, λ_maxis calculated by Eq. (6), n is the order of judgment matrix [31].

The consistency test results of the judgment matrix show that the evaluation system of drilling fluid loss control effect meets the consistency standard (Table 6).

3.3.4. Relative weight calculation

The weights of the evaluation indexes, including the comprehensive embodiment of strength, efficiency and compactness of fracture plugging zone, are calculated based on the square root method [29–31].

After normalization of each column element of the judgment matrix, the general term of its element is given by

bij=∑b_nij i=1bij

(i,j=1,2,…,n) (3)

Fig. 10.Characterization system of drilling fluid loss control effect.

Fig. 11.Hierarchy structure for lost circulation control effect.

Table 1

Judgement standard of relative importance.

Scale Meaning

1 Element i and element j are equally important 3 Element i is slightly more important than element j 5 Element i is more important than element j 7 Element i is obviously more important than element j 9 Element i extremely important than element j 2、4、6、

8 Take the median when the relative importance is between the adjacent importance

Table 2

Induced fracture loss control effect judgment matrix.

B1 Maximum

plugging pressure Initial loss volume Cumulative loss volume

Maximum plugging pressure 1 1/2 3

Initial loss volume 2 1 5

Cumulative loss volume 1/3 1/5 1

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For the normalized judgment matrix of each column, adding them as rows gives

Wi=∑ⁿ

j=1

bij(j=1,2,…,n) (4)

By normalizing the vector W = [W1,W2, ...,Wi](i =1, 2, …, n), the obtained vector is the eigenvector. The element in the eigenvector W is the weight of the corresponding parameter.

W= W

∑_n

i=1Wi

(i=1,2,…,n) (5)

The maximum eigenvalue is calculated from the judgment matrix B and eigenvector W.

λmax=∑ⁿ

i=1

(BW)_i

nWi (6)

Through the above steps, the weight proportion of the main controlling factors of drilling fluid loss control effect of natural fracture, induced fracture and extension fracture can be obtained. It is convenient for subsequent experimental results analysis and calculation (Table 7).

3.4. Fitness degree analysis between laboratory experiment and field test An example of loss control in a well is selected to clarify the loss circulation control results of plugging formula. In the laboratory, the fracture plugging simulation experiment was carried out by different

evaluation methods using the plugging formula used in the field, including different fracture module parameters (fracture module height, fracture module dip/length and fracture surface roughness) and different experimental steps (pressurized method, single pressure increment and pressure stabilization time). The laboratory and field loss control results were compared, including the maximum plugging pressure, initial and cumulative loss volumes. Combined with the characteristics of different main controlling factors of drilling fluid loss control effect of different loss types, the fitness of laboratory and field drilling fluid loss control effect was compared and analyzed (Table 8).

3.5. Optimization of experimental parameters

According to the fitness analysis in section 3.4, the experimental results of the evaluation of fracture module parameters are analyzed. It includes the analysis of the experimental results of the influence of fracture height, fracture dip angle/length and fracture surface roughness on the control effect of drilling fluid loss. The analysis of the experimental steps on the effect of drilling fluid loss control includes pressurized method, single pressure increment and pressure stabilization time. Compared with the loss control results of well A in K block, the fracture module parameters and the experimental steps with the highest fit are selected to establish the experimental evaluation method of lost circulation control effect of different loss types.

3.5.1. Experimental parameters for the evaluation of induced-fracture-type loss

Through this method, the best fracture module parameters and the best experimental steps can be selected to evaluate the drilling fluid loss control effect of induced fracture (Table 9).

3.5.2. Experimental parameters for the evaluation of extension-fracture- type loss

Through this method, the best fracture module parameters and the best experimental steps can be selected to evaluate the drilling fluid loss control effect of extension fracture (Table 10).

3.5.3. Experimental parameters for the evaluation of natural-fracture-type loss

Through this method, the best fracture module parameters and the best experimental steps can be selected to evaluate the drilling fluid loss control effect of natural fracture (Table 11).

Table 5

Relationship between RI value and judgment matrix n.

n 1 2 3 4 5 6 7 8 9

RI 0 0 0.58 0.94 1.12 1.24 1.32 1.41 1.45

Table 6

Consistency test results of judgment matrix.

Consistency Induced fracture Extension fracture Natural fracture

λ_max 3.0026 3.0649 3.0999

CI 0.0013 0.0324 0.0500

RI 0.5800 0.5800 0.5800

CR 0.0025 0.0324 0.0961

Table 7

Weight proportion of main control factors of lost circulation control effect of different loss types.

Index Maximum plugging pressure Initial loss volume Cumulative loss volume

Relative weight Induced fracture loss 0.29 0.62 0.09

Extension fracture loss 0.73 0.19 0.08

Natural fracture loss 0.75 0.08 0.17

Table 3

Extension fracture loss control effect judgment matrix.

B2 Maximum plugging pressure Initial loss volume Cumulative loss volume

Maximum plugging pressure 1 5 7

Initial loss volume 1/5 1 3

Cumulative loss volume 1/7 1/3 1

Table 4

Natural fracture loss control effect judgment matrix.

B3 Maximum plugging pressure Initial loss volume Cumulative loss volume

Maximum plugging pressure 1 7 6

Initial loss volume 1/7 1 1/3

Cumulative loss volume 1/6 3 1

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3.6. Quantitative scoring criteria and final scoring formula 3.6.1. Evaluation method of loss control effect

From the above process, the weight proportion of the main control factors of the control effect of different loss types is obtained. On this basis, it is necessary to quantitatively characterize the results of lost circulation control. Therefore, it is of great significance to make a comprehensive evaluation and classification of the lost circulation control ability of the plugging formula used in the experiment. The steps are as follows:

(1) The maximum plugging pressure, initial and cumulative loss volumes of lost circulation control are quantified (Table 12).

(2) Combine with the type of drilling fluid loss and the weight proportion of the main controlling factors of lost circulation control effect (Table 13).

(3) The comprehensive evaluation of the lost circulation control ability of the plugging formula is carried out (Table 14).

Among them, X, Y and Z represent the specific scores of the maximum plugging pressure, initial and cumulative loss volumes in the lost circulation control results, respectively. The score is obtained by

Table 9

Experimental evaluation method for loss control effect of induced fracture.

Drilling fluid loss control effect Evaluation index Optimized experimental parameters

Parameters of fracture module Fracture height When the fracture height: fracture inlet width =6: 1, the laboratory and field drilling fluid loss control effect fit well.

Fracture dip/length When the fracture dip angle is 0.5^◦, the laboratory and field drilling fluid loss control effect fit well.

Fracture roughness The rougher the fracture surface, the higher the fit of laboratory and field drilling fluid loss control effect.

Experimental steps Pressurized method The pressurization method has no significant effect on the evaluation of drilling fluid loss control effect.

Single pressure increment When the single pressure increment is 5 MPa, the laboratory and field drilling fluid loss control effect fit well.

Pressure stabilization time When the pressure stabilization time is 2 min, the laboratory and field drilling fluid loss control effect fit well.

Table 10

Experimental evaluation method for loss control effect of extension fracture.

Drilling fluid loss control

effect Evaluation index Optimized experimental parameters

Parameters of fracture module Fracture height When the fracture height: fracture inlet width =6: 1, the laboratory and field drilling fluid loss control effect fit well.

Experimental steps Pressurized method The stepped pressurization method has a high degree of fit between the laboratory and field drilling fluid loss control effects.

Single pressure

increment When the single pressure increment is 5 MPa, the laboratory and field drilling fluid loss control effect fit well.

Pressure stabilization

time When the pressure stabilization time is 4 min, the laboratory and field drilling fluid loss control effect fit well.

Table 11

Experimental evaluation method for loss control effect of natural fracture.

Drilling fluid loss control

effect Evaluation index Optimized experimental parameters

Parameters of fracture module Fracture height When the fracture height: fracture inlet width≈3: 1, the laboratory and field drilling fluid loss control effect fit well.

Experimental steps Pressurized method The stepped pressurization method has a high degree of fit between the laboratory and field drilling fluid loss control effects.

Single pressure

increment When the single pressure increment is 2.5 MPa, the laboratory and field drilling fluid loss control effect fit well.

Pressure stabilization

time When the pressure stabilization time is 4 min, the laboratory and field drilling fluid loss control effect fit well.

Table 8

Fitness degree analysis method of laboratory and field drilling fluid loss control effect.

/ Maximum plugging

pressure Initial loss

volume Cumulative loss

volume

Loss control results on site X1 Y1 Z1

Laboratory experimental results X2 Y2 Z2

Deviation between laboratory and field |X2-X1|/X1 |Y2-Y1|/Y1 |Z2-Z1|/Z1

Weight proportion of main controlling factors of lost circulation control

effect Induced fracture loss 0.29 0.62 0.09

Extension fracture

loss 0.73 0.19 0.08

Comprehensive deviation of drilling fluid loss control effect Induced fracture loss 0.29×|X2-X1|/X1 +0.62×|Y2-Y1|/Y1 +0.09×|Z2-Z1|/Z1

Extension fracture

loss 0.73×|X2-X1|/X1 +0.4×|Y2-Y1|/Y1 +0.2×|Z2-Z1|/Z1

Natural fracture loss 0.75×|X2-X1|/X1 +0.08×|Y2-Y1|/Y1 +0.17×|Z2-Z1|/Z1

The fit of laboratory and field drilling fluid loss control effect Induced fracture loss 1-(0.29×|X2-X1|/X1 +0.62×|Y2-Y1|/Y1 +0.09×|Z2-Z1|/Z1) Extension fracture

loss 1-(0.73×|X2-X1|/X1 +0.4×|Y2-Y1|/Y1 +0.2×|Z2-Z1|/Z1) Natural fracture loss 1-(0.75×|X2-X1|/X1 +0.08×|Y2-Y1|/Y1 +0.17×|Z2-Z1|/Z1)

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combining the specific values of the three indexes with Table 13.

(4) The lost circulation control ability of plugging formula is classified (Table 15).

3.6.2. Evaluation method of drilling fluid loss control effect considering multiple loss types

In practical scenarios, it is observed that field conditions often involve multiple types of drilling fluid loss, sometimes encompassing two or even three different types simultaneously. It is necessary to determine the lost circulation type and analyze the proportion of different lost circulation to identify the main lost circulation type and the secondary lost circulation type. The procedure is as follows:

(1) First of all, the lost circulation types are determined, and the main lost circulation types and secondary lost circulation types are identified.

The weight proportion of different lost circulation types is analyzed by analytic hierarchy process.

The evaluation index weight ratio analysis method is presented in section 3.3. From the analysis of the weight proportions of different lost circulation types, it is determined that the proportions of induced fracture loss, extension fracture loss and natural fracture loss are M, N and O, respectively (if there are three lost circulation types at the same time). If there are two types of lost circulation at the same time, the proportions of determined weights are M and N, respectively (M, N, O ∈[0,1]).

(2) The experimental evaluation method of lost circulation control effect for different loss types is evaluated.

After determining the main loss type, the experimental evaluation method corresponding to the main loss type is selected to evaluate the lost circulation control effect. If the main loss type is induced fracture loss, the lost circulation control effect for induced fracture loss is evaluated. The rest of the situation is the same.

(3) The lost circulation control ability of plugging formula is comprehensively evaluated and the classification results are given.

According to step (1), among the loss types of fractured formations, the proportions of induced fracture loss, fracture extension loss and natural fracture loss are M, N and O, respectively (if there are three loss types at the same time). According to Table 13, the comprehensive score

of plugging formula loss control ability R =RM +RN +RO can be obtained, where:

RM=M× (0.29X+0.62Y+0.09Z) (7) RN=N× (0.73X+0.19Y+0.08Z) (8) RO=O× (0.75X+0.08Y+0.17Z) (9)

where, X, Y and Z are obtained by combining the specific values of the three indexes with Table 11.

Finally, the R value is obtained. According to Table 14, we can determine the classification results of the loss control ability of the plugging formula.

4. Results and discussion

Take the sandstone reservoir of Bashijiqike Formation in K block of Tarim Basin as the field study case. Well D is an evaluation well located in K block of Tarim Basin. The Bashijiqike Formation in the reservoir section of the well is a salt-gypsum strata. The interlayer lithology is mudstone and dolomite, and interlayer fractures are developed. The subsalt strata are mudstone and glutenite, and micro-fractures are developed. When drilling to the depth of 5694 ~ 5819 m, the loss occurred. Lost circulation control was carried out by the plugging formula: oil-based drilling fluid (1.88) + 4 % flaky material (medium coarse) + 4 % flaky material (type II, medium coarse) + 2 % flaky material (type I, coarse) +4 % spheroidal material +4 % spheroidal material (fine) + 8 % spheroidal material (medium coarse) + 6 % spheroidal material (coarse) +0.8 % fiber material (1/2

″

). The plugging result was not satisfactory.

Table 12

Quantitative standards of maximum plugging pressure, initial and cumulative loss volume.

Score 1 2 3 4 5 6 7 8 9 10

Maximum plugging pressure (MPa) <3.5 3.5 ~ 5 5 ~ 6 6 ~ 7 7 ~ 9 9 ~ 11 11 ~ 13 13 ~ 15 15 ~ 20 ≥20

Initial loss volume(mL) ≥100 80 ~ 100 70 ~ 80 60 ~ 70 50 ~ 60 40 ~ 50 30 ~ 40 20 ~ 30 10 ~ 20 ＜10

Cumulative loss volume(mL) ≥250 200 ~ 250 170 ~ 200 150 ~ 170 120 ~ 150 100 ~ 120 70 ~ 100 50 ~ 70 30 ~ 50 ＜30

Table 13

Weight proportion of main control factors of lost circulation control effect of different loss types.

Index Maximum plugging pressure Initial loss volume Cumulative loss volume

Relative weight Induced fracture loss 0.29 0.62 0.09

Extension fracture loss 0.73 0.19 0.08

Table 14

Evaluation table of lost circulation control ability of plugging formulations with different loss types.

Comprehensive score R Induced fracture loss R =0.29X +0.62Y +0.09Z Fracture extension loss R =0.73X +0.19Y +0.08Z Natural fracture loss R =0.75X +0.08Y +0.17Z

Table 15

Lost circulation control ability grading system of plugging formula.

Comprehensive score R ＞9.5 8 ~ 9.5 6.5 ~ 8 5 ~ 6.5 ＜5 Classification results Excellent Good Average Fair Poor

0 7 14 21 28 35

0 1 2 3 4

5 Displacement pressure Cumulative loss volume

Time min

Displacement pressure(MPa)

Maximum plugging pressure 4.6MPa Cumulative loss volume 77mL

0 20 40 60 80

Cumulative loss volume(mL)

Fig. 12.Experimental results of conventional laboratory evaluation methods.

(12)

Due to the absence of directly applicable indicators and precise values suitable for laboratory comparisons, such as the initial loss and cumulative loss, an innovative approach is devised. This approach combines on-site loss data with the time elapsed and the actual fracture width, reaching the derivation of an on-site loss equivalent. This con- version is mathematically formalized by the following formula.

ΔS=S1/S1 (10)

Δt=t1/t2 (11)

Vie=Vi/(ΔS×Δt) (12)

Vac=Vt/(ΔS×Δt) (13)

where S1 is the outlet area of the loss channel on site (cm²), S2 is the outlet area of laboratory loss channel (cm²), t₁is the corresponding time of on-site loss (min), t2 is the corresponding time of laboratory loss (min), Vi is the loss volume within 1 min before on-site lost circulation control (mL), V_tis the total loss volume on site (mL), V_ieis the initial loss equivalent on site (mL), and Vac is the accumulated loss equivalent on site (mL).

To comprehensively assess both the conventional evaluation approach and the novel method proposed within this paper, an analyt- ical framework is established. Building upon the previously presented formulas, the field loss incurred by drilling fluid is systematically transformed into its loss equivalent. Subsequently, a comparative analysis is conducted between the laboratory-based losses and the loss equivalents, thereby facilitating an in-depth evaluation of the loss control effect of drilling fluid.

The analysis of field data shows that a predominant factor contributing to the observed loss is that there are many weak points in the target

strata. Micro-fractures and fractures are developed, and the load-bearing capacity of the formation is poor. The results of drilling fluid loss determination indicate the prevalence of natural fracture-related losses within the formation. The analysis of the early plugging situation reveals the extensive development of fractures within the target strata, yielding an excess of channels for loss. Notably, during pressure plugging, the single pressure increment no longer occurs after 5.0 MPa. Upon con- version using Eqs. (10)-(13), the cumulative loss equivalent is around 120 ml.

According to the evaluation idea of drilling fluid loss control effect and the deterministic method of loss type, combined with the previous pressure plugging results, fracture width and plugging formula required for lost circulation control, it is determined that the loss is a natural fractured loss. By analyzing the particle size of the bridging material and the filling material in the plugging formula, it is clear that the particle size distribution range of the bridging material (medium coarse) in the formula is [1.79 mm, 3.12 mm]. The particle size distribution of filling materials (type II, medium coarse and type I, coarse) is [0.11 mm, 1.93 mm]. The wedge plunger with a fracture width of 3 mm-1 mm was selected to carry out laboratory fracture plugging simulation experiments to evaluate the effectiveness of drilling fluid loss control.

(1) Conventional evaluation method.

①Experimental plunger: The fracture has a width of 1–3 mm. Under this width condition, the fracture height of the plunger commonly used in laboratory experiments is generally 18 mm, and the fracture dig/

length is 1.5^◦/50 mm.

②Plugging formula: The formula is used in Well D12 at the same site:

oil-based drilling fluid (1.88) +4 % flaky material (medium coarse) +4

% flaky material (type II, medium coarse) +2 % flaky material (type I, coarse) +4 % spheroidal material +4 % spheroidal material (fine) +8 Table 16

Experimental results of laboratory conventional evaluation methods.

Experimental plunger Maximum plugging pressure (MPa) Initial loss volume

(mL) Cumulative loss volume

(mL) Note

Wedge steel plunger 4.6 28.8 77 Evaluated by conventional evaluation method

0 3 6 9 12

0 1 2 3 4 5 6

Time min

Maximum plugging pressure 5MPa Cumulative loss volume 130mL

0 20 40 60 80 100 120 140

Fig. 13. Experimental results of effectiveness evaluation of drilling fluid loss control in natural fracture.

Table 17

Experimental results of effectiveness evaluation of drilling fluid loss control in natural fracture.

(mL) Note

3D wedge plunger 5.0 30.4 130 Evaluated by the evaluation method proposed in this paper

87.914

98.589

82 84 86 88 90 92 94 96 98 100

Conventional

evaluation methods Method proposed in this paper

Evaluation of fitness degree(%)

Conventional evaluation methods

Method proposed in this paper

Fig. 14.Comparison results of the fit between different evaluation methods and field drilling fluid loss control effect.

(13)

% spheroidal material (medium coarse) + 6 % spheroidal material (coarse) +0.8 % fiber material (1/2

″

).

③Experimental steps: Stepped pressurization is often used as a pressurization method in laboratory evaluation. The single pressure increment is generally 2.5 MPa, and the pressure stabilization time is generally 6 min.

④Experimental results: The experimental results evaluated by conventional evaluation methods are shown in Fig. 12 and Table 16.

(2) experimental evaluation method for natural fracture drilling fluid loss control effect

①Experimental plunger: The width of the fracture is 1–3 mm. Ac- cording to the proposed method, in terms of fracture module parameters, the evaluation results are better when the fracture module height:

fracture inlet width =6:1, so the fracture height should be 18 mm. The fracture dip angle/length should choose a plunger with a dip angle of 1.5^◦/length of 50 mm. The plunger with larger JRC coefficient is selected as far as possible for the roughness of the fracture surface.

Combined with the actual situation of the laboratory, the plunger with JRC coefficient of 20 is selected.

②Plugging formula: The formula is used in Well D12 at the same site:

oil-based drilling fluid (1.88) +4 % flaky material (medium coarse) +4

% flaky material (type II, medium coarse) +2 % flaky material (type I, coarse) +4 % spheroidal material +4 % spheroidal material (fine) +8

% spheroidal material (medium coarse) + 6 % spheroidal material (coarse) +0.8 % fiber material (1/2

″

).

③Experimental steps: According to the experimental evaluation method of drilling fluid loss control effect for natural fracture, stepped pressurization method should be adopted. The experiment was carried

out with a single pressure increment of 2.5 MPa for 4 min.

④Experimental results: The experimental results evaluated by the experimental evaluation method of drilling fluid loss control effect of natural fracture are shown in Fig. 13 and Table 17.

(3) comparison of evaluation results of different evaluation methods Based on the results of field loss control, the experimental results of conventional evaluation methods and the experimental results of drilling fluid loss control effect evaluation methods for natural fracture loss Table 18

Comparison results of the fit between different evaluation methods and field drilling fluid loss control effect.

Maximum plugging

pressure (MPa) Initial loss volume (mL)

Cumulative loss volume (mL)

Note

Results of field loss control 5 / 120 The results of field loss control have no

relevant data of initial loss.

Results of experiments Conventional

evaluation methods 4.6 28.8 77

Method proposed in this

paper 5 30.4 130

Deviation between laboratory and field

(%) Conventional

evaluation methods 8 / 35.8

Method proposed in this

paper 0 / 8.3

Weight proportion of main controlling factors of control effect of

natural fracture loss 0.75 0.08 0.17

Comprehensive deviation of drilling fluid

loss control effect (%) Conventional

evaluation methods 8 ×0.75 +35.8 ×0.17 =12.086 Method proposed in this

paper 8.3 ×0.17 =1.411

Fit of laboratory and field drilling fluid loss control effect（%）

Conventional

evaluation methods 87.914 Method proposed in this

paper 98.589

Table 19

Experimental results of laboratory conventional evaluation methods.

(mL) Note

Wedge steel plunger 4.6 28.8 77 Evaluated by conventional evaluation methods.

Table 20

Experimental results of the evaluation method proposed in this paper.

(mL) Note

3D wedge plunger 5.0 30.4 130 Evaluated by the evaluation method proposed in this paper

0 10 20 30 40 50 60

0 5 10 15 20

Time min

Maximum plugging pressure 20.8MPa Cumulative loss volume 8.8mL

0 2 4 6 8 10

Fig. 15.Experimental results of optimized plugging formula.