EXPERIMENTAL ANALYSIS OF OIL DISPLACEMENT BY HYBRID ENGINEERED WATER / CHEMICAL EOR APPROACH IN CARBONATES

The best combination in terms of oil recovery and chemical consumption was the hybrid EWASP flood in slug-wise injection mode. Reproduced with permission from Al Sofi et al., J. Studies showing reduction in required polymer adsorption of LSW.

INTRODUCTION

Background

The idea behind the hybrid EWPF is to increase oil recovery by combining oil displacement mechanisms with optimized injection water composition and polymer flooding. The result of the hybrid method is an increased yield of oil, greater than that obtained by independent methods.

Polymer Flooding

Recovery Mechanism
Viscoelastic Polymers
Challenges Associated with Polymer Flooding
Field Examples

54] performed coreflood experiments on Bentheimer sandstone cores and observed a 5% reduction in Sor by viscoelastic HPAM polymer flooding compared to viscous glycerin flooding at a constant pressure drop. The polymer floods in the Daqing and Shengli oil fields together contributed approximately 250,000 barrels per day in 2004 and added 6-12% to oil recovery.

Figure 1. (a) Chemical structure [38] (reproduced with permission from Liu et al. J. Pet

Surfactant Flooding (SF)

Recovery Mechanisms
Challenges in Surfactant Flooding

On the other hand, non-ionic surfactants have no charge and cannot reduce IFT, so they are mainly used as co-surfactants in surfactant-wetting EOR applications [80]. Therefore, the main goals of surfactant design are to achieve the lowest possible IFT with low surfactant concentration and minimal adsorption on the rock.

Figure 4. Schematic of processes involved during surfactant flooding [81].

Alkaline Flooding (AF)

Recovery Mechanism
Challenges Associated with Alkaline Flooding

Engineered Water Flooding (EWF)

Conditions for Engineered Water Flooding
Limitations of EWF
Lab-based Studies

The interaction between the injected EW and the crude oil is also key to observing the gradual recovery of oil by EWF. Field tests at the Pervomaiskoye field showed a three-fold reduction in relative water permeability and a corresponding 3.5% additional oil recovery with LSWF.

Table 1. Summary of corefloods showing incremental oil recovery by EWF in carbonates.

Hybrid Engineered Water-Polymer Flooding (EWPF)

Enhanced Oil Recovery by Hybrid EWPF in Carbonates
Potential Risks associated with EWPF
Lab-Scale Studies
Numerical Modeling Studies

Incremental oil recovery through synergy of smart water (SW) and polymer in carbonates was also confirmed by Al Sofi et al. Numerical model results show oil recovery as a function of (a) the mobility ratio for different flooding scenarios and (b) the salinity of the brine used for PF [156].

Figure 9. (a) Reduction in Sor after LSPF [5] (reproduced with permission from Lee et al., J

Hybrid Engineered Water-Surfactant-Polymer Flooding (EWSPF)

Limited use in carbonates mainly due to the high salinity formation water associated with carbonates [25-27]. Formation water This method can be applied to reservoirs containing high salinity and high hardness formation water in the ppm range [15, 157]. 202] on sandstone cores, using a high AN crude oil (2.84 mg KOH/g oil), showed about 20% of OOIP incremental recovery using anionic surfactant under low salinity conditions compared to high salinity conditions.

These results also indicate that oils with higher AN are favorable for low salinity and surfactant.

Hybrid Engineered Water-Alkali-Surfactant-Polymer (EWASP) Flooding

Despite the encouraging results of several laboratory studies, pilot and field implementation of ASP flooding remains mainly limited to sandstone reservoirs and there is hardly any large-scale application in carbonates due to their geological complexity and harsh reservoir conditions [207]. Combining conventional ASP flooding with chemically tuned EW can provide a significant improvement in terms of chemical stability in a low salinity environment. This hybrid method can expand the application range of ASP flooding to more challenging carbonate reservoirs.

The validated mechanistic model was then used to simulate ASP inundation in the field and evaluate the performance of EWASP hybrid inundation. Secondary flooding of LSW followed by tertiary flooding with ASP resulted in a 10% higher oil recovery compared to the case where HSW was injected in the secondary mode prior to flooding with ASP (Figure 26).

Problem Statement

Therefore, this hybrid method can show superior performance in the field, but needs extensive analysis to optimize the process design and maximize the project's net present value (NPV). Therefore, extensive experimental and research work is required to arrive at a successful design for a particular CBR system in order to make the most of the synergy of hybrid chemical EOR methods. Most research on hybrid EWPF and EWASP is performed for sandstones due to the heterogeneity problems and the complexity of rock-fluid interactions in carbonates.

This research study will address the above issues by conducting a systematic experimental analysis and evaluating critical design parameters to come up with the best design from different combinations of the EW and chemicals. Typical Kazakhstan reservoir conditions (Caspian sea water, Tengiz field formation water and crude oil from West Kazakhstan region) will be used as reference since the majority of the oil fields in Kazakhstan produce from carbonate formations.

Research Objectives

Main objectives
Thesis structure

During the first research phase, an optimal EW composition was designed and the most suitable polymer and surfactant were screened for the hybrid EWPF and EWSF [183, 197]. Analyze the effect of pH on the viscosity and viscoelasticity of polymer and design an optimal pH for the hybrid EOR process to obtain the maximum benefit in terms of both viscous and elastic properties of polymers. Study the application of hybrid EWPF and EWASP flooding to increase oil recovery from carbonate reservoirs by conducting oil displacement experiments.

Conducting detailed experimentation and designing optimal operational parameters and injection scheme to enhance oil recovery by hybrid methods. The existing gaps in the literature are identified to define the problems and finally the objectives of the thesis are defined.

METHODOLOGY

Materials

Crude Oil
Rock Samples
Brines
Chemicals

Three different brines were used in this study as formation water (FW), high salinity water (HSW) and engineered water (EW). Another reason for choosing this EW design is its compatibility with other EOR chemicals to be used in this study. Three chemicals are used in this study to investigate the potential synergy between different combinations of chemical EOR methods and engineered water.

The benzenesulfonic acid anionic surfactant Soloterra-113H was used to study the potential synergy between surfactant, polymer, and EW. Sodium carbonate (Na2CO3) supplied by SIGMA-ALDRICH was used as alkali to study possible combinations of chemicals and EW.

Procedures

Brines and Chemical Solutions Preparation
Crude Oil Properties
Rock Properties
Polymer Shear and Viscoelastic Characterization
Contact Angle Measurements
Design of Oil Displacement Experiments
Coreflooding

RESULTS

Shear Characterization

Effect of pH on Viscosity
Effect of Polymer Concentration and pH on Viscosity
Effect of pH on Thermal Stability

From the graph it can be seen that as the pH of the solution moved towards the acidic side, the viscosity decreased significantly (34% of the viscosity of the neutral pH solution). For solutions in the pH range 8–10, the thermal degradation in viscosity was the lowest, and the viscosity for these solutions dropped by only 4–5 cp during the one-week aging time. Viscosity loss under acidic conditions was maximal (~.60% higher than solutions in the basic range).

The solutions in the basic pH range showed the lowest viscosity loss (only 18% after 1 week) and maximum thermal stability due to higher repulsion among the molecules. Therefore, the viscosity of pH 12 solution started to decrease on the third day of aging at 80 oC.

Figure 37. Effect of polymer concentration and pH on F5115 viscosity at 25 o C.

Viscoelastic Characterization

Analysis of Amplitude Sweep Test
Analysis of Frequency Sweep Test

The elastic or storage modulus can be used to quantify the elastic character of the polymer. To calculate the relaxation time, the reciprocal of the angular frequency corresponding to the point of intersection of G' and G" is taken, as given in equation 6. Another useful information obtained from FST was the loss factor (tan) of which is a ratio of the loss and storage modulus of viscoelastic materials (Equation 9).

The loss factor for the solution in the acidic range remained above unity, indicating a lower elastic behavior. For the solutions in the base medium the loss factor was smaller than over a wider frequency range, indicating a dominance of the elastic.

Figure 41. pH Effect on storage modulus for 1500, 3000 and 4500 ppm neutral pH solutions at 25 o C

Results of Contact Angle Measurements

Effect of Aging Time on Initial Wettability
Effect of Initial Wettability on EW Performance
Effect of Temperature on Wettability Alteration by EW
Effect of Polymer on EW Performance

As the aging time increased, the EW-induced CA change also increased due to the higher adsorption of acidic crude oil components on the calcite surface after one month of aging. The change in contact angle with EW was greater for 2-week-old and 1-month-old pellets. At this stage, the effect of temperature on the extent of EW wettability modification was also evaluated by keeping one pellet from each set at 80 oC.

The results, as presented in Figure 50, show that F5115 had no adverse effect on EW performance because the change in CA with EW and EWP was almost the same for both pellets. It can be said that this CA change was caused by EW, not by the polymer.

Figure 47. Effect of aging time on wettability change by EW and HSW.

Polymer Concentration Determination for Coreflood Experiments

Results of Coreflood Experiments

Hybrid EW/Polymer Flooding in Water-wet System (Experiment-1)
Hybrid EW/Polymer Flooding in Intermediate Oil-wet System (Experiment-2) . 89
Hybrid EW-Surfactant Polymer Flooding (Experiment-4)
Hybrid EW-ASP Flooding - Continuous Injection (Experiment-5)
Hybrid EW-ASP Flooding - Slug Injection (Experiment-6)

The pressure drop and oil recovery data for the entire experiment are presented in Figure 53. Oil recovery and pressure drop profile for hybrid EWPF experiment in a weak oil-wet system. Oil recovery and pressure drop profile for hybrid EWPF test in an intermediate oil-wet system.

A significant increase in oil recovery of 8.3% of the OOIP was obtained by EWF after injecting 23 PVs, and the Sor was also reduced by 8% at the end of this phase. A significant oil recovery increase of 12% OOIP was achieved and the Sor at the end of the AS flood was only 19%.

Figure 53. Oil recovery and pressure drop profile for hybrid EWPF experiment in a weak oil-wet system

Effect of Initial Wettability on EWF and Hybrid EWPF Performance

The synergistic behavior of EW and PF is also pronounced under these conditions, especially in the intermediate oil-wet case. For this reason, higher incremental recovery is obtained by EWPF in the intermediate oil-wet scenario. It can be said that EW performed the best in strong oil-wet conditions, while the hybrid EWPF scenario showed the best results in the intermediate oil-wet condition.

Therefore, the optimal wettability condition for hybrid EOR methods using EW is an intermediate to heavily oil-wet condition. As a result, relatively lower residual oil saturation was available for the EWPF to move out of the core compared to the oil-soaked intermediate state.

Table 17. Summary of oil recoveries by hybrid EWPF under different initial wettability conditions

Comparative Analysis of Oil Recovery by Different Hybrid Methods

Effect of Hybrid EW/CEOR Methods on S or : Capillary Desaturation
Recovery Efficiency of Different Hybrid Processes
Calculation of Mobility Ratio for Hybrid Methods
Oil Breakthrough during EWF

CDC comparison results also show that EWASP hybrid flooding in slow injection mode resulted in the lowest Sor for similar Nc values. EWASP hybrid flooding in staged injection resulted in the highest E value and consequently the highest incremental oil and lowest Sor among all methods. As expected, the EWASP hybrid technique in stepwise injection mode provided the highest displacement efficiency, as can be seen in Figure 66 .

The critical relationship between mobility ratio and recovery of residual oil is represented graphically in Figure 67. It can be clearly seen that lower mobility ratio resulted in higher recovery of residual oil.

Figure 63. Oil recovery comparison for different hybrid EW/CEOR methods.

CONCLUSION AND RECOMMENDATIONS

Moosavi, Investigating the effects of low salinity flooding for enhanced oil recovery in carbonate reservoir cores. A laboratory study of the ionic effect of smart water for enhancing oil recovery in carbonate reservoirs. Fatemi, Experimental study of the influence of liquid-liquid interactions on oil recovery during low salinity water flooding.

Mechanistic modeling of the benefit of combining polymer with low salinity water for enhanced oil recovery. Numerical simulation of enhanced heavy oil recovery by low-saline water injection and polymer flooding. Austad, Wettability change and enhanced oil recovery in chalk: The effect of calcium in the presence of sulfate.

Design parameter sensitivity analysis of enhanced oil recovery by low salinity polymer flooding. Abrams, A., Effect of fluid viscosity, interfacial tension, and flow velocity on residual oil saturation left by flooding.