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Experimental methods and procedure

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3. Methodology

3.2. Experimental methods and procedure

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Sav 10 is the lightest among them. Polymers are provided in the form of powder. Figure 40 shows polymer sample in beakers.

Figure 40 - Polymer samples

53 3.2.2. Bulk scale rheological investigation

The first step in screening of suitable candidate was to investigate the rheology of candidates. The effects of temperature and concentration on the rheology provides an initial understanding to the properties of polymers. It is crucial to know the behavior of polymer’s rheology at different temperatures as it might vary in different regions of the reservoir. The highest viscosity value at the same polymer concentration among three Superpushers was measured as well. This value of viscosity should be demonstrated by polymer with the highest molecular weight. The salinity level of Uzen formation water for polymer solutions preparation was used in all cases. Thus, this investigation was mainly focused on the effects of temperature, concentration, and shear rate.

Figure 41 represents the Modular Compact Rheometer (MCR), which was used for rheological investigations at the bulk scale. Two measuring systems were used to measure the viscosity of polymers as a function of shear rate at different stages of laboratory experiments.

Preliminary analysis was required to study the effect of temperature and concentration variation on polymers performance and to design further experiments properly. The effect of temperature and concentration variation was investigated using a cylindrical measuring system (see Figure 43).

The temperature was gradually increased from 25 CΒ° to 80 CΒ°, while concentrations of polymers varied from 1500 to 3000 ppm.

The evaluation of polymer viscosity at room temperature was conducted using a cone plate measuring system (see Figure 42). The volume of sample required for the system is about 1.5 ml, which is suitable for tests with low volume of samples such as long-term thermal stability, mechanical stability experiments and effluents viscosity determination.

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Figure 41 - Anton Paar MCR 301 Rheometer

Figure 42 - Cone plate Measuring System for the MCR

Figure 43 - Cylindrical Measuring System for the MCR

3.2.3. Long-term thermal stability tests

The long-term thermal stability test was designed to evaluate the ability of polymers solutions to maintain the viscosity under high temperature conditions. 2500 ppm of each polymer was poured into 25 test tubes and placed into the oven with the temperature of 80 C˚ and oxygen-free environment. Each test tube was labeled with the date of experiment. The duration of the experiment was 60 days. After certain amount of time test tubes were moved out of the oven for the viscosity measurement.

3.2.4. Mechanical degradation

Mechanical stability of the chosen polymer solutions was evaluated using Hamilton Beach Single Spindle Drink Mixer HMD 200 (Figure 44). 3 polymer solutions with 2500 ppm polymer concentration were exposed to high shear rate of 400 s-1 for different time intervals. Viscosity retention was measured at room temperature and 10.8 1/s shear rate after 0, 1, 2, 3, 4, 5, and 10 minutes of exposure to a high shear rate. Viscosity values and degree of degradation (DR) were calculated as well.

𝐷𝑅 = πœ‡π‘–π‘›π‘–π‘‘π‘–π‘Žπ‘™βˆ’πœ‡π‘Žπ‘“π‘‘π‘’π‘Ÿ 𝑑𝑒𝑠𝑑

πœ‡π‘–π‘›π‘–π‘‘π‘–π‘Žπ‘™ (2)

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Figure 44 - Hamilton Beach Single Spindle Drink Mixer HMD 200

3.2.5. Core preparation

Berea sandstone core were used in the core flooding experiments. 4 core plug samples with diameter of 1.5 inch were cut into length of approximately 3 inches. These samples were placed into the oven at 80 ̊C over night. Core samples were then saturated with formation water of Uzen field under 1000 psi. A core saturator, which is shown on Figure 45 was used for this purpose.

Physical properties measurements were carried out in the core preparation process.

Figure 45 - Core saturator

3.2.6. Injectivity tests

The core-flooding experiments were conducted to estimate injectivity and polymer retention in porous media. The retention of polymer solution in the rock causes permeability reduction (Seright et al., 2009). It can be quantitatively expressed in terms of resistance factor (RF) and residual

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resistance factor (RRF) (Hincapie 2016). RF represents the ability of polymer to control the mobility and defines as:

𝑅𝐹 = π›₯𝑃(π‘π‘œπ‘™π‘¦π‘šπ‘’π‘Ÿ π‘–π‘›π‘—π‘’π‘π‘‘π‘–π‘œπ‘›)

π›₯𝑃(π‘π‘Ÿπ‘–π‘›π‘’ π‘–π‘›π‘—π‘’π‘π‘‘π‘–π‘œπ‘› π‘π‘’π‘“π‘œπ‘Ÿπ‘’ π‘π‘œπ‘™π‘¦π‘šπ‘’π‘Ÿ π‘–π‘›π‘—π‘’π‘π‘‘π‘–π‘œπ‘›) (3) RRF shows the effect of polymer retention and permeability of porous media and defines as:

𝑅𝑅𝐹 = π›₯𝑃(π‘π‘Ÿπ‘–π‘›π‘’ π‘–π‘›π‘—π‘’π‘π‘‘π‘–π‘œπ‘› π‘Žπ‘“π‘‘π‘’π‘Ÿ π‘π‘œπ‘™π‘¦π‘šπ‘’π‘Ÿ π‘–π‘›π‘—π‘’π‘π‘‘π‘–π‘œπ‘›)

π›₯𝑃(π‘π‘Ÿπ‘–π‘›π‘’ π‘–π‘›π‘—π‘’π‘π‘‘π‘–π‘œπ‘› π‘π‘’π‘“π‘œπ‘Ÿπ‘’ π‘π‘œπ‘™π‘¦π‘šπ‘’π‘Ÿ π‘–π‘›π‘—π‘’π‘π‘‘π‘–π‘œπ‘›) (4) RF and RRF values were calculated at steady state flow.

For this experiment, 3 core samples were prepared, and 3 concentrations of each polymer were injected into each core. Table 14 shows viscosities of the chosen polymer solutions at room and reservoirs temperature before the injection. The brine was injected before and after each polymer.

The core flooding system (Figure 46) was used to perform injectivity test at confining pressure of 1000 psi (2000 psi for Sav 10 XV), and temperature of 60 CΒ°. All polymer solutions except Sav 10 XV with 1000 ppm of polymer concertation were injected with 5 different rates of 0.5, 1, 1.5, 2, 5 cc/min.

Figure 46 - Core-flooding apparatus

The effluent samples were collected at each rate for every polymer concentration to estimate polymer degradation after the flooding.

Core absolute brine permeability was calculated by the following formula:

57 πΎπ‘Žπ‘π‘  = 14700βˆ™π‘žβˆ™πœ‡βˆ™πΏ

π΄βˆ™βˆ†π‘ƒ (5)

Table 14 - Initial viscosities of polymer solutions

Polymer solution Viscosity at room temperature and 10.8 s-1, cp

Viscosity at reservoir temperature and 10.8 s-1, cp

Sav 10 2500 ppm 8.15 5.52

Sav 10 1250 ppm 4.61 3.26

Sav 10 625 ppm 2.35 1.63

Sav 19 1800 ppm 7.07 4.57

Sav 19 900 ppm 3.12 1.89

Sav 19 450 ppm 2.21 1.30

Sav 10 XV 1000 ppm 4.96 3.63

Sav 10 XV 500 ppm 2.63 1.93

Sav 10 XV 250 ppm 1.31 0.94

3.2.7. Oil Displacement test

After all previous laboratory investigations were completed, the potential polymer for application on Uzen field was selected for oil displacement test. Determination of displacement efficiency in terms of recovery factor is mandatory stage of polymer screening. With that in mind, core-scale polymer injection into the sandstone rock was designed and implemented. Figure 47 represents the sequence of the tests.

Figure 47 - Oil displacement test sequence

Preflush brine

injection Oil injection Recovery with brine solution

Recovery with polymer soltion

Postflush brine injection

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The experiment was conducted at confining pressure of 1000 psi, and reservoir temperature of 60 CΒ°. Brine and polymer solutions for oil recovery were injected at rates of 0.5, 1, 2, and 5 cc/min.

The procedure of in-situ and effluents viscosity calculations was the same as in subsection 1.2.6.

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