Study on damage mechanism and protection countermeasures of coalbed methane reservoir

Huang Weian 1 Qiu 1 2 Ma Yongle 1 Zhong Hanyi 1 Bai 1

(1. School of Petroleum Engineering, Youshi University, China, Qingdao 266555; 2. Engineering Technology Design and Research Institute of East China Petroleum Bureau, Nanjing, Jiangsu 2 1003 1)

Fund Project: National Natural Science Foundation of China "Study on Mechanism and Method of Shale Gas Reservoir Protection" (No.:41072094); China Youshi University independently innovated "Study on CBM Reservoir Protection Technology and New Evaluation Method" (No.:10X040 10a).

About the author: Huang Weian, male, associate professor, engaged in the field of chemical engineering of oil and gas wells. E-mail: master Huang1997 @163.com.

Doing a good job of coalbed methane reservoir protection is of great significance to effectively develop and utilize coalbed methane, make up for the shortage of oil and gas supply in China, reduce greenhouse gas emissions, reduce environmental pollution and prevent frequent mine accidents. By means of X-ray diffraction, scanning electron microscope, shale swelling experiment, shale dispersion experiment, thin section analysis, mercury intrusion analysis, wettability test and sensitivity evaluation, the damage mechanism of coalbed methane reservoir in Qinshui Basin, Shanxi Province is analyzed. On this basis, the targeted protection countermeasures were studied, and the surface wettability improver SD -905 and water-sensitive inhibitor SMYZ-2 were optimized. Finally, the drilling fluid for coalbed methane reservoir in Qinshui Basin, Shanxi Province was developed: 0.4% SD-905+0.5% SMYZ-2 solution, which has the lowest damage to the permeability of wet coalbed methane reservoir rock samples.

Keywords: coalbed methane; Damage mechanism; Reservoir protection; Surface wettability; drilling mud

Study on damage mechanism and protection countermeasures of coalbed methane

Huang Weian 1, Qiu Zhengsong 1, Wang Yanqi 2, Ma Yongle 1, Zhong Hanyi 1, Bai Xuefei 1

(1. School of Petroleum Engineering, China Shiyou University, Qingdao 266555; 2. Engineering Technology Design and Research Institute of East China Petroleum Bureau, Nanjing 2 1003 1)

Abstract: Protecting coalbed methane reservoir can promote the development and utilization of coalbed methane, make up for the shortage of oil and natural gas supply in China, reduce greenhouse gas emissions, reduce environmental pollution and prevent mine accidents. Firstly, the damage mechanism of coalbed methane reservoir in Qinshui Basin was comprehensively analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), lining expansion test and hot rolling dispersion test, thin section analysis, mercury intrusion analysis, wettability measurement and sensitivity evaluation. On this basis, the targeted protection countermeasures were studied, and the surface wettability modifier SD-905 and water-sensitive inhibitor SMYZ-2 were screened out, and the drilling fluid for coalbed methane reservoir in Qinshui Basin, Shanxi Province was developed with 0.4% SD-905 +0.5% SMYZ-2 which had the least damage to component permeability in wet state.

Keywords: coalbed methane; Reservoir protection; Surface wettability; Reservoir drilling fluid

introduce

Coalbed methane is natural gas stored in the pores of coal seams. China is rich in coalbed methane resources, ranking the third in the world, estimated to be 31.46×10/2m3, which is equivalent to 45 billion tons of standard coal and 35 billion tons of standard oil, equivalent to conventional natural gas resources on land [1 2]. Coalbed methane reservoir is self-generated and stored in coal seam, which is different from conventional natural gas that needs large-scale migration to form a reservoir, and is also very different from conventional sandstone and carbonate reservoirs. It has high adsorption, low permeability and is easy to be crushed by compression [3 ~ 5]. These characteristics determine that the damage to coal seams is much greater than that to conventional reservoirs during the drilling of coalbed methane, and the damage to coalbed methane reservoirs directly affects the desorption, diffusion, migration and later drainage and production of coalbed methane [6 ~ 8]. Therefore, the problem of coalbed methane reservoir damage deserves special attention. Aiming at the technical problems of reservoir protection and wellbore stability in coalbed methane drilling in Qinshui Basin, Shanxi Province, the damage mechanism and protection countermeasures of coalbed methane reservoir are studied.

Rock composition and structure of 1 coalbed methane reservoir

Shanxi Formation (P 1s) has a stratum thickness of 34.00 ~ 63.80 m, with an average thickness of about 48.90m, which is in integral contact with the underlying Taiyuan Formation. The lower part is mainly gray, grayish black, dark gray, grayish black and black mudstone, carbonaceous mudstone, siltstone and sandy mudstone. The bottom is K7 sandstone, which is thick layered medium-fine sandstone with a thickness of 0 ~ 6.07m and an average thickness of 1.48m, mainly including No.2 and No.3 coal seams. Among them, No.2 coal seam is a stable minable coal seam in the whole region, with a CBM well depth of 936.2 ~ 94 1. 1m and an apparent thickness of 4.9m Quantitative calculation shows that the ash content of this layer is 5.59% lower, the fixed carbon content is 86.22% higher, the gas content is 7.83m3/t, and the porosity and permeability are poor. Comprehensive analysis shows that this layer is the coal seam with the largest thickness, the best physical properties and the highest gas content in this well. In addition, its argillaceous content is high, and it is carbonaceous mudstone containing gangue. No.3 coal seam is a relatively stable local minable coal seam, and the upper part is interbedded with medium-fine grained sandstone, siltstone, sandy mudstone and mudstone, including the unstable 1 coal seam.

Study on damage mechanism of coalbed methane reservoir

2. 1 coal and rock composition analysis

Rock samples from complex horizons in Tahe Oilfield are selected and analyzed by D/MAX-Ⅲ A X-ray diffractometer.

Table 1 X-ray diffraction analysis of mineral composition of whole rock

Samples No.5 and No.6 contain 3% and 2% of tobite (NH4Al3Si3O 10OH) respectively, and their contents are not calculated in the table.

From the table 1, it can be seen that the main component of coal and rock in Qinshui basin is carbon, followed by Yingshi, calcite, clay minerals and pyrite, but the content of each rock sample is quite different.

As can be seen from Table 2, the clay minerals in coal and rock in Qinshui Basin of Shanxi Province are mainly kaolinite and chlorite. Well H3 (708.73 ~ 708.438+0m) contains illite mixed layers in different degrees, and the proportion of mixed layers in the middle layer is not high, accounting for 20%.

Table 2 X-ray diffraction clay mineral relative content of coal and rock

Figure 1 Test results of hydration and dispersion properties of coal and rock

Fig. 2 Experimental results of hydration and expansion characteristics of coal and rock

2.2 Analysis of Physical and Chemical Properties of Coal and Rock As can be seen from Figure 1, the recovery rate of coal and rock hydration dispersion in Qinshui Basin of Shanxi Province is very high, reaching over 93%. Relatively speaking, the recovery rate of 708.73 ~ 708.438+0m coal and rock in Well H3 is relatively low.

As can be seen from Figure 2, the expansion rate of coal and rock in Qinshui Basin of Shanxi Province is very small, which belongs to weak/difficult-to-expand rock samples. Relatively speaking, the swelling rate of 708.73 ~ 708.9438+0m rock samples in Well H3 is relatively high.

2.3 Analysis of microstructure, porosity and permeability of coal and rock

Microstructure analysis of (1). As can be seen from Figure 3, the 504.55-meter rock sample in Well H2 is well developed with good tendency and mixed with timely and clay minerals.

(2) Porosity and permeability structure analysis. As can be seen from Figure 4 and Figure 5, there are cleats in the 502.89-meter and 504.55-meter coal rocks of Well H2, and there are "dissolution holes" in the 502.89-meter coal rocks, and the connectivity of cleats in the 504.55-meter coal rocks is good.

7) The porosity of 502.89m coal in Well 7)H2 is 5.394%, the maximum pore throat radius is 10.4385μm, the average pore throat radius is 2.3 174μm, the maximum mercury saturation is 29.25%, and the mercury removal efficiency is 6 1.778%. The test results show that the 502.89m coal seam in Well H2 is a typical fractured reservoir with low porosity, and the pore size contributing to the permeability is between 10.43 ~ 2.5 microns. ..

2.4 wettability test of coal

8) The average contact angles of 502.89m coal and rock in Well 8)H2 to deionized water and standard brine are 62.63 and 65.71respectively, which are weakly hydrophilic and have poor wettability to standard brine. The reason is that the 502.89m coal and rock test point in Well H2 is mainly gray inertinite formation. Generally, the contact angle measured in the dark inertinite group is smaller than that measured in the bright vitrinite group.

Fig. 3 Scanning electron microscope photos of 504.55m coal and rock in Well H2.

Fig. 4 Microscopic photograph of 502.89-meter coal and rock thin section in Well H2.

Fig. 5 Microscopic photograph of 504.55m coal and rock thin section in H2 well.

Figure 6 and Figure 2 Mercury injection curve of coal and rock in 502.89m well.

2.5 stress sensitivity analysis of coal and rock

The influence of confining pressure on permeability is studied by selecting 502.89 m rock samples from Well H2. The results in Figure 9 show that with the increase of confining pressure, that is, the net stress on the 502.89m coal rock in Well H2 increases, the permeability of the rock sample decreases, and with the decrease of confining pressure, the permeability recovery value is lower than that under the corresponding net stress, indicating that the coal rock sample has stress sensitivity.

Based on the above analysis, the typical low porosity fractured reservoirs of coalbed methane reservoir in Qinshui Basin, Shanxi Province can be summarized as follows: (1) There are potential water-sensitive damages to clay minerals and micro-fractures; The surface of coal and rock belongs to weak hydrophilicity, and there is potential water lock damage; There is a strong stress sensitivity damage.

Fig. 7 histogram of mercury saturation and cumulative curve of permeability contribution of 502.89m coal and rock in Well H2.

Fig. 8 Test results of wettability (contact angle) of 502.89m coal and rock in Well H2.

Fig. 9 Influence of confining pressure on coal permeability

Study on protection measures of coalbed methane reservoir

In view of the damage mechanism of coalbed methane reservoir in Qinshui basin, Shanxi Province, technical measures are taken to strengthen the inhibition and improvement of coalbed methane surface wettability and protect drilling fluid.

3. 1 surface wettability improver is preferred.

As can be seen from Figure 10, after soaking in 0.4% SD-905 solution, the hydrophilicity of 502.89m coal and rock in Well H2 is enhanced, showing strong hydrophilicity, obviously reducing the water locking effect and reducing the damage of water relative to coal and rock permeability, which is beneficial to coalbed methane reservoir protection.

Figure/Wettability Test Results of 502.89m coal and rock in Well Kloc-0/0H2 (after soaking in 0.4% SD-905 solution)

3.2 Optimization of Water Sensitive Inhibitors

It can be seen from the figure 1 1 that the expansion of 502.89m coal and rock in Well H2 in tap water is higher than that of 5% SMYZ- 1 and 2% SMYZ- 2 solution, and 5% SMYZ- 1 and 2% SMYZ-2 solution can make coal and rock in Well H2 "shrink".

Figure 1 1 Optimization results of water-sensitive inhibitors

3.3 Damage assessment of foreign fluid to coalbed methane reservoir permeability

The core flow experiment was carried out with high purity nitrogen with purity of 99.999%, and the damage of external fluid to coal permeability was evaluated. The gas permeability K0 of high-purity nitrogen in core before impurity fluid action, K0 1 of high-purity nitrogen in wet core after impurity fluid action and K02 of high-purity nitrogen in dry core (dried at 60℃) after impurity fluid action were tested.

(1) The sensitivity of coalbed methane reservoir samples in Qinshui Basin, Shanxi Province to 0.05% XC solution was analyzed.

(2) Analyze the sensitivity of coalbed methane reservoir rock samples in Qinshui Basin of Shanxi Province to 0.4% SD-905+5% SMYZ- 1.5%.

(3) Analyze the sensitivity of coalbed methane reservoir rock samples in Qinshui Basin of Shanxi Province to 0.4% SD-905+2% SMYZ-2.

(4) Analyze the sensitivity of coalbed methane reservoir samples in Qinshui Basin of Shanxi Province to 0.4% SD-905+0.5% SMYZ-2.

(5) Analyze the sensitivity of coal seam reservoir rock samples to deionized water in Qinshui Basin, Shanxi Province.

It can be seen from the test results in Table 3 that various external fluids have damaged the core permeability to different degrees, and the damage of 0.05% XC solution to the core permeability reaches 100%. The damage of 0.4% SD-905+5% SMYZ- 1 solution to core permeability is also above 90%; The damage of 0.4% SD-905+2% SMYZ-2 solution and deionized water to core permeability is about 60%. 0.4% SD-905+0.5% SMYZ-2 solution has the least damage to the permeability of coal and rock, and the permeability recovery value of rock samples after low temperature drying reaches 124. 1%. According to the above experimental results, it is recommended to use 0.4% SD-905+0.5% SMYZ-2 solution to drill coalbed methane reservoir in Qinshui Basin, Shanxi Province.

Table 3 Evaluation Results of Harmfulness of External Fluid to Coal Permeability

4 conclusion

(1) The coalbed methane reservoir in Qinshui Basin, Shanxi Province is a typical low porosity and low fracture reservoir, and its damage mechanism mainly includes: clay minerals, micro-fractures, and potential water-sensitive damage; The surface of coal and rock belongs to weak hydrophilicity, and there is potential water lock damage; There is a strong stress sensitivity damage.

(2)SD-905 can improve the wettability of coalbed methane reservoir surface, and SMYZ-2 can make coal rock expand or even "shrink" and release pores; 0.4% SD-905+0.5% SMYZ-2 solution has the least damage to the permeability of wet coalbed methane reservoir rock samples, and is recommended as the drilling fluid for coalbed methane reservoir in Qinshui Basin, Shanxi Province.

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