(China Petroleum Exploration and Development Research Institute, Beijing 100083)

Author brief introduction: Fan Mingzhu, male, senior engineer, mainly engaged in coalbed methane geological research and field work, fanmingzhu@pepris.com.

Sanjiao block is located in the middle of Hedong coalfield, which has great potential for coalbed methane development. Twenty CBM wells have been drilled for a long time. There are obvious differences in gas content, gas saturation and gas production between Linjiaping Well Group and Qikou Well Group, which are about 6km apart along the stratigraphic strike. It is found that the hydrogeological environment of these two well groups is different, and the strong groundwater runoff zone is the geological reason for these differences. The research results have guiding significance for the next exploration and deployment, and provide a typical example for the importance of hydrogeological conditions research in coalbed methane exploration and development.

Sanjiao block of Hedong coalfield; Hydrogeological conditions of coalbed methane gas

Influence of hydrogeological conditions on gas content and saturation in Sanjiao block

Fan mingzhu

(China Institute of Petroleum Exploration and Development, Beijing 100083)

Abstract: Jiao Shan block is located near the center of Hedong coalfield on the eastern edge of Ordos Basin in Shaanxi Province, and is considered as a favorable block for coalbed methane development. The two test well patterns, Linjiaping Well Group and Qikou Well Group, are arranged along the stratigraphic strike and 6 km. These models have obvious differences in void fraction, gas saturation and void fraction. It is considered that this difference is caused by the strong outflow of groundwater in coal measures strata. This conclusion is of great significance to the development plan of coalbed methane in this block, and is a typical case to demonstrate the important influence of hydrogeological conditions on coalbed methane exploration and development.

Keywords: coalbed methane; Gas content and saturation; Hydrogeological conditions; Jiao Shan block; Hedong coalfield

introduce

Sanjiao CBM exploration block is located in the middle of Hedong coalfield, north wing of Lishi nose structure (Figure 1). The structure of this area is simple, and a series of low-amplitude flexural structures are developed on the monoclinic background inclined to NW-NW (Figure 2). The buried depth of coal seam is about 500m, and the target coal seams are 4/5 coal of Shanxi Formation and 8/9 coal of Taiyuan Formation (Figure 3). The total thickness of coal seam is 7 ~ 15 m, with stable distribution and simple coal seam structure. It is 0.76% ~ 1. 18%, which is gas-rich coal. The primary coal body structure and cleavage are developed, and the coal seam permeability is good. The original permeability of formation test is (0.4 ~ 50) × 10-3 μ m2, and the pressure of coal reservoir is normal. The area of this block is 488.2km2, the geological resources of coalbed methane are 533× 108m3, and the resource abundance is 1.09× 108m3/km2, so the development potential of coalbed methane is huge.

Figure 1 Schematic Diagram of the Location of Three Intersections

Fig. 2 Structure diagram of 9 coal floor of Taiyuan Formation

Fig. 3 Comprehensive histogram of coal measures

Exploration in Sanjiao Block started from 1996. In 2006, 20 coalbed methane wells were constructed in 5438+0 * *, and two development test well groups were drilled, namely Linjiaping Well Group of 9 wells and Huangkou Well Group of 5 wells (Figure 4). The drainage time of Linjiaping Well Groups SJ5, SJ6, SJ7, SJ8 and SJ9 was as long as 24 hours. In exploration and production, it is found that there are obvious differences in gas-bearing and recoverability between two well groups about 6km apart, and Huangkou well group is obviously better than Linjiaping well group. This difference shows that the geological conditions of coalbed methane in this block are complex and should be paid great attention by explorers. Finding out the geological reasons for this difference is of great practical significance for formulating a reasonable exploration and deployment plan in this area and guiding the successful development of coalbed methane.

Fig. 4 Well Diagram of Sanjiao Block and Test Site

1 coal seam gas-bearing and gas production change characteristics

There are three data sources of coal seam gas content in Sanjiao block, namely, coal exploration wells, conventional coring coalbed methane exploration wells and rope coring coalbed methane development test wells. Among them, the exposure time of coal samples in rope coring wells is the shortest, and the gas content determination results are the most reliable. See table 1 for the gas content measurement results of three wireline coring wells representing two well groups, in which SJ1and SJ 13 are Linjiaping well group and SJ 14 is Huangkou well group. As can be seen from the table, the gas content in each coal seam of Huangkoujing Group is obviously higher than that of Linjiaping Well Group.

Table 1 SJ 1 1, SJ 13 and SJ 14 gas content data.

The gas saturation data of these two well groups are shown in Table 2 and Figure 5. Well SJ 1 1 represents Linjiaping Well Group, and Well SJ 14 represents Huangkou Well Group. It can be seen from Figure 5 that the gas saturation of Huangkou Well Group is higher than that of Linjiaping Well Group 17% ~ 36%, Huangkou Well Group is saturated or nearly saturated, and Linjiaping Well Group is seriously undersaturated.

Table 2 Gas saturation data of wells SJ11and SJ 14

Note: The data format is.

Fig. 5 comparison of average gas saturation between well SJ11and well SJ 14.

Fig. 6 shows the gas production of each well after about 10 months of test wells in Linjiaping Well Group and Huangkou Well Group. As can be seen from the figure, the gas production of Huangkou Well Group is generally high.

2 hydrogeological conditions and its control effect on gas

The groundwater flow direction in the coal-bearing section of Sanjiao block is consistent with the stratigraphic strike, with the central part flowing from southeast to northwest, and the southern part of Huangkou Town turning to nearly north-south direction due to the stratigraphic strike, and the groundwater flow direction changes from east to west. There is a high potential zone near the south side of Linjiaping Well Group (Figure 7). By mapping the chloride ion content of groundwater in Shanxi Formation and Taiyuan Formation (Figures 8 and 9), it is found that Linjiaping Well Formation is in an abnormal zone with low chloride ion content, and these data are * * *.

Fig. 6 Gas production of each well when the production is about 10 month.

Fig. 7 Isohydrograph of Taiyuan Formation

The field investigation shows that the groundwater in Well Sg3, located about 200 meters southwest of Linjiaping Well Group, overflows and the running water is milky white. After being put into the cup, the water surface ignites combustible materials, and after a few minutes, the water body turns from milky white to clear and transparent. This phenomenon shows that groundwater contains dissolved gases.

It is not difficult to see from the above that the hydrogeological environment of Huangkoujing Formation and Linjiaping Well Formation is different. The hydrogeological environment of Huangkou Well Group is relatively stable and closed, and Linjiaping Well Group is relatively poor. The strong alternation of groundwater and surface water makes coalbed methane migrate and dissipate in water-soluble state, which leads to the decrease of gas content and gas saturation, thus reducing gas production.

Fig. 8 chloride ion content map of Taiyuan Formation

Fig. 9 chloride ion content map of Shanxi Formation

Figure 10 Isogram of Groundwater Runoff in Taiyuan Formation

3 Conclusion

The exploration results in Sanjiao experimental area prove that hydrogeological conditions have obvious gas control effect, which is an important research content that can not be ignored in coalbed methane exploration and development activities. As far as the exploration work in this block is concerned, it is urgent to study the geological causes of the formation of strong groundwater runoff zone, find out the changing characteristics of groundwater hydrogeological environment in the whole area, and provide the necessary basis for the formulation of exploration and development plans.

Thanks to Dr. Victor Zhang, a former senior geologist of Arco China Company, for providing the required information for this article.

refer to

Yin Bajin, Ding Jianchun, Wang Chufeng, Xin. 2002. History and prospect of coalbed methane exploration in Hedong area of Shanxi Province. Development and Prospect of China Coalbed Methane Industry in the+0th Century (Proceedings of the Third National Symposium on Coalbed Methane), 42.