Coal Permeability Damage Caused by Production Pressure Difference

Abstract

Due to the special physical and mechanical properties and gas production mechanism of coal reservoirs, the changes of their physical properties are different from those of conventional reservoirs during the coalbed methane development process, of which the dynamic change in permeability is the most significant. During the development process of coalbed methane, production pressure difference can promote gas desorption, but meanwhile, it brings serious damage on coal permeability. This study simulated this damage caused by increasing the displacement pressure difference. Results indicate that the permeability of Duanshi and Gaohe samples declined about 71 and 63.3%, respectively, after the displacement pressure difference increasing to 2.50 MPa. Moreover, there was a critical value of pressure decline of coal. When the displacement pressure difference was lower than the value, the permeability would not change. Therefore, it would be best to control the production pressure difference lower than the critical value of pressure decline during the development. If exceeded, a reasonable production system to decrease the damage and to promote forming a high and stable production of a gas well should be adopted.

Factors controlling high-yield coalbed methane vertical wells in the Fanzhuang Block, Southern Qinshui Basin

“…EDR values that are too high indicate a fast drop in bottom hole pressure where too much gas is desorbed, causing gas locking in the reservoir which would result in far less than optimal outgassing. Furthermore, the effective stress on the coal reservoir will increase, decreasing permeability and gas production (Tao et al, 2012a). The relationship between the EDR and CBM well productivity is demonstrated in Fig.…”

“…A pressure drop rate that is too high can lead to a serious stress sensitivity in the coal reservoir, leading to a decline in permeability (Tao et al, 2012a;. A fast pressure drop will mobilize coal dust in the reservoir, blocking the gas seepage channels and reducing gas production.…”

“…A higher CRR indicates CDP being close to ORP, meaning that the wells would undergo a shorter single-phase drainage time. On one hand, a high CRR will reduce the permeability damage of reservoirs caused by effective stress in the process of decompression (Li et al, 2014;Tao et al, 2012a;Walsh, 1981;Yao et al, 2009). On the other hand, a high CRR value is inclined to shorten the gas breakthrough time, and then the range of the effective desorption would be expanded, improving gas production.…”

“…In recent years, along with exploitation of the CBM in China, the research on the controls of CBM well productivity has made great progress. Numerous studies have examined the effects of seam thickness, seam depth, gas content, permeability, gas diffusion behavior, cohesion of the coal, overburden stress, reservoir pressure, bottom hole pressure, dewatering rate, ground-water flow, sorption time, drilling fluid density, water production rate, well pattern and fracturing effects (Bachu and Michael, 2003;Gentzis and Bolen, 2008;Hildenbrand et al, 2006;Kaiser et al, 1994;Mitra et al, 2012;Pillalamarry et al, 2011;Tao et al, 2012a;Ziarani et al, 2011). However, there is still a lack of research results on quantitative parameters of high-production CBM wells on a block scale.…”

Gray Relational Analysis Optimization for Coalbed Methane Blocks in Complex Conditions Based on a Best Worst and Entropy Method

“…Then the stronger methane power field made gas evolution under the same declining pressure more easily [30,31]. It was also found that a higher critical reservoir ratio shortened the time of CBM well drainage, which reduced the damage caused by effective stress on the permeability of reservoir during the drainage and depressurization stage, weakened the negative effect of fracturing fluid on critical desorption pressure of reservoir, and increased the aerodynamic force [3,32]. The critical reservoir ratio is considered to be a profit-type index for reservoir evaluation; (6) Permeability.…”