Characteristics of in-situ stress distribution and its significance on the coalbed methane (CBM) development in Fanzhuang-Zhengzhuang Block, Southern Qinshui Basin, China

The permeability characteristics of gas‐containing coal under different radial stress, different axial stress, and gas pressure were studied by orthogonal experiments, using a self‐developed three‐axis servo fluid infiltration system with a gas‐solid coupling of gas‐containing coal, on the basis of a single‐factor influence on the permeability of gas‐containing coal. By considering the effective stress, three kinds of relationships between permeability and radial stress, permeability and axial stress, as well as permeability and gas pressure were established. The results show that radial stress, axial stress, and gas pressure have a great influence on the permeability characteristics of gas‐containing coal: (a) The influence of radial stress on gas permeability is significant, whereas the influence of axial stress is negligible. The degree of influence of radial stress, gas pressure, and axial stress on the permeability decreases in turn; (b) The permeability decreases following a power function with the increase of the radial stress; (c) The permeability gradually increases with the increase of axial stress. With the increase of axial stress, the permeability increases following a power function; (d) The increase in gas pressure will reduce the effective stress on the coal, and the number of pores and cracks inside the coal body will decrease. This will increase the effective seepage flow and gas flow rate of the gas and eventually lead to the increase of coal permeability. There is a quadratic function relationship between the permeability and gas pressure.

σ^{'} = σ - P,

where

σ^{'}

is the effective stress, MPa.…”

Section: Discussionsupporting

confidence: 88%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Study on the change of permeability of gas‐containing coal under many factors

Wang

et al. 2019

“…[1][2][3] According to the latest round of national oil and gas resource evaluation results in China, the geological resources with a depth <2000 m are as high as 36.81 trillion m 3 . [1][2][3] According to the latest round of national oil and gas resource evaluation results in China, the geological resources with a depth <2000 m are as high as 36.81 trillion m 3 .…”

Section: Introductionmentioning

confidence: 99%

“…14 According to Chen et al,36 there is a linear positive relationship between reservoir pressure measured with well testing and depth, which can be used to calculate the reservoir pressure of coal seams in each MBHW. The single-water drainage stage ranges from 0 to 180 days with an average of 75 days.…”

mentioning

confidence: 99%

Current status and key factors for coalbed methane development with multibranched horizontal wells in the southern Qinshui basin of China

Chen

Tang

Tao

et al. 2019

Self Cite

The Qinshui basin is the most successful area for coalbed methane (CBM) development in China. Since 2008, 45 multibranched horizontal wells (MBHWs) have been put into production in the Fanzhuang block, of which 10 wells have an average daily gas rate (DGR) of >10 000 m3/d. Well 4‐5 with the total gas production of 1.054 × 108 m3 achieved the China's highest DGR of 63 744 m3/d. Compared with vertical wells, MBHW is an effective way to develop CBM in low permeability reservoirs in China. However, there are still 23 wells have an average DGR of <3000 m3/d, of which 18 wells are below 1000 m3/d. The results show that shallow depth (<600 m), high gas content (>16 m3/t) and saturation (>80%), long coal seam interval (>3000 m), reasonable branch spread, and stable wellbore are the basis for achieving high production. The encountered faults connected to the aquifer will cause a high water production and an extremely low gas rate (<1000 m3/d). In the inclined coal seam, the up‐dip MBHW is more favorable for the formation water drainage and the pressure drop funnel expansion than the down‐dip MBHW, and the horizontal well should be drilled perpendicular to the high‐permeability direction to achieve a higher recovery ratio. MBHW is more sensitivity to drainage rate due to the coal fine migration in the horizontal segments, and the high pressure drop rate will lead to high frequency of stunk pump and eventually affect the gas production. Currently, the biggest challenge for CBM development with MBHW is wellbore stability. 57% of inefficient wells are caused by wellbore collapse during drilling and production processes. Here, a new type of “Roof Drilling‐Tree Shaped” horizontal well structure was presented in this work. The first “Roof Drilling‐Tree Shaped” horizontal well with a total length of 12 288 m and a coal seam interval of 9408 m was successfully applied without drilling accidents.

Swelling of clay minerals and its effect on coal permeability and gas production: A case study of southern Qinshui Basin, China

Tao

Gao²,

Pan

2019

Self Cite

The permeability damage of high‐rank coal reservoirs is the main factor affecting the development of coalbed methane (CBM) in the late production stage. Coal core samples from southern Qinshui Basin were saturated with actual formation water, 1/2 salinity formation water, and distilled water to study the swelling of clay minerals. Then, the mechanism of permeability change caused by water sensitivity and its effect on gas production was discussed. The results show that the permeability decrease caused by flow velocity sensitivity did not occur during the water sensitivity experiments. The permeability damage caused by water sensitivity is mainly the static damage from swelling of clay minerals, especially illite/smectite mixed clays and chlorite. During the process of CBM development, the water sensitivity of coal reservoirs will lead to the sharp increase of Na+ concentration and a decrease of Mg2+ concentration in the produced water. Meanwhile, the clay minerals swell due to the adsorption of more Mg2+ and Ca2+, decreasing the reservoir permeability and gas production rate.