In-situ stress measurements and stress distribution characteristics of coal reservoirs in major coalfields in China: Implication for coalbed methane (CBM) development

Abstract

Based on 232 sets of well-test results measured at depths from 135.9 to 1407.19m in 15 coalbed methane (CBM) development blocks in China, correlations between parameters including reservoir pressure (Po), shut-in pressure (Pc), breakdown pressure (Pf), in-situ stress, coal permeability, and depth (D) were determined. The distribution of in-situ stress in the eastern margin of the Ordos Basin (EMOB), southern Qinshui Basin (SQB), and western Guizhou province (WGP) were analyzed systematically, and its control on the coal reservoir permeability was also addressed. The minimum and maximum envelope curves of the stress ratio (including k ((σH+σh)/2σv)), KH (σH/σv) and Kh (σh/σv)) variability with depth was obtained using the Brown and Hoek (1978) method, and the average envelope curves were established by mathematical derivation on the condition that horizontal principal stresses increase linearly with depth.The results show that the maximum principal stress (σH), minimum principal stress (σh) and vertical stress (σv) vary with depth and location. Generally, there are two main stress regimes in major coalfields in China: σH>σh>σv (59.48%) and σv>σH>σh (34.52%). The σH>σh>σv stress regime has the smallest proportion (6%), which appears mainly in relatively shallower coal seams (<742.84m). Stress ratios, including k, KH and Kh, were plotted between two envelopes, which can be used to estimate the range of σH and σh at a certain depth. With increasing depths, three ratios decrease toward a fixed value, e.g., KH→0.86, k→0.74, and Kh→0.6. For coal seams with a depth>1000m, k is generally <1, indicating that the σv gradually becomes the dominant stress.Coal reservoir permeability decreases exponentially with the increase in effective in-situ stress (EIS, (σH+σh+σv)/3−Po). Vertically, four distinct vertical bands of coal permeability can be found in the EMOB, SQB and WGP, defined by the compression and deformation of the coal-matrix under certain stress regimes: in shallow coal seams (EMOB, <800m; SQB, <650m; WGP, <500m), coal reservoirs are within a strike-slip faulting stress regime (σH>σh>σv) where σH and σh tend to increase, and coal permeability decreases exponentially with an increasing depth; in intermediate coal seams (EMOB, 800–1000m; SQB, 650–825m; WGP, 500–750m), the σv>σH>σh stress regime plays a major role, with relatively high coal permeability. For deep coal seams (EMOB, >950m; SQB, >825m; WGP, >750m), CBM development conditions deteriorate due to a state of “extremely low permeability and extremely high in-situ stress”.