Evaluation of in situ stress changes with gas depletion of coalbed methane reservoirs

Abstract

A sound knowledge of the stress path for coalbed methane (CBM) reservoirs is critical for a variety of applications, including dynamic formation stability evaluation, long-term gas production management, and carbon sequestration in coals. Although this problem has been extensively studied for traditional oil and gas reservoirs, it is somewhat unclear for CBM reservoirs. The difference between the stress paths followed in the two reservoir types is expected to be significant given the unique sorption-induced deformation phenomenon associated with gas production from coal. This results in an additional reservoir volumetric strain, which induces a rather "abnormal" loss of horizontal stress with depletion, leading to continuous changes in the subsurface formation stresses, both effective as well as total. It is suspected that stress changes within the reservoir triggers formation failure after significant depletion. This paper describes an experimental study, carried out to measure the horizontal stress under in situ depletion conditions. The results show that the horizontal stress decreases linearly with depletion under in situ conditions. The dynamic stress evolution is theoretically analyzed, based on modified poroelasticity associated with sorption-induced strain effect. Additionally, the failure tendency of the reservoir under in situ conditions is analyzed using the traditional Mohr-Coulomb failure criterion. The results indicate that depletion may lead to coal failure, particularly in deeper coalbeds and ones exhibiting large matrix shrinkage.

Theoretical, numerical, and experimental analysis of effective extraction radius of coalbed methane boreholes by a gas seepage model based on defined criteria

“…With Equations (13), (14), (15), and (12) combined, the governing differential equation for the gas pressure function in the gas flow field of coal seam can be obtained, which varies with time and space.…”

“…It is well‐known that the methane generated in coal seams is an efficient and clean source of energy . In recent years, coalbed methane (CBM) development has received special attention in areas with multiple coal seams . Several unconventional gas resources located in coal deposits have been studied as systems .…”

“…11,12 In recent years, coalbed methane (CBM) development has received special attention in areas with multiple coal seams. [13][14][15][16] Several unconventional gas resources located in coal deposits have been studied as systems. [17][18][19] At present, gas predrainage along coal seam boreholes is mainly used in China, which is also an effective method for coal seam gas recovery and gas disaster prevention and control in coal mines, [20][21][22] as shown in Figure 2.…”

Anisotropy characteristics of coal shrinkage/swelling and its impact on coal permeability evolution with CO2 injection

“…The increase of CO 2 injectivity in this site might also be attributed to the CO 2 injection‐induced micro‐fracturing. When the coal was saturated with CO 2 at 15 MPa, the swelling strain will induce excess stresses due to the confined geological boundary . This sorption‐induced excess stresses might exceed the compressive strength of coal to cause the micro‐fracturing resulting in an overall increase of fluid injectivity.…”

Variation in horizontal stress with pore pressure depletion in coal under uniaxial strain conditions: An update on the modelling of laboratory data

“…More recently Liu and Harpalani (2014) reported horizontal stress variations for two coal samples (S1 and S2) measured under laterally-constrained (uniaxial strain) conditions. A non-adsorbing gas (helium), i.e.…”