Kang Junqiang, Fu Xuehai, Cheng Ming, Zhang Baoxin, Qi Qi, Chen Zhaoying
Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou, 221008, China.
State Key Laboratory of Coal and CBM Co-mining, Jincheng, 04800, China.
ACS Omega. 2024 May 17;9(22):23998-24008. doi: 10.1021/acsomega.4c02373. eCollection 2024 Jun 4.
The key to the variation in permeability within coal reservoirs lies in the stress-induced deformation and desorption-induced deformation during the coalbed methane (CBM) production. The differences in sample scale and measurement methods between stress-induced deformation and desorption-induced deformation significantly affect the accuracy of permeability measurements. Therefore, in order to elucidate the relationship between stress-induced deformation and adsorption-induced deformation, as well as the influencing factors, and to assess the accuracy of permeability evolution prediction, this study conducted a series of parallel experiments, including compression deformation experiments under stress loading (stress-induced deformation), methane adsorption-induced deformation experiments (adsorption-induced deformation), μCT scanning, and overburden permeability measurements.The results of the study indicate that stress-induced deformation and adsorption-induced deformation are negatively correlated but exhibit a relatively weak correlation. Stress-induced deformation encompasses deformation of coal matrix, minerals, and fractures, whereas adsorption-induced deformation primarily reflects coal matrix deformation. While there is some overlap between the two, they are not entirely identical. The main influencing factor of stress-induced deformation is the mechanical strength of coal, with minerals in coal increasing the Young's modulus of coal reservoirs. Among them, minerals that are more dispersed and have smaller particles have a more significant impact on stress-induced deformation. The primary influencing factor of adsorption-induced deformation is the deformation capability of the coal matrix, with minerals and fractures having less significant effects. Permeability changes are controlled by fracture deformation, but stress-induced deformation measurements weakly reflect this aspect, leading to an inability to accurately predict the scale of the impact of effective stress changes on permeability during CBM production and CO2-ECBM processes. In contrast, adsorption-induced deformation relatively accurately reflects the deformation capability of the coal matrix and provides a more accurate prediction of permeability rebound under the condition of almost unchanged effective stress in the late stages of mining. Therefore, deformation parameters under stress loading are challenging to directly apply to the prediction of permeability evolution, while adsorption-induced deformation parameters can be effectively utilized.
煤储层渗透率变化的关键在于煤层气（CBM）开采过程中的应力诱导变形和解吸诱导变形。应力诱导变形和解吸诱导变形在样品尺度和测量方法上的差异显著影响渗透率测量的准确性。因此，为了阐明应力诱导变形与吸附诱导变形之间的关系及其影响因素，并评估渗透率演化预测的准确性，本研究进行了一系列平行实验，包括应力加载下的压缩变形实验（应力诱导变形）、甲烷吸附诱导变形实验（吸附诱导变形）、μCT扫描和覆岩渗透率测量。研究结果表明，应力诱导变形与吸附诱导变形呈负相关，但相关性相对较弱。应力诱导变形包括煤基质、矿物和裂隙的变形，而吸附诱导变形主要反映煤基质变形。两者虽有部分重叠，但并不完全相同。应力诱导变形的主要影响因素是煤的力学强度，煤中的矿物会提高煤储层的杨氏模量。其中，分布更分散、颗粒更小的矿物对应力诱导变形的影响更显著。吸附诱导变形的主要影响因素是煤基质的变形能力，矿物和裂隙的影响较小。渗透率变化受裂隙变形控制，但应力诱导变形测量在这方面反映较弱，导致无法准确预测煤层气开采和CO2-ECBM过程中有效应力变化对渗透率的影响程度。相比之下，吸附诱导变形能相对准确地反映煤基质的变形能力，并能更准确地预测开采后期有效应力几乎不变条件下的渗透率回升。因此，应力加载下的变形参数难以直接应用于渗透率演化预测，而吸附诱导变形参数则可有效利用。
