Response in coal reservoirs and in-situ stress control during horizontal well coal cavern completion and stress release

Widely developed tectonically-deformed coal (TDC) blocks the recovery of coalbed methane (CBM). Horizontal well cavern completion and stress release (HWCCSR) is expected to extract TDC in-situ CBM efficiently. Sys-tematic investigations on reservoir response during stress release are essential for evaluating stress-relief per-formances and guiding engineering practices. However, the dynamic-controllable cavern enlargement and the influence of high stress and intense tectonic stress were mainly ignored in previous work. Additionally, the mechanism of permeability variation remained unclear under complex stress surroundings. In this work, the geological-engineering numerical model of coal cavern completion was constructed, and the equivalent method was adopted to investigate the dynamic HWCCSR. The stress, plastic deformation, and permeability responses were analyzed, and the impact of in-situ stress on reservoir response was discussed. The mechanism and model of permeability changes under complex stress were revealed. The results showed that the equivalent method could imitate the dynamic-controllable enlarging of the cavern properly with easy operation and good convergence. The reservoir responded significantly during the HWCCSR. With increasing cavern diameter (Dc), the plastic deformation zone (PDZ) increased, permeability could be expanded to 1-105 times the initial value, and the effective permeability increase zone (EPIZ) was linear with Dc. Laboratory and field data confirmed the rationality of simulation results. High vertical principal stress and significant lateral stress coefficient were conducive to generating more complex PDZ and larger EPIZ and enhancing more considerable permeability in TDC reservoirs due to more energy and a strong damage -deformation trend. Four models of permeability variation and possible division of permeability distribution were proposed according to the integrated control of elastic-plastic deformation, stress difference, and volume stress. Stress difference was the direct reason for coal plastic deformed-failure. Plastic deformed-failure coupled with volume stress-relief could significantly increase permeability. Plastic deformed-failure and volume stress concentration might cause a permeability decrease. Coal elastic deformation with volume stress concentration might induce a similar outcome. Additionally, elastic deformation coupled with volume stress-relief might enhance permeability and expand the EPIZ under tectonic stress conditions. In conclusion, larger caverns had better stress-relief performance, expected to attain a high recovery of coalbed methane. The HWCCSR was suitable for recovering TDC in-situ CBM in areas with intense tectonic stress and large burial depths. This work confirmed the feasibility of efficient recovery of TDC in-situ CBM through HWCCSR. It provided basics for evaluating reservoir responses to stress-relief and selecting layers.