Characteristics of in-situ stress distribution in Zhengzhuang Region, Southern Qinshui Basin, China and its stress path during depletion

Abstract

The in-situ stress and its behavior during depletion are critical for investigation of the reservoir permeability, wellbore stability, enhanced oil recovery (e.g. stimulation), fault reactivation, and reservoir management. However, the rules that govern in situ stress variation in coalbed methane (CBM) reservoirs and its influences on reservoir stability are still unclear due to the matrix shrinkage effect. In this study, the distribution characteristics of in-situ stress in the Zhengzhuang (ZZ) region were investigated by multi-loop hydraulic fracturing tests and a theoretical stress-depletion response model was built to reveal the dynamic rules of in-situ stress during CBM depletion. Additionally, considering the redistribution of in-situ stress and the limitation induced by fault friction coefficient, a criterion for stress-based failure was developed to analyze the stability of CBM reservoir. The results suggest that in the ZZ block, the fracturing pressure, closure pressure, and the maximum and minimum horizontal principal pressures are positively correlated with burial depth. During drainage, the horizontal principal stress reduces and the effective horizontal principal stress increases linearly as pore pressure reduction. During gas desorption phase, the horizontal principal stress decreases and the effective horizontal principal stress increases non-linearly under weak desorption, whereas both decline non-linearly in response to strong desorption. In addition, the failure criterion for CBM reservoirs indicates that reservoirs in a normal faulting stress regime are the most easily damaged, which may occur during drainage or desorption. However, reservoir damage can only be induced under strong desorption in a strike-slip faulting stress regime and cannot occur in a reverse faulting stress regime. If faults do develop in reservoirs, the reservoir stability is governed by the fault friction coefficient. Finally, by combining the data, the changes in in-situ stress and the stability of the reservoirs during depletion in the ZZ region were revealed and its implications for CBM development were discussed.