Characteristics of in-situ stress and its controls on coalbed methane development in the southeastern Qinshui Basin, North China

Abstract

In-situ stress is a key reservoir parameter to evaluate reservoir permeability, hydraulic fractures, coal seam deformation and coalbed methane (CBM) recovery. With limited CBM test wells in the Zhengzhuang field, southeast of the Qinshui basin, North China, the in-situ stress data is inadequate for CBM exploration and development. It is necessary to find a method to predicate in-situ stress through other exploration data such as geophysical well loggings. In this study, we provide a new well-logging data-based model to predicate the in-situ stress based on 17 sets of well test data and comprehensive logging data. As a distinguished characteristic of this model, different structural compartmentalization of CBM reservoirs was considered. A regional adaptive residual strain index was introduced to the model. Based on the model, the in-situ stress distribution in the Zhengzhuang field were evaluated systematically, and the influences of in-situ stress on permeability and the propagation of hydro-fractures were discussed. Results indicate that the magnitude of the maximum (SHmax,14.19–45.40 MPa) and minimum horizontal stresses (Shmin, 10.62–28.38 MPa) and the gravitational stress (Sv, 9.58–30.82 MPa) all show positive correlations with burial depths. The in-situ stress fields in the study area are characterized by 1) SHmax > Sv > Shmin in shallow layers (<700 m), indicating a dominant strike-slip faulting stress regime; 2) Sv ≈ SHmax > Shmin and SHmax > Sv > Shmin in the depth of 700–1050 m, suggesting a transformed regime; and 3) SHmax > Sv > Shmin in deep layers (>1050 m), indicating a strike-slip faulting stress regime. The SHmax in the study area is orientated by NE-SW, with a trend of 40°–49°. Resulted from the change of the in-situ stress regimes from shallow (500 m) to deep layers (∼1000 m), the reservoir permeability variation shows a typical increase followed by decrease. The presence of natural fractures significantly affect the propagation pattern of hydraulic fractures, and the length difference between the major and branch fractures increases with increasing stress anisotropy in the Zhengzhuang field.