Research on stress state in deep shale reservoirs based on in-situ stress measurement and rheological model

Accurately determining the stress state in deep shale reservoirs is the key to the efficient development of shale gas and other unconventional energy sources. An effective method to increase the evaluation and calculation accuracy of in-situ stress parameters in a deep shale reservoir is to combine different methods to obtain different stress information, such as obtaining the minimum horizontal principal stress based on the in-situ stress measurement, predicting the magnitudes of horizontal stress difference and the horizontal principal stresses by establishing the stress profile based on the rheological model, and estimating the direction of the maximum horizontal principal stress by the wellbore failure imaging logging. We applied this research idea to Well SZ1 in Hanzhong, Shaanxi Province. The minimum horizontal principal stress obtained by hydraulic fracturing ranged from 32 to 41 MPa; Then, the variation laws of rock rheological parameters with the depth were determined by the rock mechanical parameters obtained from cross-dipole acoustic logging data. And combined with the burial history of the reservoir and the strain rate of the crust, the stress profile of Well SZ1 was established. The results show that the magnitude of horizontal stress difference in the depth range of 1950~2025 m in the Niutitang Formation is between 10~15 MPa, and ranges of the minimum and maximum principal stresses are 28~41 MPa and 47~49 MPa, respectively. The predicted horizontal minimum principal stress values are in good agreement with the measured results. Based on the in situ stress measurement and predicted stress profiles, Well SZ1 is characterized by normal faulting (S_v > S_H > S_h)or a combination of normal and strike-slip faulting regimes (S_v≈S_H > S_h).The horizontal stress difference decreases with the increase of the gamma value, indicating that the stress profile has a good corresponding relationship with the formation lithology. Based on the distribution characteristics of borehole-induced tensile fractures recorded by imaging logging, the direction of the maximum horizontal principal stress in Well SZ1 is ~N74°W, which is consistent with the direction of the regional tectonic stress field. This study provides an important basis for accurately understanding the in-situ stress state of the target layer of Well SZ1, as well as the later horizontal well layout and fracturing control.