Research on three-dimensional in-situ stress testing methods in soft rock formations: A case study of the Ningjin salt cavern gas storage

In-situ stress is a fundamental parameter for seismic research, deep resource development, and underground engineering. However, in soft rock formations with strong rheological behavior, the hydraulic fracturing(HF) method cannot directly determine the maximum horizontal principal stress(σ_H), and the anelastic strain recovery(ASR) method often suffers from large uncertainties due to the difficulty of accurately determining the anelastic strain recovery compliance. To address these limitations, this study proposes a novel approach for in-situ 3D stress in soft rock formations that integrates the hydraulic fracturing method with the anelastic strain recovery method. The minimum horizontal principal stress(σ_h) is obtained through hydraulic fracturing tests, while the anelastic strain recovery compliance is back-calculated from ASR data on recovered core samples, enabling the complete in-situ 3D stress tensor to be reconstructed. This combined method has been successfully applied in the Ningjin salt cavern gas storage project, confirming its reliability and applicability in deep soft rock formations. The proposed approach provides a new technical solution for acquiring in-situ stress in soft rock formations and offers valuable support for salt cavern gas storage construction, unconventional hydrocarbon development, and wellbore stability evaluation.