Intelligent assessment and deflection patterns of in-situ stress orientations along wellbores in shale reservoirs: A case study of the Longmaxi Formation within a complexly folded area in the southern Sichuan Basin

Complex tectonic belts along the margins of the southeastern Sichuan Basin have emerged as a key replacement for the additions to reserves from reserve growth of shale gas from the Longmaxi Formation. However, the superimposition of multi-stage fold deformations with varying orientations and intensities has resulted in complex deflection patterns of the in-situ stress orientations therein, thus restricting the safe and efficient shale gas development. This study investigated folded geological structures within the Longmaxi Formation in a representative shale gas block of the tectonic belts. By integrating multi-source data from core tests, well logging, and microseismic monitoring, we analyzed the characteristic responses to in-situ stress orientation and established an intelligent assessment method to determine the stress orientation along wellbores. Accordingly, we revealed the control pattern of fold deformations on the deflection of in situ stress orientations and proposed optimization schemes for well placement and fracturing design according to the orientations. The results indicate that in shale reservoirs characterized by weak anisotropy, low coefficients of difference between the maximum and minimum horizontal principal stresses (< 0.15), and well-developed high-angle fractures, the responses of wave velocity anisotropy, wellbore collapse, induced fractures, shear wave splitting, and the strike of stimulated fractures to in situ stress orientations vary. The proposed intelligent assessment method, which integrates multi-source data with a prior probability model, enables continuous and high-precision interpretation of in situ stress orientations along wellbores, thereby revealing a pronounced vertical stratification of in situ stress orientations in the 1st member of the Longmaxi Formation (also referred to as the Long 1 Member). In the planar view, with the southern segment of the Huayingshan fault serving as a boundary, areas such as Weiyuan to the west exhibit gentle structures, with in-situ stress orientations dominated by E-W. In contrast, areas to the east are subjected to fold deformations, with in situ stress orientations varying complexly across different zones. In these areas, greater fold deformation intensity (> 0.1 km -1 ), higher rock mechanical strength ( E > 30 GPa), more pronounced flexural-slip deformation, and angles of 30°–60° between fold axes and regional stress orientations correspond to more significant deflection of in situ stress orientations. This deflection pattern is jointly controlled by the structural style, spatial superimposition relationship, and fold parts. Innovatively, a development strategy based on "stress zoning" was proposed, which reasonably defines the orientation characteristics of in situ stress zones and requires that the angle between the well placement direction and the maximum principal stress be greater than 55°,it is advisable to adopt parallel and near-fan-shaped well placement patterns laterally and two sets of three-dimensional well patterns with different orientations vertically. Furthermore, fracturing stimulation should be preferentially conducted in the fold deformation-induced tensile perturbation zones (i.e., zones with low in situ stress magnitude and small in situ stress differences). This scheme expands the stimulated reservoir volume (SRV), which is significant for improving shale gas production. • Developed an intelligent method for stress orientation well profiles evaluation by integrating multi-source data and prior probability models. • The influence mechanism of fold deformation on in-situ stress orientation rotation. • Developed a stress-partitioned well pattern optimization with parallel and near-fan-shaped layouts to enhance shale gas recovery. • Identified tensile stress disturbance zones with low stress magnitude and stress difference as optimal targets for hydraulic fracturing.