Evaluation method for in-situ stress orientation using wave anisotropy and electrical imaging logging data: A case study from the Longmaxi Formation in Yongchuan area, Sichuan Basin

In-situ stress orientation is a key parameter in studying oil and gas migration and accumulation, analyzing the stability of borehole wall during drilling, carrying out horizontal well design, fracturing reconstruction and well pattern layout in water injection development. Accurately predicting the in-situ stress orientation of reservoirs is of utmost importance for oil and gas exploration and development. It is the most effective and commonly used method to evaluate the in-situ orientation by using the induced fracture and breakout fracture of imaging logging data. However, in cases where effective induced fracture or breakout fracture cannot be formed during drilling, the in-situ orientation cannot be evaluated by imaging data alone. Taking the shale reservoir of Longmaxi Formation in Yongchuan area as an example, this paper presents a novel method for evaluating in-situ orientation. Firstly, the in-situ orientation is determined relative to the core sample marker line through the wave velocity anisotropy experiment. Secondly, the core sample is rotated to obtain an unfolded image of the core surface. The orientation of the core marker line in the electrical imaging is then determined by comparing the core image with the electrical imaging logging image. By doing so, the true orientation of the core can be obtained. This method aligns well with the in-situ stress orientation determined through induced fracture evaluation, addressing the limitation of electrical imaging data in evaluating in-situ stress orientation in plastic strata.