In unconventional reservoirs, horizontal well and fracture design ultimately affect reservoir stress evolution. To better understand how the in-situ stresses evolve in space and time and henceforth impact the future infill operations, we perform a comprehensive sensitivity analysis to uncover the impact of well spacing, perforation cluster spacing, and bottomhole pressure (BHP). A thorough understanding of the potential effects of well and fracture designs helps suggest the optimal infill well location and fracturing time window in an unconventional reservoir.
In this work, a tight oil reservoir is modeled with a pair of parallel horizontal wells in the reservoir producing at equal rates. Varying well spacing, perforation cluster spacing, and BHP generate a total of 11 combinations of depletion scenarios. The hydraulic fractures are characterized by the local grid refinement (LGR) approach, and the geomechanics response is captured using the finite element method while iteratively coupled with the fluid flow model until convergence. The spatiotemporal stress alteration in the infill region is investigated by monitoring the orientation and magnitude change of local principal stresses in the depletion process.
The in-situ horizontal stresses in our numerical studies start to reorient as reservoir depletion begins and eventually reverse directions at the infill region by the end of the 10-year simulated production time. The comprehensive sensitivity study indicates that an increase in well spacing leads to delayed stress reversal onset time. The flowing BHP directly impacts reservoir depletion, and small BHP leads to quicker depletion, which induces early stress reorientation in the infill region. Moreover, the depletion effect from each hydraulic fracture is predominant when cluster spacing is larger, and the stress reversal onset time is postponed accordingly. In contrast, the drainage zone is amalgamated under small cluster spacing; thus, stress reversal completes at the early depletion stage.
This work details the impacts of well and fracture designs on in-situ pressure and stress evolution in unconventional reservoirs. The sensitivity study delivers insightful understandings of infill well design and provides practical guidelines in the fracturing time window for similar unconventional formations.