Pipe-Conveyed Microfrac Testing and Acoustic/Image Logs Enables Gas Depleted Formation Stress Calibration in Carbonate Reservoirs

Abstract

Abstract Development plans in largely depleted carbonate gas bearing reservoirs are dependent upon having a complete understanding of reservoir mechanical behavior under change of in-situ stress state. The accuracy of 3D geomechanical models relies highly upon proper tectonic strain and stress calibration using in situ straddle packer microfrac testing conducted in vertical pilot wellbores. Pipe-conveyed straddle packer microfrac tests have become an important technology to measure in-situ fracture closure stress in depleted and non-depleted reservoir layers in order to quantify the stress contrast among multiple intervals with different reservoir pressure regimes. This case study describes the use of microfrac tests to validate and calibrate the horizontal stress profile in various reservoir intervals of a carbonate formation that had been developed to a substantially depleted condition. Well-injection plans, cap-rock integrity assessment, stress contrast, hydraulic fracture containment, and minimum horizontal stress estimations can all be quantified from multiple microfrac tests recorded at various depths of the reservoir formation. The fracture closure analysis was conducted using three different methods: (i) square-root of time, (ii) G-function and (iii) Log-Log plot. The final fracture closure measurement was obtained after consolidating the three fracture closure identification methods in all injecting cycles conducted on each microfrac station. The integrated post job microfrac analysis includes borehole acoustic processing and resistivity imaging. The borehole acoustic response is used to estimate not only formation mechanical properties but also log-derived stress profiles while borehole Imaging is used to select the microfrac points and to confirm the induced borehole fracture created during the pressurization of the straddle packer tool. Formation breakdown, fracture reopening, propagation and closure at multiple reservoir layers provide the necessary in situ measurements to calibrate the stresses change due to pore pressure depletion. This information provides a better understanding of the in situ stress state in depleted formations that reduce risk in designing future gas development strategies in the field.