Stress Profiling with Microfracturing and Sonic Logs in Antelope Shale

Abstract

Abstract In shale development, picking optimum lateral landing points, and accurately predicting the height to which a hydraulic fracture will grow, requires knowledge of the vertical stress profile for both the target reservoir and the bounding layers. Stress profiles can be estimated using modern sonic logs in conjunction with geomechanical models but they require calibration with direct measurement of stresses from diagnostic testing, such as micro-fracture testing. The organic-rich middle Miocene to lower Pliocene Monterey Formation is the main producing reservoir rock in the Southern California Offshore area. The most desirable reservoir rocks are the lower calcareous and massive chert zones due to the abundant presence of natural fractures which are necessary for these zones to be economically productive. The Antelope shale, the Monterey equivalent in the San Joaquin valley, may hold the same production potential. However, due to low permeability and less natural fracture density, the economic development of the Antelope shale will require stimulated completions. This paper discusses the first case study on microfracture testing in the Antelope Shale. Results show that direct stress measurements can be successfully acquired at multiple intervals in a few hours and the vertical resolution nearly corresponds to log scale. Therefore, microfracture testing is an appropriate choice for calibrating log derived geomechanical models and obtaining a complete, accurate, and precise vertical stress profile. Further, these calibrated models helped to identify potential fracture boundaries across wellbore and evaluate fracture growth using 3D frac simulator. This resulted in optimized perforation and frac placement in multi-stage frac completion. Microfracture tests were compared with leak off tests across adjacent wells to give more confidence in mud weight window for safe drilling. The closure stress calculated from calibrated sonic logs served as a yardstick for comparing with Leak off Test (LOT) and Formation Integrity Test (FIT) data. The closure stress along with corresponding formation pressure was used to adjust upper and lower boundaries of mud weight window for safe drilling. The case study demonstrates the effective integration of microfracture testing with sonic logs in the Antelope shale.