New geomechanical application utilizing MWD gamma ray logs: An Eagle Ford case study

Abstract

Understanding mechanical rock properties is essential for optimal well placement and hydraulic stimulation design which directly impacts hydrocarbon productivity from shale reservoirs with ultra-low permeability. Current industrial practice is to assess spatial variability of subsurface mechanical properties through analyzing core and dipole sonic logs. Seismic inversion is another commonly applicable approach. However, although numerous existing approaches have contributed to upstream operators mitigating operational risk, these individual or combined evaluations ultimately require significant cost and time for exploration. These approaches also have limitations identifying fine-scale variability within inter-well spacing or along the length of a horizontal well due to sparse pilot-well control and the intrinsic limitation of seismic resolution. In this study, we propose a new approach to capture spatial variability of Young's modulus utilizing MWD (Measurement While Drilling) gamma ray logs of horizontal wells. The MWD gamma ray log is essentially acquired for the purpose of having geometric and stratigraphic information on well position while drilling in real time. And, due to cost and time related issues, shale operators commonly acquire only MWD log rather than core or wireline log set for horizontal production drilling while the massive pad drilling has the advantage of large areal coverage. Under the circumstance, MWD gamma ray log is often only data having geological implication. In purpose of utilizing MWD gamma ray log for geomechanical approaches, we examined a relationship between gamma ray and Young's modulus acquired from pilot wells. The MWD gamma ray logs were converted into Young's modulus logs and ultimately populated for three-dimensional property modeling. To validate the property model, an integrated assessment using wireline logs, microseismic, and three-dimensional seismic data was conducted. The study results demonstrate that this new geomechanical approach utilizing MWD gamma ray logs can be supportive or even serve as an alternative to the existing approaches. The method enables identification of finer-scale spatial variability than sparse pilot-well control and conventional seismic analysis, which is attributed to the advantage of long-lateral, multi-benched, staggered, and dense production well placement. Based on the reliability and cost-effectiveness of the new approach, we expect it could help upstream operators mitigate risk and uncertainty related to geomechanical properties without excessive spending for exploration and ultimately improve hydrocarbon productivity with optimal well completion strategies.