The legacy of conventional fields has resulted in many low permeability reservoirs deemed sub-commercial without an appropriate stimulation strategy. With low permeabilities and potentially heterogeneous reservoir characteristics, an optimal development approach would highly depend on their specific reservoir properties that may well require stimulation methods other than hydraulic fracturing. In this paper, we present a fully integrated characterization and modeling workflow applied to the Kita-Akita oil field in northern Japan, demonstrating the screening process for multiple completion and stimulation methods in a highly heterogeneous, low permeability sandstone reservoir.
To select a best completion and stimulation candidate from multiple methods, we constructed an evaluation matrix including the maturity of technologies, applicability to our reservoir, productivity, and economics. Multi-branch type completions such as radial drilling and fishbone drilling, as well as hydraulic fracturing were simulated and subsequently compared based on their productivities. Especially for the radial drilling and the fishbone drilling, a 3D FEM model was built for their complex laterals, and the inflow performances were evaluated with homogenous reservoir properties, respectively. Besides, due to the highly heterogeneous nature of the reservoir, we built a full-physics subsurface model based on a pilot-hole data acquisition and legacy 2D seismic lines. The 3D model served as a canvas to assess reservoir flow and geomechanical behavior, calibrated with production history from past producing wells in the 1950's to 1970's. Based on these models, the best infill drilling location was selected and multiple well completion and stimulation practices were evaluated.
Through the screening methodology, the multi-stage hydraulic fracturing was identified as the best suited from an instantaneous productivity perspective. Yet, even though hydraulic fracturing would enhance the accessibility into multiple distinctively isolated sandstones occurring in the deepwater slope channel setting, the treatment costs exceeded the economic threshold significantly in our case. Inflow performance evaluation based on the 3D FEM modeling illustrates multi-branch type completions such as radial drilling and fishbone drilling were identified with a good stimulation skin factor. As a result of 3D simulation study, multi-branch completion was revealed as a technical and economically viable stimulation option in the heterogeneously distributed sandstone reservoirs.
The advent of recent completion and stimulation techniques now renders low permeability reservoirs with relatively large development potential. Even with the development challenges quite different from conventional reservoirs, the approach shown in this paper provides a helpful reference for the study and decision-making process when the legacy field needs an optimal stimulation strategy.