Energy Strategy Reviews (Nov 2021)
Quantitative assessment of the sweet spot in marine shale oil and gas based on geology, engineering, and economics: A case study from the Eagle Ford Shale, USA
Abstract
Shale oil and gas have become important areas of petroleum exploration and development. Successful shale oil and gas exploration depends on the accurate evaluation of sweet spots, which are the commercial targets. This paper proposes a comprehensive new method for sweet spot evaluation based on average estimated ultimate recovery in barrels of oil equivalent (EUR_BOE) in conjunction with key geological factors, engineering and technical conditions, and economic and technical indicators. Research results from 72 systematically cored wells, 991 sets of core analyses, and production data from 1317 horizontal wells in the lower Eagle Ford Shale (EFS) of the U.S. Gulf Basin show that marine shale oil and gas sweet spots depend primarily on four key factors: oil and gas resources, oil and gas storage capacity, fluidity, and fracability. These conclusions are based on quantitative evaluation models relating to EUR_BOE and key geological parameters. Model outcomes are controlled by inputs for total organic carbon (TOC), vitrinite reflectance (%Ro), original formation and hydrostatic pressure difference (ΔPf–h) or formation pressure gradient (Pindex), effective shale thickness (Heshale), clay volume content (Vclay), fracture porosity (Φf)and other geological parameters pertaining to EUR_BOE. Shale oil and gas sweet spots in non-fractured zones must satisfy certain conditions including TOC ≥2.3 wt%, Ro ≥ 0.9%, Heshale ≥ 15 m, Vclay ≤ 25 vol%, and ΔPf–h between ∼3 and 35 MPa or Pindex between ∼1.15 and 1.85 MPa/100 m. Defining shale oil and gas sweet spots in fractured zones is more complex, because developing fractures and the application of advanced engineering technology will increase EUR_BOE; sweet spots are impacted by sealing conditions, fracture and fault development, and technical engineering conditions. The proposed method for evaluating shale oil and gas sweet spots is validated through the determination of sweet spots in the lower EFS, and provides a basis for the evaluation of shale oil and gas sweet spots in similar marine deposits.
