A Novel Stage-Wise Degassing Approach to Evaluate Shale Reservoir Gas Nanoporosity and Permeability

Abstract

Evaluation of the total resource storage and productivity in shale gas and tight oil reservoirs provides critical information for shale oil/gas exploration and production. Among different evaluation parameters, porosity and permeability have often been assessed to infer the reservoir potential and the producing capacity. However, direct measurement of porosity and permeability in unconventional reservoirs is usually time-consuming and/or bears significantly uncertainty, which makes it difficult to apply to field operations demanding prompt and consistent measurements. Here, we present a novel stage-wise degassing approach to assess quantity of released hydrocarbon gases and the permeability of the shale samples. We quantify released gases from rock samples at each degassing stage via the PVT equation. We measure the gas carbon isotope ratios by gas chromatography-InfraRed Isotope ratio Analyzer (GC-IR2). For method calibration, we measured the nanoporosity, permeability and rock TOC via independent and well-established methods. We demonstrate that 1) stage-wise degassing amount implies the available gas distribution among different pore sizes, 2) δ13C1 increases during the degassing process, 3) wetness of recovered gas helps to estimate gas composition for a range of pore sizes, and 4) the largest δ13C1 change through degassing is related to the largest permeability among examined samples. Our approach provides rapid and consistent analysis on carbon isotope variations and degassing quantity. This method is applied to the Eagle Ford, the Woodford and the Sichuan Basin, and proves to be efficient and insightful in aiding the prompt decision on “sweet spot” during field operation.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015