Carbon isotope fractionation and production patterns of adsorbed and free gas during deep coal-rock gas production

Abstract

Taking deep coal-rock gas in the Yulin and Daning-Jixian areas of the Ordos Basin, NW China, as the research object, full-diameter coal rock samples with different cleat/fracture development degrees from the Carboniferous Benxi Formation were selected to conduct physical simulation and isotope monitoring experiments of the full-life-cycle depletion development of coal-rock gas. Based on the experimental results, a dual-medium carbon isotope fractionation (CIF) model coupling cleats/fractures and matrix pores was constructed, and an evaluation method for free gas production patterns was established to elucidate the carbon isotope fractionation mechanism and adsorbed/free gas production characteristics during deep coal-rock gas development. The results show that the deep coal-rock gas development process exhibits a three-stage carbon isotope fractionation pattern: “Stable (I) → Decrease (II) → Increase (III)”. A rapid decline in boundary pressure in stage Ⅲ leads to fluctuations in isotope value, characterized by a “rapid decrease followed by continued increase”, with free gas being produced first and long-term supply of adsorbed gas. The CIF model can effectively match measured gas pressure, cumulative gas production, and δ¹³C₁ value of produced gas. During the first two stages of isotope fractionation, free gas dominated cumulative production. During the mid-late stages of slow depletion production, the staged pressure control development method can effectively increase the gas recovery. The production of adsorbed gas is primarily controlled by the rock’s adsorption capacity and the presence of secondary flow channels. Effectively enhancing the recovery of adsorbed gas during the late stage remains crucial for maintaining stable production and improving the ultimate recovery factor of deep coal-rock gas.