Gases Released From Organic-Rich Shales by Crush Analysis: Implications for Gas Generation, Desorption, Storage and Deliverability

Abstract

The gases that fill nanoscale pores in organic-rich shales can be released during crushing. Our results show that both thermal maturity and gas desorption contribute to changes in the CH₄/CO₂ ratio of gases released during rock crushing. CH₄/CO₂ ratios of these gases are lower at low thermal maturities and higher at high thermal maturities because more CH₄-rich gas is generated at higher maturity levels. However, no obvious compositional fractionation occurs among C1, C2, and C3 of crushed-rock gas, and C1/C2 and C2/C3 ratios remain nearly constant during crushing although these ratios are greatly increased overall when the level of thermal maturity is high. Gas geochemical parameters (C1/C2, C2/C3, and i-C4/n-C4) of released gas are good indicators of thermal maturation of organic-rich shales. Gas yields from a set of samples of varied TOC content but uniform thermal maturity, vary directly with TOC and porosity, suggesting greater pore connectivity for high TOC samples because of the development of organic matter-hosted pores critical to gas storage and deliverability. Trends in released gas yield and gas chemistry during rock crushing relate to gas storage states and pore connectivity. The δ13C1, δ13C2 and δ13C3 values of gas released from particles of coarser size (> 250 μm) are similar to values of gas produced from Barnett shales after hydraulic fracturing. CH₄-dominated gas appears to be stored in larger connected pores and is therefore released during the initial stages of crushing. The carbon-isotope values of C1, C2, and C3 are heavier in the more thermally mature samples, suggesting that this released gas is representative of the gas chemistry of the subsurface reservoir. Released gases provide a basis for understanding reservoir gas compositions and saturations even in older cores, bridging a gap caused by the scarcity of desorption data from fresh canister core. Chemical and isotopic composition of the released gases is an indicator of thermal maturity in subsurface reservoirs, and retained gas yield is directly related to variations of lithology and pore connectivity. This new method can provide useful information to evaluate gas storage and deliverability when old cores are available.

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