Pore structure characteristics and its effect on shale gas adsorption and desorption behavior

As the largest producing shale gas field in China and had just announced its great annual production of 10 billion cubic meters capacity, the Wufeng-Longmasd (O(3)w-S(1)l) formation in Filling shale gasfield, Sichuan Basin had obtained great attention both in China and globally. Twelve samples from three different wells were collected to explore the pore types and the corresponding pore developing status, pore structure characteristics, sorption capacity and their relationship, for fundamental shale gas reservoir evaluation. The TOC (total organic carbon) content ranges from 0.21% to 6.79%, while quartz and clay mineral content made up 46.92% and 34.12% of the mineral composition. Intergranular pore, intragranular pore, fracture and different OM pore types (in biotic debris organic matter, within porous solid bitumen and in organo-mineral compound) were identified. Porosity obtained by different methods showed great difference because of detecting limitation, therefore it is necessary to combine multiple porosity detecting methods to explore the reservoir porosity more accurately. He porosity is not effective in detecting unaccessible discrete micropore in organic matter while NMR could reflect the nanopore although it is influenced by pore surface wettability. The preserved inorganic pores are affected by diagenesis, with intergranular pore volume and OM pores, without sufficient mineral framework, reduced by compaction and dissolved pores developed in unstable mineral, like carbonate. Clay mineral and other accessory mineral also influence OM pore development, as energy dispersive X-ray spectroscopy (EDS) result illustrated that solid bitumen mixing with mineral during hydrocarbon charging tend to be more porous. Surface area obtained from MIP, N-2-GA and CO2 experiment all showed positive correlation with the content of TOC, quartz and pyrite, which also are the main contributors of reservoir sorption capacity, total gas content and desorption rate.