Geological Controls and Computational Schemes of Shale Gas Sorption Capacity in Lower Silurian Longmaxi Formation From the Southeastern Chongqing, China

Abstract

High-pressure methane sorption experiments on eight Lower Silurian Longmaxi shale moisture- equilibrated samples from the Southeastern Chongqing, China, were conducted at pressure up to 20 MPa, at 20°C, 40°C, 60°C, 80°C and 100°C to investigate the effect of organic matter content, mineralogical compositions, pore structure and reservoir conditions (temperature and pressure) on the methane sorption capacity. The pore characterization of the shale samples was investigated using low pressure gas (N2 and CO₂) adsorption and field emission scanning electron microscopy (FE-SEM) observation. The total organic carbon contents (TOC) range from 0.45 wt % to 4.13 wt %. The minerals of the shale samples are dominated by clays (21 – 58 wt %) and quartz (29 – 57 wt %). For the entire shale samples the dominant clay minerals are mixed layer illite/smectite and illite. Both mineral matrix and organic matter pores are well developed with pore size from several to several hundred nanometers. The pore size distributions obtained from the combination of N2 and CO₂ adsorption data are bi- or multimodal. The methane sorption capacities of moisture-equilibrated shale samples show a significant positive correlation with TOC contents and BET surface areas. Not a simple linear but quadratic polynomial-law relationship was observed between the clay contents and methane sorption capacities. A threshold of clay content (42.3 – 44.4%) exists in this trend. The methane sorption capacities decline with increasing clay content under the threshold, and later increase with increasing clay content. The Langmuir pressure increases exponentially with temperature and the Langmuir volume decreases linearly with temperature. A computational scheme has been developed to calculate the methane sorption capacity of shales as a function of TOC content, temperature and pressure based on Langmuir sorption isotherm function. Using this algorithm methane sorption capacity of organic shales as function of depth was obtained. Due to the predominating effect of pressure the methane sorption capacity increase with depth initially, through a maximum and then decrease due to the influence of increasing temperature at a greater depth. The maximum gas sorption capacity typically occurs at a depth range between 800 and 1350 m. With TOC content increasing, the maximum methane sorption capacities of organic shales and the corresponding depths increase.

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