A New Logging Model for Assessing Adsorbed Gas Content of Continental Shale and its Application

Abstract

In a shale reservoir, adsorbed shale gas is critical to the evaluation and optimization of shale-gas exploration. However, calculating the adsorption gas content is a difficult problem in logging method. Most previous studies focused on qualitative description of field desorption and isothermal adsorption experiments and did not produce quantitative models to estimate gas content based on logging factors. Thus, this study built up a new quantitative calculation model for adsorbed gas content using logging data.

To select the controlling factors and build a quantitative model for continental shale gas, we chose the shale in the Upper Triassic Xujiahe Formation in the Sichuan Basin as target.204 samples were from the Xu5 member and a series of experiments were conducted, including isothermal adsorption experiment, field-emission scanning electron microscopy (FE-SEM), total organic carbon (TOC) content analysis, low-pressure CO₂ and N₂ adsorption, high-pressure CH₄ isotherm analysis. These experiments show that organic matter (OM) has the most specific surface area and is the most important adsorption matter in the reservoir, and gas sorption capacity is also controlled by the type and maturity of OM, mineral composition, temperature, and pressure. The adsorbed-gas volume is expressed as a function:V=((0.43×TOC+0.08×V_sh-0.013×T)×P)/(P+e^{（-2403.4/T+7.63)}), with R² =0.91. To obtain the adsorbed gas content of the whole well, the total TOC, Vsh, T and P can be calculated applying logging method prior to using the above formula. Based on the gas content logging interpretation model, the adsorbed gas contents can then be estimated. These results together with adsorbed gas contents determined using the isothermal adsorption method are relatively similar, with a correlation coefficient as high as 0.91, indicating that the method has a certain rationality and can be effectively used to calculate and characterize the adsorbed-gas content of the shale reservoir.

Key words: Adsorbed shale gas; Quantitative model; Logging methods; Western Sichuan Depression; China

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018