Experimental Investigation of High Pressure, High Temperature (HPHT) Adsorption of Methane and Natural Gas on Shale Gas Samples

Abstract

The adsorption capacity of shale is commonly measured in the laboratory under low pressures. At low pressures, the excess adsorption capacity is approximately equal to the absolute adsorption capacity. Under high pressure, however, the excess adsorption is far less than the absolute adsorption capacity. The objective of this paper is to extend the adsorption measurements to a high temperature of 275 °F and pressure up to 9000 psi. Under such a HPHT, the adsorption curve shows a characteristic of supercritical high-pressure isotherm in which a critical desorption pressure can be identified. The adsorption isotherm under HPHT facilitates a better assessment of gas reserves for an effective assessment of shale gas reservoirs. The adsorption is measured based on a volumetric method. An in-house built setup was constructed to conduct the adsorption measurement at HPHT. The volume of sample cell and reference cell was calibrated with non-adsorbed Helium gas. Methane and field produced natural gas were used as adsorbate while the shale samples at different depth from a Gulf Coast organic shale were used as adsorbent. Excess adsorption measurements were carried out at reservoir temperature of 275 °F with 500 psi incremental pressure at a time until the pressure reaches 9000 psi. The absolute adsorption was calculated from the excess adsorption accordingly. The excess adsorption isotherm shows a typical supercritical fluid adsorption behavior. The adsorption increases with pressure, reaches a peak point at which pressure is defined as critical desorption pressure (CDP), and then decreases at pressures above CDP. The CDP of methane is higher than that of natural gas while methane is a majority of its component. The maximum amount of adsorption determined from the measurement varies from 203 to 213 SCF/ton. The Langmuir model does a good job predicting the absolute methane adsorption but fails to properly describe the adsorption behavior of natural gas at high pressure. This work presents a HPHT adsorption measurement at pressures and temperatures typically seen in deep shale gas reservoirs. The supercritical fluid adsorption behavior presented may assist the assessment of gas reserve and development of gas production.