Despite the downturn in oil and gas prices, shale reservoir production continues and is expected to grow as prices normalize. The total gas in place is a measure of the total natural gas content in a shale which consists of both free gas in the porous spaces of the shales and adsorbed gas on the surface of the shale matrix. The total gas in place is dependent on the pore pressure and temperature and is vital to the profitable development of a shale reservoir. The goal of this work has been to employ NMR T2 distributions to derive gas isotherms.
Gas isotherms are derived from NMR data as follows. First, the total gas content is derived from the distributions at a series of pore pressures. Next, the volume of absorbed gas is estimated using the pore volume of the rock and the Van der Waals gas equation. The pore volume of the rock was measured in a separate experiment using the fully water saturated core. The adsorbed gas content is then calculated by subtracting absorbed gas content from the total gas content.
The NMR method has been successfully employed to derive gas isotherms. With the success of the isotherm measurement via NMR, experiments have expanded to include imaging natural gas in rock cores as a function of time and position. These experiments are difficult as the density of natural gas is approximately 1400 times lower than water at ambient temperature and pressure leading to a significant decrease in the signal to noise ratio when imaging gas as compared to water. Nonetheless, these experiments are valuable for determining where in the core gas is located and how the gas fills the core over time (i.e. into cracks first followed by occupation of pores).
Traditionally, gas isotherms are measured by exposing the core to helium and methane at ever increasing pressure while tracking the volume of gas absorbed and adsorbed. These experiments involve destruction of the core by crushing the sample and provide no information on the pore size distribution. The NMR method proves advantageous as the NMR analysis can be completed without destruction of the shale core. In addition, NMR pore distributions are obtained at each pore pressure as part of the experiment. These distributions can be further mined for information on adsorption versus absorption.