Pressure building-up methodology to measure gas content of shale samples

Abstract

The canister method for testing shale gas has been adopted from coalbed methane content testing standard. However, the method is inaccurate and may underestimate the gas content of shale samples because of poor estimation to lost gas in the process of recovering cores from the bottom to the wellhead of a well. In this paper, a new pressure buildup method is proposed to measure gas content at formation pressure and temperature. Since it avoids estimation of lost gas, shale gas content can be measured accurately. First pump methane into the canister in which a core is placed, and then pressurize the core until the pressure reaches the known formation pressure at known formation temperature. Then, after the pressure is balanced, degas and measure the gas released from the core. And finally, measure the residual gas after crushing a small part of the core when degassing rate is very low. The sum of desorbed gas and residual gas is the total shale gas. At unknown formation pressure, a linear equation with two unknowns -- pore volume at in-situ water saturation and adsorbed gas volume, is set up. Two linear equations with two unknowns are formed when the total shale gas tested at 25 MPa and 30 MPa for the same core, following the procedures for the case of known formation pressure, are substituted into the linear equation. Pore volume at in-situ water saturation and adsorbed gas volume are gained by solving the group of equations. Total shale gas can be calculated simply when formation pressure is gained. Unlike traditional methods, adsorbed gas and free gas can be obtained at the same time in the case of unknown formation pressure. Shale gas content measured by the pressure buildup method is 2–7 times as much as the standard.