Abstract
Even though the advances in hydraulic fracturing and horizontal well technology have unlocked considerable reserves of hydrocarbon contained in shale formations, quantification of the key petrophysical properties in shale reservoirs still remain challenging. The unconventional formations such as shales typically have permeability values in nano-Darcy range. It is not practical to measure the permeability of the shale samples by conventional steady state techniques because of very low flow rates and length of time required to reach the steady state condition. Therefore, unsteady state methods such as GRI and pressure pulse decay which have been extensively used to estimate permeability of the shale samples. However, the measured permeability values for similar samples by these techniques can differ by 3 orders of magnitudes. This large margin of uncertainty and non-uniqueness of the results can be attributed to the interpretations of the data obtained from transient techniques and reproducibility problems. Furthermore, the unsteady-state measurements are not performed under the reservoir effective in-situ stress and temperature which introduces more limitations.
This paper describes the design, construction, and application of a laboratory set up and test protocols for accurate and repeatable measurements of the rich organic shale porosity, permeability and sorption characteristics under reservoir stress conditions. The near steady-state measurements can be performed in minutes up to hours, depending on the characteristics of the sample. This allows the permeability measurements to be performed as many time as need to produce repeatable results. Furthermore, the absolute permeability of the sample can be calculated by applying the gas slippage (Klinkenberg) and/or gas double-slippage corrections. The monitoring of the flow rate during the experiment allows to determine when the sample is fully saturated (adsorbed or desorbed). Therefore, there is no need for gas sorption correction.
The paper also present the results of measurements on several Marcellus shale sample which clearly indicate that shale's porosity and permeability to a larger extent, are sensitive to stress and the stress impact on permeability is non-reversible. This could be attributed to flow of gas through micro-fractures, which do not contribute to porosity significantly. In addition, the analysis of the measurement results indicated that only gas double-slippage corrections can provide reliable values for the absolute permeability of the sample. The sorption characteristics, although not the primary objective of the measurements with laboratory setup, are compatible with published values.
