Characterization of multi-fractured horizontal shale wells using drill cuttings: 2. Permeability/diffusivity estimation

Abstract

There is considerable research interest in the transport properties of shales to assist in their evaluation as reservoirs for natural gas and oil. However, shales have proven difficult to characterize, in part because of the challenges of obtaining viable reservoir samples from multi-fractured horizontal wells used to produce from them. Often the only reservoir samples available from horizontal wells are drill cuttings – the sample sizes obtained from cuttings are typically too small for quantitative analysis using conventional techniques. Therefore, new, high-precision methods are required to analyze the smaller cuttings samples. Further, the physics of gas storage and transport through the multi-model pore structure of shale is complex, requiring rigorous modeling approaches to extract parameters of interest such as permeability/diffusivity.In this work, the use of a high-precision, low-pressure adsorption device is explored for extracting permeability/diffusivity parameters from small amounts (1–2 g) of artificial (crushed core sample) drill cuttings of Duvernay shale. In order to extract the transport parameters, gas flow through the complex, heterogeneous matrix pore structure of the shale has been approximated using a general dual porosity numerical model which assumes that (1) gas flows through macropores by continuum viscous flow (2) gas flows through meso and micropores by Knudsen diffusion and molecular slippage on pore walls and (3) adsorption occurs in meso and micropores. The model can be simplified into two sub-models, a macro/micropore system or meso/micropore system, depending on the measured pore size distribution of the samples of interest.The new multi-pore (bidisperse) numerical model is applied to carbon dioxide low-pressure adsorption rate data obtained from the crushed Duvernay shale core samples, and apparent permeability for each gas/sample group is calculated at different pressure steps. The low-pressure adsorption device yields pressure-time data that is of much better quality than a commercial crushed rock permeability device that requires larger sample sizes. The new bidisperse pore structure numerical model, which allows permeability to vary (at each pressure step) due to gas slippage effects, properly describes the entire adsorption rate history of the samples studied. Mesopore apparent permeabilities range from1 × 10−2–1 × 10−3 mD and micropore apparent diffusivities are in the 1 × 10−7 mD range. The calculated apparent diffusivities obtained from modeling adsorption rate data change with pressure.The results of this study have important implications for shale matrix transport characterization. The resulting data can be used for making completions decisions and in reservoir models which capture reservoir property changes along a horizontal lateral.