Comparative Analysis of Shale Permeability measurements (2015)
This MSc thesis has been written by former intern Tom Leeftink.
Gas has proved to be a successful extractable hydrocarbon resource from shale in the United States. In Europe results have so far been disappointing, but with increasing global energy demand a lot of interest remains in the potential of recoverable shale gas reservoirs. However, the way how the economic potential of shale is mapped is insufficient, because there are no industrywide measurement techniques available for measuring the flow properties of these (ultra)low permeable rocks. A proper assessment of the petrophysical properties of the cores from potential areas is therefore very difficult and the results are highly variable.
This study assesses the problems found after evaluating a round robin of experiments on selected samples. The multi-lab experiment was performed on crushed shale to compare the results between different laboratories. The permeability results from the various renowned laboratories were found to differ by multiple orders of magnitude on permeability results. For this study, a selection of core plugs and crushed material from various shale formations was analyzed using a wide spectrum of experiments and history match simulation models. The experiments were performed on the analyzed shales and consist of xenon expansion under a CT scan, as well as helium and methane expansion on linear, radial and crushed core plugs in confined and unconfined sealed core holders for different gas pressures. The measured data was history matched with multiple models including a single and multiple porosity-permeability model, with and without a high-permeability streak (e.g. fracture or silt layer) and the Klinkenberg effect. The combination of all these experiments and simulations resulted in a large dataset. After significant quality control, conclusions could be drawn from this data set, resulting in clearer insights into how porosity and permeability from shale samples can better be computed compared to the current technique that uses simulation results from a crushed GRI experiment.
Xenon flow in shales under a CT scan provided some insights into the permeabilities of the studied sample, but the accuracy is low. Propagation of the expanded gas over time could be monitored when the measured data was extracted from these images. If more information about the flow behavior of the expanded gas could be derived from the CT images of xenon invasion, this would improve the understanding and makes the simulation model more realistic.
The round robin test results for porosity were found to be similar between the laboratories. These experiments were conducted with the noble gas helium. Even though methane has a larger molecule size than helium, the computed porosity from methane expansion is larger due to adsorption on organic matter. Langmuir curve experiment results showed adsorption curves whose maxima positively with the TOC (total organic carbon) of the samples.
A more reliable approach than the round robin test results for permeability is proposed by combining the results of the experiments, such as perform a full core experiment on a radial sample drilled parallel to its lamination in an unconfined set-up, then measure a radial linear sample drilled perpendicular to its lamination in a confined set-up and finally perform a combination of these techniques. The first two of these experiments can be inverted with a single porosity-permeability model and the last one with a multiple porosity-permeability model. The first two results can thus verify the high and low case of the multiple porosity-permeability model, resulting in a more reliable result than obtaining permeability from crushed shale history matches.