Delivering A More Consistent Estimation of Tight Rock Permeability

Abstract

AbstractPetroleum exploration and production from the tight reservoirs such as shale have gained great success in United States during last decade. Producing hydrocarbons from shale is challenging due to its low matrix permeability. It is therefore, crucial to investigate shale permeability to better understand the performance of unconventional resources wells. In addition, permeability is not only one of the key parameters in modeling fluids flow in matrix in reservoir simulation, but also a key parameter regulating production decline rate, which is critical to the stimulation process and to the economics of well. Since the inference of shale permeability is time consuming and expensive due to its low range of permeability, Gas Research Institute (GRI) method is applied as a shortcut. Unfortunately, many studies have pointed out that there is significant variation in the results reported by different laboratories using the GRI technique. It is difficult to identify that whether the differences are due to apparatus or the interpretation method, since GRI report did not provide a detailed description for interpreting the raw data. Furthermore, there is no available theoretical base in the literatures for the GRI technique. Due to the high uncertainty in the GRI method, it is worth to develop the theoretical basis for this technique. In this study, starting from the basic physics laws, we derived the governing equation for the fluid transient flow behavior in the GRI permeability measurement. The rigorous derivation provides a solid foundation for analyzing experimental data. The interpretation of test data can be more reliable through solving the governing equations either analytically or numerically. In this way, we can increase the agreement of the results from different laboratories using GRI technique. A case study is used to compare the developed mathematical model with literature data. The differences may be caused by the deviation from the assumptions in developing the theoretical model. Considering the non-spherical particles and non-uniform sizes of particles involved in the experiment, the multi-variable regression proposed in this study makes the analysis of GRI permeability data more practical and reasonable. In order to reduce the interpretation uncertainty, multiple reference cell pressures are used to measure the same crushed sample and generate corresponding pressure-decay curves. The proposed equation will allow engineers to interpret experimental data in a more practical and reasonable way.