Fast Permeability Estimation of an Unconventional Formation Core by Transient-Pressure History Matching

Summary Unconventional resources are of great importance in the global energy supply. However, the ultralow permeability, which is an indicator of the producibility, makes the unconventional production challenging. Therefore, the permeability is one of the critical petrophysical properties for formation evaluation. There are many existing approaches to determine permeability in the laboratory using core analysis. The methods can be divided into two categories: steady-state and unsteady-state approaches. The steady-state approach is a direct measurement using Darcy's law. This approach has disadvantages because of the accuracy in the measurement of low flow rate and the long run time. The unsteady-state approach includes pulse decay, oscillating pressure, and Gas Research Institute methods. These approaches are complicated in terms of setups and interpretations. Both steady-state and unsteady-state approaches typically have a constraint on the maximum differential pressure. We propose a novel unsteady-state method to determine the permeability by transient-pressure history matching. This approach involves simulation and experiments. On the experiment side, the ultralow-permeability core undergoes 1D CO2-flooding experiments, during which the transient pressure is monitored for history matching. On the simulation side, the transient-pressure history is simulated using the finite-volume method incorporating real-gas pseudopressure and table lookup to deal with the nonlinearity in fluid properties and singularity during phase transition. The free parameter permeability in the simulation is adjusted for history matching to determine the rock permeability. Our new unsteady-state approach is developed for fast and convenient permeability estimation for unconventional formation cores. This approach is a valuable addition to existing permeability measurement methods.