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, along with the rock porosity and compressibility.
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 suffers from the accuracy in the measurement of low flow-rate and the long run-time. The unsteady-state approach includes pulse decay, oscillating pressure, and GRI methods. These approaches are complicated in terms of set-ups 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. On the experimental side, the ultralow-permeability core undergoes 1-D CO2-flooding experiments, during which the transient pressure is monitored for history matching. Another two rock properties that determine the transient-pressure history, namely the rock porosity and the pore-volume compressibility, are calculated based on the mass balance of CO2 at different states. On the simulation side, the transient-pressure history is simulated using real-gas pseudo pressure and table lookup to deal with the non-linearity in fluid properties. The free parameter, permeability, in the simulation is adjusted for history matching to determine the rock permeability.
Our simulation can generate high-quality transient-pressure history with the capability of handling the non-linearity and singularity in fluid properties. Our new unsteady-state method is validated by the standard steady-state method.
The advantages of this unsteady-state approach are: 1) it can be implemented with simple set-ups; 2) it can be finished within a considerably short-time period; 3) the data interpretation is straightforward; 4) it can be implemented over broad pressure ranges, even with phase transitions of the permeating fluids, not limited to CO2. This approach is a valuable addition to existing permeability measurement methods.