This study introduces a novel outlook on a shale pore system and pore compressibility values in shale gas formations. Pore volume compressibility is one of the vital reservoir properties in order to optimize the production of hydrocarbons in an accurate way in shale formations. However, the use of pore compressibility is often oversimplified in reservoir analysis. Correcting for pore compressibility allows for improved accuracy in both geomechanic aspects and hydrocarbon reserve evaluation. In this work, we analyze pore compressibility values and its impact on production performance for several major North American shale plays.
We divide porosity of the system into accessible and inaccessible pores, and incorporate inaccessible pores with grains into the part of the rock that is not accessible. Generally speaking, accessible pores contribute to flow directly while inaccessible pores do not.
We present a mathematical model using Mercury Injection Capillary Pressure (MICP) data to determine accessible pore and inaccessible part of the rock (IRP) compressibility as a function of pressure. During MICP testing in a typical shale sample, the rock sample experiences conformance, compression, and intrusion as effective pressure increases. We characterize the compressibility value based on MICP data as a function of pressure. The calculated compressibility values for accessible pores generally appear to be much greater (two to three orders of magnitude) than those of IRP.
Next, we studied how calculated accessible pore compressibility can affect gas recovery in shale gas plays. Numerical approach is used to evaluate impact of compressibility on gas production. Reservoir compaction is generally regarded as total pore compressibility, which has a value ranging within the 10−6 magnitude. By recognizing the part of the pore system that actually contributes to production and identifying its compressibility, we can substitute total pore compressibility with accessible pore compressibility. Our results indicate that substitution of accessible pore compressibility with total compressibility can significantly change the reservoir behavior prediction. Moreover, results suggest that at initial stage high compressibility value will contribute to production significantly. However, at the same time, this high compressibility in shale matrix, will lead to pore throat closure, thus loss of connectivity at later time.
The outcome of the paper changes the industry’s take on prediction of the reservoir performance, especially the rock compaction mechanism. This study finds that shale production owing to rock compaction is in fact much greater than what has been often regarded. Having a better understanding of pore compressibility effect leads to better reservoir estimation, production prediction, and less failed wells due to economic feasibility inaccuracies.