Numerical analysis of cyclic CH₄ injection in liquid-rich shale reservoirs based on the experiments using different-diameter shale cores and crude oil

In this paper, the potential to enhance oil recovery (EOR) by CH₄gas injection in shale reservoirs is investigated through numerical analysis based on the experiments using different-diameter shale cores and crude oil. The cores used in these experiments had the same length of 2 inches and six different diameters of 1, 1.5, 2, 3, 3.5 and 4 inches. The crude oil from Wolfcamp was used to saturate the shale core plugs. A Gas Chromatography/Mass Spectrometer (GC/MS) instrument and Xcalibur software were used to analyze the crude oil components. The experiments were history-matched using numerical simulation models. The calibrated numerical models were then employed to perform a series of sensitivity studies to investigate the effects of operation parameters on oil recovery in shale oil cores, such as the number of injection cycles, molecular diffusion, soaking time, and operation schedule. The experimental results show that oil recovery factor is smaller from a larger core than that from a smaller core due to the decline of surface area per volume and the drop of pressure gradient along the diameter. The simulation results show that incremental oil recovery in each of the subsequent cycle decreases as the number of injection cycles increases. In terms of EOR mechanisms, viscous displacement and relative permeability hysteresis may be important, while molecular diffusion does not seem to play an important role after the first five cycles. For the soaking time, a larger core needs a longer soaking time to achieve the maximized oil recovery than a smaller core within a single cycle. Within a fixed elapse time, a shorter soaking time results in higher oil recovery. This work provides a fundamental understanding of the key parameters that control cyclic CH₄injection to enhance oil recovery in shale rocks.