Flow behavior of N2 huff and puff process for enhanced oil recovery in tight oil reservoirs
In the present work, the potential of N2 huff and puff process to enhance the recovery of tight oil reservoir was evaluated. N2 huff and puff experiments were performed in micromodels and cores to investigate the flow behaviors of different cycles. The results showed that, in the first cycle, N2 was dispersed in the oil, forming the foamy oil flow. In the second cycle, the dispersed gas bubbles gradually coalesced into the continuous gas phase. In the third cycle, N2 was produced in the form of continuous gas phase. The results from the coreflood tests showed that, the primary recovery was only 5.32%, while the recoveries for the three N2 huff and puff cycles were 15.1%, 8.53% and 3.22%, respectively.The recovery and the pressure gradient in the first cycle were high. With the increase of huff and puff cycles, and the oil recovery and the pressure gradient rapidly decreased. The oil recovery of N2 huff and puff has been found to increase as the N2 injection pressure and the soaking time increased. These results showed that, the properly designed and controlled N2 huff and puff process can lead to enhanced recovery of tight oil reservoirs.
Effect of displacement rates on fluid distributions and dynamics during water flooding in tight oil sandstone cores from nuclear magnetic resonance (NMR)
Abstract Water flooding is widely used as an economical and recyclable method to improve oil recovery in unconventional reservoirs. In this study, the effects of injection rates on oil distribution, the seepage dynamics and the contributions of increasing injection rates on enhancing oil recovery during water flooding are investigated. Typical tight oil sandstone cores with different permeabilities are obtained from Junggar Basin, Northwest China, for experimental studies that incorporate nuclear magnetic resonance analyses. Synthetic kerosene (viscosity = 11.2 mPa s) and heavy water (D2O) are used to simulate crude oil and formation water under formation conditions. The samples are first saturated with simulation water and thereafter displaced by synthetic oil until no water is produced to simulate the fully oil-saturated condition. To simulate the water flooding process, the samples are stably displaced by simulation water at injection rates increasing from 0.01 mL/min until no oil is produced. The T2 spectra, injection pressures, and produced oil at different injection rates and water injection volumes are measured simultaneously. The transverse relaxation time (T2) is converted for the corresponding pore-throat radius (r) by combining pore-throat size distributions (obtained from constant-rate mercury injection) and the fully oil saturated T2 spectra. It indicates that the amplitude of T2 spectra for macropore (>100 μm) and mesopore (10–100 μm) spaces decreases more rapidly than those for the micropore spaces (0–10 μm) as injection rate increases. The classical capillary number theory is introduced to represent the effect of injection rate and characterize the dynamics of seepage processes. Results show that the oil recovery factor can be considerably improved by increasing the injection rate in tight oil cores, and this significant improvement is primarily attributed to mesopores and micropores.
Compositional Modeling by the Combined Discontinuous Galerkin and Mixed Methods
In this work, we present a numerical procedure that combines the mixed finite-element (MFE) and the discontinuous Galerkin (DG) methods. This numerical scheme is used to solve the highly nonlinear coupled equations that describe the flow processes in homogeneous and heterogeneous media with mass transfer between the phases. The MFE method is used to approximate the phase velocity based on the pressure (more precisely average pressure) at the interface between the nodes. This approach conserves the mass locally at the element level and guarantees the continuity of the total flux across the interfaces. The DG method is used to solve the mass-balance equations, which are generally convectiondominated. The DG method associated with suitable slope limiters can capture sharp gradients in the solution without creating spurious oscillations. We present several numerical examples in homogeneous and heterogeneous media that demonstrate the superiority of our method to the finite-difference (FD) approach. Our proposed MFE-DG method becomes orders of magnitude faster than the FD method for a desired accuracy in 2D.