CYCLIC SOLVENT INJECTION IN ULTRATIGHT RESERVOIRS BASED ON THE DIFFUSION PROCESS
Open Access
- Author:
- Cronin, Michael
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 04, 2019
- Committee Members:
- Hamid Emami-Meybodi, Dissertation Advisor/Co-Advisor
Hamid Emami-Meybodi, Committee Chair/Co-Chair
Gregory R King, Committee Member
Amin Mehrabian, Committee Member
Corina Stefania Drapaca, Outside Member
Russell Taylor Johns, Dissertation Advisor/Co-Advisor
Russell Taylor Johns, Committee Chair/Co-Chair
Mort D Webster, Program Head/Chair - Keywords:
- diffusion
shale
unconventional
phase behavior
enhanced oil recovery
EOR
hydrocarbon
ultratight
solvent
huff'n'puff
HnP
Eagle Ford
confinement
ultimate recovery
enhanced gas recovery
EGR
gas injection
cyclic injection - Abstract:
- This dissertation presents theoretical results on hydrocarbon recovery from ultratight reservoirs based on equilibrium phase behavior and diffusion-dominated transport within the matrix. The findings are applicable to both primary production and cyclic solvent injection (e.g. “huff’n’puff”) at the field scale. This study had three major elements. First, this dissertation presents a simple method to estimate ultimate recovery factors (URF) of ultratight reservoirs based on equilibrium phase behavior in which URF is only a function of changes in hydrocarbon density between initial and final states. URF is defined at infinite time and therefore does not depend on the transient behavior. Although URF may not be achievable during the life cycle of field development and production, it provides valuable insights on the role of phase behavior. Equilibrium phase behavior defines the absolute upper-bound for URF during primary production and explains the poor recovery from shale oil reservoirs compared to the high recovery factor in shale gas reservoirs in a unifying way. This study quantifies how injected solvent compositions (CH4, CO2, N2, and C2H6) during huff’n’puff enhanced oil recovery (EOR) improve recovery based on density reduction and compositional dilution and shows that the largest percentage increase in incremental recovery occurs for heavier oils. The calculations provide a practical means to define the URF from primary production as a function of reservoir fluid composition, temperature, and pressure drawdown. In addition, the calculations articulate incremental URF (IURF) of solvent huff’n’puff based on net solvent transfer into ultratight rock, which is a key design consideration. The results illustrate that solvent transfer dilutes the hydrocarbons in place, thus maximizing long-term hydrocarbon recovery. Net mass transfer can be improved by enhancing the diffusion of solvent into the matrix based on the huff’n’puff design parameters including solvent composition, drawdown pressure, and the net amount of solvent injected based on optimal frequency and cycle duration. Second, this dissertation provides for the first time a more physically realistic recovery mechanism in ultratight oil reservoirs based solely on diffusion-dominated transport. A diffusion-dominated proxy model is presented that assumes first-contact miscibility (FCM) to provide rapid estimates of oil recovery for both primary production and the solvent huff’n’puff (HnP) process in ultra-tight oil reservoirs. Simplified proxy models are developed that represent the major features of the fracture network in terms of parallel fractures connected to 1-D matrix slabs or intersecting fractures connected to 2-D matrix corner slabs. The key results show that diffusion-transport only can reproduce the primary production period within the Eagle Ford shale and model the HnP process well, without the need to use Darcy’s law. The mechanism for recovery is based solely on density and concentration gradients. Primary production is a self-diffusion process, while the HnP process is based on counter-diffusion. Incremental recoveries by HnP are several times greater than primary production recoveries, showing significant promise in increasing oil recoveries. The results show that methane injection is slightly superior to carbon dioxide for an equal mass of injected solvent (a comparative economic analysis was not conducted). It is also shown that the proxy model, to be accurate, must match the total matrix contact area and the ratio of effective to total contact area with time. These two parameters should be maximized for best recovery. Third, this dissertation presents a new semianalytical compositional model designed for primary production of multicomponent oil and cyclic solvent injection in ultratight oil reservoirs that is dependent on diffusion-dominated transport within the matrix (k < 200 nd) coupled to advection-dominated transport in the fractures. The semianalytical model consists of a well-mixed tank model for the fractures coupled to diffusive transport within the matrix. Production of oil, gas, and water from the fractures is proportional to its phase mobility. The matrix allows for differing effective-diffusion coefficients for each component. Because there are no grid blocks within the matrix, the analytical solution is computationally less expensive than numerical simulation while capturing the steep, nonmonotonic compositional changes occurring a short distance into the matrix that result from multiple injection cycles. The Peng-Robinson equation of state (PR EOS) (Robinson and Peng 1978) is used to calculate phase behavior with time within the fractures and to initialize density and mass concentrations within the matrix based on the semianalytical framework. The coupled convective (fracture) and diffusive (matrix) model is validated with several laboratory- and field-scale cases. For primary recovery, the results show that the model correctly reproduces the pressure and oil-recovery declines observed in the field. The results show that the hydrocarbon recovery mechanism for solvent huff’n’puff (HnP) is facilitated by greater density reduction and compositional changes. Two solvents are considered in HnP calculations: carbon dioxide (CO2) and methane (CH4). Recovery of heavier components is enhanced with CO2 compared to CH4 within the reservoir (matrix and fractures). Furthermore, the results demonstrate that multiple HnP cycles constrained to surface injection are needed to enhance density and compositional gradients, and therefore oil recovery. Although shorter soaks are better for short-term recovery (i.e., 3 to 5 years), longer soak periods maximize recovery over a longer timeframe (i.e., 10 to 15 years). The new approach provides a valuable limiting case based on diffusive matrix transport and convective fracture transport to determine the optimal number/duration of cycles and when to start the HnP process after primary recovery. In summary, these findings illuminate for the first-time hydrocarbon recovery in ultratight reservoirs in terms of equilibrium phase behavior and diffusion-dominated physics, which, provides much needed insight to practitioners trying to optimize recovery by primary and cyclic solvent processes.