The flow behavior in nano-darcy shales neighbored by high conductivity induced natural fractures violates the assumptions behind Arps' decline models that have been successfully used in conventional reservoirs for decades. Current decline curve analysis models such as Logistic Growth Analyses, Power Law Exponential and Duong's model attempt to overcome the limitations of Arps' model. This study compares the capability of these models to match the past production of hundred shale oil wells from the Eagle Ford and investigate how the choice of residual function affects the estimate of model parameters and subsequently the well life, pressure depletion and ultimate recovery. Using the proposed residual functions increased the tendency of deterministic models to have bounded estimates of reserves. Results regarding well performance, EUR, drainage area and pressure depletion are obtained quickly and show realistic distributions supported by production hindcasts and commercial reservoir simulators. Overall, the PLE and Arps' hyperbolic models predicted the lowest/pessimistic and highest/optimistic remaining life/reserves respectively. The newly proposed residual functions were thereafter used with the Arps' hyperbolic and LGA models. We found that the use of rate-time residual functions increased the likelihood of the value of hyperbolic exponent being less than 1 by 87.5%. The proposed residual functions can be used to provide optimistic and conservative estimations of remaining reserves and remaining life using any of the above decline models for economic analysis. The key results provided by the modified DCA models help in long-term planning of operations necessary for optimal well completions and field development, accomplished in a fraction of the time currently required by other complex software and workflows.
Gas Injection, Huff-and-Puff Enhanced Oil Recovery (EOR) technique have the potential to improve liquid hydrocarbon recovery in ultra-tight, unconventional reservoirs. This paper studies the technical and economic viability of this EOR technique in Eagle Ford shale reservoirs using natural gas injection – generally after some period of primary depletion, typically through long horizontal reach wells that were hydraulically fractured. To achieve this, three primary steps were undertaken: First, a series of multi-well, compositional simulation models were constructed, calibrated with lab data, and history matched for an extended production period. This effort characterizes a set of equiprobable combinations of fracture and matrix properties, as well as the parametric description of the stimulated reservoir volume. Second, these history matched models were then used to numerically simulate the Gas Injection Huff-and-Puff EOR process to determine a set of optimized operational variables (operating pressures, injection pressure, cycle durations, the corresponding injection rate, and slug size). The results were also sensitized to the effect of geomechanics, containment, as well as the effect of diffusion. The primary source of information that feeds the sensitivity analysis was derived from laboratory work investigating the EOR processes at the core scale. The third and last step, economic analysis was performed using calibrated rate profiles to assess the impact of initial yield and the amount of depletion on value. Resulting analysis provided insight to the economic viability of the EOR deployment at field-scale. Results show that the recovery factor uplift, all things being equal, is a function of the original yield, the amount of depletion, and the minimum operating pressure during the production cycles. In reality, however, equally as critical to the success of an EOR project is the formulation of the deployment strategy - the timing of the development start (forecasted price environment), pad selection, compressor scheduling, injection-soak-production durations, surveillance plans, and mitigation strategies (for poor containment and inefficient compressor utilization). The workflow utilized in this paper both characterizes the uncertainties in an EOR project in the Eagle Ford and provides insight into operating conditions and surveillance recommendations. This is the key for a successful demonstration pilot which can then lead to a field-scale EOR deployment.
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