Huff-n-Puff (HNP) design for shale reservoirs using local dual-porosity, dual-permeability compositional simulation

Before implementing an HNP pilot in the field, reservoir studies are usually conducted, and compositional numerical simulations performed to assess the impact of uncertainty on HNP design parameters. In a previous study by the authors, the effect of parametric uncertainty on designing a single-well HNP was demonstrated using effective single-porosity models. However, recent studies have shown that a limited region of complex fracturing is likely to be created during the hydraulic fracturing process. In this study, we expand on the earlier work and address the impact of model uncertainty on designing an optimal HNP for a Duvernay shale example. In particular, the complex fracture regions are represented by local dual-porosity dual-permeability (DP-DK) models near the primary hydraulic fractures. Further, a multi-well HNP design is utilized to assess the impact of fracture communication during the cyclic gas injection scenarios. A unified framework is required to conduct Bayesian history matching and perform HNP simulations using the Markov chain Monte Carlo process. This task is achieved by implementing new adaptive sampling designs and employing some surrogate modeling techniques (Gaussian processes) to obtain the distributions for probabilistic HNP forecasts. The simulation results demonstrate that, for an equivalent calibrated DP-DK model, the efficiency of HNP, for both lean and rich gas injection scenarios, can be substantially higher than that predicted with the calibrated single-porosity model. In particular, lean gas injection, projected to have a low efficiency using single porosity models, is predicted to result in substantial incremental recovery in DP-DK models. The history matching and HNP simulation results demonstrate that DP-DK models provide the highest efficiency during early cycles with a reduced performance for later cycles. For single porosity models, the efficiency is much lower than the DP-DK models and is relatively constant across most of the cycles. The high efficiency of the DP-DK models is related to an enhanced mixing and extraction process due to pervasive communication (contact area) between the fracture network and the matrix. Additionally, the compositional simulations demonstrate that hydraulic communication between nearby wells through primary hydraulic fractures can substantially reduce the HNP performance. This study provides a novel workflow to accurately assess the impact of model uncertainty on HNP design for unconventional shale and low-permeability light oil reservoirs.