Digital Rock Simulation: A Novel Approach for Accurate Characterization of Perforation Tunnel Damage

Abstract

Abstract Shaped charge jet perforation is the most widely used method for establishing hydraulic communication between the formation and the wellbore. A primary objective of this method is to create "clean" tunnels that can efficiently transport hydrocarbons. However, during the perforating event, the detonation of the explosive charge inevitably compresses the formation, resulting in "crushed" or "damaged" rock surrounding the tunnel. This damaged region significantly reduces permeability and severely impedes the flow efficiency of the perforation tunnel. Consequently, it is critical to understand the physical characteristics (thickness and permeability/porosity) of the damaged zone while designing and optimizing perforating jobs. The small-length scales and arbitrary surfaces that encompass the damaged zone make it almost impossible and impractical for laboratory or traditional modeling methods to quantitatively measure the damaged perforation zone. In this study, a state-of-the-art digital rock simulation approach is used to conduct a detailed study of the damage mechanism surrounding a typical perforation tunnel. As part of this work, an API RP19-B Section IV test was conducted first using a Berea sandstone core. The perforated core was then sectioned and smaller core plugs were drilled around the expected damage zone at various locations. For each core plug, the digital rock analysis was conducted in two steps. The first step relates to digital rock generation consisting of micro-scale CT scanning, sample segmentation to give the three-dimensional representation of the pore-space structure, and pore space characterization. In the second step, a Lattice Boltzmann model was used to simulate single-phase fluid flow through various subdomains of the rock and predict the absolute permeabilities. The computed results from this study included continuous profiles of absolute permeability and porosity along with the thickness of the crushed zone. Pore size distributions (PSD) of selected subdomains were used to better characterize the pore-space. The porosity, PSD and permeability data in the damaged zone quantified the damaged zone thickness and showed the trends of how rock properties were significantly altered in the damage zone. The effect of the perforation process to the anisotropy of the rock properties was investigated by simulating cross-flow permeabilities in addition to the main-flow permeabilities. Further, three-dimensional whole-field visualization of physical properties and statistics within the rock structure were also presented. This study demonstrates a novel digital rock physics simulation approach that can be used to reliably measure the complex characteristics of a crushed zone surrounding the perforation tunnel. Computed rock characteristics data provided insight into the true physical characteristics of the crushed zone. These characteristics can be used in large-scale computational fluid dynamics (CFD) models to accurately simulate productivity, produce dynamic perforation clean-up models and most importantly, evaluate and understand the performance of shaped charges.