Elastic Dislocation Modeling Helps Identify Subseismic Natural Fractures

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 222464, “Elastic Dislocation Modeling Is an Indirect Tool for Subseismic Natural-Fracture Identification for Carbonate Reservoirs in Wildcat, Exploratory, and Development Blocks: A Case Study From the Complex Compressional Tectonic Zone of Pakistan,” by Imran Khan, Iftikhar Ali, and Muhammad Ameer Nawaz Malik, Oil and Gas Development Company, et al. The paper has not been peer reviewed. _ The main goal of this research work was to determine the subseismic faults and fracture corridors and their characteristics, including density and orientation, for a Paleocene fractured carbonate reservoir. This work addresses the subseismic faults and fracture-set identification and their effect on the reservoir’s potential regarding maximum hydrocarbon flow and wellbore stability and addresses the reasons behind a well failure in a fracture-dependent reservoir. Elastic Dislocation (ED) Theory Application of ED theory means calculating fault-related deformation. This ED theory provides validated seismic interpretation, identifies fracture zones, and calculates stress perturbations around major faults and fractures by incorporation of the mechanical properties of the rock under study. The total displacement in rock volume surrounded by major faults along with strain and stress can be predicted through ED modeling. These predictions depend on the accurate mapping of fault geometry and slip distribution in 3D seismic-reflection data sets. The density of the predicted sets of subseismic faults and fractures depends on the predicted local strain or the intensity driving the shear stress to exceed the rock-failure envelope. The fault and fault system play a key role in ED theory, which helps to identify strain on the objective horizon for fracture study. This ED technique operates under the assumption that the strain distributions surrounding these interpreted faults are controlled primarily by slip-on large-scale faults. For purposes of accurate calculation, the fault surfaces divided into parts (rectangular or triangular meshes) serve as the focal point for calculations of deformation caused by slipping on offset horizons because analytical solutions exist for the developed geometries based on seismic interpretation. Boundary-element models rely on a description of the discontinuities within the medium (i.e., the faults) rather than a discretized description of the medium (rock volume) itself. The control of large faults (those that are mappable) on the strain distribution in the surrounding rock volume is addressed by ED modeling. ED modeling does not depict all variables that could affect how the small-scale deformation features (subseismic faults and fractures) are generated. Objective of the Case Study The study was initiated to predict subseismic fault and fracture sets and their characteristics for a potentially fractured Paleocene carbonate reservoir in the complex compressional tectonic zone of Pakistan, where multiple thrust sheets exist in the structure framework of the area. This study also explored why one well drilled for the same Paleocene fractured carbonate reservoir failed to be produced, whereas two other wells have been produced from it. In the future, this study will be helpful in further exploratory or development well planning in the block area. In the study area, the major targets were Paleocene fractured carbonates, having the least primary porosity and permeability and depending on the secondary porosity and permeability. A validated 3D structural model was exercised through ED modeling to highlight the importance of faults, both on reservoir behavior and subseismic faults and fracture characteristics.