Development of a Multi-mechanistic Triple-porosity, Triple-permeability Compositional Model for Unconventional Reservoirs

Open Access
Author:
Siripatrachai, Nithiwat
Graduate Program:
Energy and Mineral Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 14, 2016
Committee Members:
  • Turgay Ertekin, Dissertation Advisor
  • Turgay Ertekin, Committee Chair
  • Russell Taylor Johns, Committee Member
  • Luis F Ayala H, Committee Member
  • James Terry Engelder, Outside Member
Keywords:
  • Reservoir Simulation
  • Unconventional Reservoir
  • Compositional Simulation
  • Capillary Pressure
  • Phase Behavior
  • Shale
  • Multicontinuum
  • Discrete Fracture Model
  • Numerical Simulation
  • Tight Rock
  • Hydraulic Fracture
  • Naturally Fractured Reservoir
Abstract:
Most existing compositional reservoir simulators often model fractures using local grid refinement, unstructured-grid, or fine-grid models. Modeling different scales of fractures requires a large number of grid blocks to capture the heterogeneity of the formation. Using a large number of grid blocks presents computational challenges, even with today’s powerful processors. An enhanced matrix permeability on the grid block that hosts short-scale fractures is commonly used to eliminate natural fractures and simplify the model. Additionally, several existing multi-porosity models may not be able to capture heterogeneity and flow behavior in different porosity domains. Sequential flow assumption is frequently made in their models. Flows between different porosity types are not fully coupled, and in some model, a simplified inter-porosity transmissibility function is used for any porosity pairs. The oil and gas reserves and flow of reservoir fluids are strongly dependent on phase behavior. Large capillary pressure values are encountered in tight formations such as tight-rocks and shales. The tiny pore throats in these formations result in large capillary pressure. The effect of capillary pressure in tight formations can significantly impact the fluid phase behaviors in the reservoir during production and enhanced oil recovery (EOR) processes. Not incorporating this effect into the simulation can result in an inaccurate estimation of ultimate recovery as well as inefficient design and implementation of EOR techniques. In spite of this, the effect of capillary pressure on phase behavior in tight reservoirs has not been well studied using compositional simulation, especially for hydraulically fractured reservoirs. In this research, a fully implicit, multi-mechanistic, fully coupled, triple-porosity, triple-permeability compositional model has been developed for unconventional reservoirs. The hydraulically fractured tight rock and shale reservoir is treated as a triple-porosity system consisting of matrix blocks, natural fractures (micro fractures), and hydraulic fractures (macro fractures). Small-scale fractures are handled by a dual-continuum model. An embedded discrete fracture model is used to effectively and efficiently capture the flow dynamics of hydraulic fractures at any orientations, honoring the complexity and heterogeneity of the fracture networks. The triple-porosity model enables us to assign reservoir properties corresponding to the porosity type. The flows in three porosity types are fully coupled without making the assumption of sequential flow. The inter-porosity fluid transfer honors the geometry of the intersecting porosity pair. The development of the proposed numerical model incorporates the effect of capillary pressure on phase behavior. The transport of hydrocarbon follows a multi-mechanistic flow mechanism that is driven by pressure and concentration fields. The simulator has been validated with analytical solutions and a commercial reservoir simulator for a single-porosity model and a dual-porosity, dual-permeability model, both with and without grid refinement. With the proposed model, we can accurately capture major physics of transport phenomena that have been done to date and have the most realistic modeling of fluid flow in hydraulically fractured tight rock and shale reservoirs. The simulator is used in parametric studies to investigate the production performance from hydraulically fractured reservoirs under different modeling techniques and the effect of capillary pressure on phase behavior on reserves and ultimate recovery. The simulator is used to study the impact of inter-porosity transport on the recovery and fluid transport and phase behavior in hydraulically fractured tight rocks and shale formations under high capillary pressure. The outcomes of this project are an improved understanding of phase behavior and fluid flow in hydraulically fractured shale and tight rocks and an increased accuracy of the production prediction and ultimate recovery.