Degree
Doctor of Philosophy (PhD)
Department
Petroleum Engineering
Document Type
Dissertation
Abstract
Natural fractures play a critical role in the performance of unconventional oil, gas, and geothermal reservoirs due to their prevalent occurrence within these systems. However, modeling these fractures accurately and efficiently is still challenging. This work centers on an extensive analysis of naturally fractured systems and proposes new fracture modeling algorithms that can predict the performance of fractured tight rocks accurately and efficiently. Several researchers have explored various methodologies to accurately and efficiently model fractured reservoirs. Among the approaches that can account for each individual fracture in these reservoirs, the embedded discrete fracture model (EDFM) is one of the most popular. This is because of its ability to model fractures with any orientations and the fracture mesh does not have to align or conform to the matrix mesh. Despite its advantages, the EDFM is limited in that it cannot accurately model systems with low fracture conductivity. The projectionbased EDFM (pEDFM) was developed to address this limitation, but recent studies indicate that it still is unable to model low-conductivity fractures accurately when the fractures are not parallel to one of the three axes of the simulation domain. Consequently, reservoir fluids have been observed to bypass such low-conductivity or sealing fractures when the pEDFM model is used. Another significant challenge encountered in unconventional oil and gas (UOG) reservoirs and enhanced geothermal systems (EGS) lies in modeling hydraulic fracture propagation in naturally fractured reservoirs. The main challenge here is that it is extremely computationally expensive to simulate the propagation of the fracture while ensuring that the mesh for the simulation domain conforms to the propagating fracture. This challenge is worsened when pre-existing fractures are considered. Practically speaking, it is impossible to use standard finite element or finite volume methods to model the propagation of hydraulic fractures in realistic naturally fractured reservoirs while re-meshing the domain to conform to all these fractures. EGS and UOG reservoirs are typically fractured and have low-matrix permeability values.
The successful commercial development of UOG reservoirs using multistage fractured horizontal wells (MFHW) has resulted in the evaluation of related technology for the commercial development of EGS. The current approach for thermal recovery from EGS entails injecting cold water into horizontally or deviated wells with multiple fractures and producing hot water from an adjacent well positioned above the injector. However, the limited control over the hydraulic fracture location, size, and orientation in MFHWs could result in unpredictable and low thermal recoveries. In safe subsurface CO2 utilization and storage (CUS) projects, assessing and mitigating CO2 leakage through faults, fractures, and abandoned wells is critical. Unfortunately, most reservoir simulators only consider flow across faulted rock surfaces using transmissibility multipliers and do not capture the accelerated flow along the direction of the fracture. This is important because it can result in CO2 leakage into shallower aquifers. This study addresses the limitations of EDFM and pEDFM in modeling low-conductivity fractures of any arbitrary orientation in 3D. A robust algorithm that ensures the continuous projections of inclined fractures on cell faces was developed to overcome these limitations. This enhanced model is referred to as the "continuous projection-based EDFM" (CPEDFM). Through the simulation of various cases and the examination of selected projection faces, the effectiveness of CPEDFM is demonstrated. The CPEDFM method is further validated by comparing its results with high-resolution simulation results. To showcase the feasibility of modeling complex and realistic systems using CPEDFM, a 3D compositional simulation of an Eagle Ford shale reservoir is conducted, featuring 75 low-conductivity and 75 high-conductivity fractures. These studies reveal that pEDFM overestimates production because of the leakage of reservoir fluids across inclined low-conductivity fractures. Additionally, this work proposes an unconditionally stable sequentially implicit simulation scheme that couples pEDFM with an extended finite element method (XFEM) for modeling hydraulic fracturing in naturally fractured systems. Unlike previous attempts, this approach accommodates flow modeling in fractures using either the porous media assumption (with a high fracture porosity) or lubrication theory (which models the fractures using the cubic law for flow in parallel plates). An alternative technology is presented for naturally fractured enhanced geothermal reservoirs. As demonstrated from our numerical studies, the proposed technology uses strategically configured mechanically cut fractures to efficiently recover heat from subsurface hot rocks. Precise control over fracture location, size, orientation, and conductivity facilitates the design of intersecting fracture configurations suited for this purpose. Lastly, in the context of CO2 storage in faulted/fractured reservoirs, this work presents numerical simulation results that underscore the significance of considering CO2 flux across and along fault planes/fractures using the transient embedded discrete fracture model in corner point grid (tEDFM-CPG). We show that the modeling of only CO2 flux across faults/fractures is insufficient for accurately predicting CO2 migration. The proposed tEDFM-CPG method is demonstrated as an accurate and efficient method for modeling CO2 flow in high-conductivity fractured reservoirs.
Date
3-18-2024
Recommended Citation
RASHID, HARUN UR, "DEVELOPMENT OF ACCURATE EMBEDDED FRACTURE MODELS FOR NATURALLY FRACTURED RESERVOIRS" (2024). LSU Doctoral Dissertations. 6379.
https://repository.lsu.edu/gradschool_dissertations/6379
Committee Chair
Olorode, Olufemi