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Near-wellbore numerical model to analyze hydraulic fracture initiation and propagation through cemented sliding sleeves at Utah FORGE geothermal reservoir, A
Abdimaulen, Dias
Abdimaulen, Dias
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2025
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Abstract
Geothermal energy is renewable, abundant, and environmentally friendly, holding significant potential to become a dominant power source. Geothermal energy can be extracted from four different types of geologic formations: hydrothermal, geopressured, hot dry rock, and magma. Hydrothermal geothermal reservoirs (HGR) have been the most common source of geothermal energy production worldwide. However, HGRs have not seen substantial expansion or gained prominence in the renewable energy discussions in the past decades because of their highly localized environment. The lack of expansion in HGR can be attributed to the unique requirement for abundant natural heat, water, and permeability – conditions that are rare and geographically limited. On the other hand, Enhanced Geothermal Systems (EGS) could transform the geothermal industry by utilizing advanced drilling, completion, and hydraulic fracturing technologies from the petroleum industry, thereby unlocking vast, previously unreachable heat reserves. However, heat extraction short-circuiting (i.e., rapid injected water breakthrough) poses a major challenge in EGS that could result in diminishing the overall efficiency of heat extraction.
To address this, a next-generation cemented sliding sleeve completion system has been proposed to enable multi-stage fracturing through the cement sheath and into granitoid formations while mitigating short-circuiting during operation. Unlike traditional methods that utilize predefined fracture initiation points (i.e., perforations), this approach requires fractures to initiate directly within an undamaged cement sheath before propagating into the granitoid formation.
This dissertation employs a commercial Finite Element Analysis (FEA) numerical simulation software to investigate fracture initiation and propagation emanating in a single sliding-sleeve segment of a horizontal wellbore. Simulations utilized local stress and thermal effects (i.e., near wellbore cooling), cement sheath addition to the granitoid rock environment, and presence of pre-existing micro-cracks and natural fractures. Sensitivity analyses included the impact of various local rock mechanical properties (i.e., formation breakdown pressure, presence of cement sheath, tensile strength) and wellbore eccentricity and orientation, pre-existing fractures, and thermal expansion.
The findings reveal that initial stress states in the cement sheath play a dominant role in determining the magnitude of breakdown pressures, with unstressed cement being the most likely scenario. Cement characterized by a high Young modulus and low Poisson ratio exhibits brittle behavior, making it more susceptible to fracturing at lower hydraulic pressures. Eccentricity lowers breakdown pressures, with slight deviations from concentricity causing the lowest breakdown pressure with longitudinal fractures preferentially forming in thicker cement regions. In a granitoid formation, intact rock requires the highest breakdown pressures, while the presence of pre-existing micro-cracks and natural fractures substantially lower the required pressures, depending on the orientation and size of the pre-existing flaws. Thermal cooling of the near-wellbore region lowers fracture initiation pressures, and large thermal gradients and expansion coefficients lead to fracturing independent of hydraulic pressures. Additionally, cooling increases fracture aperture and facilitates fracture reorientation into the preferred fracture plane. This research provides a critical insight into EGS fracture initiation mechanisms in support of more effective and efficient geothermal reservoir stimulation techniques.
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