Evolution of permeability of natural fractures due to THMC processes in the context of CO2-based reservoir applications
Embargoed until 2023-10-12
- Doctoral Thesis
As reserves decline and development costs increase, especially in frontier regions, understanding and preventing formation damage is crucial to guarantee the economic feasibility of exploitation of deep subsurface geological reservoirs. Formation damage is characterized by a zone of reduced permeability, potentially caused by coupled thermo-hydraulic-mechanical-chemical (THMC) processes that arise during typical operations, such as injection of fluids that are not preexisting in the reservoir, e.g., dry carbon dioxide. In this context, carbon dioxide injection can be considered as a perturbation to the presupposed equilibrated system. Injection of supercritical carbon dioxide (scCO2) into geological reservoirs is involved in several geoengineering activities, such as geothermal energy extraction, hydrocarbon production, and geological CO2 storage. In fractured-dominated reservoirs, THMC effects arising from scCO2 injection into brine-filled formations have a high potential of impairing the reservoir permeability due to the particularities of flow through fractures. These THMC effects act in a coupled and complex manner. Despite wide research on different effects impacting reservoir permeability, there is a need to conduct further studies, especially through laboratory experiments, to determine the hydraulic conductivity of fractures under prescribed combinations of confining stress, temperature and fluid chemical imbalance. This work focuses on fracture permeability behavior subjected to coupled THMC processes, by investigating: (1) the impact of effective normal stress on fracture absolute permeability in numerical simulations, supported by laboratory experiments, and on relative permeability curves in laboratory experiments; (2) the impact of temperature on fracture absolute permeability in laboratory experiments; and (3) the impact of mineral precipitation of fracture absolute permeability in numerical simulations. All rock specimens in this work are fractured granodiorite specimens obtained from the Grimsel Test Site (GTS), Switzerland, whose matrix has a negligible porosity. Therefore, the originality of this work lies in providing results on natural fractures submitted to coupled THMC effects, which are challenging to predict but, if the morphology of the fracture is available, the investigations potentially give valuable insights. First, to evaluate the impact of effective normal stress, sigma0, on fracture permeability, laboratory and numerical studies were performed. For the numerical studies, aperture fields of six naturally fractured specimens were first obtained under zero-stress conditions, via photogrammetric scans, and afterward under effective normal stress conditions of 2–30 MPa and temperatures of 25–400oC, by means of a contact mechanics model. Both, mechanical and hydraulic apertures decreased with an increasing sigma0 and temperature, and the results show that the distributions of fracture apertures have a direct impact on the fracture permeability response. Laboratory single-phase flow-through tests conducted in a GTS fractured specimen under effective normal stress conditions of 2–15 MPa also showed decreases of fracture permeability due to increasing sigma0. Additionally, laboratory tests were conducted to investigate the drying process of brine by scCO2 injection into another GTS fractured specimen, under conditions of 5-10 MPa effective normal stresses. A novel approach was developed to delineate the evolution of brine saturation and relative permeability from measurements of fluid production and pressure gradient across the specimen. Results from the laboratory experiments revealed lower mobility of brine and higher mobility of the scCO2 phase under higher effective normal compressive stresses. The analysis of relative permeabilities and fractional flow also suggested that higher effective normal compressive stresses increased channelling and decreased brine displacement efficiencies. Finally, lowering effective normal compressive stresses seems to hinder water evaporation. Secondly, laboratory flow-through tests on naturally-fractured granodiorite specimens were performed to investigate the impact of temperature on fracture absolute permeability. Two specimens were subjected to a constant flow rate of deionized water and the pressure gradient across the fracture was measured, while step-wise and constant-rate changes in temperature were applied to the pressure cell. For different levels of confining stresses (20–40 MPa), the temperature varied from 25oC to 140oC, for 2 to 3 cycles, yielding decreases in hydraulic apertures of 20–75%. This decrease was more pronounced during the cycles subjected to higher confining stresses. The tests show hysteretic behavior during individual load cycles, indicating temperature path-dependent behavior of permeability. Chemical analysis of the effluent samples suggests that the decrease in fracture absolute permeability is caused by THMC processes such as thermal dilation, mechanical creep, and pressure dissolution, triggered by high temperatures. Thirdly, to investigate the impact of mineral precipitation on fracture permeability, a novel numerical model was implemented into the MOOSE framework to simulate the injection of scCO2 into a brine-filled heterogeneous single fracture. The numerical model captures the changes in the fracture aperture distribution due to the volume of precipitated salt, which arises from the supersaturation of brine after water evaporation into the scCO2 stream. The simulations were carried out with aperture fields under effective normal stresses of 2–10 MPa. The results indicate impairments of fracture permeability due to mineral precipitation up to 21.98%, and larger impairments were observed for the cases of lower effective normal stresses. Interestingly, despite the experienced larger permeability reductions, lower effective normal stresses promoted lower volumes of precipitated salt, relative to the initial volume, when compared to the volumes of salt observed for higher sigma0. For cases of higher effective normal stress, the salt precipitated in regions that did not impair the permeability as much as it did for the cases of lower effective normal stresses. The simulation results demonstrate the importance of considering not only the overall reduction of the fracture volume when studying formation damage in heterogeneous fractures, but also the spatial distribution of the precipitate throughout the aperture field under the considered effective normal stress state. To sum up, this thesis highlights the importance of fracture closure driven by increasing temperature and effective normal stress, as well as clogging by mineral precipitation, as these effects can compromise the long-term operation of enhanced geothermal systems or the operation of any reservoir where the performance depends strongly on the transmissivity of natural or stimulated fractures. Show more
External linksSearch print copy at ETH Library
ContributorsExaminer: Saar, Martin O.
Examiner: Kong, Xiang-Zhao
Examiner: Hellevang, Helge
Examiner: de Oliveira L. Falcao, Flavia
SubjectFormation Damage; Permeability impairment; Fractures; CO2 injection; Thermo-hydraulic-chemical-mechanical coupling; Flow-through testing; Injectivity
Organisational unit09494 - Saar, Martin O. / Saar, Martin O.
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