Study of near-borehole thermo-hydro-mechanical processes

Abstract

Near-borehole coupled thermo-hydro-mechanical (THM) processes are critical in defining the success or failure of geothermal projects. Injectivity decline due to clogging or the increased viscosity of cold water reduces the safe operational window of geothermal projects. However, near-borehole fracturing, a result of near-borehole coupled thermo-hydro-mechanical (THM) processes, is thought to possibly be able to contribute to maintaining or improving the re-injection performance. Understanding and being able to simulate the fracturing processes is therefore of great significance for both analysing re-injection and designing stimulation operations. This thesis presents a geo-mechanical tool, based on the finite element method and cohesive zone model, to simulate the fracture initiation and propagation under THM loadings. The geo-mechanical tool is used to study several stimulation scenarios, including monotonic, stepwise, cyclic, and stepwise combined with cyclic stimulation. To monitor such stimulation processes, this thesis proposes a dual-cable distributed acoustic sensing (DAS) system in a single vertical well and investigates its feasibility to localise and understand near-borehole cracking events.

In the numerical method, possible discontinuities are represented by zero-thickness triple-nodded interface elements, which allow solid elements to separate with mechanical damage and the simulation of longitudinal and transversal fluid/heat flow in the discontinuity. The cubic law is used to simulate the fracture transmissivity changes, while an elasto-damage law is used to characterise the mechanical response of the discontinuity. To simulate the fracture initiation and propagation from high-permeability intact rock, interface elements are inserted in-between all the solid elements, with high stiffness and transversal hydraulic coefficient assigned to reduce artificial compliance. An artificial heat conductivity is introduced to stabilise the numerical solution, in which high Peclet numbers lead to numerical divergence. Substantial verifications and validation are implemented to demonstrate the performance of the developed method.

A new elasto-damage law is developed by incorporating a fatigue damage variable into the tensile branch, in order to account for the fatigue effects during the simulation of cyclic (thermal) stimulation to geothermal reservoirs. The fatigue damage variable is calibrated using the number of loading cycles and fatigue life at different load intensities, with Palmgren-Miner’s rule used to account for varying-amplitude cyclic loading. The proposed model is validated against extensive laboratory tests, including cyclic Brazilian test, cyclic hydraulic fracturing test and cyclic thermo-hydraulic fracturing test. The validation results show good agreement with the experimental data, demonstrating that the proposed model is capable of handling fatigue damage under cyclic and coupled THM loadings.

The developed tool is then used to study stimulation to a synthetic sedimentary reservoir, which, according to regional experience, is assumed to be clogged in the near-borehole region. THM simulations of various stimulation strategies - monotonic, stepwise, cyclic, and stepwise combined with cyclic - demonstrate that the stepwise stimulation yields the most favourable outcomes. Specifically, it enables a significantly lower peak injection pressure with more near-borehole damage. This performance is not achievable using either monotonic or cyclic strategies (assuming same Qinj and Tinj). Conversely, cyclic-injection-rate stimulation slightly underperforms (under high injection rate) or slightly outperforms (under low injection rate) the monotonic stimulation. A combined approach incorporating both cyclic and stepwise strategies may lead to slightly better stimulation performance, showing lower peak pressure, compared to corresponding monotnic stimulation, but is inferior to the stepwise stimulation alone.

The feasibility of using a single-well dual-cable DAS to fully localise and understand the near-borehole micro-seismic events is investigated based on synthetic signals, assuming homogenous and isotropic media. A localisation method is introduced to determine the source depth, epicentral distance and azimuth. Sensitivity analysis shows that the localisation accuracy is not sensitive to source with frequency varying from 50 Hz to 200 Hz. But a low signal-to-noise ratio and/or source-to-receiver azimuth close to 0◦ can lead to decreasing accuracy. Moreover, resolvability analysis suggest that non double-couple moment tensor components Mxx,Myy and Mzz can be reliably resolved with an epicentral distance within 20 meters, showing improvement on the case of only one cable in a well. A discussion based on the geo-mechanical simulation demonstrates that the single-well dual-cable DAS can be used to understand near-borehole tensile fractures induced during thermal stimulation, with a limited epicentral distance, which implies it is well suited to monitoring stimulation operations.

This thesis contributes to the energy transition by developing a geo-mechanical model to simulate cyclic and coupled THM processes, including the development of fractures, around the near field of the wellbore which can allow the design of novel cyclic thermal stimulation and by proposing a single-well dual-cable DAS configuration that is demonstrated to be feasible to localise and understand near-borehole micro-seismic events to monitor thermal stimulation operations.