Bisheng Wu

Deep geothermal: The ‘Moon Landing’ mission in the unconventional energy and minerals space

Deep geothermal from the hot crystalline basement has remained an unsolved frontier for the geothermal industry for the past 30 years. This poses the challenge for developing a new unconventional geomechanics approach to stimulate such reservoirs. While a number of new unconventional brittle techniques are still available to improve stimulation on short time scales, the astonishing richness of failure modes of longer time scales in hot rocks has so far been overlooked. These failure modes represent a series of microscopic processes: brittle microfracturing prevails at low temperatures and fairly high deviatoric stresses, while upon increasing temperature and decreasing applied stress or longer time scales, the failure modes switch to transgranular and intergranular creep fractures. Accordingly, fluids play an active role and create their own pathways through facilitating shear localization by a process of time-dependent dissolution and precipitation creep, rather than being a passive constituent by simply following brittle fractures that are generated inside a shear zone caused by other localization mechanisms. We lay out a new theoretical approach for the design of new strategies to utilize, enhance and maintain the natural permeability in the deeper and hotter domain of geothermal reservoirs. The advantage of the approach is that, rather than engineering an entirely new EGS reservoir, we acknowledge a suite of creep-assisted geological processes that are driven by the current tectonic stress field. Such processes are particularly supported by higher temperatures potentially allowing in the future to target commercially viable combinations of temperatures and flow rates.

An Efficient and Accurate Approach for Studying the Heat Extraction from Multiple Recharge and Discharge Wells

In order to understand the thermal recovery Behavior of an engineered geothermal system (EGS), this paper develops a model in which fluid circulates in a single, planar hydraulic fracture with a constant hydraulic aperture via multiple recharging and discharging wells. The coupled equations for heat convection in the fracture plane and heat transfer into the rock are provided for steady and irrotational fluid flow conditions. By using velocity poten‐ tials and streamline functions, the temperature along a streamline is found to be only a func‐ tion of the potential. By utilizing the Laplace transformation, the analytical solutions in the Laplace space for the temperature field are found, which are numerically inverted for timedomain results. Several examples with different arrangements of injection and production wells are investigated and the comparison with other published results is provided. The semi-analytical results demonstrate that the proposed model provides an efficient and accu‐ rate approach for predicting the temperatures of a multi-well reservoir system.

An approximate solution for predicting the heat extraction and preventing heat loss from a closed-loop geothermal reservoir

Approximate solutions are found for a mathematical model developed to predict the heat extraction from a closed-loop geothermal system which consists of two vertical wells (one for injection and the other for production) and one horizontal well which connects the two vertical wells. Based on the feature of slow heat conduction in rock formation, the fluid flow in the well is divided into three stages, that is, in the injection, horizontal, and production wells. The output temperature of each stage is regarded as the input of the next stage. The results from the present model are compared with those obtained from numerical simulator TOUGH2 and show first-order agreement with a temperature difference less than 4°C for the case where the fluid circulated for 2.74 years. In the end, a parametric study shows that ( ) the injection rate plays dominant role in affecting the output performance, ( ) higher injection temperature produces larger output temperature but decreases the total heat extracted given a specific time, ( ) the output performance of geothermal reservoir is insensitive to fluid viscosity, and ( ) there exists a critical point that indicates if the fluid releases heat into or absorbs heat from the surrounding formation.

Predictions of fracture growth in Walloon coals using a layer fracture model

Individual low-permeability Walloon coal seams are separated by the stiffer beds of carbonaceous shales, mudstone, siltstone and sandstone in the Surat basin. Considering the thin nature of Walloon coals, the fracture growth across the stiffer beds becomes a key issue in assessing the fracture treatment efficiency and in determining the impacts of fracture stimulation on groundwater resources. A hydraulic fracture model considering the elastic property difference across layers is developed to obtain the fracture growth behaviours in a multiple layer formation. A case study was performed where the in situ stress across beds is of similar magnitudes to highlight the effect of property difference. The fracture initiates at the targeted lower coal seam and the complex footprints generated by fracture growth are obtained. In contrast to the assumed constant fracture height and the large fracture height to length ratio obtained by other layer models, the vertical fracture growth is limited in the propagation speed by the alternating stiff and compliant rocks. The alternating growth in lateral and vertical directions results in an oscillating pressure, which is an indicative for fracture height growth.