Modeling of geothermal reservoirs poses significant difficulties for nonlinear solvers in reservoir simulation. This research focuses on designing novel numerical algorithms to overcome these nonlinear difficulties. These difficulties arise from the strong coupling between the mass and energy conservation equations. This strong coupling results in an apparent "negative compressibility" for blocks that have both liquid and steam phases. This "negative compressibility" presents significant difficulty for the standard fully implicit approach. An example of this is shown the figure, where the Newton paths diverge depending on the initial guess to the problem. To overcome this, different sequential formulations and nonlinear preconditioners are investigated and tested using the AD-GPRS (Automatic Differentiation-General Purpose Research Simulation) Framework.
Research over the past decade includes studies of well test analysis of fractured and multiphase reservoirs, design and interpretation of tracer tests in fractured reservoirs, adsorption in vapor-dominated reservoirs, experimental measurements of fluid flow parameters, and optimization of production and reinjection strategies.
Fracture Characterization in Enhanced Geothermal Systems by Wellbore and Reservoir Analysis (2009-2012)
This project will develop systematic techniques and tools to characterize the entire fracture network in Enhanced Geothermal Systems, both in the wellbore and in the reservoir. A complete characterization of fractures in the region both near the wellbore and in the interwell reservoir regions is central to maximizing the efficiency of energy extraction from Enhanced Geothermal Systems. By allowing for the optimal design of the recovery process, the results of this research will permit the energy extraction from a given area of enhanced fractures to be maximized. Given the significant cost of producing an enhanced fracture system in the field, the improvement of energy recovery is a key to making this energy source viable economically.
EGS stimulation modeling remains a relatively undeveloped field. There are major challenges associated with modeling EGS stimulation. It involves interacting hydraulic, thermal, mechanical, and thermoelastic processes. It is complicated by the need to account for the complex geometry of the preexisting fracture network. A handful of individual fracture pathways may take most of the flow between two wells. This study uses discrete fracture networks to tackle the challenges of EGS modeling.
Analytical and Experimental Study of Measuring Enthalpy in Geothermal Reservoirs with a Downhole Tool (2015-2017)
Downhole measurement of enthalpy could provide a better understanding of the performance of geothermal reservoirs and a more accurate estimation of the amount of available thermal energy than surface measurement. However, it is difficult to measure downhole enthalpy in two-phase flow from multiple feed zones. A downhole tool has been developed to measure chloride concentration in two-phase geothermal wells in real time. In this work, an analytical model to calculate downhole enthalpy based on chloride concentration measurement was developed.
Towards a Better Understanding of the Impact of Fracture Roughness on Permeability-Stress Relationships using First Principles (2011-2017)
In this study, the DDM model was used to investigate the changes in permeability due to applied stress conditions for rough fractures. Fracture aperture maps for the different stress conditions were generated using the DDM model. Afterwards, a local cubic law model was used to calculate the fracture permeability. Results showed that the roughness of the fracture surface created stress interactions that led to local opening in the absence of a fluid within the fracture. The rough fracture surface also created heterogeneous fracture aperture and slip distributions. Overall, the fracture permeability and average slip increased with the shear stress magnitude and decreased with the normal stress magnitude. Moreover, it was demonstrated that the fracture permeability was higher in the direction perpendicular to the applied shear stress direction compared to the parallel direction.
Fracture Connectivity of Fractal Fracture Networks Estimated Using Electrical Resistivity (2011-2013)
This work investigated a method of characterizing fracture connectivity in geothermal reservoirs using conductive fluid injection and electrical resistivity measurements. Discrete fractal fracture networks were modeled and a flow simulator was used first to simulate the flow of a conductive tracer through the reservoir. Then, the simulator was applied to solve the electric fields at each time step by utilizing the analogy between Ohm’s law and Darcy’s law. The time history of the electric potential difference between the injector and the producer gives information about the fracture network because the potential difference drops as conductive fluid fills the fracture paths.
The development of a reliable and accurate method to predict thermal breakthrough time is a significant open problem in geothermal reservoir engineering. Such a method would enable more informed decisions to be made regarding reservoir management. Methods developed at present include analytical models and solute tracers, both of which have limitations. The use of particles as temperature-sensitive tracers is a promising approach due to the high degree of control of the physical and chemical properties of nanomaterials and micromaterials. This could potentially be exploited to infer temperature and measurement location, which could in turn provide useful information about thermal breakthrough.
characterization and prediction modeling have long been among the more
challenging tasks in geothermal reservoir engineering. The main reason is the
presence of fractures and faults, which control the mass and heat transport in
the subsurface. In this work, the applicability of using statistical methods
for reservoir characterization as well as prediction modeling was explored.
Three methods were analyzed and applied on a synthetic library of fracture