A fully coupled thermal-hydraulic-mechanical (THM) finite element model is presented for fractured geothermal reservoirs. Fractures are modelled as surface discontinuities within a three-dimensional matrix. Non-isothermal flow through the rock matrix and fractures are defined and coupled to a mechanical deformation model. A robust contact model is utilised to resolve the contact tractions between opposing fracture surfaces under THM loadings. A numerical model has been developed using the standard Galerkin method. Quadratic tetrahedral and triangular elements are used for spatial discretisation. The model has been validated against several analytical solutions, and applied to study the effects of the deformable fractures on the injection of cold water in fractured geothermal systems. Results show that the creation of flow channelling due to the thermal volumetric contraction of the rock matrix is very likely. The fluid exchanges heat with the rock matrix, which results in cooling down of the matrix, and subsequent volumetric deformation. The cooling down of the rock matrix around a fracture reduces the contact stress on the fracture surfaces, and increases the fracture aperture. Stress redistribution reduces the aperture, as the area with lower contact stress on the fracture expands. Stress redistribution reduces the likelihood of fracture propagation under pure opening mode, while the expansion of the area with lower contact stress may increase the likelihood of shear fracturing.

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High-resolution numerical simulations give clear insights into the three-dimensional structure of thermal convection associated with black-smoker hydrothermal systems. We present a series of simulations that show that, at heat fluxes expected at mid-ocean ridge spreading axes, upflow is focused in circular, pipe-like regions, with the bulk of the recharge taking place in the near-axial region. Recharging fluids have relatively warm temperatures. In this configuration, the system maximizes its heat output, which can be shown to be linked to nonlinearity in the fluid properties. Furthermore, we present a series of simulations with different permeability scenarios. These show that when permeability contrasts are moderate, convection maintains this pipe-like fluid flow structure. The permeability contrast has a dominant effect on flow patters only at early, immature, stages of convection, focussing upflow in high-permeability regions and downflow in low-permeability regions. In such early stages of convection, diffusive vent styles can emerge, which look remarkably similar to diffuse vent fields in natural systems. Finally, simulations in which permeability is defined as a function of temperature indicate that the brittle-ductile condition is likely to occur at temperatures not lower than 650°C. At lower brittle-ductile transition temperatures, the system cannot remove the heat delivered from the magma chamber and vent temperatures are substantially lower than 4000C. This result is in agreement with estimates of the brittle-ductile transition temperature from rock-mechanical studies and the occurrence of earthquakes in the oceanic lithosphère.@en

Realistic simulation of structurally complex reservoirs (SCR) is challenging in at least three ways: (1) geological structures must be represented and discretized accurately on vastly different length scales; (2) extreme ranges and discontinuous variations of material properties have to be associated with the discretized structures and accounted for in the computations; and (3) episodic, highly transient and often localized events such as well shut-in have to be resolved adequately within the overall production history, necessitating a highly adaptive resolution of time. To facilitate numerical experiments that elucidate the emergent properties, typical states and state transitions of SCRs, an application programmer interface (API) called complex systems modelling platform (CSMP ++) has been engineered in ANSI/ISO C ++. It implements a geometry and process-based SCR decomposition in space and time, and uses an algebraic multigrid solver (SAMG) for the spatio-temporal integration of the governing partial differential equations. This paper describes a new SCR simulation workflow including a two-phase fluid flow model that is compared with ECLIPSE in a single-fracture flow simulation. Geologically realistic application examples are presented for incompressible 2-phase flow, compressible 3-phase flow, and pressure-diffusion in a sector-scale model of a structurally complex reservoir.

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