ABSTRACT:

Fracture aperture and fluid flow are affected by poro-thermo-mechanical processes and mineral precipitation/dissolution. In this paper, we study these phenomena by development and application of a three-dimensional porothermo- mechanical model with silica dissolution/precipitation effects. The solid mechanics aspect of the problem is treated using a poro- and thermoelastic displacement discontinuity method, while the solute transport and heat transport in the fracture are solved using the finite element method. The single component solute reactivity in the fracture is considered using temperature dependant reaction kinetics. The model is applied to simulate the impact of water circulation on the dynamics of fracture permeability in enhanced geothermal systems. Injecting under-saturated cold geothermal fluid causes large silica mass dissolution in the fracture in a zone that extended towards the extraction well over time, increasing the fracture aperture in this zone. Fluid pressure near the injection well initially increases with injection and aperture reduction in response to leak-off, however, pressure decreases as cooling proceeds. Thermo- and poroealstic stresses are induced in the reservoir matrix that cause secondary fracturing and possibly induce seismicity.

1. INTRODUCTION

During extraction of the thermal energy from subsurface, fluid flows through natural fracture/fracture networks and interacts to the adjacent rock-matrix in the reservoir. This circulation of low-temperature fluid in fractures leads to the variation in the geometry of fractures which can be described as its response to mechanical, thermal and chemical processes. Different aspects of thermal and mechanical processes have been studied by the researchers [1, 2, 3, 4, 5,6]. For example thermo-elastic effects are reported dominant near the injection when compared to those of poro-elasticity and under some conditions; silica reactivity may govern permeability [3]. Furthermore, experimental studies [7, 8, 9] also show that chemical precipitation and dissolution of minerals significantly affect fracture aperture. In general, simulating poro-thermo-elastic-chemical mechanisms involves solving a set of equations each for fluid flow, heat transport, solute transport/reactions and elastic response of the reservoir and more importantly, these processes are coupled. In present work, we use a partially coupled poro-thermo-elastic approach [6] and the displacement discontinuity method [5] to compute the poro-thermo-mechanical processes. On the other hand, we use finite element method for reactive flow and heat transport to find the solution for their distributions in the fracture. The solute reactivity and solubility in fracture is considered using a temperature dependent formulation (e.g., [10, 11]) and presented in detail in following subsections.

2. GOVERNING EQUATIONS

The physical and chemical processes associated to the geothermal injection/extraction are represented and described by a number of equations, which obtained by considering constitutive models, transport, and balance laws (.e.g. fluid momentum, fluid continuity).

3. NUMERICAL SOLUTION

In this study, the system of equations((3)-(9); (10)-(13); (14)-(17)) is solved using combined finite element and boundary element method. For example, we use Galerkin's finite element method to model fluid flow, heat and solute transport in the fracture, whereas boundary element is used to compute diffusive fluid flow, conductive heat flow and mineral diffusive transport in the reservoir matrix (three-dimensional space).

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