ABSTRACT

Heat extraction from deep, hot and tight rocks for energy production is based on forced water circulation through fractures. This process will cause changes in fracture aperture distribution and may also induce seismicity. To account for these, it is necessary to consider the role of poro-mechanical, thermal and geochemical process on intact rock and fracture deformation. A model is developed in this work for three-dimensional analysis of intersecting natural and hydraulic fractures. The model considers the geometric nonlinearity of the joint deformation in shear and closure. Temporal variations of injection/extraction rate and pressure-dependent leak-off are also considered. The model uses a coupled finite element/boundary element method to solve for stress, fracture displacements as well as fracture and matrix fluid pressure. The displacement discontinuity method is used to model the mechanical response of the fractured media while considering the non-linear joint response. Examples are provided to show the impact of joint interactions on potential for slip, fracture permeability, and the flow path and their dependence on the in-situ stresses conditions and the injection rate. Poroelastic effects on joint deformation are also highlighted. The model shows to be versatile for studying the unconventional reservoir stimulation and understanding of induced micro-seismicity.

INTRODUCTION

Whilst designing geothermal reservoirs, the impedance factor, water loss rate, and availability of an adequately large heat exchange surface between the rock and the circulating fluid are considered to control the economic viability of heat extraction operation. As the fractures are the major pathway for fluid flow and heat exchange, analysis of their spatial-temporal behavior has been the focus of many investigations, which have shown that coupled poro-mechanical, thermal, and geochemical process have a large influence on fracture permeability evolution. Generally, predicting the impact of the interactions of these processes in natural and man-made fracture requires numerical simulation. Two approaches can be used for this purpose, a statistical fracture network approach in which the reservoir is simulated using a system of fractured rock blocks, and a deterministic fracture modeling approach wherein the major fractures are directly modeled. These fractures often are distinguished by direct imaging of the wellbore and geological/geophysical studies. During past decades, various numerical models in each category have been developed and used in reservoir simulators. For example, Bruel introduced a simplified thermo-poroelastic method for simulating a circulation test in Soultz-sous-Forets geothermal project in Rhine Garben, France, without explicit consideration of flow through the rock. Wessling et al. simulated water injection into a fracture using 2.D-ROCMAS finite element software which has a coupled flow-geomechanic capability. Mathias et al. modeled the problem without coupling between hydrological and mechanical processes and assumed a constant total stress and fracture aperture during injection/extraction cycles. Swenson et al. developed a finite-element model to solved the problem using a 2D finite element method by assuming 1D fluid flow and constant joint stiffness. The displacement discontinuity method has proven to be particularly effective for the class of problems involving a finite number of discrete fractures within the circulation system.

This content is only available via PDF.
You can access this article if you purchase or spend a download.