Abstract:

The temperature difference between injected fluid and surrounding rock can play an important role in initiation and propagation of fractures in completion and simulation operations. In this work, the thermal effect is investigated using DEM-based micromechanics model implemented in Particle Flow Code (PFC). The rock mass is represented by an assembly of discrete particles that are bonded at contacts. In the developed thermo-hydro-mechanical (THM) module, heat conduction across the solids in the rock matrix and heat convection between the injected fluid and solid particles are coupled. The evolution of the system is driven by a continuous point source of heat. Simulation results show that the thermal induced stress, temperature and pore pressure predicted by this model are consistent with laboratory observations and simulation results from conventional continuum mechanics models. This study indicates that thermal stresses can greatly affect initiation and propagation of fractures and assist the communication between injection pressure and pore pressure in the rock formation; thermal fracturing can be considered as an effective method to connect fracture network, enhance weak zones and create more fractures.

Introduction

The effects of thermal contrast between injecting fluid and surrounding rock can play very important roles in initiation and propagation of fractures in completion and stimulation operations. A significant temperature difference between the wellbore fluid and rock formation may introduce additional compressive or tensile stresses, depending on whether the fluid temperature is higher or lower than the surrounding rock [1]. Most existing research works related to thermal stresses have focused on the geothermal and fractured reservoirs. For example, Zeng et al. [2] investigated the potential of heat production from deep hot dry rock and indicated that produced heat relies on thermal conductivity, injection temperature and water production rate. In a fractured reservoir, the heat flow within the fracture is primarily accomplished by advection; in the area outside the fractured zone, heat flow is mainly controlled by conduction [3].

The investigation of Thermo-Hydro-Mechanical (THM) process in fractured reservoir has been developed rapidly in recent years. Both numerical simulations and laboratory experiments showed that cooling injection is an effective approach to improve reservoir permeability.

Three-dimensional structural model showed that rock shrinkage will very likely take place after several months of cooling circulation and the tractions across fractures around reinjection borehole can significantly increase [4]. The laboratory studies showed that a strong thermal gradient is generated by increasing the number of cryogenic stimulation after injecting liquid nitrogen (LN2). The fracturing can be enhanced as a result of creating new cracks and reactivating/opening preexisting cracks. In addition, this method avoids the shortcomings of water-based hydraulic fracturing, such as formation damage, water supply shortage, and contentious political climate [5].

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