The objective of this study is to analyze and describe quantitatively the effectiveness of thermal EOR processes in increasing production from multi-fractured unconventional resources such as shale oil and tight sand reservoirs. The study evaluates the efficacy of circulation of hot fluids at different temperatures through dedicated unperforated wells in a process that involves direct heat injection without the concurrent injection of fluids that could cause problems of excessive pressures and adverse relative permeability regimes in these ultra-low-permeability (ULP) systems.

In this numerical simulation study, a modified version of the TOUGH+ compositional simulator is used to represent (a) the flow of heat by all possible transport mechanisms and (b) the flow of the three phases routinely involved in these ULP reservoirs. The simulation domain is a stencil, i.e., the minimum 3D repeatable element of a hydraulically-fractured unconventional reservoir, and is discretized using a high-definition (to a mm-scale) grid. The solutions associated with different thermal treatments are compared to a reference case that involves a simple, non-isothermal depressurization-induced production. The rate and composition of the production stream, as well as the spatial distributions of pressure, temperature, phase saturations, viscosities and relative permeabilities is continuously monitored during the simulation process.

This high-resolution 3D study simultaneously considers all thermophysical processes that are affected by the changes in pressure and temperature involved during these thermal EOR operations in ULP reservoirs, as well as their evolution over time: fluid flow, heat flow and transport by conduction and advection, phase density and viscosity, gas solubility in the liquid phases, phase changes and phase production rates. The study captures in detail phenomena that can be easily attenuated in coarser grids. An important contribution of this study is a detailed analysis of the various aspects of production affected by a thermal process as well as the quantification of mass and energy balance, as well as the associated losses. The results of the study indicate that thermal processes (even ones that involve long heating periods) lead to increases in hydrocarbon recovery that (a) are practically negligible to minor even under ideal conditions (under which heat losses are disregarded) and (b) cannot even begin to compensate for the significant energy needs of these methods. The major contribution of this study is that it provides documented evidence of, and sufficient quantitative information on, the ineffectiveness of thermal processes as possible EOR methods, thus reducing them in priority (if not eliminating them from further consideration altogether) as a viable EOR option.

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