Flow Simulator Optimizes Geothermal Heat Extraction From End-of-Life Wells
Authors: Carpenter Chris 
Published: February 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 208928, “Optimizing Geothermal Heat Extraction From End-of-Life Oil and Gas Wells Using a Transient Multiphase Flow Simulator,” by David Sask, SPE, David Sask Technology; Peter Graham, Algar Geothermal; and Carlos Nascimento, SPE, Schlumberger. The paper has not been peer reviewed.

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The complete paper presents results from evaluation of the rate of thermal energy that can be extracted under various completion scenarios using a transient flow simulator. This evaluation was conducted on closed-loop systems wherein the fluids are contained within the wellbore and surface facilities and do not involve any formation fluids. The use of a multiphase flow simulator for this study provides a road map for understanding thermal energy potential and important variables when considering extraction of geothermal energy from existing oil and gas wells.

Introduction
In Western Canada, there are, at the time of writing, more than 130,000 inactive and suspended wells. This includes more than 3,000 orphan wells but excludes an additional 115,000 abandoned wells.
For some of these wells, end-of-life liability can be turned into an asset. This paper presents an overview of one system for extracting geothermal energy from a single well configuration using a closed-loop mode of operation.

Equipment and Processes
The technology described in the paper calls for inserting an insulated inner tubing inside the existing production casing immediately above the sealed base of the well. Cooler water is injected down the annulus of the coaxial configuration. As the water descends, it collects heat from zones where the adjacent rock is hotter. At the base of the well, the heated water is then redirected to surface from the open base of the insulated inner tubing. At surface, the heat is recovered, and the resulting cooled water is reinjected into the well to complete the circuit. This closed-loop process has little pump demand because there is no hydraulic head. This technology was developed in the late 1990s but did not gain much traction because of the excessive cost of drilling, inadequate and expensive insulated tubing, and poor heat and temperature recoveries.
The technology has developed enhancements to overcome the latter limitation. One key enhancement is to operate the well in two different modes. The first mode operates as a storage mode during periods when no heat demand exists. Water is pumped into the annulus of the well and returned to surface within the insulated inner tube. Once the heated water reaches the surface, it is reinjected directly into the same well without any heat being recovered at surface. As the returning hot water descends, it transfers heat to the adjacent cement and rock, which creates a heat jacket in the upper sections of the well. When heat demand returns, the second mode, referred to as extraction mode, commences. The only change is that the ascending heated water is redirected to a heat exchanger, which extracts only heat sufficient to meet demand; then, the partially heated water is returned to the well to collect additional heat.