ABSTRACT
A novel process has been developed that targets the oil that remains in medium to heavy oil reservoirs (typically 80% or more of the original oil in place in Alberta and Saskatchewan amongst other regions) and efficiently extracts the energy in the chemical bonds of the oil to produce electricity. The produced CO2 is recovered for disposal so that it is not released into the atmosphere.
The chemical bond energy in the oil is released by controlled in-situ combustion. Oxygen-enriched air is injected into the formation. The resulting combustion can result in formation temperatures in excess of 1800 °C. The heat is extracted to surface via a closed loop system of horizontal wells with circulating water as the carrier fluid. The produced water will have a surface temperature from 150 °C to over 250 °C which can generate electricity via a binary Organic Rankine Cycle ("ORC") turbine. The hot combustion product stream (a mixture of volatile oil, CO2, water vapour, nitrogen and other minor combustion products) is also used to produce electricity. The residual heat in the circulating water (which can have a temperature too low for electricity production) could be used for district heating or agriculture (greenhouses for year-round locally produced food crops). During the initial start-up of the operation, until the reservoir is sufficiently hot to produce stable electricity, any volatile oil that is produced is condensed and sold to market. Once the reservoir is sufficiently hot and there is stable production of electricity, the oil is reinjected back into the combustion zone as fuel for electricity production. The CO2 is recovered, either for use in EOR or disposal.
The subsurface process has been modeled using a commercial petroleum reservoir simulator, STARS1. The design and performance of the surface equipment, including the turbines for electricity production (modeled as a binary ORC turbine system), has been based on industry performance empirical calculations.
As the process targets medium to heavy oil reservoirs that have been developed for conventional oil production, the geologic risk is low. The drilling and completion cost is estimated to be less than C$20 million for a system that consists of 17 lateral circulation wells, 2 subsurface trunkline wells and two riser wells. The formation typically will be less than 1 km depth. A high residence time (which implies very long horizontal lateral length) is not required due to the high temperature gradient from the formation to the wellbore. The residence time of the water circulating in the lateral wells will be about 14.5 hours when the system is at plateau operation. The production riser completion has been designed to minimize heat losses to the overburden using insulation in the casing and a small riser residence time (about 0.2 hour).
Economics have been based upon the fiscal regimes and prices in Alberta, Canada. Over C$850 million NPV(10% discount) results for a one-square mile development. With a Levelized Cost of Electricity equivalent (LCOEeq) as low as C$ 42 / MW-hr, the process is competitive with other sources of electricity. Patents are pending in the USA and Canada.
