Potential Of Combining Natural Gas Production And Co2-Plume Geothermal (Cpg) Electricity Generation In Deep Natural Gas Reservoirs

Abstract

There is potential for utilizing supercritical CO2 (scCO2) as a working fluid for the dual purpose of enhanced natural gas recovery (EGR) and CO2-Plume Geothermal (CPG) electricity generation in deep and hot natural gas reservoirs. This is referred to as a combined CO2 EGR-CPG system. Clear synergy effects are observed in combining these two systems (CO2-EGR and CPG) – including shared infrastructure and working fluid – which can be exploited. The combined system can generate electricity and permanently store CO2 underground while increasing the overall system efficiency and extending the useful life of the gas field beyond natural gas depletion. We have also estimated that the system’s power output can be maximized using a combined system, whereby power is generated from the separated CO2 via a direct CO2-turbine expansion system and from the heat extracted from the CH4 by a Rankine cycle. In this study, we find the benefit of integrating these two systems in deep natural gas reservoirs with regards to power generation and associated CO2 geological storage. A summary of existing deep and hot natural gas reservoir examples and their respective reservoir properties are presented in this study. An anticlinal natural gas reservoir model was created based on these existing reservoirs. Numerical reservoir simulations, using TOUGH2, found the natural gas recovery and geothermal- energy-extraction performance of the combined system. Additionally, the reservoir model is coupled with a wellbore heat- transfer model to numerically calculate the fluid pressure and temperature decrease as it arrives at the surface. Two stages of development are considered: 1) when there is mostly methane production before CO2 injection has started (pre-CPG stage), and 2) when more than 90% of the produced gas is CO2 (CPG stage). During Stage 1, power is generated in an Organic Rankine Cycle (ORC) by extracting heat from the produced methane. During Stage 2, power is generated by directly expanding produced CO2 in a turbine at the surface (CPG). Results show a high heat-mining potential for both stages. However, geothermal power generation is significantly greater during the CPG stage than during the ORC stage, as during the CPG stage, the produced CO2 is passed directly through a CO2-turbine expansion system. Our results also show that all of the CO2 can be permanently stored in the natural gas reservoir. In addition, we observe that CO2-EGR can serve as an important transitional stage on the way to CPG operation, as the CO2 both extracts methane from a partially depleted reservoir and is increasing the CO2 plume in the reservoir to enable the CPG operation to quickly follow the CO2-EGR operation. Hence, deep, and thus hot, natural gas reservoirs (depleted or partially-depleted) may serve as particularly well-suited sites for a pilot CPG system implementation.

AAPG Datapages/Search and Discovery Article #90346 ©2019 AAPG European Region, 3rd Hydrocarbon Geothermal Cross Over Technology Workshop, Geneva, Switzerland, April 9-10, 2019