Results From Continuous Downhole Monitoring (PDG) at a Field-Scale CO2 Sequestration Demonstration Project, Cranfield, MS

Abstract

Abstract Elevated formation fluid pressure resulting from large-volume carbon dioxide (CO2) injection for sequestration is a key factor affecting storage integrity (seal and well performance) and ultimate capacity. Monitoring pressure dynamics (buildup and falloff) is a fundamental and pragmatic reservoir surveillance technique for monitoring sequestration performance. The Southeast Regional Carbon Sequestration Partnership (SECARB) field project conducted by the Gulf Coast Carbon Center with support from the National Energy Technology Laboratory (NETL) and the U.S. Department of Energy (DOE) and managed by the Southern States Energy Board (SSEB) at Denbury Resources' Cranfield Field in southwest Mississippi provides a unique opportunity to monitor large-scale (>106 tons) CO2 injection on the flank of an anticline at 3 km depth. Prior to large-scale CO2 injection, the reservoir has been shut-in for decades and was near hydrostatic pressures when injection commenced. Thus, the initial pressure buildup prior to production is similar to what would be expected for many brine sequestration projects. The evolving CO2 distribution and pressure perturbation has been monitored continuously for over one year. This paper presents the monitoring well design, including a novel dual completion for above-zone monitoring, and presents real-time pressure data recorded in the injection interval. Correlation of changes in CO2 injection rates from eleven injection wells throughout the field with observed pressure response at the observation well are used to demonstrate the sensitivity of this monitoring technique for quantifying CO2 fluxes within the reservoir, and to illustrate the sealing nature of a reservoir-scale fault in the field during CO2 injection. At Cranfield, subsurface pressure monitoring is capable of detecting CO2 fluxes (injection rate changes) corresponding to as little as hundreds of tons per day at distances up to 1 km. The reservoir appears to have a predictable response of the rate of pressure change at the observation well for injection perturbations of variable rates and distances, suggesting that rates of observed pressure change that are uncorrelated with injection changes could be used to detect, quantify, and locate unanticipated migration of CO2 out of the injection zone. Introduction Large-volume injection of carbon dioxide (CO2) for sequestration in subsurface geologic reservoirs will typically elevate subsurface reservoir fluid pressure. The magnitude and duration of pressure elevation depends on the rate/duration of injection and the geological properties of the reservoir, including any associated brine aquifer systems. Pressure elevation and propagation is a key factor affecting storage integrity1 ultimate storage capacity2,3 and long-term regional environmental effects4,5. Monitoring pressure dynamics is a fundamental technique for monitoring storage performance, and has been used in the natural gas storage industry to both characterize geologic conditions as well as identify geologic and engineering flaws in subsurface storage systems6. The research presented here focuses on the deployment and interpretation of continuous downhole pressure measurements in a CO2 injection zone for pragmatic monitoring of reservoir, seal, and well performance. Data presented illustrate the sensitivity of pressure in sequestration environments and what types of reservoir events and features can be identified with the data. The Southeast Regional Carbon Sequestration Partnership (SECARB) Phase 2 field project is conducted at Cranfield Field in southwest Mississippi by the Gulf Coast Carbon Center at the Texas Bureau of Economic Geology with support from the National Energy Technology Laboratory (NETL) and the U.S. Department of Energy (DOE) and managed by the Southern States Energy Board (SSEB). This location provides a unique opportunity to monitor large-scale (105-106 tons) CO2 injection in an anticline structure at 3 km depth. Injection of one million tons of CO2 occurred approximately 13 months after the initiation of injection. The study area is approximately 20 km east of Natchez in Adams County, southwest Mississippi, USA (Figure 1). Carbon dioxide from the natural geologic accumulation at Jackson Dome (near Jackson, MS) is transported 160 km via pipeline to Cranfield and has been injected continuously in supercritical phase since July 2008 to support enhanced oil recovery. Pressure monitoring began two weeks prior to injection initiation and has been essentially continuous throughout the ongoing injection. A total of eleven injection wells and one dedicated observation well have been involved in the experiment.