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Injection of Acid Gas (CO2/H2S) into a Devonian Pinnacle Reef at
Zama, Alberta, for Enhanced Oil Recovery and Carbon Sequestration*
Steven A. Smith1, James A. Sorensen1, Anastasia A. Dobroskok1, Bill Jackson2,
Doug Nimchuck2, Edward N. Steadman1, and John A. Harju1
Search and Discovery Article #40355 (2009)
Posted February 27, 2009
*Adapted from extended abstract prepared for oral presentation at AAPG Convention, San Antonio, TX, April 20-23, 2008.
1Energy and Environmental Research Center, University of North Dakota, Grand Forks, ND([email protected])
2Apache Canada, Ltd, Calgary, AB, Canada
Since December 2006, a stream of acid gas (approximately 70% CO2 and 30% H2S) has been injected into a Devonian pinnacle reef structure in the Zama oil field in northwestern Alberta, Canada. The injection has been conducted at an average rate of approximately 750 mcf (thousand cubic feet) of acid gas per day, which includes approximately 15 tons of CO2 per day. The project includes a variety of efforts focused on examining the effects that high concentrations of H2S can have on enhanced oil recovery (EOR) and carbon sequestration operations, particularly with respect to monitoring
, mitigation, and verification.
Research activities are being conducted at multiple scales of investigation in an effort to predict and ultimately verify the fate of the injected gas. Geological, geomechanical, geochemical, and engineering data are being used to fully describe the injection zone, overlying seals, and other potentially affected strata. Validating the integrity of the anhydrite sealing formation and determining the nature of potential geochemical and geomechanical changes that may occur because of acid gas exposure are primary goals of the research. Challenges in dealing with acid gas as a miscible fluid for EOR and sequestration have been identified and examined. Lessons regarding the use of acid gas for EOR and sequestration may be widely applicable, as the exploitation of deeper sour gas pools increases throughout the world.
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Carbon dioxide (CO2) capture and storage (CCS) in geological media have been identified as important mechanisms for reducing anthropogenic greenhouse gas emissions currently vented to the atmosphere. Several means for geological storage of CO2 are available, such as in depleted oil and gas reservoirs, in deep saline aquifers, in CO2-flood enhanced oil recovery (EOR) operations, and in coal seams for enhanced coalbed methane recovery. Studies in CO2 capture, transportation, storage, and
The Energy & Environmental Research Center (EERC), through the Plains CO2 Reduction (PCOR) Partnership, one of the U.S. Department of Energy (DOE) National Energy Technology Laboratory’s Regional Carbon Sequestration Partnerships, is working with Apache Canada Ltd., the Alberta Geological Survey (AGS), and Natural Resources Canada (NRCan) to determine the effect of acid gas (H2S and CO2) injection for the simultaneous purpose of disposal, sequestration of CO2, and EOR. The injection process, and subsequent hydrocarbon recovery, is being carried out by Apache Canada Ltd., AGS has developed baseline
The field validation test, conducted in the Zama oil field of northwestern Alberta, Canada (Figure 1), will evaluate the potential for geological sequestration of CO2 as part of a gas stream that includes high concentrations of H2S (20% to 40%). The results of this project will provide insight regarding the impact of H2S, in conjunction with CO2, on sink integrity (i.e., seal degradation), MMV techniques, and EOR success within a carbonate
As part of the EOR scheme, acid gas is being injected into the top of pinnacle reef structures (a process referred to as “top-down” injection) that have been depleted of oil through primary and secondary (waterflood) production techniques. Incremental oil is produced from a second well in the
The acid gas used in this project is obtained from the Zama gas-processing plant and injected into the
The development and execution of effective MMV operations are a critical element in conducting large-scale injection projects. Successful MMV activities will result in data sets that 1) verify that injection operations do not adversely impact human health or the environment, and 2) validate the sequestration of greenhouse gases for the purpose of monetizing carbon credits from geological storage if such a market were to be developed. There is a broad range of technologies and approaches that can be, and in some cases have been, applied to CO2 sequestration projects of various scales around the world. Early geological sequestration research and demonstration projects deployed MMV strategies that were developed based on a lack of knowledge about the effectiveness and utility of many of the applied technologies. The absence of knowledge required early projects to gather as much data as possible using a wide variety of techniques. In particular, a desire to “see” the plume of injected CO2 led to a strong emphasis on the use of geophysical data, especially 3-D and 4-D seismic, to monitor the plume. While the use of geophysical-based approaches and techniques in early projects yielded valuable results that are essential to the development of geological sequestration as a CO2 mitigation strategy, their high costs of deployment and often limited ability to identify CO2 in many geologic settings may render them as being the exception rather than the rule when it comes to developing MMV plans for future projects.
If the deployment of large-scale CO2 injection for geological sequestration is to become widespread, then MMV activities must be cost-effective. The use of existing data sets to develop background and baseline conditions should be maximized wherever possible. The use of invasive or disruptive technologies should be minimized not only to reduce costs, but also to limit the inadvertent development of leakage pathways through new
The following techniques are being employed to monitor the effects of acid gas injection at the Zama site. The preinjection state of each of these parameters has been determined either by currently available historical field data or field activities conducted in 2005 and 2006 to acquire new baseline data:
Geomechanical
A suite of activities focused on geomechanical Initial results of the geomechanical studies of the rock system in the Zama oil field indicates that both
Geological and Hydrogeological
To evaluate and predict the long-term migration of gases injected into geological formations, an in-depth knowledge of the target injection zone and surrounding area is critical. An evaluation of the geological province, fluid flow regimes, and water quality for the area that encompasses the Zama sub-basin of the Alberta Basin was completed in June 2007. Figure 4 illustrates the geographic area that includes the Alberta Basin and highlights the regional-scale study area in the northwest corner of Alberta. This evaluation includes a brief history of the Zama oil field, a detailed accounting of the basin-scale geology and hydrogeology, structural setting and tectonic framework, and water chemistry for the Zama sub-basin. A discussion of the larger Alberta Basin is also included as it provides context with regard to hydrostratigraphic units and larger regional flow systems that pass through the smaller basins it contains.
Results of the investigation will aid in the verification of this site as an appropriate candidate for CO2 sequestration or acid gas disposal scenarios. Stratigraphically, the injection zone is well contained between massive anhydrite and shale packages that will ultimately slow and/or prevent the migration of leaked gas, should it occur. Because of the number of pinnacles in the region, the Zama sub-basin is analogous to an upside down egg crate (Figure 5). As fluid and gas migration takes place over geologic time, it is likely to travel through “inter-pinnacle” region and become trapped in pinnacles along the flow pathway. Gas migrating upwards along wellbores may be introduced to a minimum of two zones of porosity above the Keg River and may be carried along through these flow systems, each already containing “sour” hydrocarbons, and acted upon by dissolution in formation water, dispersion, residual gas saturation and mineral trapping. It is important to note that the process of gas migration in these formations is acting at geological time-scales on the order of tens of thousands to hundreds of thousands of years to move through the system and is unlikely to reach the surface.
New core was collected from a well in the vicinity of the pinnacle in March 2007. The new core is approximately 55 feet long and includes portions of the Muskeg Formation (anhydrite caprock) and the Keg River (pinnacle
Geochemical evaluations are being conducted to generate data that will hopefully provide further confidence in the establishment of the Zama field as a preferred site for sequestration activities. Geochemical models are being created using existing water quality, petrographic and mineralogical datasets. These datasets will be supplemented by ongoing field and laboratory activities. Results of this work will provide an understanding of the interaction of acid gas mixtures, formation waters, and carbonate rock systems that contain hydrocarbons and will be used in future demonstrations of this type.
Acid gas injection for the combined purpose of EOR, disposal, and CO2 sequestration is proving to be a technology that can bridge the time gap between implementation of small scale CO2 sequestration demonstration projects and full scale injection for mitigation of greenhouse gases currently vented to the atmosphere from large industrial sources. Research activities are being conducted at multiple scales of investigation in an effort to validate predictions of the ultimate fate of the injected gas. Geological, geomechanical, geochemical and engineering work is being used to fully describe the injection zone and adjacent strata. Certifying the integrity of the cap rock is a critical research area with additional tests being completed on the reef to determine the nature of potential geochemical and geomechanical changes that may occur due to acid gas exposure.
Preliminary study of the rock system in the Zama oil field indicates that both Apache Canada Ltd., 2003, Resource application for approval to implement an enhanced oil recovery scheme in the Zama Keg River F Pool using acid gas as a miscible flooding solvent: EUB Guide 65 Schedule 1, March 31, 2003. Davies, G.R. and S.D. Ludlam, 1973, Origin of laminated and graded sediments, Middle Devonian of Western Canada: Bulletin, Geological Society of America, v. 84, p. 3527-3546. Buschkuehle, M., K. Haug, K. Michael and M. Berhane, 2007, Regional-Scale Geology and Hydrogeology of Acid-Gas Enhanced Oil Recovery in the Zama Oil Field in Northwestern Alberta, Canada, Client Report for the PCOR Partnership. |