Technical and Economic Benefits of Gas Injection in Steam-Assisted Gravity Drainage (SAGD) Wind Down
SAGD is the dominant technology for in situ oil sands production. After approximately 10 years, the production from a well pair (injection and production wells) will decline and eventually become uneconomical. SAGD operations would then proceed to a wind down phase whereby the well pair would be taken off steam injection and production. However, several issues may arise that would require attention. For example, the steam chamber of a shutdown well pair may act as a thief zone and affect production and steam to oil ratio (SOR) in neighboring producing well pairs. Also, buffer zones between well pairs or projects in the same oilfield may be as wide as 200 meters, resulting in a loss of reserves. Furthermore, enhanced recovery may be possible with in situ combustion in order to recover the oil-in-place not recoverable by SAGD, but only if the flue gas from in situ combustion would not negatively impact adjacent well pairs that are still producing in SAGD mode.
Recognizing the challenges posed by the SAGD Wind Down process (i.e. the end of life of a SAGD well pair), Petroleum Technology Alliance Canada (PTAC) proposed the project “Technical and Economic Benefits of Gas Injection in Steam-Assisted Gravity Drainage (SAGD) Wind Down” for ecoEII funding. The Project was awarded $97.5K from ecoEII to study the SAGD Wind Down process, generate reservoir engineering information and find solutions that will minimize negative environmental and cost impact of depleted SAGD reservoirs on adjacent producing SAGD well pairs.
The Project was undertaken by PTAC in collaboration with Devon Energy, Husky Energy, Nexen, Statoil Canada, Suncor Energy, Alberta Innovates Technology Futures (AITF), and Computer Modelling Group (CMG). To set the stage for the Project, a workshop was held to discuss the merits of various technologies in SAGD Wind Down and maintenance modes. The workshop was attended by oil sands operators and provided a unique opportunity for attendees to discuss various strategies to maximize value from old and mature SAGD reservoirs, including the injection of NCG (air, O2, CO2, CH4), or electrical heat to maintain pressure in depleted areas.
In stage 1 of the project, CMG conducted a review of available data (specifically related to SAGD Wind Down operations) to assess the usability of the data for numerical simulation. Reservoir simulation is a critical first step because it evaluates and screens potential solutions at a far lower cost and greater speed than field testing. The purpose of the data review was to address concerns raised by stakeholders (such as oil sands operators) about the quality and completeness of existing data sets used for simulation. CMG found a lack of experimental data and methodology in some areas of study, such as in phase behavior pertaining to the dissolution and ex-solution of methane into bitumen.
The results from stage 1 of the project helped to define the scope for stage 2 of the project, by identifying key areas of focus for data collection. Stage 2 involved laboratory testing of the dissolution and ex-solution rates of methane in order to determine how temperature affected these rates. The tests were successfully completed by AITF, yielding data sets that the filled the gaps identified in stage 1 of the Project and improving the accuracy of future simulation studies.
After completion of stage 2, it was determined that oil sands companies in PTAC’s Phoenix Network would be able to perform numerical simulations individually, according to their own models and specific reservoir configurations, using the information generated in stages 1 and 2 of the project. The simulations would provide the foundation for the identification of the preferred SAGD Wind Down procedure, which may include NCG injection such as methane, CO2 and flue gas in a manner that reduces costs and environmental impact.
Benefits to Canada
Given the number of existing and future SAGD wells in Canada, cost reduction during SAGD Wind Down will benefit all stakeholders and the oil sands industry as a whole. Mitigating environmental impact and reducing GHG emissions during SAGD Wind Down operations would also benefit all of Canada.
Participating companies may update their numerical models with the new information generated by the Project and perform numerical simulations specific to their context and reservoirs. This would lead to the identification of a short list of preferred solutions which will need to be field tested and proven before wide scale deployment.
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