Methodologies and Solutions to Remediate Inter-Well Communication Problems on the SACROC CO2 EOR Project – A Case Study
Authors: Larkin Rebecca , Creel Prentice 
Published: April 2008

This paper describes a case study of conformance problems aggravated by apparent direct communication channels between injector and offset producer(s) in certain patterns of the SACROC Unit Carbon Dioxide Enhanced Oil Recovery (CO2 EOR) Project in Scurry County, Texas. Such communication, frequently accompanied by limited vertical injection fluid distribution, results in rapid recycling of injectant and pattern sweep inefficiency. If excessive gas production cannot be controlled by increasing the ratio of water to CO2 (WAG) injection, offset production wells must be shut-in and/or CO2 injection stopped to avoid overloading surface handling systems. Taking this action reduces the opportunity to maintain optimum pressure support and desired mobility of the hydrocarbons in affected patterns. Unwanted communication paths are difficult to remediate due to the magnitude of the volumes of commercial materials normally required to address them.

Various technologies and methods were investigated and/or deployed in attempts to redistribute injection into desired intervals and reduce the recycling. Besides filling eroded near wellbore formation (as indicated by caliper arm log surveys or solid plug back material volumes), most of the injection wells would have to be treated with solutions capable of modifying fluid flow away from the wellbore to affect inter-well communication. The objectives of reducing unnecessary cycling of injectant between injectors and producers, improving sweep efficiency, increasing oil production and improving the ultimate oil recovery were the drivers in searching for possible solutions.

Problem identification through well testing, well logging, and volumetric analysis were guidance for the designed remediations on poor conformance patterns. The simplest diagnostic tool is the well test; monitoring the ratios of gas to oil, water to oil, and CO2 to oil provided simple exception checks to anticipated levels. While less quantitative than inter-well tracer surveys, over time, noting the apparent breakthrough time of CO2 to an offset producer after each switch from water to CO2 injection was possible when switches were staggered between neighboring injectors. Using radioactive tracer logging technologies, injection profiles run at variable rates and indicated pressure responses were analyzed through multi-rate injectivity profile analyses. Estimated injection fluid distribution using sequential injection profiles and allocated injection volumes were determined to gauge relative vertical sweep efficiency and potential wellbore intervals to target for remediation. These diagnostics established the criteria for the physical and chemical attributes needed for conformance treatments to address the identified problems.

Selection of treatments was also restricted to ones that would withstand the detrimental effects of CO2 injection and resistance to bacterial growth potential. Rigless workovers were desired because of the availability, time involvement and cost associated in performing workovers with pulling units.

Examples of the problem identification diagnostics, the selection process for the needed solution attributes and capabilities, and the placement control requirements while performing the conformance treatments are detailed. Vertical profile modifications to several of the individual injection wells using new super absorbent crystallized co-polymer systems are shown.