Sized Calcium Carbonate Particles Used in Carbon Dioxide EOR for Deep Conformance

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 231523, “Using Sized Calcium Carbonate Particles in Mature CO2 EOR Projects for Deep Conformance,” by Badar Al Busafi, SPE, Andrew Jones, SPE, and Jeffrey Jenneman, Occidental Petroleum, et al. The paper has not been peer-reviewed. _ This study presents applications involving the use of calcium carbonate particles for conformance improvement for CO2 water-alternating-gas injection in the operator’s Permian enhanced-oil-recovery projects. Deep reservoir thief-zone conformance has been one of the most challenging tasks to achieve in the oil industry, particularly with regard to identifying a stable, cost-effective, and easy-to apply conformance agent. One recent technology, borrowed from treatment of drilling-circulation losses, involves calcium carbonate (CaCO3) particles. This study presents applications involving the use of CaCO3 particles for conformance improvement for CO2 water-alternating-gas (WAG) injection in the Permian Basin. CaCO3 Conformance-Operation Overview Being an abundant, naturally occurring material, granular CaCO3 is a cost-effective bridging material commonly used by drilling rigs. In the Permian Basin, local drilling-fluid providers offer granular CaCO3 from a median particle diameter of approximately 5 μm up to 1,200 μm. Crushers are used to break the rock into sizes suitable for milling before vertical roller mills generate finer particles. An air classifier is used to separate the particle sizes and control the particle-size distribution of the packaged material. The product sourced for this project is specified to be at least 97% pure CaCO3. The acid solubility of the product is another benefit, mitigating risk because the treatment is partially, if not fully, reversible. The application of CaCO3 conformance involves a simple, rigless bullheading operation that can be applied in both openhole or cased wellbores. The particles usually are added to a viscous water-based synthetic polymer solution to transport them from the surface to the reservoir. Blender testing is used to determine the maximum particulate loading of the base fluid, typically approximately 2 lbm/gal. The application has been limited to injection wells with turbulent flow along the completion interval. In preparation for the treatment, a tentative pumping schedule is created to help estimate water needs and granular CaCO3 product requirements. Tanks are set on location and filled with water; then, an emulsion polymer and biocide are added to generate a polymer solution with the desired viscosity to carry the particles downhole. The polymer solution is slipstreamed through a tub, and granular CaCO3 is added manually to the base fluid, with a positive-displacement pump sending the particulate-laden fluid into the wellbore. Pumping crews must take special care to introduce the particulate gradually into the tub to the desired particulate loading to prevent plugging the tub outlet.