Successfully Controlling Unwanted Gas Production in a Highly Naturally Fractured Carbonate Reservoir

This paper presents the successful field implementation of a conformance polymer (CP) system used to control undesired gas production in the Cantarell field, a naturally fractured offshore carbonate reservoir in southern Mexico. Gas control is a challenge in this field because high gas-oil ratios (GORs) force the operator to prematurely abandon wells because of gas-handling facility constraints, leaving substantial oil reserves in place. Case histories are presented in which the CP system was successfully applied to reduce undesired gas production to an acceptable level. The CP treatments were tailed in with foam cement to avoid overdisplacement in these fractured, depleted formations.

The CP system is based on a copolymer of acrylamide and t-butyl acrylate (PAtBA) crosslinked with polyethyleneamine (PEI). To date, more than 700 jobs have been performed with the CP system around the world, mostly to address excessive water production problems, such as coning/cresting, high-permeability streaks, gravel-pack isolation, fracture shutoff, and/or casing-leak repair. Similarly, this system provides excellent gas shutoff properties as well. The operating temperature range of this system is from 60 to 350°F. The CP system has been successfully applied to sandstone, carbonate, and shale formations requiring a conformance treatment. This system has been successfully tested to withstand a differential pressure of at least 2,500 psi and is resistant to acid, CO₂, and H₂S environments.

One of the most demanding scenarios for this type of polymer system is stopping gas production, especially in this reservoir where the operator must deal with a combination of high matrix permeability (3 to 10 darcy) and natural fractures. These CP treatments have proven to be economical gas shutoff solutions in difficult well/reservoir environments, yielding significant reduction of gas and/or increased oil production. A detailed description of the wells treated with the CP systems is presented along with one-year production rates after the treatment.