Completed with 4.5 in tubing and 7 in Liner, the upper carbonate reservoir had previously been stimulated and produced until reaching its economic potential. To prolong the well life, the decision was made to permanently isolate the upper zone, then perforate, matrix stimulate and produce from the lower zone.
The challenge for this operation was that the existing upper zone had been stimulated, creating a large void behind the casing with known high fluid loss. It was determined that isolation with cement would have only a small chance of success due to the high leak-off in the previously stimulated reservoir. Also, this zone was at a 40° inclination, making it more difficult to place an uncontaminated cement plug with Coiled Tubing (CT) and clean out the cement after placement.
An inflatable packer was also considered to isolate the upper zone and perform acid stimulation on the bottom zone. The bottom-hole pressure and temperature (5,000 psi and 300°F) were beyond the operational limits of any inflatable element available in the region.
For this unique situation, a new method was designed and successfully implemented. The new approach included first filling the upper zone with calcium carbonate chips (CaCO3) to reduce or limit the fluid losses. Then, utilize CT to place a chemical isolation treatment. The product utilized was an organically cross-linked polymer (OCP) system capable of permanently sealing the target zone by forming a 3D rigid gel. It is effective for preventing water and gas flow in sandstone and carbonate formations from 100°F to 350°F (38°C to 177°C). A benefit of the treatment was the ease at which it could be pumped and placed via CT and then cleaned out. All that is required for cleaning is a nozzle and jetting action. The implementation of this chemical sealant procedure provided reliable zonal isolation of highly depleted upper zone and allowed full access (with CT, Slick Line and E-line) to deeper productive lower zone. The successful chemical isolation in this well has resulted in avoidance of expensive workover operations.
This paper discusses the pre-job planning, benefits, challenges, design, execution, evaluation and lessons learned for applying this novel isolation method.