Abstract
Injection of fluids continuously into the reservoir at higher flow rates can create channels for preferential movement of fluids. Zones of higher permeability, leading to higher injectivity in selective zones, can also exist because of various lithological conditions and rock structures comprising of naturally occurring fractures or fissures (Vasquez et.al. 2008). For injection wells, the entry of fluids into a set of perforation interval is governed by the quality of the perforations and the permeability of the formation at that depth. Preferential flow of injected fluids into a selective set of perforation interval results in diminished overall sweep efficiency. Water Swelling Polymers (WSP) have proved to be effective in shutting off the thief/vugular zones and thus diverting the injected fluids in the reservoir matrix. WSP swells multiple times of its original size once it comes in contact with water and thus reduces the flow area available in the thief/vugular zones.
This paper discusses a case study of the use of water swelling polymers (WSP) to shut off thief zones in an injector well. The operational aspects along with the associated methods and challenges are discussed. The selection of carrier fluid for the WSP was one of the biggest challenges. Diesel as a carrier fluid would have increased the chances of WSP settling due to its low carrying capacity, leading to an increase in tubing pressures, potential choking of pump valves, and an improper placement of treatment fluid. Water has a higher carrying capacity but would have led to premature swelling of WSP. Hence, the treatment fluid selected was an oil external emulsion of diesel and water. This enhanced the carrying capacity of the treatment fluid without leading to pre-mature swelling of WSP.
The treatment was pumped with alternate stages of WSP with carrier fluid followed by 5% KCl solution to swell WSP in the formation. Pressure increase was observed with every subsequent WSP stage, which indicated the effectiveness of the treatment. Post-job injectivity data and MPLT confirmed a considerable reduction in injectivity index, indicating placement of WSP across the desired zone.
The treatment was pumped at lower rates than expected, but no WSP settling was observed in surface lines. In a post-treatment cleanout stage, some WSP from the sump of the well was collected and found to have sufficiently swollen as designed. No settled WSP was observed in the coil, pump, or surface lines upon rig down.
The use of emulsified diesel as a carrier fluid was implemented for the first time in India. Coil Tubing (CT) was used for effective placement of WSP. The combination of emulsified diesel and CT resulted in reducing the probability of WSP settling in wellbore and pumps, which improved the economic efficiency of the job by eliminating cleanup and pump valve replacement time.
