Designing an In-Depth Conformance Gel System for Carbonate Reservoirs

Abstract

Abstract Many fields worldwide are facing conformance problems increasing with time, due to heterogeneity of the reservoir rock (fractures, features, vugs). During water injection, channeling occurs through the high-permeability streaks, causing early water breakthrough and poor waterflood efficiency. The objective of this paper is the design of an efficient and performing gel system for in-depth fluid diversion in reservoir conditions. A laboratory and simulation study was undertaken to design a gel system that can suit high-salinity high-temperature conditions. The temperature is around 99°C. Formation water salinity is very high while water injection salinity can reach up to 57,000 ppm. Vuggy intervals create a thief zone between injection well and production well. The gelation has to be delayed for at least one week to be injectable at a large distance from the well. To achieve this goal, four gel systems were investigated. The influence of three parameters has been evaluated, namely polymer concentration, crosslinker concentration and aging temperature. Aging tests were performed at reservoir temperature and gelation tests at slightly lower temperature, representative of near-well conditions (85°C). Three main criteria have been taken into account to select the best system, i.e., gel quality, gelation time and gel stability over time. From bulk tests, two gel systems were qualified. Then, coreflood experiments were performed in low-permeability matrix cores and in high-permeability vuggy cores. Injectivity of polymer and crosslinker solutions (gelant) before gelation was evaluated. After gel formation in the vuggy core, gel stability tests by water injection under increasing differential pressure aimed at determining gel parting pressure and permeability reduction after gel breakthrough. Many parameters influenced gelation time. Gelation time can be increased by decreasing polymer and/ or crosslinker concentration but a minimum concentration is needed to obtain a strong gel. Coreflood experiments in matrix rock type showed that the gelant will not invade the matrix deep in the reservoir. Corefloods in vuggy zone indicated that mobility reduction of the viscous gelant drops when flow rate increases, thus showing the ability of the gel to invade deeply the high-permeability streaks. For the best gel formulation, gelation time was 12 days. After aging, the gel generates a permeability reduction as high as 20000 and strongly reduces accessible pore volume of the vuggy core. The novelty of the lab work is the design of an optimum gel treatment to improve the water injection profile, especially in extended lateral and horizontal wells. This study also gave parameters like blend adsorption, mobility reduction during polymer injection, permeability reduction and gelation time. These parameters can be utilized in simulation work to evaluate gel treatment efficiency in a pilot.