Effective Acid Diversion and Water Control in Carbonate Reservoirs Using an Associative Polymer Treatment: Case Histories from Saudi Arabia

Abstract

Abstract Acid diversion and water control are usually addressed as two separate issues in the oil industry. An associative polymer can be used to simultaneously address these two issues in a single treatment. Associative polymer treatments were successfully applied during matrix acid stimulations in onshore and offshore oil carbonate reservoirs in Saudi Arabia. The oil producers were completed as perforated and open hole wells. The associative polymer was applied in combination with acid treatments mainly to:mitigate the preferential stimulation of water saturated zones located near oil producer layers,improve acid diversion in heterogeneous carbonate reservoirs, andrestore the productivity of the damaged wells. Extensive experimental studies were carried out to investigate potential use of an associative polymer to control water mobility and act as an acid diverter. Coreflood experiments (parallel cores) conducted on reservoir cores at downhole conditions showed that the polymer treatment has no significant effect on the relative permeability to oil. However, the relative permeability to water was significantly reduced. In addition, the associative polymer was very effective in diverting acid into oil-saturated cores. This paper presents the results obtained from several wells where an associative polymer was applied successfully during matrix acid treatments of damaged wells. All the treatments included stages of associative polymer solutions and 20 wt% HCl with additives. Post stimulation treatments production data, build-up, downhole gauges and production logging confirmed that the associative polymer was very effective in diverting the acid into oil saturated zones and resulted in a significant reduction in water production. Introduction Acid diversion is an imperative key to the success of matrix acid treatments. Various techniques are used to achieve good acid diversion, thus better zonal coverage. Mechanical and chemical means are used to improve acid coverage including: zone isolation, ball-sealers, particulate diverting agents, gelled acids, and foams. Viscoelastic surfactants have been used extensively for acid diversion as non-damaging fluids and have shown a tendency to reduce water production as reported by Nasr-El-Din et al.1 Acid stimulation treatments are conducted to enhance oil/gas production, but may result in simultaneous increase in water production. Therefore, one of the main objectives for diversion process during acid stimulation treatments is to prevent acid from attacking water-producing zones, which will significantly enhance the output of acid stimulation treatments. Associative polymers can be used to simultaneously provide an effective acid diversion and water control in a single treatment.2–4 The associative polymers have been applied successfully in oil wells in Saudi Arabia and in Mexico.2,3 The results showed that associative polymers were very effective as acid diverting agent and water control chemical. The objectives of this study are to:assess the effectiveness of associative polymers in diverting acid and reducing permeability to water in carbonate cores,design polymer-based treatments to control water and divert acid in matrix treatments, andevaluate the effectiveness of this type of polymers based on field results. Associative Polymers for Acid Diversion and Water Control Associative polymers are macromolecules with attractive groups, some of which possess hydrophobic moiety. Association in polymers can occur through interactions of polymers with other chemicals present in the solute or by interaction of groups incorporated in the polymer backbone.5,6 Association is introduced to increase substantially the viscosity and elasticity of polymer solutions, improve their tolerance to salts and enhance polymers resistance to permanent shear degradation.5,7 Hydrophobically modified polymers exhibit the second type of association in polymers. Hydrophobically associating water soluble polymers include both hydrophobic and hydrophilic moieties.8 Associative polymers have been used in water control/acid diversion, completion and drilling fluids, and enhanced oil recovery.2–10