Three different commercial formulations (A,B,C) of in-situ gelled acids are compared in detail at temperatures up to 150 °F and at acid concentrations from 5 to 20 wt% HCl. In-situ gelled acids are claimed to work by a gelation mechanism that occurs at the rock surface as the acid is neutralized. These acids contain a polymer, a crosslinker, and a breaker in addition to other additives. Detailed viscosity measurements of each in-situ gelled acid were made as a function of pH. A new experimental procedure was developed to partially neutralize the in-situ gelled acid, then the viscosity was measured as a function of shear rate in the range 1 to 3000 s-1. Relative reaction rates with reservoir rock of the three in-situ gelled acids were compared at 100 °F. Coreflood experiments were conducted with small acid volumes so that injectivity could be measured before acid breakthrough occurred.
In-situ gelled acids significantly retard the rate of acid reaction with reservoir rock. Results showed that acids B and C had similar behavior, while acid A was more reactive with reservoir rock. Viscosity measurements of partially neutralized in-situ gelled acids showed that only with acid C was a significant viscosity maximum observed as the pH of the acid increased. This pH maximum occurred at a value of approximately 2.2 with 5 wt% HCl at 100 °F. In the absence of breaker and crosslinker, acids A and B were very similar, while acid C showed as little as a tenth of the viscosity at low shear rates. Coreflood experiments showed that in some cases, the in-situ gelled acids irreversibly reduced the permeability of reservoir rock.
Acid diversion is very important when stimulating vertical wells with long target zones or horizontal wells in carbonate formations.1,2 In a heterogenous formation the injected acid will flow primarily into the high permeability zones. This poor acid distribution will reduce the overall efficiency of the stimulation treatment. Increasing the acid viscosity can overcome problems associated with acid diversion in the formation. In-situ gelled acids increase the viscosity of the injected acid and retard the acid reaction with the formation, improving treatment efficiency. In addition, the gel should break rapidly as the acid is spent, to improve well clean-up after the acid treatment.
In-situ gelled acids consist of an acid-soluble polymer, a pH buffer, a cross-linker and a breaker. It has been reported that the polymer in this system forms a gel within a narrow pH range.3,4 As a result of gel formation, the viscosity of the acid increases in-situ and acid diversion can be achieved. Ideally, the gelled acid will form wormholes evenly distributed over the entire target zone. This gel will improve acid placement, provide more uniform damage removal, and control acid fluid loss.4,5
When hydrochloric acid is injected into the formation, it has a pH of nearly zero. The pH of the acid increases as the acid reacts with the carbonate rock. At a pH value of approximately 2, it is claimed that the polymer reacts with the cross-linker and forms a very viscous gel.3,4 At pH 2, the acid concentration has decreased to approximately 0.04 wt%, and it is nearly completely spent. The viscosity of the acid can reach 1,000 mPa.s and is able to divert unreacted acid further into the well bore.4 As the acid continues its reaction, the pH will rise further. At pH values greater than 4 to 5, the viscosity of the gel is claimed to decrease as the polymer and cross-linker dissociate. A breaker is used to ensure a complete reversal of the crosslinking process.4 The decreased viscosity of the spent acid is designed to improve its removal from the formation. Each type of in-situ gelled acid uses a slightly different gelation chemistry. The cross-linker may be iron (III), zirconium compounds, or other cationic species.6 The behavior of the gelled acid will be determined by temperature, which affects viscosity and acid reaction rate. Dilution of the acid with formation brine will reduce viscosity and the effectiveness of the cross-linking reaction.