A core based comparison of the reaction characteristics of emulsified and in-situ gelled acids in low permeability, high temperature, gas bearing carbonates

Abstract

Abstract There are a number of acid formulations that will generate deeply penetrating wormholes in carbonate formations. Two of the most recent advances are diesel emulsified acid (DEA) and self-diverting polymer/HCl (in-situ gelled acid, or GA). The acid volumes required for propagation of the wormholes, as well as the characteristics of the wormholes are significantly different in each of these formulations. Two different service company implementations of GA, and one inhouse developed DEA formulation were evaluated for stimulation of a high temperature (250°F, 121°C), low permeability Saudi Arabian gas and gas condensate producing carbonate. In this paper, the reaction characteristics of DEA were compared to GA formulations using reservoir condition core floods. The characteristics of the acids that were compared included wormhole propagation rates, volumes of carbonate consumed by the acids, wormhole geometric characteristics, and pressure response during injection. Since the acid formulations called for 28 percent HCl acid concentrations, high temperature stability and corrosion control chemicals of the packages were evaluated. The core study found that the GA formulations enhanced the permeability of core samples significantly more then the emulsified acid. The DEA was the more stable of the acid formulations at 250°F. Both of the GA formulations showed some degree of separation of the corrosion control components at reservoir temperature. The GA was found to require larger volumes of acid to achieve an equivalent penetration distance versus the DEA, and logically, the GA dissolved a larger volume of rock per volume of acid injected. The DEA has a higher rate of wormhole propagation and the DEA leaves no residual material in the generated wormholes. The GA left a residuum of polymeric material in the wormholes even after high rate injection condensate. Iron precipitation was noted on both the core injection and production faces with all of the acids after spending. The two acids types showed individual characteristics that can be applied to acid frac applications, but full understanding of the behavior of each, tested with specific reservoir core is required for a correct acid selection to be made. In specific applications, both acid types can be combined in a multiple stage treatment to result in significant enhancement of acid frac simulations.