Field Success in Carbonate Acid Diversion, Utilizing Laboratory Data Generated by Parallel Flow Testing

Abstract

In order to treat the total interval in an anisotropic, heterogeneous carbonate formation, acid diversion is necessary. Many diverting agents are commercially available which will plug higher permeability portions of the formation and divert the acid treatment to lower permeability zones. For an oil-productive reservoir solid diverting agents such as wax beads, benzoic acid flakes, or polymers are used. Several problems may be associated with the use of these diverting agents: 1) too much additive may be used which may result in a severe reduction in acid injectivity and diverter may be very difficult to remove after the treatment; 2) flow may not be diverted from one zone to the other, but, rather, flow is equalized between the high permeability zone and the low permeability zone. Recently, viscoelastic surfactants have been used in acid treatments for diversion.1–3 The viscoelastic surfactant contains neither solids nor polymer so clean up is of no concern. The appropriate concentration of surfactant is determined by laboratory flow testing of formation cores under reservoir temperature conditions.2 A parallel flow set up has been constructed which allows independent monitoring of flow parameters for a higher permeability and lower permeability formation core pair. Data demonstrate, through pressure analysis and effluent volumes, that flow is diverted from the higher permeability core to the lower permeability core, until acid break through occurs. Pressure data are confirmed by acid effluent volumes which are collected at regular time intervals throughout testing. In all cases, the effluent volume through the lower permeability core is greater than through the higher permeability core. Core plugs are photographed and imaged by the CT-scan technique before and after testing. Parallel flow testing has also been applied to stage treatments in which a stage of stable foam of viscoelastic surfactant in 2 wt% KCl is injected prior to the main acid stage. Appropriate surfactant concentrations in the foamed stage are determined by flow testing with formation cores. The results of core diversion testing and fluid rheology testing of both the foamed and non-foamed viscoelastic surfactant diverting fluids are discussed. Field case histories of the application of acid treatments, which are designed on the basis of laboratory generated data, are reviewed. Introduction Effective diversion is the key for the success of carbonate matrix stimulation treatments, especially for long horizontal and multi-lateral wells. Conventional stimulation treatments include stages of regular acid with suitable diverters including: foam,3–5 gelled and/or in-situ gelled acids.6–8 However, several concerns have been expressed regarding the use of polymerbased fluids in matrix acidizing treatments.9,10 To overcome some problems encountered with polymer-based fluids, viscoelastic surfactant-based acid systems were developed almost four years ago and have been successfully used over the last few years.1–3,11–15 Simple inorganic salts force the molecules of viscoelastic surfactants to form long worm-like or rod-like micelles.11,13 Entanglement of these micelles generate a 3-D structure, which increases the viscosity of the solution. The resulting high viscosity fluid temporarily plugs higher permeability streaks within the rock matrix, forcing live acid into lower permeability zones. The viscosity breaks down when hydrocarbons (oil or condensate) contact the fluid during flow-back. For injectors, mutual solvent (ethlenegylcol monobutyl ether) is added in the preflush stage to break down the viscosity of the surfactant gel.13 Laboratory testing has demonstrated that lower concentrations of viscoelastic surfactant can provide the viscosity to achieve effective diversion. Lower concentrations obviously make it a more cost effective material and also facilitate cleanup at lower bottomhole temperatures without a breaker. Extensive laboratory testing was conducted to evaluate the diversion of both foamed and non-foamed viscoelastic surfactant-based diversion systems. Various concentrations of foamed and non-foamed viscoelastic surfactant fluids were tested at bottomhole temperatures and under pressure.

A Procedure for Measuring Contact Angles When Surfactants Reduce the Interfacial Tension and Cause Oil Droplets to Spread

“…Analysis suggests that oil droplets spread because EGMBE and VES reduce the oil/ acid IFT below the critical spreading tension. Similarly, we can attribute the spreading observed in experiments by Ayirala et al (2006), Xu et al (2008), Rao (1999) and Rao and Vijapurapu (2004) to low IFTs between the oil and the surfactant solution.…”

“…The tendency for carbonates to become more water-wet as the temperature increases was reported by Rao (1999), Wang and Gupta (1995), and Hjelmeland and Larrondo (1986). A decrease in the surface activity of surfactants in oil as the temperature increased may cause the wettability to change to water-wet (Hjelmeland and Larrondo 1986).…”

“…A US patent and the rheological properties for the VES were described by Caweizel (2007) and Li et al (2010Li et al ( , 2011, respectively. Field applications were described by Nasr-El-Din et al (2006a, 2006b, 2006c.…”

“…The low IFTs caused oil droplets to escape from the lower rock surface and to spread quickly beneath the upper rock surface. Variations in temperature (Rao 1999) and brine concentration (Vijapurapu and Rao 2004) also resulted in low IFTs that caused oil droplets to spread in receding-contact-angle experiments. One cannot determine wettability from contact-angle measurements when low IFTs cause spreading and droplet instability.…”

Synthesis, Characterization, and Viscosification of Amidosulfobutaine and Zwitterionic Gemini Surfactants

“…However, these fluids suffer from limitation such as increasing the viscosity of the acid which in turn leads to decreasing injection rate. (Lynn and Nasr-El-Din, 2001;Nasr-El-Din et al, 2002, 2004b.…”