Dual Diversion System for HT Acid Fracturing Treatments

Acid fracturing is a common stimulation technique used on low permeability carbonate reservoirs to enhance the productivity and deliverability of hydrocarbons. The success of acid fracturing treatments ultimately depends on the created fracture geometry, complexity and surface area. To maximize these parameters, it is imperative to include a fluid diversion strategy in the treatment design. The use of high viscosity fluids and solid particles is well addressed in the literature as effective diversion methodologies. In this study, the benefits of a dual diversion system composed of solid polymer-based particulate diverters suspended in a viscoelastic surfactant (VES) fluid were investigated for high temperature acid fracturing. The degradation profile of the solid particulates was examined for different temperatures (ranging from 250 to 300°F) in different fluid systems (15 wt% HCl, spent HCl and KCl brine). The selection of fluids was also made to ensure the compatibility of the resulting diverter monomer solution with expected formation fluids. The viscosity of the VES system was examined at 275°F to ensure its thermal stability. The bridging capability of the dual diverter system was assessed using a filter press apparatus with a slotted metallic disk by measuring fluid leak-off as a function of time. Results showed degradation rates increased with increasing temperature where a soaking time of 4 hours at 300°F was enough to fully degrade the solid particles as opposed to only 8% weight loss at 250°F for the same soaking time. Results also indicated faster degradation in more acidic environments where almost full degradation was observed in 15 wt% HCl at 250°F and 4 hours soaking time. The VES fluid system maintained sufficient viscosity for solid diverter carry with approximately 94 cP at 275°F and 100 s-1 shear rate. Bridging tests indicated the capability of the dual diverter system to completely plug the slots on the metallic disks and stopping any fluid leak-off up to 800 psi in a filter press apparatus. This paper presents a full methodical study of an innovative dual diversion system for application in high temperature acid fracturing treatments. The paper also presents design optimization considerations in order to achieve successful diversion without causing any permanent formation damage.