Acid Placement: An Effective VES System to Stimulate High-Temperature Carbonate Formations
- Ahmed Mohamed Gomaa (Baker Hughes Ltd.) | Jennifer Cutler (BJ Services Company) | Qi Qu (BJ Services Co. USA) | Kay Cawiezel
- Document ID
- Society of Petroleum Engineers
- SPE International Production and Operations Conference & Exhibition, 14-16 May, Doha, Qatar
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 1.6.9 Coring, Fishing, 1.10 Drilling Equipment, 2.7.1 Completion Fluids, 3.2.4 Acidising, 4.1.2 Separation and Treating, 4.2.3 Materials and Corrosion, 5.4.10 Microbial Methods, 5.2 Reservoir Fluid Dynamics, 1.8 Formation Damage, 2.5.2 Fracturing Materials (Fluids, Proppant)
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The gelling performance of VES acid systems dramatically weakens at high temperature. Therefore, these fluids are typically limited to placement in relatively low-temperature carbonate formations. This study was conducted to introduce a new VES
system that can gel and maintain useful viscosity up to 300F.
The new surfactant system is completely compatible with HCl, brine, and even high iron contamination. Rheological studies defined the operational temperature limit where viscosity was sufficient for the new VES system to be used successfully as diversion agent. It was found that initial acid concentration and the degree of acid neutralization were critical parameters affecting the new VES system diversion performance at elevated temperatures.
The effects of four corrosion inhibitors were examined. One is recommended for use with this system because it minimizes negative effects on the operational temperature range. Also, it enhances the values of elastic modulus, which enhances the VES system's diversion performance.
Coreflood studies using limestone and dolomite cores confirmed that the new VES system increased differential pressure sufficiently to achieve diversion. For limestone cores, the pressure drop increased by a factor of 10 during VES acid injection; for dolomite cores, the pressure drop increased by a factor of 100. The pressure drop changed in a cycling manner, where the crest and trough of each cycle increased with time. Cycling of the pressure drop indicates that the acid was able to change its direction inside the core. Coreflood testing also indicated that there is no need for a breaker, as 18% permeability enhancement was observed with CaCl2 brine flowback. This paper will discuss the results obtained and recommend the conditions under which the new system is most likely to be successful in the field.
The goal of acid-stimulation treatments is to remove or bypass formation damage from the near-wellbore area. The success of matrix treatments depends on the uniform distribution of the treating fluids over the entire production (or injection) interval.
Acid placement methods should not cause formation damage, should be compatible with the treating fluids (overflush or displacement fluids) and formation brines, should clean-up rapidly and completely when the well is put back on production, and should be stable at the bottomhole treating conditions.Hill and Rossen (1994) described the techniques that have been applied to improve acid coverage: mechanical (mechanical zone isolation (packers), ball sealers, coiled tubing) and chemical (foam, particulate diverting agents, and viscosified acids). Chang et al. (2007) showed that mechanical techniques are more expensive and time consuming than chemical techniques, and they are often neither applicable nor effective in open-hole
completions. Economides and Nolte (2000) showed that solid bridging materials have operational difficulties and can cause plugging of the near-wellbore area if overused.
Viscoelastic surfactants (VES) have been used in the industry for stimulation applications (Chang et al. 2001; Qu et al. 2002). VES acids exhibit remarkable viscoelastic character, which can significantly increase the viscosity of the fluid. Therefore, VES fluids can be used to improve the fluid's diverting ability in acidizing treatments. VES acid systems have been successfully applied in the oilfield industry as fracturing fluids and matrix acidizing fluids (Chase et al. 1997; Chang et al. 2001; Nasr-El-Din et al. 2003). A VES based in-situ gelled acid system can be prepared by adding surfactant to the acid. When reservoir fluid does not naturally break the VES based in-situ gel, a post flush of mutual solvent is recommended to ensure a complete gel break (Samuel et al. 1997; Yang 2002; Nasr-El-Din et al. 2006a).
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