A Self-Diverting-Acid (SDA) using a Visco-Elastic Surfactant (VES) has been developed. The viscosity of the solution does not develop until the acid reacts with carbonate in the formation. The increases in Ca2+ ions and pH due to the HCl-carbonate reaction cause in situ gelling of the acid. The high viscosity temporarily blocks the wormholes formed in the rock matrix, allowing the acid to cover the un-acidized area. The viscosity of the gelled acid can be completely reduced by post flush of solvent or by the hydrocarbon in the formation during flow back. Unlike the polymer based gelled acid systems, the new material does not leave any residue once it has broken. Multi-Core flood testing incorporating a post acidizing Computed Tomography (CT) scans showed that the VES based Self-Diverting-Acid successfully diverted acid from high permeability section into lower permeability sections. The rock face remained clean without any trace of residue. Rheology measurements showed the consistency of the viscosity development by the gelled acid upon reacting with carbonates.
When carbonate hydrocarbon bearing formations are acidized by hydrochloric acid (HCl), a few dominating channels in the matrix are created and the majority of the treatment acid will flow along these channels, leaving the rest of the rock untreated. This phenomena is undesirable, particularly when long vertical sections or horizontal open holes are to be acidized. Therefore, a diverting agent is often, if not always, needed. Polymer gels, foams or rock salt are commonly used as diverting agents during matrix acidizing of carbonates. These materials do not allow the acid to reach its full stimulation potential and may cause damage due to residue precipitation, saturation alteration, or solid invasion.
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