New Insights on Chemical Diversion in Carbonate Acidizing: Experimental and Simulation-Based Study

Abstract

Abstract Carbonate acidizing restores and enhances production by dissolving a fraction of the rock to create highly conductive channels or wormholes. Most experimental acidizing studies are focused on acid injection at Constant Volumetric Rate (CVR) rather than at Constant Injection Pressure (CIP). A recent study highlighted the benefits of each injection technique for 15% wt. HCl acid. Formations with high injectivity should be treated at CVR as efficient wormholes can be propagated with less acid volume. Aditionally, the increasing injection rate characteristic of CIP can be beneficial in formations with initial low injectivity where wormhole dissolution can change from face and conical to dominant pattern. The injection rate and injection pressure for acid and chemical diverters are important variables because they affect wormholing, treatment distribution, and therefore the overall skin reduction. The primary objective of the present work is to investigate the effect of chemical diversion when acid treatments are injected at CVR and CIP. This study focused on the behavior of a Viscoelastic Surfactant (VES) based acid diverter inside the porous media and its effect on the treatment and wormhole distribution for low and high injectivity formations. A core-flood study was conducted using Pink Desert Limestone cores with similar permeability to assess the pressure response of the diverter at CIP. Additionally, a 2-D wormhole model was used to describe the acidizing phenomena. This model captures the acid reaction and chemical diversion coupled with Darcy flow. Synthetic scenarios were created to evaluate the effect of each acid injection technique (CVR and CIP) and diversion staging on the post-treatment skin and wormhole length. The results of these simulations showed that proper staging of a the chemical diverter aided to improve the treatment placement injected at CVR in a long, highly permeable and homogenous zone as shown by smoother wormhole and skin profiles. CIP delivered nearly uniform treatment placement without chemical diverter when applied at the highest rate below fracture pressure in a long, homogenous and low permeable zone. For a synthetic case of a long and heterogeneous treatment zone, this study confirmed that CIP generated branched wormholing in the high-permeability zones in the early stages of a treatment sequence and did not significantly reduce the skin in the low injectivity layers. Fluid placement turned less efficient as the injection rate increased over the course of the treatment. Based on these results, the simulation outputs suggest that CIP is not an effective diversion technique in long treatment intervals with significant injectivity contrast. Additionally, it was observed that the inclusion of diverter stages can improve the treatment distribution.