Carbonate Matrix Stimulation with Pickering Emulsified Acid Studied by Large-Scale Radial Flow Experiments
Authors: Aly Moustafa , Aidagulov Gallyam , Sultan Abdullah , Gwaba Devon , Abbad Mustapha 
Published: March 2023

Abstract
In carbonate matrix stimulation, hydrochloric acid (HCl) is injected to partially dissolve the near-wellbore region forming a network of high-conductivity wormholes. Optimal treatments are sought to achieve the deepest wormhole penetration into a reservoir at minimum volume of injected acid. Acid-in-oil emulsions are widely used to enhance stimulation by retarding the rock dissolution reaction. In this work, performance of a Pickering emulsified acid is studied in large-scale laboratory experiments under realistic radial flow conditions.
The stable Pickering emulsified acid, studied in this work, was prepared by precise mixing of 15-wt% HCl aqueous solution with diesel in a 70:30 ratio and addition of a nanoclay emulsifier. The matrix acidizing process was simulated in large-scale laboratory experiments where acid was injected radially through a borehole drilled in a block-shaped sample of carbonate rock with dimensions of 20 × 16 × 16 inches. A true-triaxial load frame setup was used to maintain 2,000-psi pore pressure inside the water- saturated block during acid injection. A series of experiments was conducted at various acid injection rates with injection data acquired and analyzed.
Three radial acidizing experiments were conducted on natural carbonate blocks sourced from an outcrop in central Saudi Arabia. Analysis of injection data revealed interesting similarities and differences between the test results. For example, in two tests—representing intermediate (120 cm3/min) and high (240 cm3/min) acid injection rates—maximum differential pressure during acid injection was found remarkably lower than one may expect for a fluid with apparent viscosity such as Pickering emulsified acid has. In these tests, acid breakthrough to the block face was reached and the number of injected acid pore volumes (PVBT) was determined to quantify effectiveness of the acidizing process. The low-rate test (30 cm3/min) was interrupted before the moment of breakthrough but still demonstrated smoothly decelerating differential pressure decline toward the breakthrough. The latter differs drastically from the behavior observed in intermediate- and high-rate tests where differential pressure dropped abruptly from its peak values at the moment of breakthrough. The theoretical effective acidizing model was calibrated with the obtained PVBT numbers and used to predict PVBT values at untested rates. Also, we have shown how this calibrated model can be used to predict wormhole penetration depth at the field scale of stimulated openhole wellbores.
Radial acidizing experiments representing realistic flow conditions around the well are a preferred source of data for calibrating theoretical models required for field job design. However, because of experimental complexities, radial test data are rarely available and mostly limited by small-scale experiments involving plain nonretarded acid systems. This work is novel in presenting large-scale radial test results for Pickering emulsified acid on limestones from Saudi Arabia and upscaling them to openhole stimulation in the field.