Upscaling Linear to Radial Acidizing Curve in Carbonate Stimulation using Pickering Emulsified Acid
Authors: Aly Moustafa , Sultan Abdullah , Aidagulov Gallyam , Gwaba Devon , Abbad Mustapha 
Published: October 2023

Carbonate matrix stimulation involves injecting hydrochloric acid to partially dissolve the near-wellbore region creating a network of highly conductive channels known as wormholes. To enhance stimulation, acid-in-oil emulsions are commonly used as they reduce the rock dissolution rate. While matrix acidizing experiments were widely conducted to better comprehend the wormhole initiation, propagation, and breakthrough mechanisms, these studies involved small cylindrical cores in a one-dimensional flow geometry failing to represent the radial geometry of the field.

This study examines the performance of a Pickering emulsified acid in large-scale laboratory experiments conducted under realistic radial flow conditions inside 20×16×16 in. blocks from carbonate outcrop formation in Saudi Arabia. Additionally, linear experiments were conducted on typical 1.5×6 in. (Len × Dia) cylindrical core samples extracted from the same blocks to study the performance of the acid system in a one-dimensional setting as well. The aim is to determine how effective this acid is in creating wormholes, identifying the optimal injection rate and volume required for maximum wormhole penetration, and establish a correlation to predict radial pore volume to breakthrough (PVBT) values when only the linear PVBT curve is present.

A novel Pickering emulsified acid system was prepared using a nanoparticle emulsifier. The acid phase was based on 15 wt% HCl while the oil phase consisted of diesel. The prepared emulsified acid displayed excellent properties for rheology, and even corrosion inhibition. To determine the linear acidizing PVBT curve, core flow acidizing experiments were carried out at consistent back pressure of 2,000 psi and ambient temperature 25°C (77°F) conditions. CT-scan imaging was done for the linearly acidized cores and 3D images were rendered to visualize the created wormhole size, and shape. A similar acidizing curve was constructed for the radial experiments at consistent pressure and temperature. Subsequently, we developed a correlation that could forecast radial PVBT values using the linear acidizing data given the same experimental conditions (pressure and temperature). For the particular example of Pickering acid system, rock and testing conditions, our results confirm that the newly developed correlation can predict the optimum radial injection rate with 18.9% error while the optimum PVBT value can be predicted with only 0.7% error.

In this work, we proposed a novel approach to build correlation aimed to link the PVBT curve in linear acidizing experiments with the one obtained in radial experiments. In this work, this correlation was built for the Pickering emulsified acid and its accuracy was demonstrated by comparison of the actual linear and radial acidizing results experiments. Particularly, the developed correlation can be used to predict the optimum acid injection rate in a radial flow configuration given the optimum linear injection rate, and thus enhance acidizing predictions when upscaled to field conditions.