Sandstone-Acidizing System Eliminates Need for Preflush and Post-Flush Stages
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper IPTC 18571, “Retarded HF System To Deeply Stimulate Sandstone Formation and Eliminate the Need for Preflush and Post-Flush Acid Stages: Experimental and Field Cases,” by Ahmed M. Gomaa, SPE, Sergey Stolyarov, and Jennifer Cutler, SPE, Baker Hughes, prepared for the 2015 International Petroleum Technology Conference, Doha, Qatar, 7–9 December. The paper has not been peer reviewed.
The goal of sandstone-matrix acidizing is to remove siliceous particles that are blocking or bridging pore throats. This is accomplished by injecting acid formulations containing hydrofluoric acid (HF) or its precursors because HF is the only common acid that dissolves siliceous particles sufficiently. Standard treatments include preflush and post-flush stages of hydrochloric acid (HCl) to minimize the potential for secondary precipitation. This paper presents experimental and field-case studies with a sandstone-acidizing treatment designed to retard the HF reaction rate and enable single-stage treatment.
Introduction
The goal of sandstone-matrix acidizing is to dissolve damage that blocks or bridges pore throats in the formation matrix, thus ideally restoring the original reservoir permeability. HF is the only common, inexpensive mineral acid able to dissolve siliceous minerals. Therefore, any sandstone acid system contains HF in some form. Mud acid, which is composed of HCl and HF, is used commonly to remove the formation damage in sandstone reservoirs. Three main steps are involved in conventional sandstone-matrix treatment: preflush, main acid stage, and post-flush. In the main acid stage, mixtures of mud acid have been used extensively in the field. In mud acid, the role of HF is to dissolve aluminosilicates and silica, while HCl helps keep reaction products soluble in spent acid.
To overcome some of the potential problems associated with sandstone acidizing, a one-step sandstone acid system was developed. The new acid system will eliminate the need for preflush and post-flush HCl stages, reduce the treatment complexity, reduce the HCl requirements, and reduce the overall treatment rig time. A coreflood study was conducted with different sandstone cores at 180°F.
One-Step Sandstone Acid
Because of the variation in mineralogical composition of the formation being treated, an inherent risk is always involved in sandstone acidizing. In an effort to mitigate any problems, the new system contains a higher ratio of HCl to HF than conventional sandstone acid systems, which reduces the pH of the HF mixture to levels at which some unwanted reaction products have higher solubility.
Four fluid formulations were selected for testing as one-step sandstone acid systems. Selection was based on HCl-/HF-concentration ratio as follows:
Acid A is equivalent to 15 wt% HCl and 1.9 wt% HF.
Acid B is equivalent to 10 wt% HCl and 2.3 wt% HF.
Acid C is equivalent to 5 wt% HCl and 2.6 wt% HF.
Acid D is equivalent to 3 wt% HCl and 2.8 wt% HF.
Regular Sandstone Acid
Coreflood Experiments 1 through 4 were conducted to investigate the effect of using preflush and post-flush stages of HCl when using the regular sandstone acid. In these experiments, Bandera sandstone cores were used. The regular sandstone acid system evaluated in this study contains 3 wt% HF.