A New Acid for True Stimulation of Sandstone Reservoirs
Authors: Di Lullo Gino , Rae Phil 
Published: December 2007

Abstract
Hydrochloric:hydrofluoric acid mixtures have been extensively used to treat sandstone reservoirs, however, their application has been restricted to restoring the natural permeability of the formation. A combination of rapid spending with clays and silicates, matrix unconsolidation in the near wellbore area and subsequent precipitation of various reaction by-products has limited the usefulness of mud acid for matrix stimulation treatments.
Several existing "retarded HF acids" mixtures have been tried with mixed results. The marginal reduction in reaction rate of these acid systems could not overcome the huge contrast in surface area between clays and quartz, which dominate HF's reaction rate. Also, various "in-situ-generated" acids were developed and pumped, with questionable to poor results due to premature or improper mixing of the solutions, both in the tubulars and in the formation.
This paper discusses the chemistry and development of a new acid that can completely replace HCl in HCl:HF mixtures. Compared with available HF acid systems, the new HF acid has lower reaction rate and limited solubility with clays, but higher reaction rate and dissolving power with quartz. As a complement, various field treatments are discussed and a complete database with production data is presented.

Introduction
The framework of sandstone formations is normally made up of quartz, silicate grains, feldspars, chert and mica. These minerals are bonded together by secondary minerals that precipitated from connate water, occupying partially the original pore spaces, for instance, over growth quartz, carbonates and pore lining clays. The porous fraction and hence the permeability of the formation rock is sometimes reduced by invasion of water base filtrates, either from drilling, completion, workover or treating fluids. These fluids can damage the matrix by swelling and dispersing clays or even by inducing the precipitation of scales. Also, in high permeability formations under high differential pressure some particles can invade the formation and plug some pores.
Hydrofluoric and hydrochloric acid mixtures (generally described as mud acids) have been chosen to treat these formations since they dissolve clays from drilling mud and react with most constituents of naturally occurring sandstone. The dissolution of these materials will enlarge and interconnect pores, allowing the produced fluids to move easily. Kinetics of mud acid reaction with sandstone is heterogeneous, complex and difficult to predict. However, the overall reaction rate depends mainly on the nature of the surface reaction within the pore. Unlike quartzitic fines, clays are unstable and easily protonated thus very reactive and therefore dissolved at a faster rate by HF acids. Nevertheless, when reacted with HF acids under isolated condition, both clays and quartz have similar solubility. The solubility of these minerals with 3% HF mixtures is close to 8 times lower than the amount of limestone dissolved per gallon of 15% HCl. Thus to produce in sandstone reservoirs equivalent stimulation results to those obtained in limestone, the acid volume must be several times larger.
During matrix stimulation treatments, the main factors that contribute to matrix dissolution by injected acids are the availability of minerals and their surface area. Therefore pore lining and flocculated clays, carbonates, scales and invading particles are more vulnerable to acid attack than any framework minerals. Moreover, clays have much higher surface area than quartz, hence increasing their dissolution rate, leaving quartz minerals virtually untouched. Thus, as more acid is injected, more clays and cementitious materials are dissolved especially in the big pores and the formation becomes progressively weaker until it eventually re-compacts to lower porosity and permeability levels.
The by-products of sandstone dissolution by HF acids are amorphous compounds and complex fluosilicates, fluoaluminates and fluoride salts which by nature have extremely low solubility.
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