A New Technique for Generating In-Situ Hydrofluoric Acid for Deep Clay Damage Removal
Authors: Hall B.E. 
Published: December 2007

This paper discusses a new retarded clay-dissolving system that preferentially dissolves clays instead of sand. This process is a result preferentially dissolves clays instead of sand. This process is a result of the ion-exchange properties of clay minerals that allow the in-situ generation of hydrofluoric acid on the clay surfaces when exposed to the treating solutions. Theory is described and laboratory comparisons are presented. presented.

Introduction
Hydrofluoric acid (HF) solutions have been used to stimulate sandstone formations since the early 1930's. In this type of application, acid is injected radially at pressures below fracturing pressure in the matrix of the rock. pressures below fracturing pressure in the matrix of the rock. HF acid then can react with and dissolve sand and clay in the formation. The most successful stimulation treatment with HF acid usually occurs in clay damaged formations when enough HF acid is used to penetrate and dissolve clays in the damage zone. Studies have shown that HF acid reacts rapidly and, in an average sandstone formation, active HF may not penetrate deeper than 1 or 2 ft into the formation. Consequently, clay damage at depths greater than 1 or 2 ft may not be easy or economical to remove conventionally.
Sequential injection of HF-acid solution followed by fluoride-ion solution is a new technique for generating an in-situ, clay-dissolving acid system (SHF). Laboratory tests indicate that SHF will dissolve clays in a sand-clay mixture deeper than conventional RF acid. In addition, SHF primarily reacts with clays, rather than sand. Therefore, the dissolving capacity of the SHF process is used primarily for clay removal, thereby providing a more primarily for clay removal, thereby providing a more efficient system for treating clay damage.

Effects of Clay Damage
Sandstone formations usually contain mixtures of various minerals that can include quartz, feldspars, calcite, dolomite, and such clays as illite, kaolinite, mixed-layer clays, chlorite, and smectite. During production, sandstone formation permeability can be damaged because of physical migration and chemical ion exchange of clay physical migration and chemical ion exchange of clay minerals. In addition, formation clays can be altered during drilling, completion, workover operations, and/ or stimulation treatments. The degree and depth of clay damage can vary greatly, depending on the damaging mechanism and formation parameters, such as permeability and clay content and type. permeability and clay content and type.Clay damage usually results from a combination of two processes. One damaging process involves imbibition processes. One damaging process involves imbibition of water between clay crystals that results in physical swelling of the clay. Another damaging process can occur when "loosened" clay particles are transported in fluids through pore channels of the formation to a constriction, where the clay may bridge and block the pore. This is a rather simplified discussion of clay damage. Other references treat this subject in more detail.
The depth of clay damage in a formation is an important factor in determining how to stimulate a clay-damaged formation most effectively. Usually, this parameter is not known and cannot be determined easily. parameter is not known and cannot be determined easily. Therefore, in most circumstances, economic considerations govern the size of the simulation treatment and the resulting depth of damage removal.
One approach for maximizing stimulation results on a cost basis is to design the treatment so that only the damaging element is removed.
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