STUDY OF FACTORS FOR SELECTING TYPE OF ACID IN SANDSTONE ACIDIZING: AN OVERVIEW

Mud acid matrix acidization is a stimulation technique commonly applied to sandstone reservoirs to remove near wellbore permeability damage and improve well productivity. For any acidizing process, the selection of acid (Formulation and Concentration) and Additives (Corrosion Inhibitors and Mutual Solvents) is very important.
Mud acid has been successfully used to stimulate sandstone reservoirs for a number of years. It is a mixture of hydrofluoric (HF) and hydrochloric (HCl) acids designed to dissolve clays and siliceous fines accumulated in the near-wellbore region to re-open or enlarge microscopic flow paths and thus improve formation permeability [1].
The success of acidizing depends to a great extent upon proper acid selection and good treatment design. This can be accomplished only with a good understanding of the fundamental factors which affects acid stimulation. This paper reviews these important factors and includes descriptions of the basic concepts of Damage removal, Types of treatment, Combination of Acids, Function of Additives and Acid reaction mechanisms.
As HF enters a sandstone core, almost all the minerals present begin to dissolve, but at different rates depending on the intrinsic rates of heterogeneous reactions and the exposed surface area. The reacting mineral can roughly be divided into two distinct categories: slow and fast reacting. Detrital quartz and clay fragments tend to react at a slower overall rate, whereas feldspars, authigenic clays, and amorphous silica tend to react at a faster rate. So, the mineralogy is one of the factors that contribute in acid selection design [2].
We have to consider other factors also on which the concentration and the ratio of HCl/HF depend. Percentage of carbonate, type of clay, temperature of reservoir and cementing material are certain factors which controls the type of acid and additives. So, before the selection of acid and additives the brief knowledge of these factors is necessary.