Factors controlling the reaction rate of acid are: area of contact per unit volume of acid, formation temperature, pressure, acid concentration, acid type, physical and chemical properties of formation rock, and flow velocity of acid.
Effect of area-volume ratio on acid spending time
Reaction time of a given acid is indirectly proportional to the surface area of limestone or dolomite in contact with a given volume of acid. Extremely high area-volume rations are the general rule in matrix acidizing. Thereforce, it is very difficult to obtain a significant acid penetration before spending during matrix treatments.
Effect of temperature on acid spending rate
As temperature increases, acid spends faster on limestone or dolomite.
Effect of pressure on acid spending time
Pressure has very little effect on acid reaction rate, except at very low pressures.
Effect of acid strength on spending time
As concentration of HCl increases, acid spending time increase because the higher acid concentration dissolves a greater volume of carbonate rock. This reaction release greater volumes of CaCl2 and CO2 which further retards the HCl acid.
Effect of formation composition on acid spending time
Physical and chemical composition of the formation rock is a major factor in determining spending time. Generally, the reaction rate of limestone is more than twice that of dolomite, however, at high temperatures reaction rates to be nearer equal.
Effect of acid velocity on spending time
The effect of velocity of acid on reaction rate appears to be slight, except possibly with high strength HCl. In fracture acidizing, an increase in pumping rate increases fracture width during pumping. This decreases area-volume radio, thereby increasing acid reaction time.