Much of the worlds oil and gas comes from limestone (CaCO3) and dolomite (CaMg(CO3)2) formations, either in their relatively pure form or in the form of carbonate or siliceous sands cemented together with calcareous materials (CaCO3).
Dolomites are similar to limestones with the exception that they generally react more slowly with hydrochloric acid.
The primary method of stimulating wells drilled into these formations is to inject an acid treating solution. The acid dissolves part of the formation and may also dissolve other acid soluble material (mud damage, scales etc.), which is restricting or blocking the flow of oil or gas from the formation. Matrix acidizing increases the flow capacity of a producing formation when these restrictions are removed.
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Limestone and Dolomite
When either limestone and/or dolomite formation are stimulated, acid enters the formation through pores in the matrix of the rock or through natural or induced fractures. The type of acidizing used depends on, the injection rate and the number and size of the fractures present. Most limestone and dolomite formations produce through a network of fractures, though both formations can exist in an unfractured state. Normally, an interval will accept acid through the fractures more readily and at lower pressure than through the pore spaces. The acid solution reacts with the walls of the flow channels, increasing the width and conductivity of the fractures.
Most limestones and dolomite formations vary in acid solubility. Acid will attack the surface of the formation at varying rates, leaving an unevenly etched face. The existence of natural fractures, that occur at random intervals and in random sizes, contribute to the final uneven etching configuration.
The type of acid and strength are equally important factors in influencing the etch pattern. . The use of various types of acid (such as chemically retarded or emulsified acid), ensure that the volume of limestone or dolomite dissolved, will occur in an uneven pattern across the face of the fracture.
Gelled and cross-linked acids can also be used effectively. These fluids will create wider fractures and have reduced leak-off, resulting in less “worm holing” and deeper penetration due to the retarded reaction of the acid.
Chemically retarded acids are made effective by preceding the acid treatment with a hydrocarbon preflush containing an oil-wetting surface acting agent (surfactant) Due to the variable composition of the rock, the surfactant leaves a discontinuous oil film on the fracture face. The resulting acid break-through is irregular, creating an
improved etch pattern.
With emulsified acid, the resulting etch patterns are influenced by the rate at which acid penetrates the hydrocarbon outer phase of the emulsion and reacts with the The temperature of the formation should also be considered to ensure that the selection of either chemically retarded acid or delayed reaction acid is the one that is most suitable for the treatment recommended Acid volume and pump rate determine the acid contact time, during which the
fracture faces are exposed to live acid. Contact time has a direct bearing on the amount of etching obtained. However, increasing the volume of an acid treatment does not appreciably increase the depth of penetration. Thus, the benefit of a treatment with a contact time greater than the spending time of the acid, can be attributed to acid etching, which results in additional flow conductivity.
The “shut-in time”, or the length of time a well is closed in after a stimulation treatment, is determined by the type of acid used and by such downhole factors as:
· Type of formation.
· Bottom-hole temperature.
· Bottom hole pressure.
After an acid solution has been neutralised by reaction with the formation, it is no longer a stimulation agent. However, it may become harmful to the formation permeability if allowed to remain downhole.
Hydrochloric acid reacts so rapidly with limestone formations that it is essentially neutralised by the time the acid has been completely placed. This neutralisation generally occurs at all ranges of temperature and pressure. Limestone formations incorporate varying amounts of insoluble impurities, which can plug permeability if allowed to come to rest. Therefore, it is important to remove the neutralised hydrochloric acid as soon as possible. The shut-in time with such formations is zero.
Figures 2 to 5 show the relative reaction rates of 15% hydrochloric acid with limestone and dolomite formations at different temperatures. When chemically retarded acids like super retarded acids (SRA), delayed reaction systems (Super Sol Acid (EQH)), Sta-Live and emulsified acids like SRA-3 are used, the reaction time exceeds the displacement time. This is also true for gelled and cross-linked acids (Gelled Acid, Gelled Acid XL, XL Acid II). Here, the shut-in time may be extended if there is sufficient bottom-hole pressure to promote rapid cleanup.
For reaction times of retarded acids consult the engineering product bulletin pertaining to the acid system used.