Wormholes matrix acidizing

Carbonate matrix acidizing is, realistically, a method for bypassing damage. Stimulation response potential is much greater when damage is present that can be bypassed by flow channels (wormholes) created by formation dissolution by acid. 

In the matrix acidizing of carbonates, HCl is so rapidly and completely reactive with carbonate minerals that macroscopic channels, or wormholes, are formed through the rock matrix. The formation of wormholes is also... 

However, penetration of acid in a carbonate formation is not uniform. In most formations, the pores are different sizes and shapes. The porosity maybe present in the form ofvugs, natural fractures or fissures, or tortuous, capillarylike pores. Such heterogeneities in the porous structure cause channeling or wormholing of acid through the formation. The effect of wormholing is the penetration of acid much deeper than expected into isolated portions of the matrix, which may be sufficient to overcome skin damage. 

Actual responses of tuo carbonate petroleum reservoirs to matrix injection of hydrochloric acid are compared with a recently proposed experimental model for wormholing. This model is shown to be applicable in undamaged primary porosity reservoirs, and should be useable in damaged double porosity ones. Formations of no primary porosity are shown to respond very differently. 

In dolomite, add spends differently than in limestone formations. The readion of HC1 with dolomite is rate-limited at formation temperatures under 50 °C. Wormhole development, which is common in limestone acidizing, is not charaderistic in dolomite addizing (32). As pores become connected during matrix addizing operations, dolomites form caverns. The use of foamed acid ensures that the acid is spent in the primary channels and allows deeper penetration. Foams are good acid extenders. Deeper penetration can be achieved with foamed add as opposed to an equal volume of nonfoamed acid. 

Retarded acid systems can extend the length and number of wormholes. Such systems include slightly gelled acid, chemically modified acid, surfactant-retarded acid, emulsified acid, and foamed acid. However, the time it takes for acid to spend is still short in most cases. Usually, only the formation near the wellbore can be treated effectively. Thus, effective uniform matrix treatment beyond several feet from the wellbore is exceptional. 

Strong acid, such as HCl, will typically form longer, single wormholes extending from the perforations. If add is not effectively diverted, a matrix treatment with conventional HCl may create only one wormhole. This wormhole will then accept all add injected, as it will be the increasingly conductive path of least resistance for add as it continues to be injected. 

Leak-off is more severe in formations with high permeability and in gas wells (high reservoir fluid capacity). Leak-off occurs through acid-created channels, or wormholes, branching off from the fracture to the matrix. Leak-off can be partially controlled by reducing wormhole creation (through acid retardation) or physically blocking those wormholes that are created. 

Retarded acid systems can increase acid penetration depth by slowing or blocking acid reaction. They also can reduce the rate of acid leak-off to the matrix surrounding wormhole channels during their creation, providing deeper penetration and extension of flow channels formed. 

Physically retarding acid reaction is accomphshed by thickening (viscosifying) the acid used. Viscous acids include polymer-gelled, surfactant-gelled, emulsified, and foamed acids. Combinations can also be used in addition, surfactant-retarded acid can be gelled or foamed. The intent of viscosifying acid is to slow the rate of acid diffusion outward, to the rock surfaces, and to reduce the rate of fluid loss from wormhole to unreacted matrix. Both of these effects work to increase live acid penetration distance. 

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