Live acid penetration distance

In fracture acidizing, response depends on final fracture length and conductivity. Length depends on live acid penetration distance. Conductivity depends on the etching pattern (e.g., see figs. 10-1 and 10-2) imparted on the fracture walls by acid and its resulting flow-channel stability. 

Live acid penetration distance is enhanced by reducing the mass transfer or diffusion of acid in the fracture to the reactive fracture wall surfaces. This slows or partially blocks the acid reaction itself, reducing fluid loss (leak-off) from the fracture to the matrix. Fluid-loss reduction has the greatest effect. Fluid loss is controlled by several factors, including formation permeability and porosity, viscosity of the lost fluid, compressibiHty of the reservoir fluids, and the difference in pressure between the fracture and the matrix. 

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. 

Formation con osition. The chemical and physical compositions of the formation are very important in defining the acid spending time, and, subsequently, the acid penetration distance. Acid spends very rapidly in highly reactive (>95%) carbonates. Acid spending time can be much slower in formations with lower HCl reactivity (65%-85%). As mentioned previously, the reaction rate of acid in limestone is about twice that in dolomites (at lower temperatures). Therefore, live acid penetration can be deep in low-solubility, lower-temperature dolomites. 

One of the difficulties with add fracturing is delivering the live acid to the fracture tip without it leaking off into the formation or spending. Hydrochloric add is a strong acid, and the mass transfer is often the limiting fador in the readion with limestone (22). Consequently, the rate of spending is dependent upon the injedion rate. At low injection rates, the acid penetrates only a short distance into the fradure before consumption. This condition causes the limestone in the area near the wellbore to be dissolved excessively and prevents deep stimulation. 

Importantly, in sandstone acidizing, more acid is not necessarily better. In fact, in most cases, the opposite is true. The formation penetration distance of live acid is usually much less than one foot. However, near-wellbore... 

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