Well Stimulation Using Acids
Authors: Milligan Mike 
Published: March 2010

Abstract
The fundamental aim of well stimulation treatments is to enhance the revenues from oil and gas wells by improving their productivity or, in the case of injection wells, their injectivity, This can be accomplished by removing or bypassing any damage or impairment from around the wellbore, or by creating conductive fractures, emanating from the wellbore, which enhance the formation's ability to flow.
Although perhaps a simplification, stimulation treatments can be divided into those using acids and those employing proppants such as sand, sintered bauxite, or ceramics. This article will concentrate on stimulation of well performance using acids. Acid treatments can generally be divided into acid washes, matrix treatments and acid fracs. However before reviewing these techniques, the relevant factors affecting the productivity (or injectivity) of an oil gas (or injection) well need to be understood. The productivity index or PI for oil and gas wells can be expressed by:
Equation (1) (Available in full paper)
Equation (2) (Available in full paper)
Where:
Q = production rate
re = drainage radius
p = average reservoir pressure
rw = wellbore radius
pwf = flowing bottomhole pressure
S = skin factor
k = permeability
Z = compressibility factor for gas
u = viscosity
h = formation height
Bo = oil formation volume factor
CI, C2 = constants for units
T = temperature
In damage removal/bypass treatments, in which acids are generally used, we increase well productivity by decreasing the parameter (S) - the skin factor. In fracturing treatments, we can increase productivity by increasing the wellbore radius (rw) to some fictitious radius called the effective wellbore radius (rw).
FIGURE 1: Effect of skin. (Available in full paper)
The skin factor (S) is a term which reflects a zone of altered permeability around a wellbore. Invasion by drilling fluids, cement filtrate and completion fluids together with perforation damage can effectively decrease the permeability of the formation rock near the wellbore. This decrease in permeability results in a pressure drop in excess of that predicted by the radial flow (diffusivity) equation. Thus in the damaged case, the additional pressure drop is accounted for by a 50-called positive skin factor. On the other hand, stimulation techniques such as acidizing will normally increase permeability in the near well bore region, Thus in the stimulated case, we should see less pressure drop than predicted by the radial flow equation. In the stimulated case, a negative skin is sought. These concepts are illustrated in Figure 1. The degree of damage, or lack of it, can be determined by analyzing pressure build-up (PBU) data and deriving a skin factor. If the skin factor is positive and attributable to damage mechanisms, as against turbulence or other limited entry effects stimulation treatments such as acidizing may be required.

Increase in Effective Wellbore Radius
Fracturing is often used to increase the productivity of low permeability formations. The effect of fracturing can be interpreted as an enlargement of the wellbore radius (rw) to some imaginary radius called the effective wellbore radius (rw).