Controlling leakoff of reactive fluids is the key to more successful stimulation by acid fracturing treatments. Results of stimulation treatments using alternating stages of pad fluid and acid, in conjunction with 100 mesh sand as a fluid loss agent, show that leak off of the reactive fluids in carbonate reservoirs can be controlled to the extent that actual productivity increases closely approximate those predicted by computer design programs.
This paper describes the evolution of currently used acid fracturing techniques and materials. Leak off mechanisms and methods of controlling the leak off are discussed. Case histories show the effect on leak off of each advancement that has led to the present alternate stage pad and acid technique. present alternate stage pad and acid technique
Acidizing as a method of well stimulation was introduced on a commercial basis in 1932. For many years recommendations for treatment of carbonate formations were based on zone thickness and solubility. Increased productivity was believed to be due to the amount of rock dissolved rather than where and how it was removed. Little thought was given to deep penetration and highly conductive flow channels. penetration and highly conductive flow channels. Improvements consisted of additives such as surface tension reducers, deemulsifiers, iron sequestering agents, silicate control agents and many others.
Although fracturing or "Rock Busting" was theorized as early as 1936, the hydraulic fracturing process was not recognized as a stimulation technique for carbonates at the time. In the early 1950's, as hydraulic fracturing became a basic method of stimulation, it became apparent that fracturing was occurring in most acidizing treatments. Later in the decade research developed fracture acidizing design calculations and computerized design programs. Although the application of fracturing principles improved treatment design, results of fracture acidizing treatments fell far short of results predicted by the computerized design programs. Most predicted by the computerized design programs. Most researchers felt this discrepancy was due to failure of "live acid" to penetrate as far into the reservoir as calculations showed it should. Research was then directed toward increasing penetration. Some of the approaches taken have included:
Increased acid concentration.
Increased injection rates.
Increased fracture width to decrease area-volume ratio.
Improved matrix leakoff control.
Increased pad volumes.
Improved computer programs.
Although each action helped, either separately or when used in various combinations, still a large gap remained between predicted and actual results.
At this time it was felt that leak off of the acid it the formation was greater than originally believes due to the rapid reaction of the acid along the fracture faces, The acid reaction exposed worm holes through which fluid loss was increased. Thus if a non-reactive pad fluid were used ahead of the acid, this pad fluid could create fracture area and fill some of the voids. The following acid could then penetrate farther into the formation before reacting. penetrate farther into the formation before reacting. This type of stimulation treatment, termed Frac Pad and Acid, apparently originated in the Deep Delaware, Basin, and probably was based as much on economics as fluid loss theory. The deep formations here were calculated to require such large volumes of acid to achieve satisfactory productivity increases that treatment would be prohibitively expensive. Treatments were designed, therefore, using large volumes of pad fluid (gelled water) to establish fracture area and penetration. The pad fluid was followed by smaller volumes of acid to etch the fracture faces and provide conductivity. These treatments were highly successful and the Frac Pad and Acid Technique gained wide acceptance. It did not, however, substantially close the gap between computer-predicted and actual acid fracturing results.