Laboratory and Theoretical Studies for Acid Fracture Stimulation Optimization

Abstract

Abstract The success of an acid fracturing treatment is determined by two characteristics of the fracture formed by the acid reaction: length of the conductive fracture and conductivity of the fracture. The distance that reactive acid moves along a fracture is controlled by the competition between the acid fluid loss and the rate of acid reaction with the formation versus the acid flow rate along the fracture. Acid conductivity experiments were performed with a unique conductivity instrument using limestone samples designed to simulate this complex process. Special emphasis was placed on scaling the processes described. Two different acid systems were compared: inhibited 28% HCl and emulsified 28% HCl at 130 F and 215 F. Results show that emulsified acid is 8.5 times more retarded than straight acid and that emulsified acid provides a more efficient use of the acid capacity, allowing for longer fracture length without sacrificing fracture conductivity. An acid fracturing simulator was used to illustrate the effect of acid retardation on the resulting etched fracture. The simulator computes many operating parameters which enables the systems to be compared under realistic field conditions. The emulsified acid is most beneficial at high temperatures, where it generates greater etched half-lengths than HCl even at substantially lower injection rates. P. 231