Acid Filtration Under Dynamic Conditions To Evaluate Gelled Acid Efficiency in Acid Fracturing

Abstract

Summary During the acid-fracturing process, part of the fluid enters the formation, dissolving the rock and creating channels, named wormholes which enhance considerably the volume of fluid leak-off and acid consumption into the formation. To increase the efficiency of the acidizing process, the fluid leakoff must be reduced to improve the fracture acidized length. This paper is a contribution to the study of the phenomenology of the acid fracturing process. An experimental approach, which includes the measurements of wormhole propagation velocities, leakoff volumes, and the observations of the dissolution patterns, has been developed to evaluate acid fluids performance under various representative conditions of acid flow into a fracture. The tests are performed in a tangential cell which allows the solution to circulate in a slot and to penetrate the core at constant pressure drop, thus respecting the geometry encountered in the field process. The experiments herein described have been conducted on limestone cores of different permeabilities. The tests have been performed at room temperature, 50 and 80°C with different overbalance pressures. Fluids are gelled and regular acid. The results are discussed in terms of acid fluid efficiency. The fluid leakoff is evaluated for different initial conditions and linked to the dissolution pattern. The results show that etched patterns suitable for acid fracturing are not developed at the core surface with the gelled acid. However, viscosifying the acid reduces the water filtration by a factor ranging from 3 to 10. Etched patterns are favored by high strength straight acids at low overpressure.