Acid Filtration in Dynamic Conditions to Mimic Fluid Loss in Acid Fracturing.

B. Bazin*, C. Roque*, G. Chauveteau* and M. Bouteca*

SPE Members

Abstract

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 leak-off must be reduced to improve the acid penetration into the fracture. 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 the performance of acid fluids in various conditions.

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 C and 80 C with different overbalance pressures.

The results are discussed in terms of acid propagation rate and interstitial injection rate. The fluid leak-off is evaluated for different initial conditions and linked to the dissolution pattern. In a second part, the efficiency of gelled acids to reduce fluid loss is presented. The results show that the wormhole tip velocity depends only on the acid flux. 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 straight acids at low overpressure.

Introduction

Our purpose is to examine acid filtration in relation to the acid fracturing process. To optimize the acidizing process, the best situation is to provide sufficient acid concentration to maximise the length of the acidized fracture with a minimum leak-off in the reservoir. In addition, surface roughness on the fracture wall is required to maintain a sufficient conductivity after the fracture closure. The rate of fluid leak-off through the formation during an acid fracturing stimulation treatment is one of the most critical factors affecting fracture geometry, etching and conductivity. Different models have been proposed for the design of acid fracturing; in most cases, the leak-off coefficient is determined from experiments as usually done for the evaluation of leak-off of conventional fracturing fluids. As the physics of leak-off for acidic fluids differs significantly this method is not satisfying. Recently, a more sophisticated approach, using a new model, has been presented. However, even if the leak-off value for acid fluid can be calculated, experimental data are required. The objective of the paper is to provide an experimental methodology for the evaluation of acid fluids under conditions representative of the acid fracturing process.

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