Acid Filtration Under Dynamic Conditions To Evaluate Gelled Acid Efficiency in Acid Fracturing
- Brigitte Bazin (Inst. Français du Petrole) | Claude Roque (Inst. Français du Petrole) | G.A. Chauveteau (Inst. Français du Petrole) | M.J. Bouteca (Inst. Français du Petrole)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 1999
- Document Type
- Journal Paper
- 360 - 367
- 1999. Society of Petroleum Engineers
- 3 Production and Well Operations, 5.1 Reservoir Characterisation, 1.11 Drilling Fluids and Materials, 4.1.2 Separation and Treating, 5.3.3 Particle Transportation, 2.5.2 Fracturing Materials (Fluids, Proppant), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 1.6.9 Coring, Fishing, 1.8 Formation Damage, 2.2.3 Fluid Loss Control, 3.2.4 Acidising, 4.2.3 Materials and Corrosion
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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 leakoff 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.
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 maximize the length of the acidized fracture with a minimum leakoff in the reservoir. Excessive fluid loss is prevented by using gelled acids or by pumping a pad of viscous fluid ahead of the acid. Whereas in the first case the viscous acid reduces the rate of fluid leakoff, in the second case, high strength acid is supposed to erode some patch on the fracture face where etching occurs, with the risk of wormholing and high leakoff.
The need to control fluid loss has led to the investigation of the way the acid consumption can be handled to favor an etched pattern at the fracture face with low wormholing and leakoff. In this respect, the knowledge of the mechanisms leading to the formation of an etched dissolution pattern with low leakoff and the characterization of acid fracturing fluids in terms of: (i) wormhole formation, (ii) leakoff volumes, and (iii) dissolution figures, are of major concern for designing the operating parameters of an acid fracturing treatment.
The experimental investigations of acid fluid properties have been performed in most cases at a constant flow rate in a Hassler cell. This design is suitable for the evaluation of wormhole velocities of high strength straight acids. However, leakoff volumes cannot be evaluated. Measurements of the wormhole propagation rate are not sufficient to characterize the leakoff rate.1 Since filtration occurs at the wormhole walls, it is necessary to evaluate whether or not the gel hinders the acid filtration (Fig. 1). In addition, with gelled acids, the formation of a viscous layer on the core face, which increases in thickness as the injection proceeds, raises an important question about the validity of the measurements made at constant injection rate in a Hassler cell. In this paper we present a tangential cell for the measurement of leakoff at constant pressure drop. To our knowledge, leakoff measurements in tangential flow conditions representative of field conditions have never been made for acid fluids.
The primary objective of this paper is to provide an experimental method for the comparison of acid fluid properties in various reservoir conditions. These properties are the wormholing rate, the leakoff rate, and the dissolution pattern on the core face. First, we describe the experimental devices and procedures. We point out the differences obtained from the experiments performed in a Hassler cell and in the tangential flow cell. Next the results of experiments with straight HCl are reported giving a reference case for the understanding of the dissolution mechanisms. This is followed by a presentation of the properties of acid-gel systems and a discussion on the effect of a gelling agent. The characterization of the dissolution pattern at the core surface and the extent of the wormholing inside the core is achieved under CT scanning. Various experimental conditions are investigated including variations in temperature, acid concentration, and nature of core, i.e., limestones and dolomite.
State of the Art
Special cells have been designed for leakoff studies. Three types of equipment are schematically described below. These are:
- The hollow core: the fluid circulates inside the core through a hole and leaks off radial; sometimes the flow applies in the opposite direction from the outside of the core to the inside to increase the surface area to fluid volume ratio; this geometry has been recently used for the evaluation of acid leakoff. 2
- The slot flow cell: the core cylinder is cut into two halves and flow is set between the two halves of the core. Acid reaction rates and conductivity have been studied.3,4
- The tangential cell: the fluid circulates in a slot parallel to the core face and the flow enters the core at a constant pressure drop.5
The main drawback of the hollow core and the slot flow cell is the small distance allowed to acid filtration which may prevent the formation of filter cake. On another hand, the tangential cells have been specially designed to accept longer flow paths and have been successfully used for the evaluation of leakoff of conventional fracturing fluids.6 They have also been used for examining filtration properties of drilling fluids. Indeed, the tangential flow cell is particularly well adapted for the evaluation of gelled fluids when both filtration and cake buildup occur. One purpose of this paper is to use such equipment for acid fluids and to make possible an evaluation of gelled acids in representative conditions of flow at a fracture wall.
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