A Laboratory Evaluation of Acid Propagation in Relation to Acid Fracturing: Results and Interpretation.

Abstract

SPE Members Abstract To design acid fracturing treatments, acid fracturing models require acid leakoff values including experimental coefficients such as acid propagation rate and acid penetration distance into the formation. This paper presents an experimental study aiming at defining a methodology for the acquisition of these parameters. The pressure variation curve obtained during a conventional acid injection experiment in a Hassler cell is analysed in relation with the dissolution mechanism. X Ray Computerized Tomography is applied to characterize the dissolution patterns. Influence of core length, acid concentration and acid flow rate are considered/critical values are found above which the filtration process is dominated by the wormholing mechanism. Critical lengths and rates are interpreted in terms of the Damkohler Number, i.e. the ratio of the acid brought by transport to the acid consummed by the chemical reaction. In addition, from the interpretation of the experimental results, a new methodology is proposed for the measurement of acid propagation rates and distances. Introduction The rate of fluid leakoff to the formation during fracturing treatment is one of the most critical factors involved in determining fracture geometry for a given treatment design. In acid fracturing treatments, the acid reacts with the formation, inducing the creation of channels (wormholes) that in turn increase the fluid leakoff Attention has been recently paid to the modelling of acid leak-off. Settari proposed to multiply the leakoff velocity predicted by the conventional leakoff model by a factor determined in the lab as the ratio between the leakoff velocity for the acid and the leakoff velocity of an inert fluid. This factor is correlated with one or two quantities that are internally computed in the model -i.e. cumulative masses of acid spent and leaked off. Hill modified a conventional leakoff model by introducing a layer with no pressure drop. This layer corresponds to the region penetrated by wormholes. The leakoff is thus computed along several regions: the filter cake, the invaded zone that contains the wormhole region and the compressed reservoir region. In both cases Settari's model or Hill's model the wormhole propagation evaluation will play a major role and has to be fully understood and described. Depending on the injection rate, three different mechanisms have been proposed: compact dissolution, dominant wormhole and multiwormholes. At very low injection rate, the dissolution pattern is compact, most of the acid being spent at the entrance of the pores. As the injection rate increases, a dominant "wormhole" develops inside the core which propagation is controlled by the mass-transfer of acid on the pore walls. At higher flow rates, the pattern becomes more homogeneous resulting in severe branching and high acid consumption. P. 63