Anisotropic-Wormhole-Network Generation in Carbonate Acidizing and Wormhole-Model Analysis Through Averaged-Continuum Simulations
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SummaryThe optimal design of matrix-acidizing operations in carbonate reservoirs is a discussion in progress. Although several models are available to the industry for predicting wormhole propagation, most of them are not sufficiently practical to be used in real treatment designs, or they were developed to represent coreflood data and cannot be simply scaled up to represent wormhole formation in complex well geometries. This problem is addressed by Furui's wormhole-propagation model (Furui et al. 2012a), which is a modification of the Buijse and Glasbergen (2005) empirical correlation, including a scaleup procedure to represent field carbonate-acidizing operations using laboratory coreflood data. It is a practical engineering tool that can be used for treatment designs in horizontal wells, including barefoot and perforation-cluster completions in fairly isotropic and homogeneous reservoirs.In this work, an analysis of Furui's model is performed, including the effect of anisotropy (x, y, z permeability) in the carbonate reservoir. The analysis includes both radial and elliptical wormhole propagation that forms from an openhole completion, and the spherical or ellipsoidal wormhole propagation that emerges from each perforation in a perforation-cluster completion that makes the use of a limited-entry technique for achieving an acceptable acid placement.The development is made using extensive 3D numerical simulations with a two-scale continuum model and a finite-volume method to represent the dissolution of the porous medium. The numerical model is tuned to represent real results through matching experimental coreflood data and dissolution patterns.Conclusions are obtained regarding both isotropic and anisotropic formations. In isotropic formations with the radial propagation of wormholes, simulations indicate that a number from four to six wormholes propagates radially in each plane. When the propagation is spherical, simulations result in a number from 16 to 24 wormholes, propagating spherically from the point of acid injection.In anisotropic formations, the radial stimulated zone might become an elliptical stimulated zone, depending on the acid-injection rate and the permeability-heterogeneity magnitude. The major axis of the elliptical stimulated zone coincides with the direction of a higher permeability and a longer permeability-correlation length, and it is longer for larger acid-injection rates. In the same manner, the spherical wormhole-propagation pattern might become an ellipsoidal stimulated zone in anisotropic formations.
