Abstract

It was recently shown that anisotropic wormhole networks may arise from the acidizing of anisotropic carbonates. In openhole or cased and densely perforated completions, where in isotropic formations the wormhole network would be expected to be radial around the well, the actual stimulated region may be elliptical in anisotropic formations. Analogously, in completions where the limited entry technique is used, the wormhole network is expected to be spherical in isotropic formations, but it may actually be ellipsoidal in anisotropic formations. That has an impact on the well performance and should be taken into account when designing the acidizing treatment and the completion. At the same time, the use of a limited entry technique may result in better stimulation coverage and also longer wormholes, but it may also result in a partial completion skin factor, impairing the productivity from the stimulated well. This should be taken into account when estimating the stimulated well productivity.

In this study two main topics are analyzed: the impact of wormhole network anisotropy and the impact of a limited entry completion. Both radial and spherical wormhole propagation patterns are considered, to be applied in both openhole and limited entry completions. The differences in well performance is studied for each case, and analytical equations for the skin factor resulting from each scenario are presented.

The anisotropic wormhole networks are obtained from numerical simulations using the averaged continuum model, and the results are validated with experimental data. The analysis of the well performance is made through simulation of the flow in the reservoir with the different stimulated regions.

The results show that for highly anisotropic formations the wormhole network anisotropy may have a great impact on the acidized well performance and this should be taken into account in the acidizing treatment design. It was observed that the anisotropic wormhole networks present lower productivity than equally sized isotropic stimulated regions. Hence, equations like Hawkins formula should not be used for estimating the skin factor from anisotropic wormhole networks, and the equations proposed in this work should be used instead.

Specifically, the impact of anisotropic wormhole networks is large when the limited entry technique is used. It is shown that for this type of completion there is an optimum stimulation coverage of about 60 to 70%, and the perforation density required to obtain for a given acid volume depends strongly on the wormholes' anisotropy. The skin factor equations proposed in this work for the stimulation with limited entry completion should be used for obtaining the optimum perforation density for a given scenario.

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