Skin-Factor Equations for Anisotropic Wormhole Networks and Limited-Entry Completions
- Mateus Palharini Schwalbert (Petrobras and Texas A&M University) | Ding Zhu (Texas A&M University) | Alfred Daniel Hill (Texas A&M University)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- August 2019
- Document Type
- Journal Paper
- 586 - 602
- 2019.Society of Petroleum Engineers
- carbonate acidizing, anisotropy, wormhole modeling, wormholes
- 3 in the last 30 days
- 161 since 2007
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It was recently shown that anisotropic wormhole networks might 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 might be elliptical in anisotropic formations. An analogy would be in completions where the limited-entry technique is used and the wormhole network is expected to be spherical in isotropic formations, but it might actually be ellipsoidal in anisotropic formations. This has an effect on well performance and should be considered when designing the acidizing treatment and the completion. At the same time, using a limited-entry technique might result in better stimulation coverage and also longer wormholes, but it might also result in a partial-completion skin factor, impairing the productivity from the stimulated well. This should be considered when estimating the stimulated well productivity.
In this study, two main topics are analyzed: the effect of wormhole-network anisotropy and the effect of a limited-entry completion. Both radial and spherical wormhole-propagation patterns are considered, to be applied in openhole and limited-entry completions. The differences in well performance are 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 might have a significant effect on the acidized well performance, and this should be considered in the acidizing treatment design. It was observed that the anisotropic wormhole networks present lower productivity than equally sized isotropic stimulated regions. Hence, equations such as the Hawkins (1956) 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 effect 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 optimal stimulation coverage of approximately 60 to 70%, and the perforation density required to obtain this for a given acid volume strongly depends 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 optimal perforation density for a given scenario.
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