(319b) Analysis and Simulation of Reactive Dissolution of Carbonate Rocks with Gelled and Emulsified Acids

Authors: 
Maheshwari, P., University of Houston
Balakotaiah, V., University of Houston

The phenomena of reactive dissolution of carbonate rocks is a common technique used to stimulate oil and gas wells. In this process, an acidic solution (typically mineral acid, organic acids or chelating agents) is injected into the porous rock. The acid dissolves some of the rock and creates highly conductive channels called wormholes. These wormholes facilitate the flow of hydrocarbons from reservoir to wellbore, which leads to enhanced production (Maheshwari, et al., 2013). In field operations, hydrochloric acid is the most commonly used acid for carbonate acidization because of its low cost and high dissolving power. However, for high temperature carbonate reservoirs it cannot be used as the high surface reaction rate of acid with rock limits the acid penetration in reservoir and corrosiveness to well tubulars. In such cases, polymer based (i.e. gelled and in-situ gelled acids) and emulsified acids are normally used. These acids retard the reaction as well as reduce the leak-off rate from the wormhole walls, resulting in deeper penetration of acid. In-situ gelled acids are normally used for acid diversion from high permeability to low permeability zone. In the literature, numerous experimental studies have been performed to analyze the effect of acid gelation and emulsion on carbonate acidization. However, there is no qualitative or quantitative theoretical study that presents results on the reactive flow of gelled/emulsified acid in a porous media. More specifically, an analysis of transport and rheological properties (i.e. shear thinning behavior) of gelled/emulsified acid on the acidization process is not available in the literature.
We use the 3-D version of a two-scale continuum model (Panga et al., 2005) to present and analyze carbonate acidization with gelled and emulsified acids. From the 3-D numerical simulations, it has been observed that acidization remains in the optimum dissolution regime for a large variation in acid injection rate for gelled/emulsified acid when compared to HCl. In addition, wormholes are thinner and minimum pore volume to breakthrough (PVBT) is lower for gelled/emulsified acid when compared to HCl, which is experimentally verified by Gomma et al., 2012. From the sensitivity analysis of power-law index, it has been observed that acidization becomes more efficient (as minimum PVBTdecreases) with decrease in power-law index or acid exhibiting higher shear thinning effect. Finally, we develop a wormholing criterion for acids whose rheological behavior can be described by the power-law. This criterion can be used to estimate the optimum injection rate for the vuggy and non-vuggy carbonates.

References:
1) Gomaa, A. M., and Nasr-El-Din, H. 2010. New Insights into Wormhole Propagation in Carbonate Rocks Using Regular, Gelled and In-Situ Gelled Acids. SPE: 133303-MS.
2) Maheshwari, P., Ratnakar, R.R., Kalia, N. et al. 2013. 3-Dsimulation and analysis of reactive dissolution and wormhole formation incarbonate rocks. Chem. Eng. Sci. 90 (7 March 2013): 258-274.
3) Maheshwari, P., and Balakotaiah, V. 2013. Comparison of Carbonate HCl Acidizing Experiments With 3D Simulations. SPE Production & Operations,28(04), 402-413.
4) Panga, M.K.R., Ziauddin, M., and Balakotaiah, V. 2005. Two-scale continuum model for simulation of wormholes in carbonate acidization. AIChE J. 51 (12): 3231-3248.