Carbonate Matrix Acidizing with Gelled Acids: An Experiment-Based Modeling Study
- Ram Ratnakar (U. of Houston) | Nitika Kalia (Halliburton) | Vemuri Balakotaiah (U. of Houston)
- Document ID
- Society of Petroleum Engineers
- SPE International Production and Operations Conference & Exhibition, 14-16 May, Doha, Qatar
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.1.2 Separation and Treating, 1.10 Drilling Equipment, 5.1 Reservoir Characterisation, 1.6.9 Coring, Fishing, 3.2.4 Acidising, 5.8.7 Carbonate Reservoir, 4.3.4 Scale
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In-situ gelling acids are used in carbonate reservoir stimulation to reduce the leakoff rate of the acid and to divert acid to low-permeability zones. Understanding the mechanism of flow diversion and the impact of fluid rheological properties on diversion is critical to developing good matrix stimulation designs. A literature review shows that few studies present models for stimulation with in-situ gelling acids. Specifically, there are no theoretical or empirical rheological models for gelling acids that describe the effects of pH on viscosity and hence flow diversion. The objective of this work is to develop, analyze, and validate a model to represent carbonate acidizing with gelling acids.
In this work, a semi-empirical rheological model based on experimental data for gelling acids that accounts for viscosity as a function of temperature, shear rate, and pH is presented. A two-scale continuum model is developed for non-Newtonian fluids, such as gelling acids. Using the model, the effect of rheological parameters on flow diversion is analyzed and the conditions for maximum diversion are identified. The predicted results are compared with the available experimental data and field observations. A scaling analysis is also presented that leads to a criterion for the optimum injection rate for attaining maximum stimulation for a fixed amount of acid (or a minimum in pore volumes to breakthrough (PVBT)). It is shown that the optimum injection rate for gelling acids depends strongly on rheological parameters of the acid. It is found that the presence of gel leads to more branching of wormholes and more uniform stimulation compared to Newtonian acids.
Flow diversion from a high-permeability region to a low-permeability region is the key to achieving successful stimulation of carbonate reservoirs, especially for long horizontal and multilateral oil wells. Traditionally, acidization treatment involves the injection of regular acids that flow preferentially through high-permeability regions, leading to overstimulation of high-permeability zones and understimulation of the less-accessible low-permeability zones. Therefore, in-situ gelled acids are used to achieve more uniform stimulation. These acids are prepared by adding polymers, crosslinkers, and breakers that use pH to control the crosslinking of polymers and form gel in certain pH ranges where viscosity is increased enormously. Experimental studies (Rose 2004) show that, at very low pH, these acids have very low viscosity. As pH increases to a certain threshold value, the polymer chains begin crosslinking in the presence of crosslinkers, leading to gel formation that increases viscosity by one to two orders of magnitude. After the pH exceeds a certain limit, polymer links are broken as a result of solubility limits or the presence of breakers. As a result, viscosity is decreased to its base value, as can be seen in Fig. 1. Thus, when in-situ gelled acids are injected, initially, most of the acids travel preferentially to high-permeability regions. But, as dissolution occurs, pH increases and gel is formed that offers very high resistance in the high-permeability zones, which forces the acid to divert to lower-permeability zones.
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