A Semiempirical Model To Calculate Wormhole Growth in Carbonate Acidizing

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe success in matrix acidizing carbonate formations depends entirely on the ability to create long wormholes that bypass the damaged near wellbore region. During the last decade, much progress has been made in the understanding of the physical and chemical processes that determine the final wormhole structure. Several different wormhole models have been proposed in literature. These models are usually successful in predicting wormhole growth under idealized conditions in the laboratory: linear core flow tests, one main wormhole, simple fluids like HCl with known rheology and reactivity, etc. In the design of an acid treatment, however, these models are less useful because they cannot handle the complex field conditions such as: radial flow, multiple wormholes, complex fluids such as emulsified acids or foams, etc.In this paper, we describe an alternative approach to model wormhole growth. A relatively simple wormhole growth model is developed, that still captures the essential physics and chemistry, and is therefore qualitatively correct. The model is semi-empirical, in the sense that the accuracy depends on two parameters that can be measured in a simple (linear) core flow test. Alternatively, the value of these two parameters can be taken from literature data. In developing the model, the intend was to create a relatively simple tool that can be used by the field engineer to design and optimize an acid treatment, and answer basic questions such as: what type of acid must be used and what is the optimum acid volume and pump-rate?The paper will describe the details of the wormhole growth model. The wormhole model was embedded in a comprehensive near wellbore flow simulator to analyze wormhole behavior in more complex environments, such as multi-layered reservoirs and long horizontal wells. The model has been used with good success in the design of many carbonate acid treatments. Several examples will be discussed in the paper. IntroductionIn matrix acidization of carbonate formations, the acid generates highly conductive channels, or "wormholes", that bypass the damaged near wellbore region and lower the skin. Creation of wormholes and optimization of the wormhole length is one of the main goals in acid treatment design. A proper understanding of the parameters that affect wormhole growth is therefore important. Acid wormholing has been the subject of numerous studies and many papers 1-17 . These studies revealed the complexity of the wormholing process. Moreover, although much insight has been gained into the fundamental chemistry and physics underlying the wormholing process, gaps in the knowledge still remain. For example, the mechanism behind the wormhole density is only poorly understood.Modeling wormholing has proven to be equally difficult. Many published models start with basic chemistry and physics 9-16 . Acid flow in a pore, or in a network of pores, is coupled with acid diffusion and reaction at the pore wall. Some models exploit the fractal behavior...

From pore scale to wellbore scale: Impact of geometry on wormhole growth in carbonate acidization

“…Daccord (1987) experimented with linear and radial water injection on plaster core, from which he showed the fractal nature of wormhole growth and observed a correlation in power −1/3 between flow rate and wormhole velocity. This study was extended in Daccord et al (1993a) and its conclusions were implemented in several empirical models used to simulate full acid treatments (Buisje, 2000;Buisje and Glasbergen, 2005) at the reservoir scale. Fredd and Miller (2000) proposed a detailed description of dissolution patterns observed in core acidizing.…”

“…This definition of the optimum is adapted to laboratory conditions (core-scale) but is not necessarily valid for field conditions. However, it is common practice to consider optimum conditions defined at the core scale as similar to the optimum at the well scale (Buisje and Glasbergen, 2005). At the reservoir scale, other parameters will define the optimum and not only the breakthrough time: fluid placement, depth of formation damage, etc.…”

An Experimentally Validated Wormhole Model for Self-Diverting and Conventional Acids in Carbonate Rocks under Radial Flow Conditions

“…A more practical approach was proposed recently by Buisje et al 1 whereby the rate at which r wh grows can be determined using the mean radial Darcy velocity at the vicinity of the wormhole front, <V r >(r wh ): (10) h is the thickness of the layer or radial core in which the flow occurs. If valid, Eq.…”

The Accuracy of Carbonate Matrix-Acidizing Models in Predicting Optimum Injection and Wormhole Propagation Rates

“…The results of their study suggest that the injection rate has no major effect on the dissolution patterns. Buijse and Glasbergen (2005) took a different approach by describing a relatively simple model to capture the essential physics and chemistry of the wormholing process. The growth rate of the wormhole front was modeled as a function of the interstitial velocity of the acid, which is a function of acid injection rate.…”