Abstract

Wormholing is a common phenomenon in carbonate acidizing. These macro-sized channels, which are several orders of magnitude larger in diameter than natural pores, provide a magnified flow path, and hence enhance productivity. As the acid mainly attacks the larger pores, acid can penetrate much farther into the formation and bypass the formation damage rather than dissolve it. There is experimental evidence that wormholing on a smaller scale can occur in sandstone formations. The purpose of this study was to determine the treatment conditions and formation characteristics required for such wormholes to be created in sandstone through experimental and modeling work.

We have conducted extended experiments with samples of varying properties at different treatment conditions. Meanwhile, a theoretical model was built to simulate the process and visualize the acid etching pattern in the laboratory core samples. The findings from this study show that there are many parameters that affect the etching pattern. The most important factor that determines etching pattern is the heterogeneity of the rock sample. The spatial variation in permeability, porosity, and mineralogy are the driving force for the acid to channel through certain paths, developing highly permeable channels or wormholes in sandstone. The simulation results show that high permeability channels result when the initial porosity distribution is non-uniform, especially with existence of high porosity streaks. Such wormholes are not likely generated in homogenous rocks, even with an extremely high concentration HF acid solution at a high temperature. Uniform dissolution patterns are created in such samples as supported from our experimental work.

Introduction

Acidizing is a common stimulation technology used to remove damage around near wellbore region and to enhance well performance. The success of the acidizing treatment depends strongly on the dissolution pattern of the matrix. While acidizing of carbonate formations results in creations of highly permeable, deep penetratingmacro-sized channels, sandstone acidizing usually results in limited acid penetration, and therefore limited productivity enhancement, due to uniform dissolution of formation.

In order to overcome precipitation problems in sandstone acidizing, lower concentration HF solutions (1 – 3 wt%) are generally used. However, studies have shown that it is possible to achieve wormhole-like dissolution patterns using high concentration of HF at high temperatures[1–4]. Most sandstone acidizing models are based on the assumption of homogenous formation and cannot predict wormhole-like dissolution patterns. Recently, Li et al developed a fine-scale sandstone acidizing model that accounts for heterogeneities in the formation[5]. The fine-scale simulation makes it possible to detect the acid flow path inside the formation and the impact of precipitation on the flow field.

Using the fine-scale simulator, we studied the conditions that would lead to wormhole-like dissolution patterns in sandstone acidizing. Further, we determined the primary factors that control dissolution patterns and also created a matrix of acidizing conditions that would lead to wormhole-like dissolution patterns.

We also conducted series of laboratory experiments under similar acidizing conditions in order to compare results to our simulator output. In a few test cases, small channels or wormhole-like structures were created on the inlet of the core. The results of the laboratory experiments are compared with the simulation results.

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