The Optimum Injection Rate for Wormhole Propagation: Myth or Reality?

Abstract

The success in matrix acidizing carbonate-formations depends entirely on the ability to create long wormholes that bypass the damaged near-wellbore region. Recently, much work has been performed to understand the wormhole propagation. This has led to various criteria and models to optimize matrix-acid treatment design for volumes, fluids, and flow rates. In many publications, the existence of an optimum flow rate for wormhole propagation has been described and efforts have been made to design treatments around these criteria. This paper will discuss several of the criteria and models that can be used to determine the optimum flow rate in both radial and linear flow and will introduce a new and simple criterion for both radial and linear flow that is based on diffusivity, reactivity, porosity, and surface area. Most of the criteria have been developed based on laboratory results or simulations in controlled and idealized environments. Under field conditions, many additional factors must be considered. Because of certain parameters such as significant heterogeneities in the formation, uncertainties in formation data, uncertainties in fluid placement, and operational constraints, the environment is neither controlled nor ideal. This paper will discuss the impact of these field conditions on the criteria and models to determine the optimum injection rate and verify their validity under field conditions. Examples and a case history are included to emphasize the need for consideration of these parameters. In addition, constraints are given to the validity of the criteria for the optimum flow rate under field conditions. This paper will further discuss the best practices and recommendations for cases where these constraints cannot be easily met or in cases where the data is uncertain. Introduction The major goal of acid stimulation under matrix conditions into carbonate formation is to create conductive flow channels known as wormholes that bypass the damage in the formation. These flow channels make the connection between the hydrocarbon-containing formation and allow the hydrocarbons to flow into the wellbore when the well is put back on production. The wormholes are formed when the matrix of the porous and permeable rock is dissolved by reactive fluids, such as hydrochloric acid (HCl). In contrast, during sandstone stimulation, only the damage inside the pores can be removed and typically wormholes are not formed. The practical implications are that matrix stimulation in sandstones can only restore a well's natural productivity (skin = 0) by removing damage if the damage skin is acid-soluble; whereas, matrix stimulation in carbonates can truly stimulate the productivity in a well because the highly conductive wormholes can penetrate beyond the damaged zone and result in a negative skin. To obtain this negative skin, the treatment must be performed in such a way that deep penetrating wormholes are generated. Three important parameters that can be controlled need to be considered: fluid volume, injection flow rate, and fluid type. The fluid volume is an import consideration because more volume will generally lead to deeper fluid invasion and to potentially deeper wormhole penetration. The second important aspect is the injection flow rate. Under some conditions, typically when the injection flow rate is too low, wormholes are not formed. Instead, only some of the permeable rock is dissolved in the vicinity of the wellbore. At the other end of the spectrum, when the injection flow rate is extremely high, one consequence could include the development of very thick wormholes that do not penetrate deep enough. The results of these two flow extremes have been observed in core tests and led to the observation that there is an optimum flow rate. The third parameter that can be controlled is the fluid type. Some fluids will behave better for wormhole propagation under known reservoir conditions, such as temperature, mineralogy, and permeability. Variations in acid types, concentrations, additives such as gelling agents, and emulsions affect the wormhole propagation.

Comparison of Carbonate HCl Acidizing Experiments With 3D Simulations

“…There exists an optimum acidinjection rate at which the least amount of acid is required. This injection rate corresponds to the wormhole-formation rate and is a quantity of interest for field operations (Glasbergen et al 2009). …”

Fluid Temperature as a Design Parameter in Carbonate Matrix Acidizing

“…Several theoretical and experimental studies have been conducted in the past to understand the effect of each of these factors on wormhole formation. A comprehensive review of these works has been done previously by Glasbergen et al (2009a). In addition to the factors mentioned above, temperature also influences pattern formation (Wang 1993;Frick et al 1994;Fredd and Fogler 1999;and Bazin 2001).…”

Reaction Kinetics and Coreflooding Study of High-Temperature Carbonate Reservoir Stimulation Using GLDA in Seawater

“…When the retention time of acid in the rock is small, narrow dissolution channels are formed and propagate through the rock and form more branches with continuous acid injection [8,9]. An optimum channel formed during acid injection at the minimum volume of acid injected and yield the highest permeability increase is known as a wormhole [4,10,11]. Other dissolution patterns may present, such as conical wormholes, at an injection rate between face dissolution and the optimum wormhole, and ramified wormholes at an injection rate between the optimum wormhole and uniform dissolution patterns.…”