The Effects of Corrosion Inhibitors and Mutual Solvents on Matrix Acidizing Treatments

Abstract

SUMMARY The addition of corrosion inhibitors is necessary to protect the tubular goods during acid treatment in wells. Together with the acids, they also enter the formation, where their effects on matrix permeability and acidising results vary with the type of inhibitor. An experimental study has been carried out to evaluate the impact of these materials on matrix acidising treatment results. Simple bottle and core flushing tests showed that recommended recipes could be incompatible with precipitates being formed. The texture of these precipitates was a function of the particular recipe used. The precipitates may block perforations or filter out at the formation surface and hence have a large impact on acid placement. Single phase flow tests with compatible solutions showed that filtercake-forming inhibitors have a stronger effect on high-permeability rock, whereas the effects of adsorbing inhibitors are greater in low-permeability rock where they can reduce acid reaction rates. Filtered out or adsorbed inhibitors can be removed from the formation by mutual solvent. However, in the absence of an oil phase, the effects of the corrosion inhibitors tested were so small that they do not appear to justify the use of expensive mutual solvents (assuming acid formulation compatibility). The experimental programme was extended to evaluate the effects of corrosion inhibitors and mutual solvents in the presence of an oil phase. It was shown that the stimulation of oil and water flow are, unfortunately, frequently not related – an increase in water cut being measured. In addition, adsorbed inhibitor appears to alter the wettability of formation fines, which then help stabilise crude oil/spent acid emulsions. The emulsions formed during acid treatments can severely reduce the effective permeabilities in the treated zones. Core-flooding experiments indicate that mutual solvents, which are frequently used to destabilise these emulsions, are less effective than expected from beaker experiments. The impact of these observations on field design are evaluated. It has been shown that the use of a mutual solvent – a frequently used co-additive – can compound the permeability impairing effect of the corrosion inhibitor via selective stimulation of water flow. This work needs to be extended to better understand the processes at work, an understanding which will no doubt stimulate the identification if improved products.