Inorganic Scaling and Chemical Inhibition Challenges Associated with Production of Reservoir Equilibrated Spent Acid Stimulation Fluids

Abstract

Hydrochloric acid (HCl) is commonly used to remove wellbore damage and to enhance near-wellbore formation permeability in carbonate formations. Although in most cases the spent acid is recovered rapidly during well flow back after the treatment, in heterogeneous reservoirs some of the spent acid can remain trapped for long periods becoming fully saturated with CaCO 3 under downhole conditions. These reservoir-saturated spent acids then lead to scaling when eventually unloaded into the production system. This paper describes a modelling and laboratory study to replicate the system, to allow selection of inhibitors which are effective against carbonate scales in spent acid solutions containing extremely high levels of Ca 2+ /Mg 2+ , and also remain stable at elevated temperatures in the spent acids. The work also examined chemical retention and release via coreflood testing followed by field application modelling to select effective scale inhibitors (SI) which possess poor retention properties on the carbonate substrate thereby remaining present in the "spent acid". One significant challenge associated with this study was the ability to reproduce the mildly oversaturated field scaling environment in the laboratory. When using these fully saturated (with respect to CaCO 3 ) partially acidic brines, very small changes in the brine chemistry or preparation procedures had a significant impact on scaling. A detailed evaluation of brine preparation, stabilisation and buffering was therefore required prior to evaluating generic scale inhibitors for performance under these extreme conditions leading to selection of appropriate species. The second stage of the work then involved core test procedures to determine those chemicals that offered minimal retention properties on the reservoir along with performance. This paper will present the field scaling challenges observed due to unloading of trapped spent acids; describe challenges faced within the laboratory in reproducing these conditions and present results from generic chemical types which are effective at preventing scale under these extreme conditions. From the shortlisted products further results are presented demonstrating those which offer poor retention, which is generally the opposite of what is required for conventional scale inhibitor squeeze treatments, allowing selection for upcoming field trials in the selected field system.

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“…Scale inhibitors have been proven to work at sub-stoichiometric quantities and are commonly referred to as threshold inhibitors. Depending on the type of scale formed, and the type of inhibitor applied, performance of these threshold inhibitors has been observed on the order of less than 1ppm (Shaw et al 2013 andMorrow et al 2013). As a result of this low treatment rate, the inhibitor chemistry can be placed in the formation, or near wellbore utilizing squeeze techniques, or by placement of solid inhibitor.…”