Ester Cross-Linking of Polycarboxylic Acid Scale Inhibitors as a Possible Means to Increase Inhibitor Squeeze Lifetime
Authors: Bache Øystein , Nilsson Svante 
Published: December 2007

Abstract
Ester cross-linking of various classes of organic acid scale inhibitors has been investigated as a possible means to enhance inhibitor retention in the near wellbore reservoir. Three generically different polycarboxylic acid based inhibitors have been successfully ester cross-linked. Inhibition tests indicate that the cross-linking slightly reduces the inhibition efficiency. After desorption, the inhibition efficiency increases back to approximately the same level as before cross-linking, indicating that the cross-linking has been broken. Core-flooding tests indicate that ester cross-linking has a potential to enhance inhibitor retention in the formation and thereby increase squeeze lifetime.

Introduction
Downhole scale prevention is achieved, almost without exception, by so-called scale inhibitor squeeze treatments. Such treatments involve injection of the inhibitor into the near wellbore reservoir and subsequent back-production when the well is restarted, thereby protecting the near well bore formation and wellbore against scale precipitation. The lifetime of the treatment for a given well depends on the amount of inhibitor that can be placed, the inhibitor retention and release characteristics, and its inhibition performance. Squeeze treatments are expensive, especially for subsea and horizontal wells. Improved squeeze technology is continuously demanded as the wells become more and more advanced, water production increases, and profit margins become smaller.
The inhibitor can be retained in the reservoir by adsorption from solution on the rock surface or as a finely distributed solid, which is usually precipitated in situ. The inhibitor is produced back through desorption from the rock surface or dissolution of the solid inhibitor into the water phase. In the last few years, ways to prolong treatment lifetime have included:precipitation squeezes using calcium [1–4] and/or pH increasing additives [5–9]stimulation and/or precipitation enhancing additives [10–13]use of inhibitor particle suspension [14]cross-linking the inhibitors through ester bonds [15] and by chromium [16].
The main objective of the present project is to pursue the ester cross-linking concept [15] as a possible means to enhance retention in the reservoir and thereby increase the treatment lifetime. Low retention through adsorption is a particular issue for low molecular weight polymer based products. The concept should therefore have the largest potential for this kind of inhibitors.
Potential benefit and field application of ester cross-linked inhibitors
Enhanced retention will in principle occur because of the increased molecular weight and the stronger adsorption to the rock surface achieved by the cross-linking. Each inhibitor molecule will be more strongly attached to rock surface through the additional indirect adsorption of inhibitor molecules in the cross-linked network. Therefore, in the adsorbed state, the inhibitor should stay partly cross-linked as long as possible. Release of inhibitor will take place through ordinary desorption in combination with hydrolysis of the ester groups that attach it to the inhibitor network. The ester hydrolysis is acid or base catalysed and should therefore be relatively slow at pH values of most reservoirs (pH=5 to 6), but will also depend on the ester concentration. After ester hydrolysis the inhibitor regains its basic non-cross-linked structure and performance. Ester cross-linking could therefore be a way of combining the benefits of high and low molecular weight inhibitors in the same treatment, the former and latter having superior adsorption/retention and inhibition properties, respectively.