ABSTRACT

Oilfield scale formation is dependent upon the production of water and naturally most scale inhibitors are water-soluble and are usually deployed in aqueous media. Squeezing low watercut wells with aqueous and oil miscible inhibitor treatments can cause problems, however for low watercut wells that are water sensitive, squeezing may result in significant and costly damage to the formation.

This study describes the development of a range of truly oil soluble scale inhibitors, which precludes any aqueous phases. The products have been screened for use in two fields that are known to exhibit sensitivity to the application of both aqueous and oil miscible scale inhibitor squeeze formulations. The paper describes chemical development and blending issues and also the inhibitor evaluation protocol - which differs significantly from conventional aqueous screening techniques, and includes a new coreflood procedure for oil soluble chemicals. Finally the paper discusses the implications of the test work for field application.

INTRODUCTION

Oilfield scale is a key issue in the petroleum industry where vast amounts of water are used and co-produced with hydrocarbons. The formation of mineral scale can create a range of problems to production including; reduction in pipe carrying capacity, increase in operational hazards, localization of corrosion attack, impedance of heat transfer and increases in operating costs due to down time and system maintenance. Prevention and control of mineral scale are the aims of water treatment. The mineral ions in water can be divided into three groups; ions in solution, as fine precipitates dispersed in the fluid phase or as precipitate depositing on solid surfaces as scale. Common oilfield scales include calcium carbonate and the sulfates of calcium, barium and strontium.

Squeezing is the most common method for scale control downhole. Scale inhibitor, diluted in carrier solvent (usually brine), is propogated out to an optimised radial distance into the oil producing formation where it is retained and then released slowly back into the aqueous phase during normal well production. In water sensitive formations, application of aqueous treatments can result in increased water cut and also formation damage via clay hydrolysis, swelling and subsequent constriction of pore throats. Near wellbore wettability can also be significantly altered resulting in impaired hydrocarbon production that may take weeks or even months to clean-up. Non-aqueous squeeze technologies that allow the active scale inhibitor to readily partition into the aqueous contact phase have been researched and applied with some success1,2,3,4,5,6,7, however the majority of commercially available products feature blends of aqueous based inhibitors with solvent systems or emulsification of aqueous inhibitors in an oil based solvent. This paper describes the next generation in oil soluble technology whereby completely water free, crude oil soluble scale inhibitors are proposed. The novel chemicals comprise a complex blend of non-aqueous scale inhibitor and organic solvents, where the defining property of the package is that it contains absolutely no aqueous phases whatsoever.

The following paper details the results of a laboratory test programme specifically designed to examine the key features of a new range of water free crude oil soluble squeeze inhibitors for squeeze application in low water cut and water sensitive wells. The laboratory work focussed on chemical compatibility with production fluids, thermal aging, oil/water partitioning, inhibitor performance and core flood studies to evaluate formation damage potential and retention and release characteristics. The paper also pro

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