Scale Inhibitor Adsorption/Desorption vs. Precipitation: The Potential for Extending Squeeze Life While Minimising Formation Damage

Abstract

Abstract In this paper, results are presented from a series of scale inhibitor core flood experiments using oil reconditioned reservoir cores from four North Sea fields. Phosphonate, poly phosphino carboxylic acid, poly acrylic acid and sulphonated acrylic acid scale inhibitors were used in order to evaluate their performance prior to field squeeze application. Different methods of application were considered including both (i) adsorption/desorption and (ii) precipitation/redissolution type squeeze treatments. The return profiles for the different types of applications and the different chemistries are compared as are the observed formation damage effects. The advantages and disadvantages are outline for these two mechanism in terms of the extending squeeze lifetime vs potential for formation damage. It is intend that this paper contributes to the development of better designed and safer field applications of both adsorption and precipitation scale inhibitor squeeze treatments in the light of individual reservoir petrography and petrophysical properties. Introduction Scale inhibitor "squeeze" treatments provide one of the most common and efficient methods for preventing the formation of sulphate and carbonate scales in producer wells. Two types of inhibitor squeeze treatment can be carried out where the intention is either:to adsorb the inhibitor by a physical- chemical process using an acid phosphonate or a polymeric material; orto extend the squeeze lifetime by precipitation (or phase separation) which is commonly carried out by adjusting the solution chemistry ([Ca2+], pH, temperature) of a polymeric inhibitor solution e.g. poly phosphinocarboxylic acid. The procedure for applying such chemical treatments normally involves the following six stages;a "spearhead" package (a demulsifer and/or a surfactant) is injected which is thought to increase the water- wetness of the formation;a dilute inhibitor preflush is often applied to push the spearhead into the formation and, in some cases, to cool the near wellbore region;the main treatment is injected which contains the inhibitor chemical, normally in the concentration range 2.5% to 20%;the brine overflush is applied which is designed to push the main treatment to the desired depth in the formation away from the wellborea shut-in or soak period (usually 6 - 24 hours) is allowed which is the time when the pumping of the overflush stops and the inhibitor adsorbs (e.g. phosphonate) or precipitates (e.g. poly phosphinocarboxylic acid) onto the rock substrate;finally, the well is brought back on production. During the squeeze process, several chemical and physical processes may affect the inhibitor adsorption and phase separation characteristics. In addition, these same factors may be responsible for various types of damage in the reservoir formation. Adsorption of scale inhibitors is thought to occur through electrostatic and van der Waals interactions between the inhibitor and formation minerals. For phosphonates or polymers, it is known that this process may be described by an adsorption isotherm, (C), which is a function of pH, temperature, mineral substrate, molecular weight and cation concentration. The precise form of (C) determines the squeeze lifetime, as has been described in detail in a number of previous papers. The "precipitation squeeze" process is based on the formation of a gel-like inhibitor-cation complex, usually of a polymeric scale inhibitor, within the formation. P. 287