SPE 31125 The Correct Selection and Application Methods for Adsorption and Precipitation Scale Inhibitors for Squeeze Treatments in North Sea Oilfields M.M. Jordan, SPE, K.S. Sorbie, SPE, G.M. Graham, SPE, Department of Petroleum Engineering, Heriot-Watt University, Edinburgh, U.K., K. Taylor, Shell Exploration and Production (Aberdeen), K.E. Hourston, SPE, Total Oil Marine (Aberdeen), and S. Hennessey, LASMO North Sea (Aberdeen). Copyright 1996, Society of Petroleum Engineers, Inc.

Abstract

Over the past three years, the Oilfield Scale Research Group at Heriot-Watt has conducted a number field studies to evaluated scale inhibitors for both downhole squeeze application and topside continuous injection for a number of North Sea operating companies. This paper presents an approach for screening commercial sulphate and carbonate scale inhibitors for field application. The screening results, which include data from static/dynamic inhibitor efficiency. static adsorption, compatibility and thermal stability are used to rank the performance of commercial scale inhibitors. From this short list, a small number (1 to 3) candidate products are taken on to reservoir condition coreflooding. In the screening of topside scale inhibitors, no adsorption tests are conducted.

Results from adsorption and precipitation type corefloods will be compared for polymer and phosphonate chemistries selected using these screening procedures. Such corefloods serve both to evaluate the squeeze lifetime performance and to assess the levels of formation damage caused by the scale inhibitor package. The strategy of deriving a dynamic isotherm which can be utilised in computer modelling of the coreflood data to produce a "Field Squeeze Strategy" will be outlined. This systematic approach provides a set of effective and economical methods for the chemical screening of scale inhibitors. This results in an improved field application strategy with longer squeeze lifetimes, while minimising formation damage potential.

Introduction

The downhole and topside formation of both sulphate and carbonate inorganic scales can be a serious problem in oilfield production operations. One of the most common and efficient methods for preventing the formation of such deposits is through the use of chemical scale inhibitor "squeeze" treatments. Two main types of inhibitor squeeze treatment can be carried out where the intention is either (a) to adsorb the inhibitor on the rock substrate by a physical-chemical process using a phosphonate or a polymeric material; or (b) to extend the squeeze lifetime of poorly adsorbing scale inhibitors by precipitation (or phase separation) which is commonly achieved by adjusting the solution chemistry ([Ca2+], pH, temperature) of a polymeric inhibitor such as poly phosphino carboxylic acid (PPCA).

The central factor governing the dynamics of the inhibitor return curve in adsorption/desorption treatments is the inhibitor/rock interaction as described by the adsorption isotherm, (C). This is a function of the inhibitor type, molecular weight, pH, temperature, mineral substrate and the brine strength and composition. 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 calcium salt, usually of poly phosphinocarboxylic acid scale inhibitor, within the formation. "Precipitation" (or phase separation) is controlled either by temperature and/or pH although it will generally involve a coupled adsorption process.

In this paper, our objective is to present a general methodology for the screening of chemical scale inhibitors for both downhole and topside applications. This is illustrated by results generated for application in four North Sea fields, although the Oilfield Scale Research Group have actually applied these methods to over 20 fields.

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