The production of mobile silver films at the interface between an aqueous silver sol and a solution of a transition-metal complex in an organic solvent is reported. The films exhibit very similar physical properties to those described in a recent report (J. Phys. Chem. 1988, 92, 5754) but are produced by a simpler method of shaking together the sol and a solution of metal complex in a nonaqueous solvent. The phenomenon has been observed for several complexes of Cu1, Fe11, and Fem and affords a novel method of investigating Raman scattering of water-insoluble complexes near a metal surface. In a number of the examples investigated, significant enhancement of scattering is observed similar to the surface-enhanced (resonance) Raman scattering from silver sols.

Downhole mineral scale prevention in oilfields is usually achieved through the use of chemical scale inhibitors applied in "squeeze" treatments. These species are thought to be retained in the reservoir formation by one of two main mechanisms, viz. adsorption and precipitation. In a previous modelling and experimental study of scale inhibitor "precipitation" squeeze treatments, a mechanistic view of the precipitation/dissolution process was proposed and several predictions were made on the design of such processes. The principal conclusions were that the solubility of the precipitated inhibitor complex (Cs) and the dissolution rate (r4) governed the dynamics of the inhibitor return curve. This paper presents an experimental confirmation of some of these previous predictions by analysing core flooding experimental results using both outcrop and reservoir cores. Results are presented from a series of resin coated core floods conducted at 70 C and reconditioned reservoir conditioned corefloods at 90 C to 110 C. Both adsorption and precipitation floods have been carried out using the same generic scale inhibitors. It is shown that precipitation of a generic scale inhibitor, either a polymeric or a phosphonate species, will give a longer squeeze lifetime at higher inhibitor concentrations than the same product when applied purely as an adsorption treatment. However, depending on the solubility of the precipitate and the composition of the postflush brine, an adsorption treatment may have a longer lower concentration return curve. In the outcrop core floods, modification of the precipitation formulation (principally in the calcium level) changed the precipitate solubility (Cs) and this was shown to have a significant influence on the inhibitor return concentration during back production stage of the floods which was in accord with previous predictions. In a reservoir application of a precipitation squeeze process, the produced brine composition will generally be very different from the brine in which the inhibitor was applied. For example, the produced brine may have much higher or lower levels of calcium which will strongly affect the solubility of the precipitated inhibitor complex and will consequently have a significant impact on the return characteristics during back production. These more practical issues are examined in the reservoir condition core floods. The combination of the more basic mechanistic studies along with the applied results presented in this paper will help us (a) to design precipitation squeeze treatments with solubility tailored to the inhibitor concentration required for a particular reservoir and (b) to be aware of issues which affect lifetime of a precipitation squeeze which are connected with the composition of the produced brine. Introduction and Background Scale inhibitor "squeeze" treatments are the most frequently applied 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 inhibitor retention mechanism in the reservoir is due to (a) adsorption of the inhibitor onto the rock mineral substrates or (b) "precipitation" or phase separation of an inhibitor complex. The idea in a precipitation process is to extend the squeeze lifetime. Precipitation is generally induced by adjusting the solution chemistry ([Ca2+], pH, temperature etc.) such that an insoluble - or partially insoluble - inhibitor complex is formed; for example, this is often an inhibitor/calcium complex. Polymeric species such as polyphosphino carboxylic acid (PPCA), have often been applied in precipitation squeeze treatments although, more recently, phosphonates have also been proposed for precipitation treatments by a number of service companies. P. 641^

Total 011 Marine; Kari Ramstad, Norsk Hydro as.; and Paul Griffin, Enterprise Oil q SPE Members -W9ht IWS. SocMY of Petmbum Engin@rs, Inc. This paper was pmpamd for presentation at the SPE Intwnathnal Sym~um on Oilfield CMW w in an A~Onb, TX, u. S.A -14-17 *~V 1* This paper was aekcted lor prawntattcm by an SPE Program committee Wowing review 01 information c+mtairmd in an 8Mmct submltbd by ttw authuts). COntenta of the P9PW, * -d. haw nor hen rfMewd by the SocretY of Petroleum Er@near'a M we subw to cmectmn bythOaurhOrts). ThOmatOM, as Prwem@, do-m~* wry poaitbn of the SaciMY of Petrotoum Engirwwa, w offiim, or members. P8PWS pmnnbd at SPE rnostings am wbjwt to publicdon reviw by Edhorial CommHton9 ot ttw Society d~m EW-. mxnmqbtiMd tomti-titi me~=-.ll~rMymtbOwPiBd. ThOabmmcttic=-n -=F@-=kõ r Mere and by wlwn the paper is rxwenbd. Write Ubrwian, SPE. P.O. Sox 8SSSSS, FMwOaOm TX 7SCWH4M, u.S.A. hbX, 1SS24S SPEUT. ABSTRACTIn this paper, results from static tests have been used to establish scale inhibitor adsorption mechanisms and levels in ... ...Lconsolidated reservoir cores and to ratm mmcmors for their adsorption behaviour and, in some cases, squeeze return lifetimes. The purpose of this rapid and simple type of bulk adsorption measurement is to assist in the selection of inhibitors for further coretlooding which should be carried out on a minimum number of inhibitors.A bulk adsorption sensitivity study can be canied out very rapidly compared with aref~]!v carried out reservoir condition core floods. The , ---------. . . value of such rapid screening tests is evident although we show that it is not always possible for all factors concerning squeeze lifetime to be determined in this way. It is still often necessary to carry out a much smaller number of reservoir condition core floods for a few (usually between 1 and 3) selected inhibitor products. This is necessary if the dynamic adsorption isotherm, I'(C), is to be derived in order to develop the "Field Squeeze Strategy" or for the assessment of formation damage which might occur in the squeeze treatment. A field example of this is presented briefly in this paper aithough cietaik can "Mfound eiseiviiere.

The problem of downhole mineral scale formation is most commonly remedied by carrying out scale inhibitor "squeeze" treatments. The success of this process depends on there being an appropriate level of interaction between the scale inhibitor species and the rock formation. This interaction is described by an adsorption isotherm, F(C), in adsorption/desorption type squeeze treatments and the nature of this isotherm governs the dynamics of the inhibitor return profile. The isotherm depends on the factors relating to (i) the chemical species itself e.g. acid phosphonate, phosphinocarboxylic acid, polymer etc.; (ii) the conditions in the fluid e.g. [Ca2+], pH, iron content, temperature etc.; and (iii) the nature of the adsorbing surface i.e. mineralogy, surface charge, wettability etc. In reservoir formations, all of these factors may be important in determining the inhibitor/rock interaction and,, hence, the squeeze lifetime which is defined as the "time" - in days or in terms of produced fluid - that the squeeze lasts before the scale inhibitor concentration falls below the "threshold concentration", Ct, required to inhibit scale in that particular case. In this paper, we focus on the effects of the reservoir mineralogy and surface conditioning of the rock on the squeeze lifetime. Commercial barium sulphate scale inhibitors (acid phosphonate and poly phosphinocarboxylic acid) were tested in core flooding experiments using oil-reconditioned Brent Group Sandstones (North Sea) in order to evaluate their performance prior to field squeeze application. Representative results are presented from an extensive series of phosphonate and poly phosphinocarboxylic acid (PPCA) scale inhibitor core flooding experiments using sandstone cores from different formations within the Brent Group as the adsorbing substrate. In order to isolate the mineralogical effects, results from comparative floods using identically oil-conditioned, clean (highly quartzitic) outcrop sandstone cores are presented. The mineralogy of these cores is described and is found to be significant when inhibitors are being selected for field application. From the experimental work presented, the presence of clay minerals (principally kaolinite) is particularly important because of their influence on the adsorption of inhibitor species. Variation in mineralogy between individual formations within a single reservoir must be addressed prior to inhibitor and core selection so that more appropriate experiments that can support the modelling of field squeeze treatments can be carried out. The results from this experimental work are then used in the computer modelling of the data in order to develop the "Field Squeeze Strategy". An example of this is presented for a field case from the North Sea where the computer predictions were used to design an improved squeeze strategy. These recommendations were implemented by the operator and the subsequent field observations are compared with the design predictions. We find that this experimental/modelling approach can result in extended lifetimes of field squeeze treatments.

In the application of chemical inhibitors in field squeeze treatments for the prevention of sulphate and carbonate mineral scale formation, it is very important that the chemical species involved can be accurately assayed. When the inhibitor Concentration drops below a predetermined threshold level for scale inhibition (Ct) then the well may need to be resqueezed. The accurate assay of scale inhibitors down to concentration levels of a few ppm in real field brines can be a difficult task. In this paper, we examine a number of interferences which often make assay techniques very difficult to apply in field produced brines. The inhibitors examined in this work include phosphonates (PH), polyacrylates (PAA) and phosphinopolycarboxylates (PPCA). The main objective of this work is to develop suitable pre-treatment/purification techniques which allow the standard wet chemical techniques to be applied effectively after appropriate modification. Successful techniques - all based on careful modification of existing methods - have been developed by which these common inhibitors can be assayed very accurately at ppm and sub-ppm levels in a variety of North Sea field produced waters. This paper examines some of the major problems and interferences associated with poor analysis and introduces modified methods which can be applied in the field without the use of expensive equipment. It is also shown that different detection methods can often be employed in order to avoid more extensive clean-up strategies. Finally, instrumental methods such as ICP analysis (commonly used for phosphonates) are examined and pre-treatment methods are developed which allow phosphino-polycarboxylic acid based inhibitors to be assayed very accurately by this method. The results from an independent assessment by a North Sea operator, using spiked field produced water, are also presented as an independent verification of the accuracy of the techniques which have been developed in this work. Introduction Oilfield scale has long been recognised as one of the major chemical problems in the oil production industry. The formation of mineral scales may result in greatly reduced well performance as the rock pores, tubulars and topside machinery become choked by a build up of insoluble inorganic precipitate. In many cases, the most effective measure in dealing with the problem is through the treatment of the near wellbore area of the producers with chemical scale inhibitors applied in a "squeeze" process. In a successful squeeze treatment, the injected inhibitor is required to return to the production tubing at levels above the threshold concentration, Ct, over long periods of time (typically 3 - 12 months). Since threshold concentrations are often in the region of 5ppm or less, it is important to be able to accurately monitor the returning inhibitor at these low levels in order to determine the approach of the end of the squeeze lifetime. Monitoring of the scaling ions themselves, for example [Ba2+], will indicate when re-treatment is essential, but this approach provides no lead time in order to prepare for the operation. Furthermore, if successful computer modelling is to be carried out in order to either optimise the conditions for future squeeze treatments or to predict the end of the current squeeze lifetime, accurate low concentration return analysis is essential. In this paper, we are concerned with the detection and accurate assay in field produced waters of a variety of different chemical inhibitors commonly applied using the squeeze process. P. 543