Abstract

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

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.

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