As the industry moves towards increasing deepwater production, the very high cost of well intervention places large constraints upon the performance of scale control options. In particular, where scale is managed by squeeze treating, it is an absolute requirement that the treatment lasts as long as possible and efficiently utilizes the applied chemical. This paper discusses using enhanced oil recovery-type surfactant systems in conjunction with scale inhibitors to dramatically extend the squeeze treatment lifetime. It is believed that the treatment lifetime is extended by the miscible displacement of organic material from formation surfaces thereby increasing the surface area available for the scale inhibitor to adsorb onto. The efficacy of this approach, using simple to formulate packages, is demonstrated by coreflood data in which the lifetime of the treatment is extended by up to four times.
The economic production of crude oil is based upon effective management of flow assurance. In essence, flow assurance can be considered to be the ability to produce petroleum fluids economically from the reservoir to a production facility over the lifetime of a development. Whilst hydrate control and corrosion control are important issues, scale control is also one of the key aspects of the flow assurance. The industry faces increasingly difficult challenges in scale management: for example, we now have subsea fields, deepwater production, complex new well completions (horizontal and multi-lateral), subsea tiebacks and commingled flow. Any intervention for scale inhibitor treatments in such complex wells is very costly indeed.
There are two main types of scale problems associated with hydrocarbon production. The first type is associated with natural depletion, where the consequent scale problems are normally restricted to calcium carbonate formation. Carbonate scale formation occurs when connate water or aquifer water passes through the bubble point and carbon dioxide is evolved. As carbon dioxide is evolved, the solubility with respect to carbonate declines rapidly and a precipitate forms with divalent ions such as iron, and more commonly calcium, as outlined in the following equation (Equation 1)
Where water injection (seawater, aquifer) is used sulfate scales can form when the injection water contains sulfate ions (Equation 2).
Sulfate scales have a wide range of solubility depending upon which divalent cation is present. In a production operation where barium is present within the formation water and seawater is the pressure maintenance fluid, the principal flow assurance risk is barium sulfate scale. This is due to its very low solubility relative to calcium carbonate and the sulfate scales of strontium and calcium.
Prevention of scale formation from the onset is by far the easiest and most economical way to treat scale problems. Scale inhibitor chemicals have been developed which prevent scale precipitation by two mechanisms. These chemicals either prevent the formation of scale crystals by nucleation i.e. disrupting clusters of scaling ions before crystallization can occur, or, by disrupting and preventing crystal growth once initial crystallization has occurred. They do this by attaching onto the faces of established crystals and sterically distorting growth sites such that further crystallization is inhibited.
Inhibitor chemicals can be of several varieties, the main ones being either phosphorus compounds e.g. phosphonates, phosphonic acids, phosphate esters, or, they can come under the general class of polymer inhibitors, such as polyvinylsulphonates (PVS) and polyacrylates.