Modelling of scale inhibitor squeeze treatments is routinely performed to assist with chemical selection and to optimise treatment design, many examples having been presented in the literature previously. However, the modelling techniques are not always used to best effect, due to lack of experience, time or a methodical procedure for calculating sensitivities.
This paper presents a systematic approach to the use of squeeze models that makes use of laboratory data and field experience to assess, simply and effectively, the options for treatment design. Examples are presented that demonstrate the use of such models in aiding the selection of an appropriate inhibitor and the design of the first treatments as part of an integrated scale management philosophy. Very good accuracy in modelling the core flood is usually achieved. While the match between the model prediction and the first squeeze treatment is typically less accurate, history matching of the model parameters based on the first treatment is shown, by means of examples from two North Sea fields, to enable accurate predictions of numerous subsequent treatments in the same formation. The ability to accurately model treatments means that squeeze performance can be predicted with a high degree of confidence, and thus the treatment design may be optimised. This ability to accurately predicted treatment life is critical as wells mature, and the focus on cost per barrel of treated fluid becomes more critical. The most sensitive parameters are shown to be inhibitor type, inhibitor volume and overflush volume, and the paper discusses how they should be optimised to achieve the desired protection while striking a balance with chemical cost and deferred oil production.
The use of scale inhibitor squeeze treatments to prevent downhole deposition of carbonate and sulphate scales is a well-established procedure in onshore and offshore oil production facilities.
In general, the scale inhibitor squeeze process, illustrated in Fig. 1, comprises pumping a preflush solution (0.1% inhibitor in KCl or seawater), followed by the selected scale inhibitor (normally in the concentration range of 5% to 20% v/v in KCl or seawater) and finally an overflush stage (using inhibited seawater or KCl). The well then remains shut-in for a period allowing the inhibitor chemical to be retained on the reservoir rock, before the well is flowed back into the test separator and the main process vessels.1–4
There are two types of mechanism that are exploited to ensure inhibitor retention within a reservoir. One process is adsorption, which is a physical/chemical interaction of the scale inhibitor molecule with the reservoir mineral surfaces.5–9 The amount of adsorption is a function of the chemical type, formation water composition, formation water pH, application pH and temperature, reservoir wettability and reservoir mineralogy (principally clay types and abundance).7–9 Precipitation squeeze treatments entail the adsorption of inhibitor followed by precipitation onto mineral surfaces.10–12 The precipitation process is controlled by inhibitor chemical type, application pH and temperature, and divalent ion levels within the precipitation formulation.11,12 Precipitation squeeze treatments can offer advantages over adsorption treatments where the system requires a high (> 15 ppm) minimum inhibitor concentration (MIC) value. However, this squeeze life extension with precipitation treatments is not always observed for cases where the MIC is lower.6
The control of scale via chemical treatments both topside and downhole can result in a significant chemical expense for an operator. The total cost of operation (TCO) for a scale squeeze can be defined as all the cost elements involved that result from technical challenges caused by the chemical interactions of the produced fluids.
It is essential to reduce this expense while maintaining production and not compromising safety. The components of the TCO may include manpower (i.e. pump crew), chemicals, tank rental, transport, analysis, and the value of deferred oil production (lost production during operation), and penalties associated with excessive water in oil or oil in water concentrations.
The ability to accurately model scale squeeze treatments allows the following challenges, which directly impact TCO, to be addressed: