INTRODUCTION Scale Squeeze Process
This paper outlines how developments in the placement software are able to predict the squeeze lifetime when using different overflush fluid types (hydrocarbon overflush vs. seawater overflush). The study present field data from two production wells, each treated with the same aqueous scale inhibitor. The initial squeeze treatment for each well used a diesel overflush to displace the chemical (low water cut wells). Subsequent squeeze treatments to these wells utilised the same inhibitor but with seawater displacement. It is clear from field returns data that the seawater rather than marine diesel improved chemical placement and extended treatment life. The theory behind this phenomenon is be outlined along with the changes to the placement software that can now predict this effect, so allowing more accurate treatment designs to be generated.
Scale inhibitor squeeze treatments for preventing carbonate and sulphate scales are well-established procedure in onshore and offshore oil production facilities.1-8
In general, the squeeze process, illustrated in Figure 1, comprises pumping a preflush solution (0.1% v/v inhibitor in KCl or injection quality seawater), followed by the selected scale inhibitor (normally in the concentration range of 5% to 20% v/v in KCl or injection quality seawater), and finally an overflush stage (using inhibited seawater or KCl). The well then remains shut-in for a period (6-24 hours) allowing the inhibitor chemical to react with and be retained by the reservoir rock, before the well is flowed back into the test separator and the main process vessels. 1-8
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The function of each stage is described below.
This stage is, in its simplest form, designed to displace the tubing and production interval fluids back into the formation. This creates a buffer zone between the formation fluids and the treatment chemicals. This is often desirable due to chemical and produced fluid (oil, brine) compatibility concerns. The preflush stage also reduces the tubing and near wellbore temperature, which reduces the scale inhibitor adsorption rate and reduces the risk of premature precipitation of a treatment designed to phase separate at elevated temperatures. The preflush stage may contain a small concentration (<0.5% v/v) of scale inhibitor along with a small concentration of surfactant or demulsifier to reduce emulsion risk as the produced fluids are displaced back into the formation.
In more complex treatment programs mutual solvents are applied at higher concentration (10-100% v/v). These chemicals perform two functions; they clean oil films from the mineral surfaces, increasing the surface area for chemical adsorption, and during reflow, the chemicals can remove much of the residual water saturation left in the near wellbore region following an aqueous scale squeeze treatment. The result of such preflush treatments is longer squeeze lifetime and faster well clean up rates.9,10,11
A spacer stage may also be present in squeeze programs between the preflush and the main treatment. Such stages are normally included when the preflush itself can react with the main treatment stage.
