The oilfield industry is familiar with the challenges associated with controlling inorganic scale precipitation in production wells. There are well-established techniques and procedures that may be followed, including modelling and test work in the laboratory, and prevention and removal in the field. While the rate of scale accumulation is not routinely predicted as yet, know-how developed over decades of scale management means that engineers have access to the expertise and tools to be able to manage scale effectively in a large range of production scenarios.
However, similar expertise and tools have not yet been fully developed and applied for controlling scale precipitation in Produced Water Re-Injection (PWRI) wells. This is a particular concern in scenarios where the injection brine has a scaling tendency, for example if it consists of a mix of incompatible brines.
Some of the existing tools used for scale management in production wells may be used, such as thermodynamic predictions. In addition, of late there has been significant work on modelling rates of deposition in such systems. However, there are key differences between conventional production scenarios and PWRI systems in terms of scale inhibition. In production systems the reservoir may be considered as an upstream batch reactor, in which, if the precipitation reactions are occurring far enough from the wells, there is a benefit in terms of a reduced scaling tendency at the producers; hence the demands on inhibitors are lowered. In injection systems, however, there is no such sacrificial upstream reactor, and so the demands of scale inhibition may be more stringent.
These issues are discussed in depth in this paper, with detailed reference to field examples where PWRI has been used, including occasions where scale damage to injection systems occurred. Oxygen and corrosion control are identified as key interactions with scale management. As well as a comparison of scale control in production and PWRI wells, this paper will present a strategy for identifying and managing the scale risk in PWRI wells.