Introduction
In the early 1990s, a number of technologies were evaluated that could resolve the environmental issues associated with the offshore use of oil-based drilling fluids. (1) Of these, cuttings re-injection (CRI) was identified as one of the few that could be utilised at the offshore drilling location and would provide a complete disposal solution. CRI had been pioneered in Alaska and Offshore California (2) and had received regulatory approval in both the UK and Norwegian sectors of the North Sea. Projects were commissioned in both sectors (3) and by the mid 1990s this approach had been shown to be both technically and economically viable. (4) Since then, CRI has been adopted from Alaska to Argentina, from California to Sakhalin Island, north of Japan, and in most regions in-between.
When oil-based drilling fluids are utilised in the drilling operation, the rock cuttings are covered with a residual layer of the oily fluid and have to be disposed of in such a way as to avoid environmental damage. With CRI operations, these cuttings are mixed with sea water with some form of mechanical action, such that the mixture forms a stable viscous slurry. This is then pumped down a dedicated disposal well, or through the annulus between casing strings, and forced into the sub-surface rock formations under pressure (see Figure 1). This process creates a hydraulic fracture in the formation that effectively contains the slurry and, at the end of the injection programme, the well or annulus is sealed with cement.
Figure 1: Schematic of Cuttings
Re-Injection Process
In most cases the injection programme proceeds without serious mishap. However, there are many instances of the annuli or casings becoming blocked and of blockages of the injection perforations. Additionally there are reported cases of slurry migrating through natural fractures, or poorly cemented sections of the well, back to the surface and, in a very few cases, of the fractures propagating back to the seabed with consequential release of the slurry. These failures have not resulted in significant environmental damage to date but they have been costly in terms of remediation of the injection well, clean-up programmes and, in some cases, have necessitated the drilling of new injection wells. Operators have been fined for their failure to control the CRI process and, on occasion, injection permits have been revoked, impacting the whole drilling programme.
In addition, because the CRI process is online, any mechanical failure of the surface equipment prevents continued drilling progress and under-capacity slows down the drilling operation. Both of these lead to increased project costs.
From this, it is evident that a more thorough engineering and assurance approach is required. Inherent risks in the process need to be identified and managed proactively. Additionally, continuous monitoring of the CRI process needs to be improved such that any developing failures can be identified at a very early stage and mitigation steps be taken.
Category:
Environment
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