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The Management of Surface Risks

Delays to the drilling operation can be caused by mechanical failures of the CRI equipment or be due to under-capacity. To obviate these risks it is important to de-couple the drilling and CRIoperations. This can be achieved through the installation of additional processing capacity. Because space is always at a premium offshore this is often impracticable. Preventative maintenance reduces the risk of failure but does not address the under-capacity issue during rapid drilling phases. However, recent advances in the pneumatic collection, transportation and storage of oily cuttings (5) mean that equipment failure issues and under-capacity limitations can now be overcome. Figure 2 is a simplified schematic of an installed system designed to allow continued drilling during periods of equipment repair and to provide buffer storage during fast drilling operations.

Figure 2: Buffered CRI Unit Installation
Figure 3: Solids Setting

Cuttings and the solids from the centrifuges can feed directly to the CRI unit, as usual. In the event of failure or excess cuttings production, part or all of the volume can be pneumatically transferred to the storage tanks. Once the repair is complete or the cuttings generation rate slows down, cuttings can be transferred from the storage tanks back to the CRI unit for processing. This effectively de- couples the drilling activity from the CRI process allowing for optimum drilling performance. Additionally, since the pneumatic system can transfer the cuttings over distances in excess of 100 metres and vertically between decks of the platform, the storage tanks can be sited remote from the drilling package, where space allows, overcoming the crowding problems otherwise associated with additional equipment. Also, since the cuttings are completely contained during thetransfer, storage and return phases, there is no risk of spillage and no manual handling is required.

The Management of Risks Associated with Plugging of the Tubing, Annulusor Perforations

Plugging of the tubing, annulus or perforations is a result of solids settling from the slurry, normally during periods between successive slurry injection phases, when the slurry is static. In any suspension of solids in a viscous fluid, solids will settle. The rate of settlement is a function of particle size, the low shear rate viscosity of the fluid and time. Larger particles, typically greater than 300 microns, are screened out at the surface, reducing the risk of settlement. However, in CRI operations the situation is complicated because of the elevated temperatures downhole and because the injection well is often deviated. Under these circumstances, particles settle on the low side of the well bore, forming a bed which, ultimately, becomes unstable and slides downhole (see Figure 3). When injection is recommenced, the solids are compacted and form a solid plug preventing further injection. Attempts then have to be made to clear the blockage, re-perforate the tubing at a shallower depth or move to another injection well. All of these steps are time-consuming, often impact the normal drilling operation and are costly. In the worst case, where an alternative injection well is not available, this may even entail the drilling of a completely new well with the resultant delay to the overall project and significant costs.

Quality control of the slurry is normally achieved through measurement of the fluid density and funnel viscosity. The former is a measure of the total solids loading, the latter of the suspension characteristics of the slurry. However, the slurry is thixotropic, i.e. shear-thinning, hence the viscosity measured is only relative, rather than one that can be used to model the suspension characteristics of the fluid.

In order to fully characterise the slurry’s viscosity a viscosity profile at various shear rates and temperatures is needed. That can then be used, within a solids transport model, to predict the rate at which the particles will settle. This in turn will dictate the safe period for the slurry to remain static and, hence, the point at which displacement of the slurry out of the tubing or annulus has to be undertaken. This information can then be used to develop a slurry design specification and a pumping procedure aimed at minimising the risk of plugging (6) In the optimum case, a bottom hole pressure sensor is installed. The changing pressure readings, under static conditions, give direct indications of developing settling problems, which can then be managed proactively.

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