This paper summarizes the efforts made to date by the Brent Field Unit of Shell UK Exploration and Production to arrive at a cost-effective and environmentally accepted method to dispose of oily cuttings. The use of Oil Based Drilling Fluids is a pre-requisite to drill the complex wells in the Brent field and with tighter environmental legislation an alternative to the discharge of oily cuttings to sea was required. Cuttings Re-Injection was identified as the most attractive disposal method for Brent.
The paper describes the problems, related to both surface equipment as well as to the sub-surface disposal process, that were successfully overcome since the introduction of CRI in Brent in 1994.
Shell Expro's Brent field is a mature oil field in the northern part of the North Sea. An extensive drilling programme is, however, being executed as part of the redevelopment of the field. From 1997 onwards the field will be depressurised and will thereafter progressively transform from an oil field to a gas field. The purpose of de-pressurisation is to liberate gas, in economic quantities, from the oil left behind in the reservoir. As de-pressurisation progresses, the production of oil will gradually diminish due to the lower reservoir pressure.
One of the key challenges facing Brent today is how to drill the remaining wells quickly enough to maximise the field's oil production prior to de-pressurisation. At the same time drilling cost needs to be reduced while well complexity is increasing (Figure 1). Low Toxic Oil Based Mud (LTOBM) is seen as one of the Critical Success Factors to meet the challenges and drill the complex well trajectories successfully while continuously improving drilling performance. From January 1st 1997 onwards, cuttings containing more than 1% by weight of LTOBM cannot be discharged to sea. Prior to this date the Oil on Cuttings (OOC) limit was 10%, which could just be met by the conventional solids removal equipment and as a consequence cuttings drilled with LTOBM could be discharged to sea. The more stringent environmental legislation with respect to oil to sea discharges was anticipated in 1994 and the decision was taken by Brent to investigate alternatives to discharge of oily cuttings to sea.
Several alternatives to the discharge of cuttings to sea are available:
ship to shore (and treat onshore)
clean offshore and discharge clean cuttings to sea
use Synthetic Based (environmentally friendly) Mud (SBM) systems
re-inject the oily cuttings
use newly developed Water Based Mud systems.
Each of these alternatives has its own merits and application; for Brent alternatives 3 and 4 were shown to be the most attractive. The choice between alternative 3 and 4 is primarily governed by the number of wells drilled per year per platform and the perceived risk associated with more stringent discharge limits for SBM in the future.
Based on economic grounds and on the anticipated tighter legislation with respect to discharge of SBM in the future, CRI was favoured over SBM. Further studies were initiated to evaluate the available surface equipment for CRI and the sub-surface aspects of the re-injection process.