Modeling Options for Drill-Cuttings Management

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 164983, ’Modeling the Options for Managing Drill-Cuttings Piles on Decommissioning,’ Sean Hayes, SPE, Genesis, and Liz Galley, CNR International, prepared for the 2013 SPE European HSE Conference and Exhibition, London, 16-18 April. The paper has not been peer reviewed. Advances in modeling techniques allow quantitative prediction of long-term trends in cuttings-pile characteristics and environmental risks, providing firm direction in mitigating risks. Modeling involving fluid dynamics, soil mechanics, scientifically verified 3D dispersion modeling, contaminant degradation, and seabed recovery was used in several scenarios: the extant pile, moving the pile across the seabed using suction pumps, backflush discharges related to retrieving the pile, and disturbance from remaining jacket footings ultimately falling into the pile after several hundred years. Introduction The Murchison platform is located in 156 m of water in the northern North Sea and is operated by CNR International. Drilling operations had been conducted at the field since the 1970s, ending in 2008. Discharges of drill cuttings and drilling mud have resulted in the formation of a drill-cuttings pile beneath the platform jacket structure. Between 1980 and 2000, oil-based muds were used and oil-contaminated discharges took place in line with normal permitted operations, and the cuttings pile now in place is consequently oil-contaminated. Fig. 1 shows the location of the Murchison platform and the local bathymetry, and Fig. 2 illustrates the jacket base. The Convention for the Protection of the Marine Environment of the North- East Atlantic (OSPAR) Commission Recommendation 2006/5 sets out a management regimen for such oily-cuttings piles, depending largely on thresholds against which the level of pollution attributable to a historical cuttings pile may be measured in order to determine whether the level of pollution could be significant. An assessment of the Murchison pile was submitted to the commission in 2008 as part of the regulator’s implementation report, concluding that the Murchison pile did not exceed the relevant criteria and that it could be left to degrade by natural processes. For those piles that can be demonstrated to have characteristics below OSPAR thresholds, the recommendation states that no further action is required and that the cuttings piles may be left in situ to degrade naturally. The Murchison field is in the process of being put forward for decommissioning, and the assessment made in 2008 regarding the rate of oil loss and the persistence footprint has been re-evaluated. By use of records of the discharges made from the platform, survey data from the Murchison cuttings pile, and industry data sources on the composition of drill-cuttings piles, a model of the existing pile has been constructed and used to understand the present condition and long-term fate of the pile.