Summary
The discharge of drill cuttings to the seabed can significantly reduce
offshore drilling cost. However, the formation of cuttings piles on the seabed
near the rig site can present problems for drilling and production operations.
When the problem is likely to occur in the areas of low-current velocity, the
cuttings are not discharged to seabed, but ground and injected to formation or
transported to the onshore for disposal, which increases drilling cost. It is
highly desired to have an accurate method to predict the location and
configuration of the cuttings piles in the design stage of well drilling. This
paper presents a solution to the problem.
Starting from Newton's second law of motion, a mathematical model has been
developed in this study to predict the location, configuration, and
characteristics of cuttings piles. Factors considered include water depth,
current velocity, cuttings properties (e.g., size distribution, density, and
sphericity), and water properties (such as density and viscosity). This
mathematical model has been computerized. It provides well planners a useful
tool for predicting the characteristics of cuttings piles during drilling.
Application of the tool can reduce the cost of drilling through better handling
of drill cuttings such as discharging to water vs. injecting into the
formation.
Introduction
There are three basic types of fluids that are currently used for drilling
oil and gas wells. They are water-based mud (WBM), oil-based mud (OBM), and
synthetic-based mud (SBM). Historically, most wells in U.S. waters have been
drilled with WBM. Water-based mud and its associated cuttings usually are
permitted for discharge in federal waters. Because of their adverse
environmental effects, OBM and its associated cuttings have never been
permitted for discharge in U.S. waters. Synthetic-based muds are designed to be
less toxic and to biodegrade in marine sediments faster than OBM. Discharge of
SBMs is not permitted except for small amounts associated with cuttings.
At offshore platforms, most WBM- and SBM-associated cuttings are discharged
to the seabed whenever feasible. If the discharge of cuttings is not an option,
cuttings handling will become extremely complicated. The volume of slurry to be
re-injected to the formation may increase by a factor of 3 to 6, however, in
several cases the formation may not be able to receive such a large volume
during a short time interval. Land-based handling of cuttings contaminated with
WBM is difficult. Burning off the chemicals and distillation of water are power
consuming. All processes involved in the disposal or recycling of these
residual drilling materials will have an environmental impact, not necessarily
from discharges to the sea or ground, but resulting from the emission of
different gases like CO2 to the air.
The formation of cuttings piles on seabed near the rig site can present
problems for drilling and production operations. If the water is too shallow,
or the current velocity is too low, cuttings piles will grow up to the
discharge point and plug the discharging device. When these problems are likely
to occur, the cuttings are not discharged to seabed, but ground and injected to
the formation, or transported to onshore for disposal. Both options increases
drilling cost. Currently, engineers predict whether the problem is likely to
occur or not on the basis of very rough calculations. Because the decision
between discharging cuttings to the seabed vs. injecting into the formation, or
transporting to onshore all have strong impacts on drilling cost, it is highly
desirable to have an accurate method to predict the location and configuration
of the cuttings piles on the seabed under different conditions.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
14 April 2006
- Meeting paper published:
13 November 2006
- Revised manuscript received:
29 June 2007
- Manuscript approved:
17 August 2007
- Version of record:
20 March 2008
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