Journal of Canadian Petroleum Technology
Managed Pressure Drilling (MPD) is an alternative to overbalanced and
underbalanced drilling in conditions where pore pressures and fracture
gradients are so close to each other (depleted reservoirs, deep and ultra-deep
offshore reservoirs) that it is not possible to drill significant depths
without setting a casing. While MPD enables an operator to drill longer
footages without setting a casing, it requires precise estimation of equivalent
circulating density (ECD) during drilling and static bottomhole pressure (SBHP)
during non-drilling times.
General practice in the drilling industry is to use rheological and volumetric
properties of drilling fluids measured at surface to estimate ECD and SBHP.
Consequently, ECD and SBHP measured using MWD and LWD tools in the field do not
match the theoretical calculations.
This study shows the importance of introducing the effect of downhole
conditions to hydraulic equations in order to estimate ECD's and SBHP's
accurately. Paraffin-based synthetic drilling fluid is used for this purpose.
The effect of pressure and temperature on density of fluid is determined using
PVT cell experiments. An equation relating the density of the fluid to pressure
and temperature is determined using linear and non-linear regression
techniques. Rheological characterization of the fluid was obtained on a Fann 75
HPHT rotational viscometer. A Bingham plastic model was used to define shear
stress - shear rate relation of the fluid in all pressures and temperatures.
The effect of pressure and temperature on plastic viscosity and yield point are
determined using linear and non-linear regression techniques, similar to the
ones used in PVT analysis. Both onshore and offshore cases are investigated and
the effect of incorporating downhole effects to density and rheological
parameters on ECD are analyzed.
As a result of the depletion of most of the known reservoirs around the globe,
companies are searching for oil and gas in more challenging areas such as deep
and ultra-deep offshore locations. In addition, high oil prices motivate the
industry to produce the last measure of oil from mature oil fields where the
pressure is depleted. The conventional overbalanced drilling technique creates
a major drawback to drilling in ultra-deep and depleted reservoirs. In
ultra-deep offshore locations, pore pressure and fracture pressure gradients
are very close to each other, and with conventional drilling, it is hard
(sometimes impossible) to drill a hole up to the target depth(1). In
the case of depleted reservoirs, pore pressure is so low that it is not
possible to drill without damaging the formation.
These challenges create the need for a new technology to drill in such hostile
environments. Managed Pressure Drilling allows drilling of longer intervals by
drilling overbalanced while maintaining near constant bottomhole pressure,
using a combination of drilling fluid density, equivalent circulating density
(ECD) and casing back pressure in a closed system(2, 3). While MPD
will enable operators to drill longer sections and use light drilling fluids,
it does require better wellbore pressure management. Only by managing the
wellbore pressure, will it be possible to decide on which type of drilling
fluid to use and how deep it can be used.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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- Original manuscript received:
27 May 2006
- Meeting paper published:
13 June 2006
- Revised manuscript received:
26 November 2008
- Manuscript approved:
22 December 2008
- Version of record:
1 February 2009