Abstract
Downhole motors are widely used to drill vertical, directional and horizontal
wells in conjunction with Polycrystalline Diamond Compact (PDC) bits. When a
bent housing Positive Displacement Motor (PDM) is oriented for slide drilling
to manipulate a well's trajectory, the drill string does not rotate.
Consequently, the rate of penetration (ROP) typically decreases. It is
therefore important to optimize bottomhole assembly (BHA) performance in
conjunction with PDC drill bits. This paper discusses how motor performance
data, coupled with an ROP model, can predict the optimal weight-on-bit (WOB)
required to derive maximum ROP for a given section of a hole to be drilled.
This approach solves the ROP model and determines the ideal WOB with respect to
any restrictions that PDM performance equations apply on it. Bit wear is
included in the ROP model and an analysis is performed to optimize a given
interval of wellbore. The optimization approach is illustrated with two
examples for different formation types and one field case comparing the
performance of two motors with PDC bits. The optimum WOB, maximum average ROP
and differential pressure values are the outputs from the analysis. This
analytical approach can be used to determine the optimum PDM/PDC bit
combination to achieve maximum ROP through a wide range of operational
conditions.
Introduction
Positive Displacement Motors (PDMs) have gained widespread use in vertical,
directional and horizontal drilling applications. In directional and horizontal
mode, bent housing PDMs are used to manipulate well trajectory (inclination and
azimuth) to intersect bottomhole targets. Slide drilling occurs when the bend
in the PDM is oriented in a certain direction. During slide drilling, the drill
string does not rotate. In slide drilling mode, bit rotation is generated only
from the motor as drilling fluid is pumped through the drill string. Drilling
in this mode can significantly reduce ROP and increase well costs. Accordingly,
overall performance of bit and motor combinations can have an extremely
significant impact on drilling costs. In comparison to using a simple approach
like mechanical specific energy (MSE) which is a relative 'local' value as a
function of instantaneous operating parameters like WOB and RPM
only(1), the approach herein can do a global bit run optimization in
the pre-planning and follow-up phases, which include bit selection and detailed
design parameters, bit wear throughout the bit run as a function of operating
parameters and motor selection and performance. MSE does not consider any of
these parameters and is not an overall 'global' ROP or $/m optimization
tool.
In a PDM, the power section converts hydraulic energy of mud flow into
mechanical rotary power ? the reverse action of the Moineau pump
principle(2). Each PDM has a helical rotor assembled inside a
helical stator. The rotor has one less spiral or lobe than the stator, which
results in a continuous seal line between the two. Likewise, the length of
helical pitch for the stator is greater than the rotor, which forms cavity
spaces between them. These cavities move along the power section from the inlet
to outlet by rotating the rotor.
© 2009. Petroleum Society of Canada (now Society of Petroleum Engineers)
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History
- Original manuscript received:
26 March 2007
- Meeting paper published:
12 June 2007
- Revised manuscript received:
13 April 2009
- Manuscript approved:
4 May 2009
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