Cairn India Limited - Ravva | Innovating Development | Polycrystalline Diamond Compact Bits

1
 
   
   
   
   
   
   
RAVVA Celebrating 16 Years of Technical
Excellence Innovating Development
Polycrystalline Diamond Compact Bits
2
Ravva - Innovating Development Polycrystalline
Diamond Compact Bits The use of Polycrystalline
Diamond Compact (PDC) bits in Ravva has
consistently resulted in attaining higher Rates
of Penetrations (ROPs) over long intervals and
without significant wear as compared to Tricone
bits in the same region. The PDC bit can also be
re-used in a greater number of wells than
the Tricone bit. A PDC bit has no moving parts
(i.e. there are no bearings) and is designed to
break the rock in shear and not in compression as
is done with the roller cone bits. Rock breakage
by shear requires significantly less energy than
in compression hence less weight on bit can be
used, resulting in lesser wear and tear on the
rig and the drill string. A PDC bit employs a
large number of cutting elements, each called a
PDC cutter. The PDC cutter is made by bonding a
layer of polycrystalline man-made diamonds to a
cemented tungsten carbide substrate in a high
pressure, high temperature process. The diamond
layer is composed of many tiny diamonds which are
bonded together at random orientation for maximum
strength and wear resistance. In general, the
PDC bits provided ROPs ranging from 25 to 50 m/hr
compared to the Tricone bit, which provided ROPs
ranging from 5 to 30 m/hr in different hole
sizes. Also, the Tricone bit has a limitation
on the bearing life, leading to bearing failures
requiring the unwanted pull out of hole (POOH)
activity to change the bit unlike PDC bits. This
also leads to higher wear and tear in Tricone
bits. The PDC design is affected by 1.
Body design steel-bodied or tungsten carbide
(matrix) 2. Cutters Geometry
? ?
Number, size and spacing of the cutters Back
Side Rake
3. Geometry of the bit in terms of number and
depth of blades 4. Diamond table
? ?
Substrate interface Composition shape
PDC Bits Applications 1. PDC bits have been
typically useful for drilling long, soft to
medium shale sequences which are a low
abrasive. In such formations they typically
exhibited high ROP and extended life,
enabling entire sections to be drilled in one
run. 2. PDC bits are not usually appropriate
for highly abrasive well cemented sand sequences.
When drilling tight siliceous formations the
incidence of PDC chipping and breaking is
dramatically increased resulting in less than
expected ROP and bit life. 3. When drilling
heterogeneous formations containing alternating
shales and or shale limestone sequences, the
use of PDC bits has proved to be encouraging.
This bit incorporates the use of back-up
diamond studs behind the PDC cutter. When
drilling harder abrasive strings, the
diamond stud absorbs the increased weight
required to drill the stringer and prevents
premature damage and wear to the PDC cutter.
3
Ravva - Innovating Development 4. With the
use of mud motors, the use of long tapered
profile bits is considered. In addition,
radial jetting bits reduce the potential for
friction induced high cutter temperatures when
run on a motor, thereby reducing the
temperature degradation of the cutter. Logging
While Drilling Technology and Rotary Steerable
System The Logging While Drilling (LWD)
technology and Rotary Steerable System (RSS) has
been used in the drilling industry for some time.
These techniques were introduced for the first
time in India in the Ravva block. Triple combo
LWD tools and RSS tools were run in the RX-10
well successfully for the first time and opened
the way for their confident use in future wells.
Triple combo uses three LWD tools in conjunction
for acquiring three kinds of petrophysical data,
i.e. resistivity, porosity and density measurement
s. Logging While Drilling Technology The LWD
technology was developed primarily for use in
high angle wells, as the conventional
wireline logging tools would no longer fall under
their own weight or free fall. Generally, in
wells with inclination up to 45, the wireline
logging tools reach the bottom on their own. When
the inclination exceeds 60 in an open hole (70
in cased hole), the logging tools do not reach
the bottom and the ability to get logging
tools becomes a function of hole conditions such
as how clean the hole is and the roughness of the
wellbore wall. As the number of high angle wells
increased, the need for developing new logging
methods was felt and as a result the LWD
technology was invented. In this technology, the
logging tools are an integral part of the drill
string and acquire data when the drill string is
actually drilling. This is completely opposite to
the conventional wireline methods, where data
is acquired after drilling and a separate run is
required. Logging data is transmitted to the
surface using mud pulse technology and a computer
interprets it. Some distinct advantages of the
LWD are as follows
? ? ? ? ?
Logging data is available while drilling to help
pinpoint target formations, allowing more
accurate placement of directional wells LWD can
provide early reservoir evaluation and
Delineation Casing or core point selection is
made easy LWD logs can be used for pore
pressure prediction LWD saves rig time as it
eliminates separate logging runs
However, the technology also has its
disadvantages. In areas where drilling speeds are
high and the directional Measurement While
Drilling (MWD) is being used to orient down hole
motors, the logging data will be limited. Another
limitation of LWD is that the data can only be
transmitted at a certain rate. Hence, the use of
LWD tools is situation specific. Sometimes their
use is justified by the cost savings in terms of
saved rig time but sometimes their rental costs
and risk associated with their use renders
them uneconomical. The LWD data can also be
stored in the memory chip of logging tools and
downloaded into a computer on a trip. The
rationale behind their use in Ravva was to save
rig time and thus, the cost of the operation.
Since RX- 10 was an exploration well and was
drilled using Synthetic Oil Based Mud (SOBM), all
this data was required to evaluate LM 100-40
reservoir, LM 10 sands and geological prospective
target of the well. Hence, to acquire all this
data in a single run and save time, it became
necessary to run triple combo LWD in 8.5 hole.
The LWD run was successful and provided good
quality data to fulfil the reservoir requirements.
The same LWD triple combo was run in the RX-8
exploratory well again to evaluate MM30 M20SB
sands in real time.
4
Ravva - Innovating Development
Observing the success and cost saving in the
formation evaluation in the two previous wells
(RX-10 RX- 8), it was decided to run the
triple combo again in the RB-4 appraisal well.
The angle was high for the well and it was
difficult to run wire line after drilling, so it
was an added advantage to use the triple combo
LWD in the well.
Rotary Steerable System (RSS)
RSSs are different from other drilling systems as
they allow active steering of the bit while
continuously rotating the drill string. They
permit the guidance of well trajectory in terms
of inclination and azimuth, while rotating the
drill string. As a result, the directional well
can be placed within optimal reservoir position
and orientation. Conventional drilling techniques
include drilling with motors in which
hole placement is not easy in complex reservoirs
due to their inability to navigate and rotate at
the same time. The figure shows different types
of Bottom Hole Assembly (BHA) using conventional
Positive Displacement Motors.
On one hand the RSS has applications in the
high-cost extended reach market and on the other
it provides a cheap system for onshore marginal
properties where directional control is critical.