severely depleted field, with known drilling challenges, was drilled
successfully utilizing ERD techniques to access reservoirs farther away from an
existing platform by means of slot recovery. Complexity of long open-hole
intervals and near-horizontal wellbore was mitigated with an engineered
solution utilizing MPD, optimized ERD techniques and fit-for-purpose drilling
ANANTHAN SRIDHARAN, Petrofac (PM-304) Malaysia Ltd.
PM, on the east side of Peninsular Malaysia, has four fields that produce oil
and gas. These fields cumulatively produce an average 10,500 bopd and flare
close to 12 MMcfd of gas. The drilling and workover campaign planned for 2021/2022
involved both the WD and I fields in this block. The objective of the campaign
was to perform slot recovery of two wells and sidetrack to new objectives.
well (Well 9) is an oil producer; the other (Well 13) is a water injector. In
total, 18 wells were drilled and completed during 2013 to 2015, using three
different rigs from the wellhead support structure. Most of these wells
have a build-and-hold profile, with TD depths between 1,100 m to 4,465 m and
inclination between 30° to 80° in a water depth of 64.8 m. Drilling challenges
include well collision risk (resulting from crowded wells underneath the
platform), extended reach drilling hazards, wellbore instability driven by coal,
and narrow drilling margin, due to reservoir depletion.
WD and I fields were known to be severely depleted, due to ongoing production.
The known drilling challenges that were seen from the 2017-2018 drilling
campaign were narrow mud weight margin, wellbore stability, kick and loss
circulation cycles, drilling performance issues and pressure ramps. These
challenges contributed to NPT on the previous campaign.
wells drilled were extended-reach to access the reservoir objectives upon
recovering the existing slots. Together with the complexity of long open-hole
intervals near horizontal, the wells were engineered to mitigate issues seen in
the previous campaign. As with the deployment of any technology, the
application of MPD and wellbore strengthening in an ERD well architecture needed
to be fully understood, together with its benefits, challenges and limitations.
MPD in an ERD well architecture was meant to reduce wellbore instability
related to the pressure cycling effects during drilling and making connections
and narrow pressure margin management within a severely depleted formation
9 was designed to have two new hole sections, Fig. 1. The
well was sidetracked from a shallow depth of 353 m, utilizing a mechanical-set,
single-trip 9⅝-in. whipstock. Upon running in and orienting the whipstock, the
system was set in place by means of a set-down weight on top of a 9⅝-in. bridge
plug that was set during the wellbore clean-out operations, post-plug-and-abandon
operations of the mainbore. The successful whipstock anchor was set, and the lead
mill, together with the middle and upper watermelon mill assembly, was released
from the whipstock ramp.
operations then began, and a successful 6-m window was created, together with a
10-m rathole drilled with the same assembly. Mill gauge and wear evaluation on
surface showed good window creation, together with the corresponding metal
swarf collection. A BOP cleaning tool, by means of vortex creation, was used to
avoid the cycling of BOP rams while cleaning. The 8½-in. and 6-in. hole
sections were then drilled, and the 7-in. and 4½-in. hydraulic set liners were
set and cemented in place, together with pressure point evaluations being
performed in open hole. Completions with retrievable tubing-conveyed
perforation (RTCP) were run, and the well was completed successfully.
13 was designed to have three new hole sections, Fig. 2. As
with well 9, the initial drilling start-up operations were almost the same, with
some changes. A 13⅜-in. whipstock was set on a shallow cement plug at 421 m,
which was dressed and tagged during the P&A operations of the mainbore. The
successful whipstock anchor was set, and the lead mill, together with the
middle and upper watermelon mill assembly, was released from the whipstock
ramp. Milling operations then began, and a successful 6-m window was created, together
with a 10-m rathole drilled with the same assembly.
gauge and wear evaluation on surface showed good window creation, together with
the corresponding metal swarf collection. A BOP cleaning tool, by means of
vortex creation, was used to avoid the cycling of BOP rams while cleaning. The
12¼-in., 8½-in. and 6-in. hole sections were drilled, and the 9⅝-in. casing, 7-in.
and 4½-in. hydraulically set liner were cemented in place. Pressure point
evaluations were performed in open hole. Completions with integrated tubing-conveyed
perforation (ITCP) were run, and the well was completed successfully.
through the operations, ERD techniques and MPD methods were implemented and
executed to ensure the drilling objectives were met. Overall, the use of MPD
successfully delivered two wells that would otherwise be much more challenging
following drilling hazards were identified on the basis of design stage, and
mitigation plans were carried out during planning and operations.
instability. A wellbore stability study was performed for the WD field, and
empirical evidence from the previous wells suggests a higher mud weight is
required to drill the well and is determined by the wellbore stability,
especially in high- and long-angle wells. In a certain identified weak group
sand, the shear failure of this formation is evaluated as being sensitive to
the inclination of the well.
drilling through this sand, the recommendation was to have a minimum ECD and equivalent
static density (ESD) of 10.5 ppg, to keep the wellbore stable at minimum shear
failure and to keep the wellbore cyclic pressure effect at a minimum during
drilling and connections. The recommended MW and ECD is meant to be above 12.0 ppg
for drilling the production hole section. This is mainly to reduce the risk of
drilling through unstable formations. Therefore, the mud weight strategy was to
start with a low static MW that covers the pressure ramp high pressure
formation and apply MPD to keep the bottomhole pressure constant, to stay
within the pressure window.
while drilling. As risk mitigation action for drilling past the weak sand, the
ECD management is now crucial and, therefore, the best practices for ECD
management were adhered to. The synthetic-based mud (SBM) density for drilling
this section was at 9.0 ppg, with lower high-end rheology being maintained for
optimized ECD control to avoid losses. With the MPD system in place, losses are
detected early and adjustments in terms of surface backpressure or mud pump
flowrates are made. With the low static mud weights, the stress on the wellbore
was also reduced, together with maintaining a constant bottomhole pressure
while cementing. Given the potential losses while cementing, due to ECD during
displacement, especially for the surface hole section, a low-weight cement
slurry was used to mitigate this risk. Losses during cementing would affect the
cement quality behind casing and might jeopardize the well’s integrity.
Therefore, the cementing for the well was designed and executed with two
slurries for both surface and the production hole section to give an assurance
of having a good cementing job for the wells. The usage of managed pressure
cementing in these wells was planned and executed as per plan. High ECDs were
seen when displacing the well from kill weight mud back to drilling mud.
avoid the possibility of fracturing the depleted zone gradient and keep the
wellbore pressure constantly overbalanced, the MPD system was used to compensate
for ECD while:
MPD system also allowed higher flowrates while pumping the spacer for mud
ramp. Leveraging the MPD system to be used for drilling the pressure
ramp section, MPD surface back pressure was applied to keep the bottomhole
pressure constant and overbalanced to the pore pressure, thus reducing the
possibilities of taking in an influx. Even in sections where the static MW was
underbalanced to the pore pressure and minimum stability requirement, MPD
applied sufficient pressure to maintain constant bottomhole pressure,
overbalanced to the pore pressure and achieving wellbore stability requirements.
9 had a 957-m horizontal departure (ERD depth ratio 1.33:1), and well 13 had a 3,922-m
horizontal departure (ERD depth ratio 2.77:1) from vertical depth. Well 9 was
drilled first, followed by well 13. All ERD and MPD practices and design were
implemented in well 9 as a learning curve before drilling the more crucial well
13. From the identified drilling hazards, both the wells were equally
challenging in their own ways.
13 was predominantly in the stationary zone, Fig. 3. Therefore, in terms
of engineering and operations, it was assumed that no effective hole cleaning
will take place, unless both flowrate and drill-string rotation are greater
than hole cleaning thresholds. This was the basis for the conveyor method.
12¼-in. section drills through the avalanche zone, landing out within the stationary
flow regime. The section was drilled with a bent motor housing (1.5° bend) and
as such, hole cleaning could become challenging, due to limited surface
rotation. To compensate for the reduced rotations, flowrates and fluid rheology
was maximized within system limits. Tripping practices also were adjusted to
ensure good hole condition for installation of the 9⅝-in. casing string.
8½-in. section held tangent at 81° inclination before dropping inclination to
62° at 2°/30 m to land within the reservoir. Given this trajectory, cuttings
accumulation would appear at the top of the drop section and was considered a higher
risk during tripping operations. The section largely remains within the stationary
regime, only dropping back to the avalanche regime towards well TD.
this, good hole cleaning parameters and fluid rheology were required to
effectively clean the hole and reach section TD. A rotary steerable system was utilized
to greatly help with hole cleaning efficiency, lifting cuttings off the low
side of the wellbore and into the high-velocity flow stream. With the use of
managed pressure drilling techniques, flowrates were maintained above the ERD minimum
recommended annular velocity (45 m/min.) throughout the section. This aided in
hole cleaning efficiency and improved drilling performance.
6-in. section was drilled with a rotary steerable assembly, and it provided
sufficient parameters for effective hole cleaning across the open hole and 7-in.
liner section. However, lower annular velocities were observed within the 9⅝-in.
casing (above 7-in. TOL) and would have resulted in cuttings accumulation at
the base of the avalanche zone. With the high flow rate, pipe rotation and
controlled ROP, this issue was mitigated. An intelligent circulation sub
(telemetry based) included within the BHA facilitated higher flow rates for
wellbore clean-up and remedial hole cleaning at the top of the 7-in. liner
which alleviated any cuttings loading within this area.
use of MPD mitigated the risk of losses to the major fault zone, as observed in
offset well 18. MPD was identified as a technology to mitigate the past
campaign issues. The objectives of utilizing MPD were to:
wells were designated as sidetrack ERD development wells. The approach for the
application of MPD is to have maximum flexibility to control and manage the
wellbore pressures during the drilling of the 12¼-in., 8½-in. and 6-in. hole
sections. With the expected narrow margin available between pore and fracture
pressure upon the introduction of the ECDs, all the drilling sections were
drilled with a low mud weight, either at/near balance to the pore pressure or
even underbalanced. This means that when circulation is stopped, the
hydrostatic pressure (ESD) of the mud will either be at balance or
underbalanced to the pore pressure.
MPD was utilized to keep the well overbalanced to pore pressure by utilizing
the surface back pressure (SBP). As a minimum, 150-psi overbalance was required
to be maintained above the pore pressure. This maintained well integrity and kept
well control in check.
pressure while drilling (PWD) in the drilling BHA was used to correlate to the
MPD well model for automated choke control. Gas readings also were used as an
indication that sufficient overbalance was being maintained. Site-specific
communication procedures were developed to ascertain the limits and alarm at
the wellsite, together with MPD wellsite training within the rig crew.
section TD for the wells, the kill mud weight chosen was based on the density
required to maintain the wellbore stability, due to the ERD nature of the well and
the coal stringers present in the lithology. The concept of maintaining a
constant bottomhole pressure between drilling, making connections, tripping in,
tripping out and killing the well was maintained all through the operations, to
avoid cycling the wellbore pressure and imposing fatigue on the wellbore.
For the ease of operations, the
elected method used for MPD influx circulation was that any measurable influx
detected by the MPD system will be controlled by the automated MPD system, by
increase of the SBP and within the limits of the SBP in relation to the
equipment and well. By doing so, the influx is kept small, avoiding its further
expansion. Once the influx has been controlled, the well control operations of
circulating the influx out safely will be performed by the rig.
Entrained gas in the system, by
identification of connection gas, background gas and trip gas, will be
considered as a non-measurable influx. These events can be safely circulated
out through the MPD system, if identified properly and the appropriate limits
on surface backpressure are adhered to following the MPD operations matrix.
As part of the design and well
control strategy, the mud gas separator on the rig was evaluated to ascertain
the operating envelope, based on the highest drilling MW. These results tied into
the creation of the MPD well control operations matrix.
Both wells were designed, as per the well delivery process. Being a development
well, the known pressure was taken as a basis for design, and the issues
encountered in the offset wells were taken into consideration for the well
design. The planned and actual pressure profiles are shown in Figs. 4, 5, 6 and
8½-in. hole section for well 9 was drilled with a 500-gpm flowrate on a motor
assembly, with low MW’s of 8.8 ppg, and MPD SBP was maintained constant above
the minimum stability requirement of 10 ppg. The 6-in. hole section was drilled
with a 250-gpm flowrate, utilizing an RSS assembly. The section was split into two
parts, where the first half of the section before the depleted reservoir was
drilled with 10.5ppg MW (in anticipation for losses), and the second half was
drilled with 10.8 ppg MW. Initially, 11.4 ppg was planned, however,
anticipating the reservoir was depleted even more, the MW was reduced to 10.8 ppg
and then 10.5 ppg in the reservoir section. All through the section, constant
bottomhole pressure was maintained to be equal to drilling ECD. On both
sections, a deviation of 0.5 ppg between drilling ECD and static EMW (ESD) was
seen, as shown in the ECD plot.
section TD, without killing the well and maintaining stability with constant
bottomhole pressure, formation pressure tests were performed with the required flowrates
for the tool to operate. This proved to be very beneficial in terms of
operational timing and wellbore condition.
12¼-in hole section for well 13 was drilled with a 1,200-gpm flowrate,
utilizing a motor assembly with a low MW of 9.5 ppg, and MPD SBP was maintained
constant above the minimum stability requirement of 10.5 ppg. The 8½-in. hole
section was drilled with a 700-gpm flowrate on an RSS assembly, with low MW’s
of 10.2 ppg for the first half of the section before the Fault I (Major Fault)
and then increased to 10.8 ppg for the second half after penetrating the fault.
surface backpressure was maintained constant above the minimum stability
requirement of 12.0 ppg throughout. The 6-in. hole section was drilled with a
250-gpm flowrate, utilizing an RSS assembly and with a low MW of 10.2 ppg that
was underbalanced to the pore pressure, Fig. 6. Prior to drilling out
this section, a dynamic formation integrity test (FIT) was performed to
ascertain the formation strength. With MPD in place, 490 psi of SBP was applied
to achieve the 13.5 ppg EMW, using an underbalanced MW.
section TD, formation pressure tests were performed with the required flowrates
for the tool to operate without killing the well and maintaining stability with
constant bottomhole pressure. This again proved to be beneficial in terms of
operational timing and wellbore condition. All through the section, CBHP was
maintained to be equal to drilling ECD, overbalanced to the pore pressure. On
all three sections, a deviation of 0.5 ppg between drilling ECD and static EMW
was observed, Fig. 7.
proven point that was seen during operations, the ECD’s in the 8½-in. hole
section for well 13 were maintained constant and trending below the well 18
total loss ECD of 13.5 ppg, maintaining the required stability requirement of
12.0 ppg, Fig. 8. Flowrates required for good hole cleaning practices
also avoided the annulus from loading up and creating ECD-induced losses. ECD’s
in the 6-in. hole section were maintained constant and trended, maintaining the
required stability requirement of 12.0 ppg.
required for good hole cleaning practices also avoided the annulus from loading
up and creating ECD induced losses, Fig. 9. For an ERD well, good
drilling practices, high flowrates, rotation and optimum low-end rheology of
the mud maintain the conveyor belt method while drilling these wells. This proved
to be beneficial, as the friction factors while drilling the wells were low and
trending within expected simulated factors, Fig. 10. This was attributed
to good hole cleaning practices, good hole conditions and stability.
also proved to be a great success for both well 9 and well 13. Out of the five hole
sections between the two wells, three were cemented in place, using the MPC
method. The risk of fracturing the depleted sands and the fault sections was a
motivation to engineer the MPC method to fit within the well limits. The
objective of maintaining the EMW in the well, to be within the wellbore
stability limits and the ECD that the well has been exposed to, was also the
prime objective. Displacing the well from kill MW to drilling MW and
introducing the cement slurry conventionally would fluctuate the wellbore
pressures a great deal and risk inducing losses.
the cement job, six pump-off events were identified, which would put the well
in an underbalanced condition without some SBP. Thus, a SBP schedule for each
pump off event was prepared and followed through. MPD successfully applied the
required SBP to maintain CBHP, which maintained wellbore stability. Even with a
low FG at the depleted sands, there were no measurable losses during
displacement of the cement. An LCM spacer was also used in the cement slurry
recipe to aid the MPC method. The cement pump job playback showed a gradual
pressure increase, as cement was displaced up-hole, Fig. 11. A good
volume of cement spacer was circulated back to surface after setting the liner
top packer. All indications support a successful casing and liner cement job
with cement at, or very near to, the top of liner and planned top of cement.
rollovers between drilling mud and kill mud, and back to drilling mud, proved
to be advantageous with the MPD system. Constant bottomhole pressure was
maintained, and wellbore fatigue was eliminated. The anchor points for MPD
control were set at the depleted sand zones. This method was applied whenever
the MPD system was online.
MWs chosen to drill the hole sections were finalized, based on limitations in
terms of the wellbore pressures and required overbalance to be maintained. Note
that with MPD in place, the circulation system becomes a closed loop system,
and the addition of SBP becomes apparent in the hydraulics.
drilling sections were engineered with a low MW to be used to keep minimum static
overbalance against the pore pressure and be within the limitations of the FIT
value with ECD in considerations. The kill MW was designed to support minimum
wellbore stability values and induced drilling ECDs to keep pressure
fluctuation in the wellbore as minimal as possible. With MPD in place, the
downhole pressure was maintained at a constant pressure throughout the drilling
sections, Fig. 12. The drilling fluid selections were based on the well
requirements, formation pore pressures and wellbore stability. The mud system
utilized was based on the obtained offset wells data, BOP in place and MPD mode
were also in place to drill the depleted sections with an engineered wellbore
strengthening solution, using agents whereby suggested formulation was mixed
into the SBM system and tested across a 508-um slotted disc to simulate the
fracture width, Table 1. The wellbore stability issues while drilling
through coals and fault zones were also addressed by addition of bridging
materials into the mud system as a preventive measure prior to penetrating
these zones, Table 2.
fluids and cementing-related NPT throughout the drilling phase of the campaign
were encountered. Effective fluids planning and execution, as mentioned above
for lost circulation prevention and wellbore strengthening methods during
drilling of depleted zones, were applied to the mud system, as the coal and
fault zones were being approached. The stable SBM fluids system properties (Table
3), maintaining low shear rate viscosity at optimum levels for good hole
cleaning while keeping the LGS content below the 7% level, provided good
cuttings carrying capacity for hole cleaning. They also provided excellent hole
stability, as well as achieving desired friction factor levels. An LCM decision
tree (particle type) was in place to combat potential losses while drilling.
ECD profile while cementing the 4½-in. liner in well 9 is shown in Fig. 13.
By using the MPD system, the ECD profile of the well will be maintained
overbalanced by compensating for a pump-off scenario between operations, thus
maintaining integrity and the 150-psi overbalance. The ECD is also below the
limits of the fracture.
and spacers played fundamental roles to achieving zonal isolation in the
wellbore. For a cement slurry to create a proper and effective bond, the
surface of the casing and formation must be clean of drilling fluids. A spacer
is used to displace fluid from the annulus, while a pre-flush is used to thin
and diffuse any leftover drilling fluid particles. With a 4-bpm pre-flush and
spacer displacement rate, the mud removal was effective. The rotation of the
liner also helped with effective mud removal. In conclusion, the MPC method,
together with an LCM spacer technology-achieved successful cementing for the
a combination of extended-reach drilling techniques with managed pressure
drilling achieved the following:
objectives of well 9 and well 13 were successfully achieved. The utilization of
MPD, together with ERD engineering and operational practices, mitigated all the
issues seen with the previous campaign wells, namely well 18 where narrow mud
weight margin, wellbore stability, kick and loss circulation cycles, drilling
performance issues and pressure ramps were seen. The improved drilling
performance and operational efficiency proved that MPD, utilized in a complex
ERD well, enabled the operator to mitigate drilling challenges and reach
previously unproduced reservoirs. WO
ANANTHAN SRIDHARAN is a Kuala Lumpur-based drilling engineer with 14 years of
industry experience in drilling and well engineering. He has depth of
experience in well planning, operations and management. Mr. Sridharan focuses
on the complex well technical domain in MPD, deepwater, HPHT and ERD, together
with conventional exploration, development and appraisal of offshore and
onshore wells. He has worked in an office-based environment, in addition to wellsite
operations on semi-submersibles, drillships, jackups, and tender and land rigs.
He has a degree in mechanical engineering from University of Sunderland and
holds an MSc degree in drilling and well engineering from Robert Gordon