8/10/2019 SPE 130316 ECDmanagementGulfofThailand http://slidepdf.com/reader/full/spe-130316-ecdmanagementgulfofthailand 1/18 SPE/IADC 130316 Managed Pressure Drilling Avoids Losses while Improving Drilling and ECD Management in the Gulf of Thailand Paul Fredericks, Greg Garcia, Ossama Sehsah P. Eng., At Balance Copyright 2010, SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition This paper was prepared for presentation at the 2010 SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition held in Kuala Lumpur, Malaysia, 24 –25February2010.This paper was selected for presentation by an SPE/IADC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers or the International Association of Drilling Contractors, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE/IADC copyright. Abstract A number of oil fields in the Gulf of Thailand are characterized by severely depleted gas sands and unstable coal and shale. Operators drilling high angle and horizontal wells in those fields often have to contend with problems associated with high equivalent circulating density (ECD) and the narrow operating margin between fracture gradient and borehole stability. Reaching total depth in those fields without losing returns is possible but it requires careful management of the ECD, vigilant hole cleaning practices, and controlled drilling. Unfortunately, success comes at the expense of drill time which is unavoidably reduced by those same conventional practices. Automated Managed Pressure Drilling (MPD) was a solution used by one operator to improve drilling efficiency while still avoiding losses and maintaining wellbore stability. By taking advantage of the MPD system’s ability to manage the BHP when the mud pumps are off the operator was able to reduce the static mud weight below wellbore stability and the ECD below the fracture gradient. With the reduction in mud weight the operator was also able to change the mud rheology in other ways that contributed to improved hole cleaning. Managed Pressure Drilling was initially used in a three well program in 2007 and later in an extended six well program in 2009. For most of the wells the drilling plan was the same: conventionally drill out of the 9-5/8” casing with an 8-1/2” bit down to the top of the depleted zones, lighten the mud density and rheology, bring the MPD system on line and drill through the depleted zone to the next casing point. However, midway through the second program the well plans changed and MPD was used to drill the entire 8- 1/2” section.For all nine wells, the MPD system remained the same: a dynamic pressure control system with an integrated real-time hydraulics model, an automated manifold and backpressure pump, and early kick detection equipment. Even though the trajectory changed from well to well, surface and downhole conditions remained relatively constant in terms of borehole and drill string geometry, stratigraphic sequence, mud rheology, drilling practices, MPD system, and rig type. Such relatively constant conditions created favorable circumstances for the comparison of conventional and MPD practices. Results show that MPD enabled the operator to avoid losses and achieve noticeable improvements in drilling efficiency and ECD management. In addition, there were noticeable improvements in MPD performance in terms of transition times and BHP control. Introduction Oil reservoirs in some areas of the Gulf of Thailand are overlain by sedimentary strata well known for their troublesome drilling characteristics. Alternating layers of shale, depleted gas sands and thin inter-bedded coal seams present alternating zones of instability and lost circulation. To be successful in those areas with conventional drilling, operators have to follow carefully prescribed ECD management practices and be willing to sacrifice drilling optimization. In those areas, losses have been known to occur while drilling 8-1/2” holes with 10.5 ppg mud which is required for wellbore stability. However, with 10.5 ppg mud the ECD can be as high as 11.9 ppg and greater the fracture gradient (Figure 1). In the drilling campaign that preceded the use of MPD in the Gulf of Thailand three wells were drilled to TD without losses.
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Managed Pressure Drilling Avoids Losses while Improving Drilling and ECDManagement in the Gulf of ThailandPaul Fredericks, Greg Garcia, Ossama Sehsah P. Eng., At Balance
Copyright 2010, SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition
This paper was prepared for presentation at the 2010 SPE/IADC Managed Pressure Drilling and Underbalanced Operations Conference and Exhibition held in Kuala Lumpur, Malaysia, 24 –25February2010.
This paper was selected for presentation by an SPE/IADC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed bythe Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society ofPetroleum Engineers or the International Association of Drilling Contractors, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of theSociety of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not becopied. The abstract must contain conspicuous acknowledgment of SPE/IADC copyright.
Abstract
A number of oil fields in the Gulf of Thailand are characterized by severely depleted gas sands and unstable coal and shale.
Operators drilling high angle and horizontal wells in those fields often have to contend with problems associated with high
equivalent circulating density (ECD) and the narrow operating margin between fracture gradient and borehole stability.
Reaching total depth in those fields without losing returns is possible but it requires careful management of the ECD, vigilant
hole cleaning practices, and controlled drilling. Unfortunately, success comes at the expense of drill time which is unavoidably
reduced by those same conventional practices.
Automated Managed Pressure Drilling (MPD) was a solution used by one operator to improve drilling efficiency while still
avoiding losses and maintaining wellbore stability. By taking advantage of the MPD system’s ability to manage the BHP when
the mud pumps are off the operator was able to reduce the static mud weight below wellbore stability and the ECD below the
fracture gradient. With the reduction in mud weight the operator was also able to change the mud rheology in other ways thatcontributed to improved hole cleaning.
Managed Pressure Drilling was initially used in a three well program in 2007 and later in an extended six well program in
2009. For most of the wells the drilling plan was the same: conventionally drill out of the 9-5/8” casing with an 8-1/2” bit
down to the top of the depleted zones, lighten the mud density and rheology, bring the MPD system on line and drill through
the depleted zone to the next casing point. However, midway through the second program the well plans changed and MPD
was used to drill the entire 8-1/2” section.
For all nine wells, the MPD system remained the same: a dynamic pressure control system with an integrated real-timehydraulics model, an automated manifold and backpressure pump, and early kick detection equipment. Even though the
trajectory changed from well to well, surface and downhole conditions remained relatively constant in terms of borehole and
Such relatively constant conditions created favorable circumstances for the comparison of conventional and MPD practices.Results show that MPD enabled the operator to avoid losses and achieve noticeable improvements in drilling efficiency andECD management. In addition, there were noticeable improvements in MPD performance in terms of transition times and BHP
control.
Introduction
Oil reservoirs in some areas of the Gulf of Thailand are overlain by sedimentary strata well known for their troublesome
drilling characteristics. Alternating layers of shale, depleted gas sands and thin inter-bedded coal seams present alternating
zones of instability and lost circulation. To be successful in those areas with conventional drilling, operators have to followcarefully prescribed ECD management practices and be willing to sacrifice drilling optimization.
In those areas, losses have been known to occur while drilling 8-1/2” holes with 10.5 ppg mud which is required for wellbore
stability. However, with 10.5 ppg mud the ECD can be as high as 11.9 ppg and greater the fracture gradient (Figure 1).
In the drilling campaign that preceded the use of MPD in the Gulf of Thailand three wells were drilled to TD without losses.
To do so the operator had to manage the ECD below 11.3 ppg which he accomplished by:
Cutting back on ROP and flow rate
Restricting string rotation speeds
Back reaming frequently
Using mud with higher plastic viscosity and solids concentrations
Though successful, those ECD management practices made it difficult and more expensive to drill high angle and longer hole
sections. In the search for an alternative solution to manage ECD, avoid losses, reduce cost, and increase drilling efficiency the
operator turned to Managed Pressure Drilling and selected the Dynamic Annular Pressure Control* (DAPC*) system.
MPD Overview
In conventional drilling, the annulus of a well and the returning drilling fluid are open to atmospheric pressure. In managed
pressure drilling they are not. The annulus is sealed by a rotating control device (RCD) mounted on the BOP from which the
mud flows through a choke manifold. As the mud circulates through the manifold, backpressure is added or released by
closing or opening the choke and therein lies the benefit of MPD to ECD management and drilling efficiency (Figure 2).
The MPD system that was used in these wells included an automated control and data acquisition system that monitored and
controlled the pressure in the annulus. The primary objective of that process was to control the BHP at a fixed point in the
well, between limits that if crossed could lead to influx, instability, and mud loss.
To accomplish that objective the pressure control process starts with an analysis of the hydraulics relative to the specific pressure limits. Multiple hydraulics scenarios were modeled for the operator and the conclusion was that the optimum mud
was one statically underbalanced relative to wellbore stability. Wellbore stability was maintained with the DAPC system by
adding and controlling backpressure whenever a pump transition occurred, that is when the rig pumps were shut down orstarted. During a transition the control system automatically synchronizes choke closure with rig pump stoppage to add
backpressure at the same rate at which ECD is lost to ensure the BHP stays above wellbore stability.
Conventional and MPD scenarios were modeled and are compared in Figure 3. The pressure boundary conditions for both
scenarios were wellbore stability at 10.5 ppg EMW and fracture gradient at 11.5 ppg EMW. To maintain wellbore stability
under static pumps-off conditions the mud weight in the conventional scenario had to be 10.5 ppg. The model of the
conventional scenario predicted that with a 10.5 ppg mud the dynamic ECD would be 12.8 ppg which would exceed the
fracture gradient in the depleted sands.
However, an alternate model of the dynamic pumps-on conditions in the MPD scenario predicted that by dropping the mud
weight to 9.0 ppg EMW an ECD of 10.5 ppg could be achieved which would put the BHP above wellbore stability and belowthe fracture gradient. However, in order to ensure wellbore stability during static pumps-off conditions the DAPC system
would have to maintain the static BHP at 10.5 ppg by creating and managing surface backpressure between 450 and 650 psi,depending on well depth, geometry, and hydraulics.
In practice, the mud weight that was used during MPD operations on each of the nine wells drilled was 9.0 ppg and the actual
amount of backpressure varied between 265 and 790 psi.
The DAPC system included an automated control system, a choke manifold, a backpressure pump, a Coriolis flow meter, a
control cabin, generator, and all the necessary surface piping and isolation valves to connect it to the rig equipment (Figure 4).
Operational Reviews
Nine wells were drilled with inclinations that varied between 40 and 90 degrees. In total, over 14,500 meters of 8-1/2” hole
were drilled with MPD during which the BHP was managed in over 450 connections, a connection represents a pressure
transition from pumps-on to pumps-off and back. However, other transitions were also managed by the MPD system, e.g.during MWD surveys and down linking, rig repairs, RCD element change out, and trips.
The operator’s objectives in this well were the same as every previous and subsequent well – manage the ECD to avoid lossesin the high risk 8-1/2” hole section and minimize the ECD with good hole cleaning practices. Managed pressure drilling was
used to achieve those objectives by reducing the static mud weight below wellbore stability and maintaining a constant BHP
whenever the rig pumps were shut down.
In general, the plan that was implemented for MPD in the initial drilling program became the blueprint for most of the wells
that followed. Drill a conventional 8-1/2” hole with 10.2 to 10.5 ppg mud just above the depleted sands, switch over to 9.0 ppg
mud, install the rotating control device (RCD), and rig-up and operationally certify the MPD system. If required, final training
would take place on the rig during which the crew would practice implementing the appropriate procedures for various
contingency scenarios. Once training was completed and the equipment certified as ready drilling would proceed.
However, after the fifth well, the drilling procedure was changed and MPD was used to drill the entire 8-1/2” section. The
reason for that decision was due to the time it was taking to rig up the RCD and the secondary trip tank and make final
connections to the MPD system. It was felt that leaving the hole open for that length of time posed a risk to well stability.
In adopting MPD the operator was able to use a lighter mud, reduce the yield point, plastic viscosity, and the solids content,
and increase the flow rate through most of the MPD intervals (Figure 5). With the implementation of these and other changesthe operator was able to avoid losses and improve his drilling efficiency.
Drilling Performance
A comparison was made of depth versus time achieved with conventional and managed pressure drilling. All of the drill timedata is from the high risk 8-1/2” hole sections some of which were directional, some horizontal. In an effort to establish a
benchmark for drill rate, depth versus time data from an offset horizontal well was plotted and presented in Figure 6. In that
offset well it took a couple of days to reach the depleted sands ( 2226 m MD) at an average drill rate of 372 m/day. Afterencountering the sand the drill rate was slowed down to an average 234 m/day. Overall the section was drilled at an averagerate of 279 m/day. Over 80 connections were made during both sections and in accordance with established procedures for
managing high ECD the pipe was back reamed before each connection which amounted to over 2 days of rig time.
Drill time data from the first well is plotted in Figure 7. That well was drilled conventionally to 2655 m, just above the
depleted sand, from which point MPD was used to manage the BHP down to the heel. The average drill rate in the MPD
section was 378 m/day which represented an improvement of over 60% compared to the 234 m/day average that was achieved
in the offset well in the section the depleted sand. As part of the ECD management practices, back reaming was being done
with the same frequency as before.
The third well was a 40 degree directional sidetrack of the second well. The sidetrack was drilled with MPD down to 2745m atwhich point a trip was made to make repairs. Down to that point the average drill rate was 525 m/day which was a further
improvement in the drill rate (Figure 8). Its moderate inclination might have contributed to its higher drill rate and better hole
cleaning compared to the previous horizontal wells. But a point to make is that the MPD section below the depleted sand was
drilled at over 500 m/day which was not the case in previous wells below the depleted interval.
The fifth well was another horizontal well which was drilled in three sections. Originally, the intention was to drill this well in
the same way as the previous wells – conventional down to the depleted zone and MPD to section TD. Everything went
according to plan until two days into the MPD section (Figure 9) when RCD malfunctions prompted the operator to drill the
rest of the horizontal section conventionally (Figure 10). In going from MPD to conventional drilling there was a significant
reduction in the drill rate from 400 m/day with MPD to 87 m/day with conventional drilling. It was unfortunate to obtain such
a comparison in this way but it highlights the difference in drilling efficiency between MPD and conventional practices.
In all the subsequent wells the operator changed their procedures to protect the well and improve efficiency. Their standard
practice was to drill conventionally down to the depleted sands then change over to MPD. However, that meant leaving thehole open during the time it took to roll-over the mud system, complete the installation of the RCD, and make final
connections to the MPD system. To reduce the risk of unnecessary instability in the wells it was decided to drill the entire 8-
1/2” hole section with MPD.
That practice was implemented in the next horizontal well in which both the pilot hole and the lateral were drilled with MPD
from start to finish (Figure 11 & 12). Throughout the pilot hole, above and below the depleted sands, the drill rate averaged of
674 m/day, a considerable improvement compared to the average achieved in earlier wells drilled with conventional and MPD.
In the horizontal section drilling averaged 330 m/day which was greater than what could be achieved under conventional
conditions.
ECD Management
Data from the annular pressure while drilling (PWD) tool was used to compare the effectiveness of ECD management in
conventional and MPD hole sections. Figure 13 contains the PWD log from the same offset well from which depth versus timewas plotted in Figure 7. It shows the nature of the ECD management problem in the high angle wells in this field. The ECD
varied widely throughout the well and in the section between 1500m and 3000m the ECD is greater than the fracture gradient
of the depleted sand.
The PWD data from the first well is plotted in two separate charts, one for the conventional section drilled above the depleted
zone (Figure 14) and one for the MPD section drilled through the zone (Figure 15). The ECD in the conventionally drilled
section is similar to the offset well in the manner in which it varies across the section and the magnitude it reaches towards the
end of the section. The BHP in the MPD section was lower and nearly constant for the entire interval which made the ECD
easier to manage.
Pressure data from the third well (Figure 16 & 17) and fifth well (Figure 18 & 19) were plotted in the same manner as the first
– the upper section above the depleted sand was drilled conventionally and lower section below the sand was drilled with
MPD. The conventional and MPD sections of both wells are similar to the first and the ECD varied significantly more in the
conventional section than in the MPD section. In the third well the ECD was managed particularly well and stayed below 11
Managed Pressure Drilling 8-1/2" Hole - Summary of Mud Properties
Well Type MW (ppg) PV (cP) YP (lb/100ft2) Flow (gpm)
#1 Horizontal 9 18 16 647
#2 Directional 9 14 14 649
#3 (#2 ST) Directional 9 17 13 641
#4 Directional 9 17 10 650
#5 Directional 9 20 10 650
#6 Hor. Pilot 9 22 10 627
Hor. Lateral 9 22 10 627#7 Hor. Pilot 9 22 12 672
Hor. Lateral 9 22 11 624
#8 Directional 9 20 13 669
#9 Directional N/A N/A N/A N/A
Total Hole Interval
Figure 5.Tables comparing mud properties used during conventional and MPD phases of drilling.
Figure 6. Plot of depth versus time from a conventionally drilled offset well. In this and each of the subsequent drill time plotsthe depth data is plotted versus time on a 24 hour scale. That allows each day of drilling to be plotted as a separate line across
the graph which makes it easier to see differences in drill rate. In this well, during the first two days of drilling the average drill
rate was 372 m/day. The depleted sand was encountered after the 2nd
day of drilling at which point the drill rate was slowed
down. The average drill rate for the section was 281 m/day.
Figure 13. Plot of annular pressure from an offset well showing the difficult nature of managing the ECD while drilling high
angle wells in this Gulf of Thailand field.
Figure 14. Plot of ECD versus time from the conventionally drilled section of the first well. In each of the pressure graphs the
red curve is depth data and the blue curve is ECD data from the PWD tool. The spikes on the depth curve represent off-bottom bit movement, typically back reaming before each connection. This section was drilled above the depleted sand. Note the