Casing Drilling

Casing-while -Drilling: The Next Step Change in Well ConstructionWorld Oil, Oct 1999 Article Published in World Oil, October 1999 issue Casing-while-Drilling: The Next Step Change in Well Construction Brian Tarr and Richard Sukup, Mobil Technology Company Background and Business Impact Casing-while-Drilling (CWD) involves integrating the casing running process with the drilling process. The CWD process allows casing strings (or liners) to be installed in a well during the hole making process where all, some parts of, or none of, the drilling tools required can be recovered after the casing reaches the required depth. Since there are a variety of possible configurations for CWD systems, the casing (or liner) may or may not be rotated during the drilling process and the drilling tools may be integrated into the casing string or may be part of an assembly that extends below the casing shoe. The Casing-While-Drilling concept is not new, but there is obviously renewed interest based on the number of patents granted to E&P operators and service companies during the last twenty years that relate to drilling with casing (full strings or liners). In the 1950’s it was a common practice for onshore operators to drill production hole sections to TD with the tubing and then cement it in place (without recovering the bit) and this practice is still thriving1,2 . During the last ten years, the industry flirted with, and then abandoned, slim-hole continuous coring (from the mining industry) for low cost exploration3,4,5 . Time was saved, as the drill rods were sized to be cemented as casing, without pulling the coring bit. The next smaller size of drill rod was then used to drill out. Wireline-retrievable core heads have recently been introduced so that the drill rods no longer need to be tripped. Hence, the slim-hole mining systems have evolved into the first commercial CWD systems. Over the last ten years, in at least one heavy-oil area, a common practice of drilling to TD with the slotted production liner and then gravel packing has been established6 . A disposable bit and under-reamer are run on the slotted liner to enlarge a pre-drilled pilot hole or to extend the production hole, once lined up with the target completion interval. The liner sections are often welded together as they are run (no threaded connections), and foam is used as the preferred fluid to minimize formation damage during the drilling and gravel-packing process. In a similar time period, operators have taken advantage of casing reamer shoe devices to aid in getting casing to bottom under difficult hole conditions7 . In the last five years Mobil and BP Amoco have taken advantage of these incremental developments in CWD liner technology and further refined them to drill across pressure transitions into severely depleted reservoirs. Today, CWD has become the standard way of installing production liners in in-fill and/or re-entry sidetrack wells in Mobil Oil Indonesia’s Arun field 8 and BP Amoco Norway’s Valhall field 9 . Last year, Tesco Corporation progressed a complete CWD rig concept to the field-test stage 10 and is now offering a complete turnkey type CWD solution for onshore applications, that includes directional-drilling capabilities. They expect to be able to realize savings of up to 30% over traditional methods of well construction. The potential prize for the wider application of CWD on all types of rigs for every hole section is estimated to be approximately 15% of current drilling costs based on eliminating the majority of drillpipe tripping related costs (including tripping-accountable trouble time). Results of a 1998 Mobil Technology Company CWD study11 indicate that longer liners and full casing strings (of all sizes) can be used for drilling – so the prize is attainable. Potential savings from CWD applications include: • Reducing drilling flat time; • Getting casing to planned depth; • Getting casing set through troublesome zones (water flows, shear zones, fluid-loss zones); • Extending hole sections beyond traditional hole section limits; • Reducing the starting hole size required by using lean-profile casing schemes. The near-term focus for CWD development activity in many of the major operating companies is on the surface casing strings for offshore wells, primarily for deepwater in the Gulf of Mexico (GOM). Some operators have focused attention on the challenge of drilling the entire 17-1/2” hole section with a 16” liner CWD system. This size was chosen for its potential to save a casing string if the CWD system can be pushed deeper than to the point where 16” can be set conventionally. Extending the 16” hole section (essentially drilling with the 16” liner as far as physically possible) has been shown to have the largest impact on reducing the number of casing strings required in GOM deepwater wells. Projected cost savings from drilling with 16” liners in deepwater wells are estimated to be 1.4 $MM per well. Page 1 but CWD liner systems and larger pipe sizes add further complexity to these challenges: • How to prevent the casing from getting stuck (stabilization and mud system innovations)? • How to compute and manage the effects of drilling dynamics induced loads on casing connections? • How to optimize hydraulics. several operators are looking to CWD to reduce the trouble time associated with subsalt drilling. jack-ups and floating rigs were included. In both cases they were not only looking for trip-time savings but also for trouble-time savings associated with being able to better manage shallow formation hazards (e. the flat time associated with a casing point on a deep water well would be approximately six days. This cost-saving projection is based on just reaching the same depth with the 16” casing as was done in the past. 10-3/4”. if five more CWD liners and/or casings are used for drilling (13-3/8”. (47 %) at 42 ft/hr. as individual well tripping time savings ranged from 3. In each of these classes. For comparison. 20” and 16” strings for deepwater. Typically. and an average of 220 hours were required to drill. which makes this the more valuable business driver for using CWD in deepwater wells. It includes two major classes of CWD systems.10 provide more detail on each of the configurations summarized in Fig.980 ft. from Mobil data the average saving from using the 16” drill-in liner is estimated to be approximately $270M per well. With input from a wider group of interested parties. eliminating a casing point is worth $1. Technology development plans for 1999 Several technology challenges still remain before the full potential of CWD can be realized. at $200M per day spread cost. Based on drilling with the 16” liner. 1).. Hence.5 hours and 6. proposed for use in similar wells in the GOM.g. this classification system is expected to evolve into an industry standard nomenclature for referring to CWD technology. tripping took 37 hours (17%) and drilling required 47 hours. Oct 1999 One operator has already used CWD technology on at least two wells to drill-in 13-3/8” surface casing from a jackup rig and another has engineered CWD systems for the 26”. Both vertical and deviated wells drilled from platforms. then the potential savings could increase to approximately $2. case and cement. there are a number of equipment configuration options that have already been used or are being considered. and at a deepwater rig spread cost of $200M per day. This difference can be explained by the wide scatter in both the Mobil and the Shell data. Hence. but the time-related costs are the most significant. Shell data12 on estimated cost savings for a similar 16” drill-in liner system. 8-5/8”.5 to 96 hours. Hence. Adding the potential savings of using a 16” CWD liner system for deep water wells yields: Trip time savings + Eliminating a casing point savings = 1.4MM per deepwater well. at similar rates and saving all the tripping time. Mobil has proposed a CWD classification system (see Fig. Savings in materials and tool rentals would also be realized. The savings associated with reaching deeper with the 16” liner are harder to estimate as no historical data exists.Casing-while -Drilling: The Next Step Change in Well Construction World Oil. Figure 2 . 6-5/8” and 4-1/2”) to reach TD that save $0. especially for smaller annular clearance applications? • How to manage well control (kicks and lost circulation)? • How to cement smaller annular clearance CWD systems? • How to integrate formation evaluation? Page 2 . but the probability of saving a casing string decreases with each casing string set. Of this total average time.5 to 100. This time-related cost saving would be offset by the additional cost of tools and service personnel required to drill and cement the 16” liner: estimated to be $38M per well.2MM per hole section. NCR . respectively.where there is No Casing Rotation required. rounding off the projected average potential tripping time savings to $200M is realistic. 1. indicates average potential savings of approximately $193M per well. water flows). Classification System for CWD Equipment Configurations To provide a starting point for selecting the appropriate CWD system configuration for a particular application. an average saving of $308M per well could be realized. Example potential cost savings for drilling with 16” liner in deepwater The potential cost savings associated with drilling with the 16” liner were evaluated based on a population of 29 GOM wells drilled by Mobil where 16” liners had been run during the last 10 years. The average length of hole section drilled for 16” liner was 1.2MM per well.4 $MM per well Similar potential savings are possible with additional CWD liners in deep water wells. Many of these challenges had to be addressed for slim-hole mining systems when used for exploration drilling. The savings from eliminating the flat time associated with a casing point that would otherwise be required can provide a ball-park estimate. Longer term. Therefore. and CR – where Casing Rotation is required. ) 9. September 1993. References 1. The necessary experimental work will be conducted at the Baker Huges Experimental Test Area (BETA) test well facility near Tulsa.com) Page 3 . McCarthy and Lilley. “Drilling liner Technology for Depleted Reservoirs” SPE paper 36827 presented at the European Petroleum Conference. Slimmer Prospects” JPT. CA. hole cleaning and ECD for a wide range of CWD applications. Issue 58.Casing-while -Drilling: The Next Step Change in Well Construction World Oil. Personal communication from Landale Cranfield. 1998” Mobil Technology Company report. page 816 5. John Peters (Chevron) and Manny Gonzalez (Texaco). BP Amoco. Acknowledgement The authors would like to thank Mobil Technology Company for permission to publish this paper and to members of the MoBPTeCh Casing-While-Drilling team for their help in reviewing the paper. “Refined Slimhole Drilling Technology Renews Operator Interest” Petroleum Engineer International. (Describes Amoco’s learning experiences with various sizes and configurations of drill-in liners at Valhall. June 1992. Vogt.mobptech. as part of the MoBPTeCh Technology Co-Operative Program13 . 1992 7. 3-6 March 1998. Special thanks to Allen Sinor (Hughes Christensen) for his review comments. “Drilling. “Casing Drilling – A Revolutionary Approach to Reducing Well Costs” SPE/IADC 52789 presented at the SPE/IADC Drilling Conference. including Warren Winters (BP Amoco). Tybero. Makohl. to investigate the hydraulics requirements for drilling with largediameter casing in the minimum acceptable hole sizes. “Reamer Shoe gets Casing to TD” BP Downhole Talk. 11. Included in the annular hydraulics model will be the effects of pipe eccentricity and rotation in narrow annular clearances.) 10. 22-24 Oct. 1996. “Casing While Drilling. BP Amoco. “Slim Holes Fat Savings” JPT. Texaco and Chevron that was chartered in 1996. Littleton. 4. For example. 2. Sinor. Texaco and Chevron have decided to join forces with Hughes Christensen. (On the Web at http://www. page 949 6. Villahermosa. Novenber 1995. Dallas. (Describes early Mobil experience in the Arun field drilling with 7” liners. 1998 3. “Slim Holes. 3-5 March. so it is important to develop and validate hydraulics models for large-diameter. Amsterdam. Shell Deepwater Development Inc. de Leon Mojarro. By the end of 1999. narrow-annulus CWD systems. June 1996 8. Personal communication with Manny Gonzalez.. Casing and Cementing in One Trip” 13. Sukup et al. Tessari and Madell. South Texas. the MoBPTeCh CWD Team plans to validate a hydraulics model that will permit analyzing the trade-offs between bit cleaning. Oct 1999 Mobil. Hudson and Dobson “One-Step Drilling System” BESTLINE Liner Systems. Terrazas and Eljure. Oklahoma. validated for hole sizes up to 12-1/4” and extrapolated for larger hole and pipe sizes. Texaco re drilling with tubing at the King Ranch. MoBPTeCh is a US registered E&P Technology Cooperative Program between Mobil. “Rotary Liner Drilling for Depleted Reservoirs” IADC/SPE paper 39399 presented at the IADC/SPE Drilling Conference. 12. 9-11 March 1999. Milan. Bakersfield. Suwarno and Quitzau. High effective equivalent circulating density (ECD) are unacceptable in many applications (including deepwater). Eide and Wenande. including using configurations that evolved from Mobil work in the Arun field. “Breaking a Paradigm: Drilling with Tubing Gas Wells” SPE paper 40051 presented at the SPE International Petroleum Conference and Exhibition of Mexico. page 19. it is already clear that some of the mud flow will have to be by-passed above the running tool to optimize the hydraulics for CWD systems using largediameter liners. Increasing shareholder returns against a background of sustained low oil prices and maturing assets will not be achieved without bold and innovative management. We will assemble the best talents from inside and outside our industry and focus their energy on common goals. Yet we continue to duplicate spending. Chevron.A Shared Vision for the Future The E&P industry of the late 1990's faces tough challenges. David Jenkins BP David Clementz Elwyn Griffiths Chevron Mobil Ron Robinson Texaco Page 4 . We. The performance levers available are limited. BP. and risk abdicating decisions over the pace and direction of developments. An efficient global technology marketplace ultimately delivers equivalent tools and techniques to all the players. Mobil. fail to communicate our common needs to the market.Casing-while -Drilling: The Next Step Change in Well Construction World Oil. Oct 1999 MoBPTeCh’s 1996 Charter: MoBPTeCh: E&P Technology Co-Operative Program . will work to secure this Prize by: taking a leadership role in technology cooperation encouraging our staffs to pool their creativity freely and openly creating a simple open process that encourages others to join us We commit to the pursuit of this vision through the devotion of the time and resources necessary to turn our common intent into reality by the end of 1996. The Prize will be radical new technologies that materially impact business performance at lower cost and with shorter lead time. resolve: to advance the availability of broadly applicable technology and services while continuing to compete vigorously in the application of technology to our separate businesses Co-operation will allow us to tackle challenges that we could not assume individually or through traditional mechanisms. By accelerating the pace of change we will permanently shift the industry to a more entrepreneurial approach and attract the best talent to solve the challenges of the future. and Texaco. We. Technology stands out as the single most powerful opportunity to improve performance. pulled on wireline. latched into bottom joint of casing string. 7-5/8”. Swab cup above motor assy. Built & Run 13-3/8” casing. Shoe Core Bit both turned by rotated from surface. Assy. pulled on wireline. pulled on wireline. Assy. Bit pulled on inner string or left downhole. spear. pulled on inner string. Assy. UnderReamer and Csg. Steerable motor assy. Casing to Surface Bit. Oct 1999 Figure 1 – PROPOSED CLASSIFICATION OF CWD OPTIONS NCR 1 NCR 2 CR 1 CR 2 CR 3 CR 4 CR 5 CR 6 CR 7 NO CASING ROTATION NCR Csg. Bit pulled on wireline or left downhole. Bit and fixed Csg. Shoe Core Bit turned by surface rotation only. Bit and free rotating Csg. Assy. DP to Surface Bit and UnderReamer turned by motor assy. Drillpipe to surface Bit and UnderReamer turned by motor assy. Planned 20”casing. Bit pulled on inner string. Unocal Indonesia Built & Run 11-3/4” liner. Csg. Shoe Core Bit (or underreamer) both rotated from surface. CASING ROTATED – SPEED DEPENDS ON WHETHER MOTOR INCLUDED or NOT CR Casing/Liner to Running Tool. Csg. Assy. Amoco Valhall Built & Run California Ops./Liner to Running Tool. 7” csg. Amoco Hod Built & Run 7” & 9-5/8” lnr Mobil Arun. Tesco test well sidetracks . Assy. Cementing possible before POH with bit. pulled on wireline. run on inner string. and UnderReamer latched in csg. 5-1/2”. Shoe Core Bit with Steerable motor assy. Shoe Core Bit turned from surface.Casing-while -Drilling: The Next Step Change in Well Construction World Oil. Shell Deep Water Team Casing to Surface Bit and UnderReamer turned by motor assy. latched in csg. Mobil Arun & High Island Page 5 Built & Run 5-1/2” & 7” lnr Amoco Valhall (16” Concept) Concept Only Up to 16” Built & Tested 4-1/2”. Shoe Core Bit sub turned by motor run on inner string. pulled on inner string. run on inner string below csg. Bit and Csg. 9-5/8” Tesco test well Built & Tested 5-1/2”. Cementing possible only after POH with bit. Use of a steerable motor assembly to allow directional drilling.NCR 1 Special Tools Target Application Advantages vs Normal Operations On bottom circulation can be maintained at all times which permits shallow water flows to be controlled effectively. To permit conventional cementing a float collar has to be run on wireline after pulling BHA. casing and cementing in one trip. Well type: Centralizers on casing Wireline retrievable latch-in Sub just inside casing shoe MWD Mud motor Under Reamer Bit (casing drift size) Reason for application: Save tripping time. Potential Refinements Size Csg. Push casing point deeper into unstable drilling environment.Casing-while -Drilling: The Next Step Change in Well Construction World Oil. Reason for application: Address potential shallow water flows seen in deep water by drilling. Centralizers on casing for cementing. No cementing float collar so must balance cement with displacement fluid and wait on cement before pulling BHA. Page 6 Potential Refinements Drilling rig equipped to run casing. Risk of casing becoming stuck before reaching planned setting depth. Oct 1999 Figure 2 . Adequate annular velocities for hole cleaning can be achieved at lower mud flow rates compared with drilling with drillpipe in the hole.DETAILS OF CWD OPTION . .NCR 2 Special Tools Target Application Built and Run Size Csg. pump into it and run wireline inside it.: 13-3/8” Surface casing Surface BOP. Vertical Use of a steerable motor assembly to allow directional drilling.: High torque running tool Well type: Swab cup for cementing. Disadvantages vs Normal Operations Assembly more difficult to trip if tool problems occur. Once a flow has started serious hole enlargement can compromise the stability of the conductor in the well and in any adjacent wells. Disadvantages vs Normal Operations Potentially higher ECD’s. Vertical Advantages vs Normal Operations No drillpipe tripping required. after pulling bit inside csg. In normal operations flows can start during tripping when control is impossible. Stabilizer inside casing Mud motor Under Reamer Bit (casing drift size) 20” Subsea.DETAILS OF CWD OPTION . Fishing BHA if stuck below the end of the casing. BHA more difficult to trip if tool problems occur. Risk of casing becoming stuck before reaching planned setting depth. Figure 3 . Drill far enough into reservoir to case off the interface effectively.CR 1 Special Tools Target Application Built and Run Size Csg. Assembly more difficult to trip if tool problems occur.CR 2 Special Tools Target Application Built and Run Size Csg. Drilling distance limited by fatique life of liner connections. (Spear used had by-pass area for return flow inside the liner. Potential Refinements Pack-off activation tool Liner hanger with pack-off Use of high torque liner hanger running tool with flow path for return mud flow inside the liner. Cementing limited to squeezing shoe after pulling drilling tools and isolating liner top with packer.Casing-while -Drilling: The Next Step Change in Well Construction World Oil.: Well type: Bumper subs Internal casing spear (LH relaease) Reason for application: To drill liner a short distance into a depleted reservoir prone to lost circulation. 11-3/4” Deviated Get required liner size into the top of the target depleted reservoir. Motor landing sub Mud motor Core bit drive spline Core bit sub (free rotating) Bit (casing drift size) Reason for application: Drill short distance into depleted reservoirs with lost circulation.: Well type: 7” & 9-5/8” Deviated.) Use of casing connections with higher resistance to drilling related fatique. Disadvantages vs Normal Operations Assembly more difficult to trip if tool problems occur. Drilling distance limited by fatique life of liner connections.DETAILS OF CWD OPTION . Drill far enough into reservoir to case off the interface effectively. (Less than 100 ft of drilling planned. Page 7 Potential Refinements High torque running tool High by-pass hanger Integral liner top pack-off (All hydraulic set) Bumper sub Use of casing connections with higher resistance to drilling related fatique. Sidetracked Advantages vs Normal Operations Avoid struck drilling assemblies. No directional control possible.DETAILS OF CWD OPTION . Oct 1999 Figure 4 . Disadvantages vs Normal Operations Potentially higher ECD’s. Get required liner size into the top of the target depleted reservoir. Advantages vs Normal Operations Avoid struck drilling assemblies. . No directional control possible.) Cementing limited to squeezing shoe after pulling drilling tools and isolating liner top with packer. Stabilized mud motor inside casing shoe Under Reamer Bit (casing drift size) Figure 5 . CR 4 Special Tools Target Application Concept Only Size Csg. Fixed core bit casing shoe. Figure 7 . up to 7” Well type: Vertical. Hydraulics may require bypass above liner to provide enough flow for hole cleaning. Nodeco has continued development. Use of drillable bit to permit normal cementing without pulling bit. BHA more difficult to trip if tool problems occur.) Reason for application: Drill long distance into depleted reservoirs with lost circulation. (Less than 100 ft of drilling planned.DETAILS OF CWD OPTION . has a drillable bit concept that looks promising. Also. Deviated.CR 3 Special Tools Target Application Advantages vs Normal Operations Install production liners and gravel pack in one trip.: High torque running tool. British Bits Ltd. Assembly more difficult to trip if tool problems occur.Casing-while -Drilling: The Next Step Change in Well Construction World Oil. Bit (casing drift size) .DETAILS OF CWD OPTION . Avoid struck drilling assemblies in pressure transitions associated with depleted reservoirs. Page 8 Potential Refinements By-pass flow nozzle sub High torque running tool High by pass hanger Integral liner top pack-off (All hydraulic set) Casing connections suitable for drilling conditions Use of casing connections with higher resistance to drilling related fatique. Save drilling related tripping time. Production liner (may be slotted pipe) Fixed core bit casing shoe or under-reamer on casing Bit & Stabilizer inside casing (casing drift size) or fixed Bit on casing. Drilling distance limited by fatique life of liner connections.) Cementing limited to squeezing shoe after pulling drilling tools and isolating liner top with packer.: Well type: Up to 16” Vertical Advantages vs Normal Operations No drillpipe tripping required for drilling. or Sidetracked Reason for application: Drill and under-ream to install production liner Drill short distance into depleted reservoirs with lost circulation. No directional control possible. Drill far enough into reservoir to case off the interface effectively. High by pass hanger and integral liner top pack-off optional (all hydraulic set). Wireline retrievable latch-in sub just inside casing shoe. Disadvantages vs Normal Operations Potentially higher ECD’s. (Amoco built such bits for 7” drill-in liners. Get required liner size into the top of the target depleted reservoir. Use of casing connections with higher resistance to drilling related fatique. Push casing point deeper into unstable drilling environment. Risk of casing becoming stuck before reaching planned setting depth. Disadvantages vs Normal Operations Potentially higher ECD’s. For conventional cementing a float collar has to be run on wireline after pulling bit. Oct 1999 Figure 6 . Potential Refinements Size Csg. Well type: Casing connections suitable for drilling conditions Reason for application: Save drillpipe tripping time. Wireline retrievable latch-in mud motor just inside casing shoe. Also. British Bits Ltd. Risk of casing becoming stuck before reaching planned setting depth. Disadvantages vs Normal Operations Potentially higher ECD’s. BHA more difficult to trip if tool problems occur. (Amoco built such bits for 7” drill-in liners. has a drillable bit concept that looks promising.DETAILS OF CWD OPTION . Drilling rig equipped to rotate casing. Potential Refinements By-pass flow nozzle sub High torque running tool High by pass hanger Integral liner top pack-off (All hydraulic set) Casing connections suitable for drilling conditions Fixed core bit casing shoe. Bit hydraulics limited by ECD considerations.DETAILS OF CWD OPTION . 7-5/8”. Reason for application: Drill long distance into depleted reservoirs with lost circulation. Add LWD for logging while drilling. For conventional cementing a float collar has to be pumped down after pulling bit.) Add LWD for logging while drilling. Save drilling related tripping time. Hydraulics may require bypass above liner to provide enough flow for hole cleaning.CR 5 Special Tools Target Application Concept Only Size Csg. Risk of casing becoming stuck before reaching planned setting depth. Bit (casing drift size) Use of casing connections with higher resistance to drilling related fatique. Potential Refinements Use of casing connections with higher resistance to drilling related fatique. 5-1/2”. Figure 9 . Oct 1999 Figure 8 . preferably rotary steerable. Use of drillable bit to permit normal cementing without pulling bit. Under-Reamer Bit (casing drift size) Page 9 . Push casing point deeper into unstable drilling environment. BHA more difficult to trip if tool problems occur.Casing-while -Drilling: The Next Step Change in Well Construction World Oil. Wireline retrievable latch-in sub just inside casing shoe. Disadvantages vs Normal Operations Potentially higher ECD’s. Adequate annular velocities for hole cleaning can be achieved at lower mud flow rates compared with drilling with drillpipe in the hole. Push casing point deeper into unstable drilling environment. Nodeco has continued development. 9-5/8” Vertical Advantages vs Normal Operations No drillpipe tripping required.: 4-1/2”. May be integrated into one of the cementing plugs.: Well type: Up to 16” Vertical Advantages vs Normal Operations No drillpipe tripping required for drilling.CR 6 Special Tools Target Application Built and Tested Size Csg. pump into it and run wireline inside it. Use of a steerable motor assembly to allow directional drilling. Fixed core bit casing shoe. For conventional cementing a float collar has to be run on wireline after pulling bit. Potential Refinements Drilling rig equipped to rotate casing. pump into it and run wireline inside it. Under-Reamer Bit (casing drift size) Save drillpipe tripping time. Use of casing connections with higher resistance to drilling related fatique.: Well type: 5-1/2”. Oct 1999 Figure 10 . Page 10 . BHA more difficult to trip if tool problems occur. Bit and motor hydraulics limited by ECD considerations. Pump down running and retrieving system for hole angles above 60 degrees inclination. May be integrated into one of the cementing plugs. Casing connections suitable for drilling conditions Reason for application: Fixed core bit casing shoe. For conventional cementing a float collar has to be pumped down after pulling bit. Risk of casing becoming stuck before reaching planned setting depth.CR 7 Special Tools Target Application Built and Tested Size Csg. 7” Directional or Sidetrack from vertical wellbore Advantages vs Normal Operations No drillpipe tripping required. Adequate annular velocities for hole cleaning can be achieved at lower mud flow rates compared with drilling with drillpipe in the hole. Wireline retrievable latch-in MWD/steerable mud motor just inside casing shoe. Disadvantages vs Normal Operations Potentially higher ECD’s. Employ rotary steerable system to permit continuous rotation to enhance ROP and minimize sticking potential during sliding.Casing-while -Drilling: The Next Step Change in Well Construction World Oil.DETAILS OF CWD OPTION . Add LWD to BHA for logging while drilling.