Casing-while-drilling: The next step change in well construction
DRILLING TECHNOLOGYCasing-while-drilling: The next step change in well constructionCase histories, progress of a co-operative industry group and a proposed classification system show how integrating casing running and drilling offers both cost savings and operational advantagesBrian Tarr and Richard Sukup, Mobil Technology Co., Dallas, Texas
Organization and current plans of the CWD Team within MoBPTeCh Technology Co-Operative Program, comprising Mobil, BP Amoco, Texaco, Chevron, and special-project partner Hughes Christensen, are explained. The group’s charter statement from its 1996 organizational effort describes what the group is about.
Background And Business Impact CWD involves integrating casing running with drilling. The resulting 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 required drilling tools 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 be part of an assembly that extends below the casing shoe. The CWD 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 20 years that relate to drilling with casing (full strings or liners). In the 1950s, it was 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 thriving.1,2 During the last 10 years, industry flirted with — and then abandoned — slimhole, continuous coring (from the mining industry) for low-cost exploration.3,4,5 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 slimhole mining systems have evolved into the first commercial CWD systems. Over the last 10 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 established.6 A disposable bit and underreamer are run on the slotted liner to enlarge a pre-drilled pilot hole or to extend the production hole, once it is 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 conditions.7 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 field8 and BP Amoco Norway’s Valhall field.9 Last year, Tesco Corp. progressed a complete CWD rig concept to the field-test stage10 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 about 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 Co. 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:
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 deep water in the Gulf of Mexico (GOM). Some operators have focused attention on the challenge of drilling the entire 17-1/2-in. hole section with a 16-in.-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 in. can be set conventionally. Extending the 16-in. hole section — essentially drilling with the 16-in. 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-in. liners in deepwater wells are estimated to be $1.4 MM per well. One operator has already used CWD technology on at least two wells to drill-in 13-3/8-in. surface casing from a jackup rig, and another has engineered CWD systems for the 26-, 20- and 16-in. strings for deep water. 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.g., water flows. Longer term, several operators are looking to CWD to reduce the trouble time associated with subsalt drilling. Drilling With 16-In. Liners In Deep Water Potential cost savings associated with drilling with the 16-in. liner were evaluated based on a population of 29 GOM wells drilled by Mobil, in which, 16-in. liners had been run during the last 10 years. Both vertical and deviated wells drilled from platforms, jackups and floating rigs were included. The average length of hole section drilled for 16-in. liner was 1,980 ft, and an average 220 hr was required to drill, case and cement. Of this total average time, tripping took 37 hr (17%), and drilling required 47 hr (21%) at 42 ft/hr. Based on drilling with the 16-in. liner at similar rates and saving all the tripping time, and at a deepwater-rig spread cost of $200 M per day, an average savings of $308 M per well could be realized. This time-related cost savings would be offset by the additional cost of tools and service personnel required to drill and cement the 16-in. liner — estimated to be $38 M per well. Hence, from Mobil data, the average savings from using the 16-in. drill-in liner is estimated to be about $270 M per well. For comparison, Shell data on estimated cost savings for a similar 16-in. drill-in liner system proposed for use in similar wells in the GOM indicates average potential savings of about $193 M per well.12 This difference can be explained by the wide scatter in both the Mobil and the Shell data, as individual well tripping-time savings ranged from 3.5 to 100.5 hr, and 6.5 to 96 hr, respectively. Therefore, rounding off the projected average potential tripping-time savings to $200 M is realistic. This cost-saving projection is based on just reaching the same depth with the 16-in. casing as was done in the past. The savings associated with reaching deeper with the 16-in. liner are harder to estimate, as no historical data exist. The savings from eliminating the flat time associated with a casing point that would otherwise be required may provide a ballpark estimate. Savings in materials and tool rentals would also be realized, but the time-related costs are the most significant. Typically, the flat time associated with a casing point on a deepwater well would be about six days. Hence, at $200 M per day spread cost, eliminating a casing point is worth $1.2 MM per well; making this the more valuable business driver for using CWD in deepwater wells. Adding the two potential savings of using a 16-in. CWD liner system for deepwater wells yields: Trip-time savings + Eliminating a casing point Similar potential savings are possible with additional CWD liners in deepwater wells, but the probability of saving a casing string decreases with each casing string set. Hence, if five more CWD liners and / or casings are used for drilling to reach TD — say 13-3/8-, 10-3/4-, 8-5/8-, 6-5/8- and 4-1/2-in. — that save $0.2 MM per hole section, then potential savings could increase to about $2.4 MM per deepwater well. CWD Equipment Classification System To provide a starting point for selecting the appropriate CWD system configuration for a particular application, Mobil has proposed a CWD classification system, as illustrated in the accompanying figure and Table 1 and Table 2. It includes two major classes of CWD systems — "NCR" where there is No Casing Rotation required, and "CR" where Casing Rotation is required. In each of these classes, there are a number of equipment configuration options that have already been used or are being considered. The accompanying figure provides more detail on each of the configurations. With input from a wider group of interested parties, this classification system is expected to evolve into an industry-standard nomenclature for referring to CWD technology.
Technology Development Plans For 1999 Several technology challenges still remain before the full potential of CWD can be realized. Many of these had to be addressed for slimhole mining systems when used for exploration drilling; but CWD liner systems and larger pipe sizes add further complexity to these particular challenges of how to:
Mobil, BP Amoco, Texaco and Chevron have decided to join forces with Hughes Christensen, as part of the MoBPTeCh Technology Co-Operative Program,13 to investigate hydraulics requirements for drilling with large-diameter casing in the minimum-acceptable hole sizes. High-effective equivalent circulating density (ECD) is unacceptable in many applications, including deep water, so it is important to develop and validate hydraulics models for large-diameter, narrow-annulus CWD systems. For example, it is already clear that some of the mud flow will have to be bypassed above the running tool to optimize the hydraulics for CWD systems using large-diameter liners. By the end of 1999, the MoBPTeCh CWD Team plans to validate a hydraulics model that will permit analyzing the trade-offs between bit cleaning, hole cleaning and ECD for a wide range of CWD applications. Included in the annular hydraulics model will be the effects of pipe eccentricity and rotation in narrow annular clearances, validated for hole sizes up to 12-1/4 in. and extrapolated for larger hole and pipe sizes. The necessary experimental work will be conducted at the Baker Hughes Experimental Test Area (Beta) test well facility near Tulsa, Oklahoma. Acknowledgment The authors would like to thank Mobil Technology Co. for permission to publish this paper, as well as members of the MoBPTeCh Casing-While-Drilling Team for their help in reviewing the paper, including Warren Winters (BP Amoco), John Peters (Chevron) and Manny Gonzalez (Texaco). Special thanks to Allen Sinor (Hughes Christensen) for his review comments. Literature Cited
 The authors
Copyright © 1999 World
Oil |
his article overviews
the concept of casing-while-drilling (CWD), starting with a background discussion describing
the evolution and more recent advances of the practice and potential positive cost /
operational impacts on drilling. Example cost savings from applications with 16-in. liners
in deep water are noted. A proposed classification system for selecting an appropriate CWD
system is presented. And several remaining technology challenges facing the industry are
listed. 
Brian
Tarr, engineering consultant, Mobil Technology Co.,
Dallas, has an MS degree in PE from Heriot-Watt University, Edinburgh, Scotland, and he is a
registered professional engineer in Texas. He has 22 years of industry experience that
includes direct involvement in managing drilling / production offshore operations in the
North Sea and, since his transfer to Dallas in 1987, providing drilling engineering support
to Mobil’s worldwide operations. His primary focus is supporting development /
cost-effective implementation of emerging technologies, including underbalanced drilling,
drilling with casing and other unconventional drilling systems. He is also one of Mobil’s
well-control experts and regularly provides training / consulting related to well-design
contingency planning and well control. 
Richard
Sukup, associate engineering advisor, Mobil
Technology Co., Dallas, holds a BS degree in engineering from Oklahoma State University
(1972) and an MS in management from Texas Christian University (1977). He has 27 years’
experience in global exploration and field development, providing technical consulting and
operational supervision to Mobil affiliates worldwide. He is presently Team Leader for Mobil
E&P Technical Center’s well-construction equipment technology, and project leader
for casing drilling. Mr. Sukup is a member of SPE and API. 
