Directional Casing while Drilling eases path through Nahr Umr shale off Qatar
Michael Avery
Todd Stephens
Occidental Petroleum of Qatar Ltd.
Ali K. Al-Hadad
Qatar Petroleum
Eric Moellendick
Mounir Turki
Tesco Corp.
Malek Abed
Bill Lesso
Schlumberger
Casing drilling has evolved quickly since gaining acceptance as a safe and cost-effective technique for well construction. However, overcoming the initial hurdles of acceptance was a long, tough, fight; one that many thought would never succeed.
Since the industry, and particularly the drilling community, has come to grips with the concept of drilling with casing, a collaborative effort of experienced drillers and talented engineers has produced several enhancements that have improved drilling efficiency and safety as well as widening the casing drilling capability window to include directional drilling. Improvements have been made in every facet of the technique, including bits, bottomhole assemblies (BHA), reamers, tool joints, and the myriad surface handling equipment required.
The casing drive system grips and supports the casing during drilling acting like a combination Kelly and elevator.
Due to recent advancements in tool design, it is now possible to circulate, rotate, and reciprocate the casing during high angle BHA retrieval and setting operations, all without rig modification. These advantages, coupled with the versatility of rotary steerable systems (RSS), allow for a much more flexible process and enable defensive measures to be taken in the event of unforeseen complications.
What about foreseen complications?
One of the biggest advantages of casing drilling is its ability to protect depleted or unstable formations while drilling to reservoir targets lying beneath them. Mitigating wellbore instability and lost circulation events have been drivers of casing drilling’s acceptance. Unstable formations tend to cave-in or wash out during or immediately after drilling, sticking the BHA and impairing the ability to run casing conventionally. The problem is exacerbated when directional well trajectories, particularly building to horizontal, are involved. However, given a big enough prize, innovative minds will seek and implement a solution. The prolific Shuaiba limestone formation offshore Qatar is just such a prize.
As anyone familiar with the region knows, a major complication exists in the form of the notorious Nahr Umr shale formation. Lying directly above the target Shuaiba formation, the extremely unstable Nahr Umr shale has plagued well construction efforts for years. Occidental Petroleum of Qatar Ltd. (OPQL) has been developing the Idd Shargi North Dome using 12 ¼-in. diameter horizontal production and injection wells. Typically, the 12 ¼-in. wellbore is landed about 80 ft MD (20 ft TVD) into the top of the Shuaiba at up to 86º inclination, and 9 5/8-in., 47 lb/ft casing is set to protect the well. Nevertheless, the unstable Nahr Umr has played havoc with conventional drilling and casing operations, largely due to its highly conductive faults, which cause sudden loss of circulation leading to severe caving of the shale with subsequent collapse onto the BHA. So severe has been the borehole instability issue that often the drillpipe and BHAs are LIH (lost in hole).
Top-bottom protection
The challenges facing OPQL’s Idd El Shargi project are well suited to a Directional Casing while Drilling (DCwD) solution. Several experiences with casing drilling have been cited in support of this application. An obvious advantage is the fact that casing is protecting all but the bottom few feet of hole at all times. If a sudden loss of circulation occurs at the dangerous Nahr Umr/Shuaiba interface, less than 100 ft (30.5 m) of drilling would be required to get the casing to its landing point.
Even if the unstable shale caves in, the BHA can still be retrieved through the casing, allowing landing of the casing at that point, cementing it in place, and drilling out. However, experienced casing drillers are of the opinion that the narrow casing-to-formation annulus, coupled with continuous rotation of the casing and application of hydrostatic pressure on the shale using NaCl brine drilling fluid, reduces the chance of hole problems commonly encountered with conventional drillpipe drilling.
The advantages of BHA retrieval while leaving the hole protected are well documented. This capability allows bits and reamers to be changed out as needed. It also ensures that the MWD/LWD string can be retrieved and changed in the event of a failure. Casing can be rotated and/or reciprocated during these trips and circulation can be maintained to resist the possibility of sticking.
For DCwD, drill bit energy is achieved by a combination of casing rotation from the rig’s top drive (or rotary table) and a mud motor. For the latter, two possibilities exist: an adjustable bent-housing steerable motor can be used, or a non-directional motor can be used in conjunction with an RSS. Both approaches have advantages. However, after a thorough analysis, OPQL concurred that the advantages of using the RSS were more compelling. These included:
- Constant rotation of the casing. This breaks static friction and allows even weight-on-bit. It also facilitates hole cleaning by moving drill cuttings into the flowstream
- Directional and inclination control is much easier with an RSS, and a smoother borehole trajectory results.
RSS tools can use either the push-the-bit technique, whereby hydraulically actuated pads on the RSS push the bit in the desired direction, or the point-the-bit technique, where a flexible internal drive shaft points the bit at its target. OPQL chose the latter technique, citing prior experience in achieving the desired dogleg through the target formation.
Several innovative technologies were applied and proven effective in this project:
Casing Drive System (CDS). Actuated hydraulically, a trip and torque grapple grips the inner circumference of the top joint of casing to facilitate running, retrieving, and rotating the casing string. An integral cup packer hydraulically seals the pipe. The top end of the CDS attaches conventionally to the bottom of the top drive using a standard 6 5/8-in. drillpipe connection.
Threaded Casing Drive (TCD). Addressing the different aspects of well control while implementing DCwD, a special device was developed for use while tripping the BHA with drillpipe. To retrieve the BHA involved tripping in with a work string, engaging the top of the BHA and tripping out of the hole with it. While this operation is going on, the casing is caught and sealed by casing rams occupying the upper slot of the BOP. A false rotary table is created atop the casing string just above the drill floor. Should it prove necessary to control the well while circulating out a gas influx, the TCD is screwed onto the top casing connection, thus sealing the drillpipe/casing annulus. Circulation can be achieved by pumping through the top drive swivel and TCD and down the drill pipe. Any gas that accumulates in the upper annulus can be relieved through a ported sub just below the top drive system (TDS). A safety valve is located atop the TCD to close off the drill pipe. Thus the casing with the work sting inside it can be rotated and/or reciprocated while circulating. The TCD is not required during normal drilling operations since circulation can be established through the CDS. The mud-cross below the BOP rams allows circulation down the outside of the drillstring and up the outer annulus to circulate out a kick.
Drill Lock Assembly (DLA). Connecting the BHA to the bottom of the casing is achieved by the DLA, which works equally well whether a directional mud motor or RSS is used. The DLA locks into the casing profile nipple (CPN) and allows both torque and weight-on-bit (WOB) to be applied by the casing during drilling. The DLA/CPN lock into a special lower casing shoe that shrouds and protects the mud motor and internal stabilizers during shipping and rig-up.
During drilling, using the chosen RSS BHA, has several advantages. Firstly, the 8 ½-in. PDC bit is employed to drill and steer a pilot hole, with steering applied by the RSS immediately above the bit. At-the-bit MWD and Gamma Ray measurements can be transmitted to the LWD string by means of a short-hop electromagnetic telemetry link. The LWD system comprises all necessary geosteering and formation evaluation measurements along with mud pulse telemetry, and these can be conducted in the freshly drilled, optimally sized pilot hole. The next step is borehole smoothing and enlargement using a series of roller reamers, followed by the 12 ¼-in. collapsible under-reamer tool.
Rotation is achieved via a combination of mud motor and casing rotation. The correct ratio is determined to keep the under-reamer within its design speed. Critical factors that must be balanced include mud flowrate, which governs motor speed and optimizes cuttings transport, and casing rotation to reduce friction and support hole cleaning. Using flowrate to maintain equivalent circulating density (ECD) of 1 lb/gal above static mud weight is another critical factor to help control borehole stability. In addition, external casing centralizers were crimped onto the casing as it was run to help reduce sliding friction and maintain annular uniformity so correct cement distribution could be achieved after the casing point was reached and casing was set.
Details of the drilling operations can be reviewed in SPE/IADC Paper 119446 (2009) to which this article is referenced. The 2,025-ft (617-m), 12 ¼-in. build-and-land section was successfully drilled to TD of 5,820 ft (1,774 m) MD and landed in the Shuaiba reservoir. Inclination was built from 28º to 76º and direction was turned from an azimuth of 14º to 41º with maximum dogleg severity of 2.5º/100 ft (30.5 m). This engineering and operational success paves the way for safe, efficient, and uneventful penetration of the Nahr Umr shale on subsequent projects.
Author’s note
SPE/IADC 119446-MS, High Angle Directional Drilling with 9-5/8-in. Casing in Offshore Qatar, was originally presented at the SPE/IADC Drilling Conference and Exhibition in Amsterdam, March 17-19, 2009.
