Operator tests limits of coiled tubing drilling

With 1,000 wells to drill in the San Juan Basin of New Mexico and Colorado, BP wanted to improve rate of penetration (ROP), directional control, rig mobility and footprint and re-entry performance. This prolific gas region has recoverable gas reserves of almost 13 Tcf, with 8.5 Tcf being coalbed methane reserves. Although the sedimentary basin bottoms out at about 14,500 ft (4,421 m), hydrocarbon production comes from reservoirs shallower than 7,500 ft (2,286.5 m). These challenges posed a tough problem. The solution came from an unusual source — CTD.

BP America initiated the “San Juan Coiled Tubing project,” a drilling program designed to challenge CTD technology to the limit. Each well in the program was progressively more difficult, so lessons learned on one well could be applied on the next. Of high importance was the realization that CTD is inherently safer than drilling with conventional jointed pipe. Rig floor hands do not have to remain in high hazard areas making connections over and over, so they are able to perform other important tasks that improve the rig’s uptime.

The test
The first four wells of an eight-well program were to be drilled in the South San Juan Basin of New Mexico, an area of moderate difficulty. The first three were to be drilled vertically, and the fourth was to be an S-shaped build-and-drop well with maximum inclination of 16.65º. Moving to the North San Juan Basin in Colorado, the final four wells were more demanding. Wells 5 and 6 were S-shaped with maximum inclinations of 27º and 35º respectively, and wells 7 and 8 were to be re-entry horizontal sidetracks. It was believed that these wells represented a wide variety of technical challenges for CTD that would prove its suitability. The challenge for the team was not the new technology, but using proven, reliable technology in an innovative way.

First steps
Wells 1, 2 and 3 were “plain vanilla” vertical wells; the objective was to prove the efficiency of CTD compared to traditional drilling using jointed pipe. Immediately, two major benefits were realized. ROP was four times that of conventional jointed pipe drilling, and the safety improvement was obvious. On Well 3, the Schlumberger PowerDrive vorteX running PowerV was used. The vorteX tool is a rotary steerable system (RSS) driven by a powerful thin-wall positive displacement mud motor (PDM). The PowerV tool automatically steers the well to keep it vertical, sensing the slightest deviation and taking instant corrective action. Closed loop logic maintains the well very close to vertical thus reducing the stepout. In comparison to wells 1 and 2, well 3 had a vertical closure of less than 1 ft (8.6 in.) compared to 90.5 ft and 85.3 ft (28 m and 26 m), respectively for the other two wells.

A variety of telemetry devices was used. In addition to transmitting well data to surface, these contain several important measurement-while-drilling (MWD) sensors that allow the directional driller to know how the downhole tools, bit and mud system are performing and which direction the borehole is going. The service company’s SlimPulse and E-Pulse XR retrievable telemetry tools were used to provide instantaneous direction and inclination of the borehole, toolface (the direction the bit is pointing), shock, stick and slip and real-time gamma ray measurements. E-Pulse XR uses low-frequency electromagnetic wave high-speed telemetry through the earth to transmit its data to surface; SlimPulse XR tool uses traditional mud pulse telemetry.

On well 6, PowerPulse telemetry was used. This mud pulse device is housed in a drill collar. In addition to the measurements mentioned earlier, the PowerPulse device provides downhole weight-on-bit (DWOB) and downhole torque (DTOR) as well as four-axis vibration options. In well 4, the DWOB sensor was used to prove that the coiled tubing was not packing off and in fact was transferring weight to the bit. This helped confirm that the hydraulic system was effectively cleaning cuttings from the deviated hole.

Upping the ante
Wells 4 through 6 were S-shaped, deviated wells. Both 4 and 6 had azimuthal turns in addition to their deviation. Because they were drilled with an RSS/PDM combination system there was no downtime needed for orienting as there is when drilling with a PDM and orienter. On well 6 the RSS/PDM system easily achieved 4º/100-ft dogleg severity. An innovative technique was developed to get the required doglegs without washing out the borehole. A flow restrictor was added to the RSS to keep minimum operating pressure on the pads while the bit nozzles were opened up to reduce hydraulic shock on the formation.
Drilling slant hole sections with the coil in sliding mode raised the risk of inadequate hole cleaning. A rigorous hydraulic model was developed during pre-drill planning. The model showed that with a maximum mud flow rate of 500 gpm, hole cleaning would not be a problem. At first, the drillers proceeded cautiously, making several short trips while drilling to keep the hole clean. Later, experience showed that short trips were unnecessary, and they abandoned the practice without adverse result. However, they learned that it was beneficial to keep the mud pumps going while tripping out of the hole to prevent packing off around the bottomhole assembly. This technique is not possible when drilling with jointed pipe.
Drilling fluids were problematic at first. With no pipe rotation to help, borehole stability was at risk. Although the rig pumps were able to maintain 500 gpm flow rate, they were operating at their maximum limit and there was little margin for error should cuttings transport become a problem. Muds with greater carrying capacity are needed. Local experience came to the rescue when we were advised by others drilling in the area to add lubricity to the mud to get the best ROP. On well 4, adding 2% lubricity
to the mud raised ROP from 20 fph
to 50 fph.

Figure 1. Hybrid rigs that can operate with jointed pipe and coil are breaking efficiency records in the Rockies and in Canada. (Photo courtesy of Schlumberger)