CNOOC outlines procedures for ERD program

First, the crew skidded the rig to template slot B3 and ran an EZSV drillable bridge plug on the top of pay zone to permanently abandon the original wellbore. Next they laid down and moved the cap of the subsea christmas tree.

It was necessary to cut and retrieve the original casing. First, crew members cut the 95⁄8-in. casing at 460 m (±60 m below 30-in. shoe). After pulling the pack off, they retrieved the 95⁄8-in. casing hanger and casing. They pulled the running tool and milled a 20-in. x 133⁄8-in. crossover swage. Afterwards, they cut and retrieved the 133⁄8-in. casing at 455 m and created a ±60 m openhole window below 30-in. shoe for following kick-off operation.

Finally, they set a balanced cement plug across the open hole and into 30-in. shoe and dressed off the plug.

Drilling sidetrack

The 171⁄2-in. section of the sidetrack had a dogleg severity (DLS) requirement of no more than 5°/30 m. The water depth is 311 m in the area and the reservoir depth is around 1,220 m TVD subsea, so that the kickoff point had to be set at 400 m MD, which was just 100 m below the mud line. The section was drilled through soft, homogeneous claystone with seawater as drilling fluid, without a marine riser, in open water. The bottomhole assembly (BHA) was designed to steer 80-100% of time the time during to build up to certain angles and avoid microdoglegs.

The buildup section was finished at 983 m with the hole inclination at 85°; the average buildup rate (DLS) was 4.47°/30 m. The hole azimuth was turned from 84.2° to 107.57° to avoid a collision; then drillers maintained the direction to 1,032 m. In the actual operation, the motor bend house was set to 2.12° to achieve the satisfied dogleg severity. But after sliding for 160 m, the bend angle was changed back to 1.83° in order to avoid bigger doglegs and reduce the torque and drag.

The BHA was as follows: 171⁄2-in. mill tooth bit + A962M5630SP mud motor + float sub + 8-in. pony nonmagnetic drill collar (NMDC) + measurement while drilling (MWD) system with gamma ray (GR) tool + 8-in. NMDC + crossover + 3 x 5-in. heavyweight drill pipe (HWDP) + hydraulic jar + 5 x 5-in. HWDP + crossover + 5.5 HT55 drill pipe (DP).

In order to keep the hole clean and stable, drillers intermittently pumped high-weight pills and a high-viscosity sweep before every three to five stands. For each stand of pipe, drillers reamed and back-reamed to keep the hole clean. Drillers set 133⁄8-in. casing after the 50-m tangent section to protect the buildup section (Table 1).

At the casing point, the hole was swept with high viscosity mud and back reamed to the 30-in. casing shoe. After pulling out of the hole, drillers laid down wear bushing and then ran and cemented the 133⁄8-in. casing. In order to run casing to bottom, drillers ran a BBL reamer shoe from Weatherford on the 133⁄8-in. casing string; the long tapered profile gave a better chance of riding over ledges.

Finally, the crews ran the BOP stack and 20-in. marine riser. The conditions for circulating drilling fluid were set accordingly.

Drilling with RSS

CNOOC took the following steps to drill the long tangent 121⁄4-in. section with rotary steerable tools:1

• Made up the motor assembly. Tagged top of cement and drilled out of the shoe track to 40 m new formation at 1,072 m to provide some space for the under-reamer operation. Adjusted bend angle to 0.39° on the surface.

• Drilled the 121⁄4-in. long tangent section. Made up and ran the 9-in. PowerDrive RSS assembly.

• Changed the mud system to KCl drilling fluid.

The bottomhole assembly was as follows: 121⁄4-in. PDC bit + RSS tool PowerDrive900 + PowerDrive extra stabilizer + compensated dual resistivity (CDR) with annular pressure while drilling (APWD) sensors + MWD with GR + 8-in. NM DC + 8-in. stabilizer + 8-in. nonmagnetic (NM) pony + 121⁄4-in.-13-in. underreamer + crossover sub + three stands of 5-in. HWDP + 6.5-in. hydraulic jar + five stands of 5-in. HWDP + crossover + 5.5 HT55 DP.

Next, the 121⁄4-in. tangent section was drilled to 5,360 m MD and reached top of the “A” carbonate. The crews took care to maintain a smooth wellbore and performed proper hole cleaning. The drilling parameters were optimized as follows: rotary speed 150-200 rpm; flow rate around 950 gpm; standpipe pressure 1,600-3,000 psi; and average weight-on-bit (WOB) 4,000-6,000 psf. The rate of penetration (ROP) was controlled at 50 m/hr.

A smooth well path was drilled successfully with partially hydrolyzed polyacrylamide (PHPA) drilling fluid; the hole was stable and clean. A lesson learned from on site operations was that as the equivalent circulating density (ECD) is increased, you must control the ROP-increase the flow rate, pump high-weight mud, then make a wiper trip or back ream.

Mud requirement- the mud weight was controlled around 9.0-9.6 ppg. The current industry best practices are to maintain high-low shear rate rheology values, shear thinning behavior, maximum the flow rate, and use minimal sweeps. During drilling of the 121⁄4-in. hole section, the mud system needs to provide adequate hole-cleaning rheology to ensure high flow rates and pipe rotation. A flow rate of 950 gpm is recommended based on previous experience. After finishing this section and before running casing, the hole cleaning was enhanced by increasing the mud weight to 10 ppg.

Drillers monitored the ECD to guide back reaming and the short wiper trip operation. The ROP was controlled to 40-50 m/hr and the ECD was less than 10.15 ppg. Ran a sweep of high weight mud to clean the hole. Finally, made two wiper trips with back-reaming during this run. The first wiper trip ran from 2,995 m to two stands below the 133⁄8-in. casing shoe. The second wiper trip ran from 4,612 m to 3,800 m. ECD increased sharply; mud weights were monitored at 9.75 ppg and 10.2 ppg.

In order to open up the wellbore to 13-in., drillers ran an underreamer. This helped to ensure that the upcoming 95⁄8-in. casing could be run smoothly to bottom by providing a slightly larger wellbore.

It was essential to optimize appropriate drilling parameters. Techniques such as high flow rate, low SPP, and appropriate rpm and WOB were applied in the section. Finally, a wiper trip BHA was run to ensure the hole quality.

Flotation casing solution

A flotation collar system was used to reduce the drag while running the 95⁄8-in. casing string. Drillers used a reamer shoe to help get the casing to bottom. A roller blade centralizer is recommended to run in the cased hole. For the openhole section, a spiral-glider centralizer would help the casing to rotate down while only contacting the wellbore. In the Liuhua oil field, all ERW casing operations were performed successfully in a single run.

After cementing the 95⁄8-in. section, crews pulled the riser and BOP stack. Next, they ran and tested the subsea christmas tree; reran the riser and BOP stack, and finally tested the system.

Drilling horizontal section

A rotary steerable drilling system was also used to drill the 81⁄2-in. horizontal section.

First, the crew made up the 81⁄2-in. PowerDrive 675 RSS with LWD tools. Then the drilling fluid was changed from PHPA to seawater.

The bottomhole assembly was as follows: 81⁄2-in. insert bit + RSS tool PowerDrive675 + PD extra stabilizer + PD ILF (flex joint) + MWD ARC6 tool with APWD + MWD + ADN6 + 5-in. NM HWDP+ three joints of 5-in. HWDP + hydraulic jar + five joints of 5-in. HWDP + crossover sub + 5.5 HT55 DP.

The 81⁄2-in. section was drilled with seawater with the weight controlled to about 8.6 ppg. A building operation was required after drilling out of the 95⁄8-in. casing. The hole inclination was increased from 83.6° to 89.91° in a 100 m interval. The azimuth was turned from 103° to 99°.

After drilling reached the target zone of Layer B1, the hole was drilled horizontally, maintaining the trajectory within the top of the layer until reaching TD at 6,300 m MD. Drillers circulated bottom up and pulled out of the hole.

The drilling parameters were as follows: flow rate 550 gpm; WOB 12,000-16,000 lb; rotation speed 110 rpm; surface torque 32,000 ft-lb; average ROP 51 m/hr.

The higher torque and drag problem and 95⁄8-in. casing wear are expected while drilling 81⁄2-in. section with seawater. A low drag joint was used with higher torque capacity of 51⁄2-in. HT55 drill pipe to reduce the casing wear and torque and drag.

Completion

The crew ran the tubing string with the electrical submersible pump (ESP) assembly. The completion string is as follows: tubing hanger + 41⁄2-in. tubing joint + adjustable union assembly + safety valve + 5-in. pup tubing + annular circulation valve assembly + 41⁄2-in. tubing + packer assembly + 41⁄2-in. tubing + ESP assembly + downhole multisensor assembly.

They installed the cap of the christmas tree and started up the ESP unit to begin flow. The well was handed over to the production department.

Torque; drag reduction

Torque and drag are very important factors in extended-reach drilling operations. For example, the plan must consider the maximum allowable torque of the top drive, maximum allowable drag for running casing, etc. The drag formed by running casing and the torque generated by drilling the hole are the two foremost indicators which show whether or not the extended-reach well can be completed successfully.

Theory and practice indicate that the maximum torque and drag generated by drilling and running casing exceeds the maximum allowable value in the Liuhua ERD project. The maximum torque limit of the top drive is 40,000 ft-lb, but the maximum torque in drilling 81⁄2-in. horizontal section to a total depth of ±6,000 m with seawater theoretically is more than 44,000 ft-lb. Therefore, torque and drag reducers have to be used in casing sections.

It’s impossible to run 95⁄8-in. casing in a high-angle, long step-out 121⁄4-in. hole with conventional drilling methods. The flotation collar, Ezee glider rigid centralizer, and low-drag roller were all used to reduce 10-20% of drag on running 95⁄8-in. casing based on theoretical calculations.

Downhole steerable tools

• Rotary steerable system. The RSS PowerDrive provided by Schlumberger was a very reliable tool during the Liuhua ERD project. The RSS tool experienced no failures since its introduction in Liuhua oilfield. The PowerDrive900 drilled 4,380 m (13,796 ft) in one run, which broke the Schlumberger world record in a single run by a PD900. The PD675 and PD475 tools also broke records for total footage in one run in China as they can easily geosteer in high porosity (20-35%) reservoirs.

Additionally, the trajectories drilled with the RSS tools had small tortuosity and the holes were very clean, which has the benefit of reducing torque and drag, good for drilling and running casing.

• Steerable mud motor. The PowerPak steerable mud motor provided by Schlumberger was predominantly used to build up angle with a 4-5º/30 m DLS in the buildup section. It proved that it’s reliable and economical from its use in drilling five extended-reach wells in the Liuhua field.

• Adjustable gauge stabilizer. The AGS tool provided by Andergauge was not proven useful in the Liuhua oil field because the formation is very soft and washing easily enlarges the hole.

Survey, logging instruments

Highly accurate gyrometer, MWD (PowerPulse, ImPulse), and LWD (ARC, ADN, et al.) measurement and transmission technologies were used in the Liuhua 11-1 field. The mud pulse telemetry system employed by Schlumberger in the MWD tool is a continuous wave pulse, or “mud siren” system. The continuous wave telemetry creates continuous wave pressure pulses, which form a sine wave of 1, 12, or 24 Hz (depending upon the chosen configuration).

The MWD tool can measure downhole weight on bit (WOB), downhole torque at bit, collar RPM, temperature, resistivity, and gamma ray, etc.

The ADN tool provides borehole compensated formation density, neutron porosity, PEF, and ultrasonic caliper measurements.

The ARC tool measures formation resistivity at multiple depths of investigation using an antenna array of five transmitters and two receivers, it also measures total formation gamma ray activity with a plateau gamma ray NaI scintillation detector. The ARC also has APWD (annular pressure while drilling) measurement sensors. The annular pressure can be converted to ECD (equivalent circulating density) to expose hydraulic problems, such as the need for hole-cleaning, lost circulation, pipe washout, etc.

Casing running, other tools

Flotation collars were used to separate casing, making the lower section hollow and floated, and reducing the downward force and corresponding friction.

High-torque rotatable liner hanger, which has enough strength and connects between casing and drill pipe, can transfer torque to the casing so as to rotate the casing being run in hole when casing slide-in is impossible.

Other tools used included:

• An underreamer, which opens up the hole while drilling and greatly benefits casing running.

• A hydraulic whipstock, which can be packed into high angle casing sections and tightly fixed inside casing to avoid rotation when cutting casing.

• Drilling office software-a drilling expert system including torque and drag modeling, well design, hydraulic calculation modeling, casing running schedule modeling, and other tools.

Achievements, benchmarks

Integrated extended-reach drilling was used in the Liuhua 11-1 field. ERD with long step-outs and multiple targets in a single well had been previously tried in the Xijiang 24-3 field in the South China Sea in recent years. Extended-reach drilling in the Liuhua 11-1 field achieved the following benchmarks:

• First time to apply integrated ERD technology successfully. Extended-reach wells with extremely high ratios, high angles, and long step-outs are rare in China. Its successful use in the Liuhua field provides a new way to develop the field, and also shows great strides in introducing and implementing new technologies. This has contributed to boosting new high-tech research and testing in China.

• The ERD ratio is 3.885 in the Liuhua 11-1 A4ERW3 well, a new record in offshore oil drilling. Shortly thereafter, the record was broken by drilling the Liuhua 11-1 B3ERW4 well, with its ERD ratio of 4.58-the maximum reached in China to date.

• The actual drilling period of the A4ERW3 well was 33.77 days, which created a record of the shortest drilling period among the offshore wells with a depth of more than 5,000 m in China.

• One PDC bit and one under-reamer drilled 4,380 m in a single run, creating the longest footage record in a single run in China.

• PowerDrive900, a Schlumberger rotary steerable tool, reached a world record of 4,380 m footage in a single run.

• Four extended-reach wells in No.3 block became the main producers in the Liuhua field, producing 46% of the total oil from all 25 wells. Drilling and completion costs were fully recouped after a half-year of production.

Looking ahead

ERD technology was used to arrest declining production in the Liuhua field. With increasing understanding of the field production and reservoirs, it may now be possible to develop deeper potential of the No. 3 block and to jointly develop the circumjacent small oil fields.

There are many small and separate reservoirs near the current production equipment in the Nanhai East fields. It is not economically feasible to develop them individually, but the economics are improved by use of existing, nearby production platforms and subsea equipment. The successful implementation of integrated ERD technology in the Liuhua 11-1 field has demonstrated this.

With further development and enhancement of drilling and completion technologies, extended-reach drilling could be introduced to other onshore and offshore oil fields in order to develop reservoirs efficiently. It will not increase the cost; the Liuhua 11-1 field development demonstrated that the investment was quickly recouped. Consequently, extended-reach drilling will be used to develop more fields in China.

Acknowledgments

The authors thank Schlumberger Ltd., China Oilfield Services Ltd., Weatherford International Ltd., MI Overseas Ltd., and other contractors and participants for their contributions.✦

References

1. Zhang, Wu Nian, “Xijiang To Create ERD World Record In South China Sea,” Oil Drilling and Production Technology (Chinese), 1998, Vol. 20 (supplement).

2. Zhang, Wu Nian “95⁄8-in. Casing Cementing Technologies,” Oil Drilling & Production Technology (Chinese), 1998, Vol. 20 (supplement).