Rates of penetration for the DwC wells (blue bars) substantially beat penetration rates for a locally manufactured drilling-with-casing product (red bars). (Graph courtesy of Weatherford)

The Dagang Oil Field Shallow Water Co. division of PetroChina adopted Weatherford’s Drilling-with-Casing (DwC) service to batch-drill 10 wells with 133?8-in. surface casing at the Zhuang Hai-4 area of Dagang field. By the time the project was finished, the company had saved some US $320,000 compared with drilling the wells with a conventional drilling string and later running casing in the hole.

Given the lower cost of Chinese rig rates and crew wages compared with Western costs, that reduction represented a significant savings for the operating company and a coup for the technology.

Dagang is a large onshore field southeast of Tianjin in Hebei Province. The field is expanding offshore into Bohai Bay, and the easiest way to drill offshore wells in the shallowwater zones is to build artificial islands. Zhuang Hai-4 is one of those artificial islands.

According to Scott Beattie, global business leader for DwC with Weatherford, the history of the technique in Chinese oil fields started in 2003 when Apache China contracted the company to drill 22 wells with the technology for its offshore wells in Bohai Bay.

One of the Chinese nationals on the Apache project later moved to a senior position on the Dagang team. He remembered the success of the project, suggested it to the Dagang team, and the management contacted the service company to try the technique at Zhuang Hai-4, Beattie said.

These were typical development wells for the field, so comparative performance was easy to gauge. The procedure called for the use of the DrillShoe II drillbit assembly and 133?8-in, 68 lb/ft casing to drill 17-in. surface holes to depths between 942 and 971 ft (287 and 296 m) through clay and sand. The team could have drilled deeper if the well design had allowed it.

The system has a lot of advantages. From the safety point of view, the drilling team makes fewer trips and reduces the handling of heavy bottomhole assemblies.
Although this job involved batch drilling the top-hole section, the system reduces risk in many jobs. From that viewpoint, drilling with casing eliminates problems with lost-circulation zones, depleted reservoir sections and unstable formations.

Efficiency improves because the technique eliminates flat spots in the drilling curve. In addition, the technique gets casing to the bottom of the hole, lowers costs and accelerates time to first production from the well.

When the operator anticipates a problem zone, it can use the technique to isolate the problem and continue drilling. All it has to do is drill through the aircraft aluminum drillable nose of the bit without dealing with the harder steel body and shoulder.

The technique includes cement-in-place hardware. The operator can cement and secure the section at the total depth of that section of the well.

The bit design and casing size minimize axial movement through the formation. The smaller annular space increases fluid pressure and velocity for better hole cleaning. That means the operator needs no wiper trips for the well. With conventional drilling, the operator must make sure the hole is in good condition before running casing, Beattie said. A wiper trip requires the operator to pull the bit out of the hole to at least the last casing point and run it back in with rotation and circulation to clean up and true up the borehole. Since DwC already sets the casing, it eliminates the wiper trip.

The higher fluid velocity offers another advantage, he added. When the hole is cleaner, the drilling crew can get a better rate of penetration (ROP) without damaging the formation or collapsing the hole.

The service company sent a crew to the drill site for all of its jobs. This crew included one engineer for each 12-hr shift and a lead engineer to work with the company representative from Dagang.

The drilling team didn’t require any special training to handle the job, but a special briefing session at the beginning of each shift let the crew know what operation was in the works and prepared crew members for the proper actions. That briefing emphasized the need for safe on-the-job practices.

“Communication is key for optimal performance and safety,” Beattie said.

The Dagang organization also drilled three wells using a locally manufactured casing bit. In nearly every case, the system significantly outperformed the local product.
Supervision was not a factor, Beattie said, because service company crews supervised the drilling of all of the wells.

All of the wells reached the same depth, but the ROP was significantly lower with the local product. Engineers also reported that the locally made bits were severely worn at total depth. “Technical superiority is the reason the systems performed better than the local product,” Beattie said.

During the batch job at Zhuang Hai-4, the company saved 96 hours of rig time by eliminating the separate drilling-with-drillpipe trip and by
using the system.

Time spent on bottom averaged

2.84 hours per well, and the average on-bottom ROP was 386.6 ft/hr (117.9 m/hr).

Drilling with casing is a regular part of PetroChina’s drilling program, Beattie added. Since this success,
the company has used the system in other fields, including the Jidong complex.

“Asia Pacific is the largest single market for Weatherford’s DwC technology, and China is one of the most important,” Beattie said.