Recent technological advances are driving casing/liner drilling from a niche market into the mainstream drilling environment. Escalating deepwater costs and the need to further reduce onshore drilling expenses in low-cost fields continue to push the technology forward. Improved connections and tubulars as well as advances in rig technology and pipe handling have enabled operators to consider drilling with casing/liner as an option on many new wells.

Contractors have utilized casing/liner drilling to effectively seal off depleted pay zones or sloughing formations for years. However, the need to further extend open well bores and drill beyond differential pressure zones is fast becoming a reality because of the increase in mature fields and the need to maximize production from existing assets. With a potential reduction in flat time and increased confidence in reaching section total depth (TD) in a single run, casing while drilling has become a viable option.

To better serve this expanding market, a new drillable casing bit solution called EZCase has been introduced (Figure 1). The system is engineered so operators can ream and drill casing/liner to bottom every time. The bit utilizes advanced 3-D design methods and is manufactured from a specialized steel alloy. This unique material allows technicians to braze polycrystalline diamond compact bits (PDCs) directly to the one-piece bit, ensuring a robust cutting structure capable of drilling new formation as well as reaming existing hole. The system allows operators to run casing/liner to section TD and then drill out with no damage to the bottomhole assembly (BHA). This non-retrievable casing/liner drilling system has been successfully applied in onshore and offshore environments.

Current technologies

Current technologies vary greatly in both complexity and ease of implementation. The increasing degree of complexity has generally meant increasing application flexibility, but also an increased learning curve and the need for detailed planning. In general terms, the systems can be characterized as retrievable or non-retrievable. The relatively slow uptake in more complex systems led the service company to focus its efforts on improving the current technology offered in "simple" drill-down or ream-down systems.

Understanding when to apply drilling or reaming with casing is still a challenge to engineers today. The very fact that there are still problems running casing and liner conventionally point to a need for wider application knowledge. The applications for casing while drilling are well documented, but the combination of application and system choice also needs to be understood. Current applications include:

• Insurance to get the casing to TD of an existing hole;

• As a contingency when casing running has been unsuccessful;

• Drilling through depleted or weak zones;

• Drilling down casing or liner into and through a rubble zone; and

• Minimizing flat time.

Planning time for these applications can vary from a few hours to detailed hydraulics and drilling programs, depending on the type of system chosen and the current capabilities of the drilling rig. Improving performance and extending the application range through improved technologies is seen as the next step in generating wider acceptance and confidence in casing while drilling. There are two distinct groups of retrievable and non-retrievable systems. The more complex systems can simply be categorized by the presence of an inner string and/or a retrievable section from the BHA.

Improvements in casing bit design

The need for a new design of bit that is capable of drilling on the end of casing or liner is driven by two distinct economic considerations:

• The need to drill longer intervals and harder formations on liner and casing to address the drilling of trouble zones.

• The need to avoid a dedicated drill-out run by drilling out with the next drilling assembly.

These two needs had been thought to be mutually exclusive, as conventional drill bits are typically constructed from non-drillable materials.

Balling resistance

The application of casing bits is somewhat different from conventional PDCs. Optimization of hydraulics is near impossible due to the large inner diameter (ID) of the casing or liner and the need to make sure that the reduced annular gap does not pack off. This makes the design of the bit for balling resistance a real challenge. In high balling applications, two nozzle configurations were settled upon; a four-bladed bit with four and six ports was studied both with and without drillable nozzles.

Hydraulics modeling of the bit enables multiple scenario iterations to be performed prior to manufacture, thus saving in time and investment. The objective of the analysis is to balance the flow of cuttings from each blade with the volume of the respective junk slot. This helps to minimize cuttings recirculation, a primary cause of bit balling (Figure 2).

The drillable drillbit

The design, modeling and testing of the EZCase bit had always taken into account that drilling out with a PDC bit was a necessary requirement. Casing shoe manufacturers had considered PDC elements to be "non-drillable" and had actively avoided placing PDC cutting elements on the bit face or had developed casing shoes with a displaceable PDC cutting structure.

A test protocol was set up to fully understand the destructive nature of drilling out the EZCase bit. Each generation of casing bit was tested for PDC and roller cone drill-out. Two casing bits were drilled out initially:

• With a steel tooth roller cone bit; drilled through in 20 minutes at 80 rpm and weight on bit (WOB) increasing from 3,000 to 5,000 lbs. The bit was graded 4-1-BT and debris varied from "chunks" of steel to powdered PDC cutters.

• With a 5-bladed PDC bit; drilled through in 35 minutes at 80 rpm and WOB increasing from 1,500 to 2,000 lbs. As expected, significant damage was experienced in the cone, nose and shoulder of the PDC bit.

These initial drillout tests encouraged the design team to explore techniques being developed for another new product aimed at milling casing and drilling on into virgin formation. The milling process was accomplished through the strategic placement of tungsten carbide cutters placed across the PDC bit profile. These milling cutters are over-exposed from the primary cutting structure to give a dual-purpose (hybrid) bit. A PDC bit was developed to include this new hybrid feature. Drill-out testing continued on EZCase bits with the latest drill-out test involving drilling through a float assembly as well as the bit. A 121⁄4-in. EZCase bit and a 95⁄8-in. Davis Lynch float valve were cemented in rock and drilled out with a hybrid 81⁄2-in. matrix body PDC bit with tungsten carbide (TC) milling cutters. The test was successful. Results are show in Figure 3.

Case studies

Gulf of Mexico - Depleted zone casing drilling. An EZCase casing bit drilled a trouble zone for a major independent operator in the Gulf of Mexico. In order to isolate a 120 ft (36.6 m) thick "thief sand," the operator determined that casing drilling was the best option to eliminate the 4 days lost time experienced on a previous well. The objective was to drill down 7,842 ft (2,391.8 m) of 95⁄8-in. casing through the problem zone. To achieve these goals, a 121⁄4-in. EZC304 was selected in an attempt to improve rate of penetration (ROP, 7 ft/hr) and durability (269 ft or 82 m) achieved with a competitor's system on the previous well.

The 121⁄4-in. EZC304 drilled a total of 398 ft (121 m) from 7,444 ft (2,270 m) to TD at 7,842 ft (2,391 m). The bit drilled through the potential problem zone at over 30 ft/hr (9.2 m/hr) before slowing in the gumbo shale below. The section was drilled and cased in just 21 hours for an average ROP of 18.95 ft/hr (5.8 m/hr). The EZCase bit helped increase ROP by 170% while drilling 50% further relative to a competitor's product used on the offset well.

Onshore Europe - Rubble zone liner drilling. After experiencing communication problems with the payzone, a major international operator decided to try to drill down to clear a rubble zone below a salt section. Severe losses were encountered and thus it was decided to pick up a drill-in liner system and 6-in. EZCase casing bit. In conventional drilling, the interval would normally be drilled with an impregnated bit and turbine indicating a hard and abrasive formation. The liner was rotated down on drill pipe from 7,306 ft to 7,497 ft (7,306 m to 2,286 m) at 33 ft/hr (10 m/hr). Returns and pump pressure were closely monitored throughout the run to ensure any pack off was detected as early as possible. This technique proved successful as the liner was set at the required depth without any major operational issues.

Onshore South Texas - Fractured formation liner drilling. A drill in liner and 61⁄2-in. EZCase bit were selected to drill through rock that had fractured and failed due to compaction from a depleted zone below. The section is characterized by a tight pore pressure fracture gradient margin - if the mud weight is below 16.4 ppg, the well will flow; above 16.6 ppg, the well will lose returns. The rock used to be strong enough to support these mud weights, but the "cracks" have grown further apart along the major fault plane, causing lost circulation problems that didn't exist before. The 51⁄2-in. liner drilled down 667 ft (203.4 m) from 10,683 ft to 11,350 ft (3,258.3 m to 3,461.75 m) at 12.1 ft/hr (3.7 m/hr). Flow rate was held back to 130 gallons per minute to enable the mud weight to be cut back enough to drill with the ECD alone. No losses were encountered in the liner drilling operation.

Future applications

The applications for casing/liner drilling are becoming increasingly more obvious, with several deserving special attention. Extending the length of open well bores is one potential application. Extended-reach wells are typically limited by the calculated risk of wellbore collapse or approaching the torque limit of the drillstring. What if the section was drilled conventionally until one of the limits was approached? A liner drilling system could be picked up and used to drill additional footage, limited either by frictional drag of the system or ultimately by wellbore collapse. Either way, the additional reach can be translated into additional production.

Mitigating shallow gas or water hazards in deepwater drilling is another potential application. Currently, most operators invest significant sums of money performing seismic evaluations to detect these hazards. They also employ the use of weighted mud which is mixed on the fly with sea water while drilling the surface holes. This is an expensive process since returns exit at the sea floor. Testing has shown that ECD increases are easily achieved with slight increases in flowrate. By properly designing the drilling system, it is quite possible that liner drilling could be used to control shallowwater hazards.

As operators look for more ways to decrease drilling expenses in expensive deepwater applications, liner drilling offers an interesting alternative to conventional drilling. Rate of penetration in these applications is less important in most cases. The drilling challenge is generally completing the section and casing off the well bore, where pore pressure and fracture pressure differences can be quite small.

Conclusions

As shown in the numerous examples, casing and liner drilling is finding its way into a number of niche operations. In many cases, the overall concept of casing/liner drilling hasn't changed much from the original concepts of the 1960s. The big difference in non-retrievable systems is in the improvements in the bits, casing connections and tubulars. These enabling technologies are allowing casing and liner drilling techniques to be more widely applied at lower cost and reduced risk to conventional methods. Casing/liner drilling isn't a one-size-fits-all solution to drilling challenges. Each drilling application requires careful planning and evaluation to determine if casing/liner drilling offers potential for cost or risk reduction.

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