Deepwater drilling operations are costly, so technologies that reduce non-productive time (NPT) and increase efficiency are of significant value to operators.

One of the methods introduced to drill efficiently through challenging zones is Tesco’s Casing Drilling process. The process delivers the ability to drill in tough places, saves time and money, and increases safety and efficiency while reducing NPT. The company believes these benefits can be greatly magnified in deepwater applications through the use of advanced liner drilling systems with retrievable bottomhole assemblies (BHAs).

The objectives for implementing the Tesco level III retrievable casing drilling system depend on the benefits necessary to complete the section as effectively as possible. On the rig floor, automated casing drive system improve the safety and efficiency of casing running and drilling operations. Below the rotary table, the benefit of always having the casing on bottom even while tripping BHAs can assist in reducing NPT associated with the open hole, such as sloughing, influx, losses, and surge and swab problems. In many cases, improved drilling windows help to mitigate the above issues as well as potentially allow for casing seats to be pushed deeper, reducing the number of casing or liner strings required to reach total depth (TD).

This drawing shows how liner drilling can be carried out from a deepwater floating production system. (Images courtesy of Tesco Corp.)

Most recently, a casing drilling campaign was launched offshore Malaysia in deviated sections drilled through soft, unconsolidated (late Miocene) formations. When these sections are drilled conventionally, losses and inferior hole cleaning necessitate the use of controlled drilling techniques to limit the equivalent circulating densities (ECDs) associated with loading up the backside with cuttings, restraining the rate of penetration (ROP). When drilled with the casing string, the small annulus created between the casing and open hole increases the annular velocity for a given flow rate. This effect vastly improves hole cleaning, eliminating the need for controlled-drilling techniques, and ultimately results in higher penetration rates.

Success in the field

On a project in Brunei, Tesco assisted in drilling a 13 3/8-in. casing string through an 1,800-ft (549-m) interval at an average penetration rate of more than 360 ft/hr (110 m/hr) from the 30-in. conductor while pumping 800 gal/m seawater with pumping high-viscosity sweeps.

When drilled conventionally, this section was control-drilled to avoid the aforementioned hole cleaning issues. The operator’s engineers found that casing drilling improved hole cleaning enough to allow for higher ROPs, as they were no longer confined by the techniques needed to prevent high ECDs due to cuttings buildup in the annulus. Additionally, the section was gamma-logged to pick casing setting depth. Upon reaching TD, the BHA was successfully retrieved, followed immediately by cementing operations.

Another Tesco first was achieved on a recent job on a semisubmersible rig offshore Malaysia where the first casing drilling job on a subsea well was completed. The drilling program for this exploration well in 340 ft (104 m) of water consisted of a 13 3/8-in. casing string installed through the 30-in. conductor to approximately 2,200 ft (671 m). Regulations mandate an 8½-in. pilot hole is drilled to probe for shallow gas, and the use of Tesco’s level III retrievable casing drilling system allowed the operator to meet this requirement, eliminating the two runs typically required to open the hole and subsequently install the 13 3/8-in.-diameter casing. This section also was simultaneously logged. Had shallow gas been detected, the well could have been killed dynamically with the gas circulated out at the source simply by increasing the pump rate. Overall, the operator realized a savings of more than 1 1/4 days and US $1 million.

The next level — retrievable BHAs

The next level involves moving to level IV liner drilling with a multiset hanger and retrievable BHA.

The benefits derived from casing drilling as demonstrated in the case studies can be applied to advanced liner drilling systems. In cases of drilling the liner through incompetent formations, the wellbore strengthening provided by rotating the liner casing against the open hole is anticipated to assist in the reduction of losses and hole sloughing, the widening of PP/FG windows, and a potential improvement in the section’s formation integrity test, any of which can permit deeper casing points to be realized versus drilling these sections conventionally. The BHA also is designed to drift through the liner, giving the operator the ability to retrieve it to the surface while leaving the liner either temporarily parked in tension anywhere in the well bore or permanently hung once TD is attained. In the event the liner becomes stuck, the BHA can still be pulled safely to surface, leaving a usable well bore through which to drill the next section.

Deep well applications

Open hole time is limited because the hole is simultaneously drilled and cased. A smaller outside diameter BHA minimizes swab and surge forces, and if the hole does slough in, a passageway exists to retrieve the BHA and leaves a useable well bore, allowing seats to be pushed deeper, possibly eliminating casing or liner strings.

In some cases of drilling deep wells, the ability to deploy casing drilling can be limited by the loading of the casing connections near the surface, whereas the liner drilling system may be adopted as drillpipe is situated from the top drive to the liner top. This allows the transfer of higher surface loads to the drillstring, as the peak loading is seen on the drillpipe at the rig floor. The maximum load applied to the liner string is at the casing connection just below the liner top and is limited to the torsional and axial loads required to rotate and convey the liner pipe alone, as all drilling loads are transmitted to the BHA via an inner string.

The liner drilling BHA is required to drift through the liner when conveyed both into and out of the well bore, necessitating the use of the next-size-down BHA. There is some bypass clearance between the liner pipe and BHA from its design; however, additional bypass exists when conveying the BHA through the liner due to the annular gap present between the liner and parent casing. The reduced BHA outer diameter is especially advantageous once inside the parent casing above the liner top, where both the bypass around the BHA and the distance traversed during the trip are greatest. This substantial bypass around the BHA provides a significant reduction in swab and surge forces throughout the BHA tripping process, allowing faster tripping speeds and quite possibly eliminating the need to set a heavy pill, resulting in a vast reduction in NPT associated with tripping BHAs.

Features of the multiset system

During the liner drilling process, a standard hole size is concurrently drilled and cased off, followed by a cementation run similar to the installation of a second-trip liner top isolation packer but without the requirement of a mill run to dress and polish the polished bore receptacle. The entire BHA, including all cutting structure, drilling, measurement, and logging tools, can be changed out multiple times to reach TD without ever setting the liner on bottom through the use of a multiset hydraulic hanger that provides the option to either park the liner across open hole or keep it tucked completely inside the parent casing string.

During the BHA reinstallation process, any gas migration into the liner or annulus can be safely circulated to surface once the BHA is stung into the liner.

Once the liner has been drilled to TD, the hanger is set and the BHA changed out for the cementation assembly installation at the liner top. The hanger’s pressure requirements are negated throughout the entire process as it remains isolated from internal flow at all times. Additionally, the liner top equipment is designed specifically for drilling and cementation applications by maximizing flow through and around the tools while preserving substantial wall thickness, minimizing the effect on ECDs without sacrificing the mechanical properties of the equipment.

Current stage of design

The complete 7-in. by 9 5/8-in. liner drilling system has been tested to prove both its ultimate load capacities and its multiset functionality. The company is currently preparing to drill its first section with the system at test facilities. The 9 5/8-in. by 13 3/8-in. system is in the detail design phase, with receipt of prototype tools scheduled for Q2 2010.