To compete successfully in the global energy market, exploration and production companies must operate efficiently, economically, and safely in increasingly difficult environments. Subsalt applications, unstable coal and shale layers, tar zones, depleted zones, and formations with variable flows and pressures are among the problems routinely encountered that must be overcome.
In these problem zones, nonproductive time (NPT) of 30% in drilling operations has become commonplace in many areas, with 45% reported in some wells. Wellbore instability appears to be the main cause of NPT. Some operators report that it accounts for more than 40% of their NPT and approximately 25% of drilling costs in difficult wells. NPT related to wellbore stability in these applications ranges from several hours to several weeks in just one section, and costs run from thousands to millions of dollars.
Other causes of NPT in these wells include narrow drilling margins and lost circulation. In many situations, operators must compromise ideal well placement to avoid troublesome formations, leaving millions of barrels of oil in the ground.
Baker Hughes recently developed the SureTrak steerable drilling liner (SDL), the industry’s first SDL system that has been used to drill, evaluate, and place a liner to total depth (TD) in complex, 3D directional wells in a single run (Fig. 1). The advent of SDL technology mitigates the risk of drilling through problem zones or formations and creates opportunities to maximize reservoir potential by reaching reservoirs that were previously unreachable. The SDL system was recently used in an innovative openhole patch application that saved drilling time and significantly increased oil recovery in a problematic North Sea well.
Technology Evolution
The concept of drilling with a string of casing rather than conventional drillpipe has existed since Reuben Baker’s original 1907 patent for a casing shoe used in cable tool drilling. In its most basic form, casing is driven into soft formations by circulating and rotating. The addition of basic cutting structures on the casing shoe facilitates the procedure. It also allows harder formations to be drilled and enables the shoe to be drilled out conventionally afterward so that drilling can continue. Various casing drilling technologies and approaches have been used throughout the decades; in general, the main goal was to reduce drilling time by combining the casing and drilling into a single operation.
As the need arose for more power to the bit in high-torque operations, a system was developed with the motor installed inside the casing. However, only minimal steering was possible with this system, which led to the emergence of steerable casing while drilling (CWD).
Over the past 2 decades, as oilfield operations focused increasingly on tapping new reserves from mature fields, CWD and liner drilling were used to drill and complete wells through depleted pay intervals with very high pore-pressure differentials. Both techniques offer alternatives to expensive contingency measures, such as managed pressure drilling, and to conservative drilling programs with reduced flow rates, lower weight on bit, and diminishing penetration rates or extra casing points.
Today, numerous wells are drilled using steerable CWD and liner drilling technologies, and wireline-retrievable rotary steerable systems (RSS). The drawback to these systems is that they require modifications to the rig to allow the casing to be rotated. Additionally, tolerances around the casing result in much higher equivalent circulating densities and potential blowout preventer (BOP) issues in deepwater projects.
Steerable Drilling Liner
The SDL is the most recent evolution in casing and liner drilling technology. Developed to address narrow drilling margins, borehole collapse, lost circulation while drilling, and completion through trouble zones, the method combines advantages of RSS and liner drilling systems.
A standard drillpipe is used as the inner string to handle drilling torque and to trip the drilling bottomhole assembly (BHA). A closed-loop RSS with modular logging-while-drilling (LWD) and measurement-while-drilling tools placed below the liner system allows the operator to accurately drill, evaluate, and place a liner to TD in a single run in 3D well profiles. This capability reduces NPT and openhole exposure and ensures wellbore stability, while eliminating the need to pull the drillstring to run the casing.
By minimizing the annulus size, compared with conventional drilling methods, the SDL system also improves the compression of the cuttings into the wellbore and formation face, which reduces fluid loss and cuttings volume and helps to mitigate formation damage.
Because the liner is isolated from the reamer shoe, it can be rotated at much lower revolutions per minute than the pilot and reamer bits. This design lessens the load on the liner and improves its fatigue life. Drilling with a liner instead of a long string of casing requires minimal rig modifications and no special rig equipment, which reduces pipe handling and the rigsite footprint. The SDL system discussed herein has a latching at the top of the liner and another at the bottom. This creates two connections between the liner and the pilot BHA, which reduces vibration levels and provides a more stable drilling system.
Locating the drive, power, and communication components inside the liner minimizes the BHA stickout while maximizing the amount of formation covered by the liner during drilling. The modular design of the LWD tools provides the flexibility to acquire various types and amounts of real-time wellbore data, including pressure, resistivity, gamma, neutron, and density measurements. These enable operators to characterize the reservoir and make critical, real-time decisions. With LWD, critical data can be gathered without incurring NPT.
Deepwater Applicability
SDL technology holds particular promise in deepwater applications that limit or preclude CWD techniques. For these applications, the rig load capacity may be insufficient to handle the potential weight of the casing string. Additionally, subsurface BOP modifications and time-consuming operations on the rig floor would be required to accommodate the constant rotation of the casing through the BOP stack. During well control situations, the BOP must be able to shear the casing being used for drilling. On the other hand, SDL technology requires less rig load capacity, minimal rig-up time, and no BOP modifications.
Case History
Baker Hughes and Statoil collaborated to develop the first steerable liner systems in 7-in. and 9⅝-in. sizes for 8½-in. and 12¼-in. hole sections, respectively. Statoil needed to address operational issues in the North Sea, where pressure uncertainty and narrow drilling margins often make well planning and execution a challenge to achieve with proper assurance for safety. The project was initiated in late 2006 with generation of the idea, and was followed by a feasibility study, system and component design, manufacturing and testing, full-scale field tests, and offshore deployment on the Norwegian Continental Shelf (NCS).
The system was subjected to extensive onshore testing to guarantee that specifications and functions were qualified before the first offshore deployment. In April 2009, the 9⅝-in. system was field tested in the Brage field. The 7-in. system was tested in January 2010 in the Statfjord field.
The first commercial deployment of the SureTrak service occurred in spring 2013 in the Grane field on the NCS to isolate an unstable shale section that protruded into the reservoir. In the past, the trouble zone had been bypassed by numerous sidetracks, which had led to suboptimal placement of the wellbore close to the gas/oil contact and had severely limited production.
After drilling directionally through the shale, the SDL system was deployed as an openhole patch in the shale interval. Before the system was run, extensive reservoir navigation steering technologies, including deep-reading resistivity and azimuthal gamma ray, were used to evaluate the shape and lengths of the shale interval. These results were confirmed with a pilot hole, thus allowing the accurate planning of the SDL run.
The required liner length and inner string of the SDL were assembled on the surface, and the system was lowered into the hole on standard drillpipe. The pilot BHA was then buried 6.6 ft (2 m) in the formation, drilling parameters were established, and the system was drilled to the target depth.
The establishment of optimal drilling parameters through the 2° to 4.4°/30 m doglegs enabled drilling to proceed with a maximum rate of penetration (ROP) of 115 ft/hr (35 m/h) and an average ROP of 62 ft/hr (19 m/h).
At TD, the upper and lower liners were released without any problems. Subsequent drilling through the 6-in. hole confirmed that the liner had been dropped off where required (Fig. 2), and no problems were encountered during tripping, drilling the BHA, or running production screens. The operation was completed in fewer than 3 days.
The use of the SDL technology in the Grane field resulted in a possible increased recovery of nearly 350,000 bbl from the well. It is estimated that the technology could increase recovery by up to 1 million bbl per well. The technology is now being adopted at additional fields on the NCS. The operator and service company are beginning to develop the second generation of the technology, which will allow drilling, evaluation, setting, and cementing of the liner in place in a single run.