As operators have moved into more challenging areas, conventional drilling systems have fallen short of reaching total depth (TD) efficiently, safely and economically. Service providers have responded by developing specialized technologies and techniques that enhance efficiency, safety and economics. Drilling with casing (DwC) and drilling with liner are among the specialized techniques that have emerged and are increasingly being deployed to overcome challenges encountered in mature fields, deepwater basins and wells with depleted or unstable intervals. Coupled with closed-loop drilling (CLD) systems and techniques such as managed-pressure drilling and underbalanced drilling (UBD), operators can navigate difficult wells and problem zones to reach reservoirs that would otherwise be inaccessible or economically unviable.

This integrated multidisciplinary approach harnesses the benefits of both CLD and DwC techniques. First, the use of a CLD system enables precise control over the annular pressure profile. This capability enables drillers to avoid unnecessarily high overbalanced levels that often lead to mud losses, differential sticking and formation damage. CLD systems also help prevent formation influxes by adjusting the bottomhole pressure (BHP) as needed to suppress a kick or control flow.

Additionally, CLD systems reduce HSE risks as well as nonproductive time (NPT) related to fluid kicks and losses. DwC systems further optimize CLD operations by reducing the number of trips and associated surge and swab effects, improving borehole wall sealing through the smear effect and in some cases eliminating the need to kill the well prior to cementing. Current casing-bit technologies also eliminate the requirement for a dedicated drill-out trip.

Under a conventional drilling philosophy, challenging sections with narrow pore and fracture pressure windows are generally addressed with high mud densities and additional casing and liner strings. However, these strategies often lead to NPT due to mud losses, differential sticking or severe kick-loss scenarios. By drilling such challenging sections using a DwC system in an underbalanced state, wellbore pressures can be maintained below the pore pressure, allowing the formation to produce while drilling, and by mitigating well-control issues, the technique enables operators to evaluate the formation productivity potential as the interval is being drilled.

Salt Creek Field

The value of combining these two techniques has been demonstrated in several fields across North and South America. One such operation took place in the mature Salt Creek Field in Natrona County, Wyo., where an operator encountered multiple well control problems while attempting to conventionally drill a gas injection well and eventually abandoned the well. While drilling previous wells in this field, the operator had experienced issues related to formation pressure uncertainties resulting from CO2 injection activity. To resolve the abnormal pressures encountered, the operator had used high mud weights, which caused numerous problems during well control operations.

Weatherford was contracted to reenter the abandoned well and drill from the top of cement at 147 m (483 ft) inside the 75⁄8-in. surface casing to the planned TD at 701 m (2,300 ft). The team selected 5-in. 18-lb/ ft casing, a 6¾-in. polycrystalline diamond compact (PDC) bit and an initial mud density of 8.6 parts per gallon (ppg). The main objective was to avoid killing the well between tripping out the drillpipe and running the casing string, even if abnormal pressures were encountered. Secondary objectives included minimizing formation damage, maximizing post-completion injectivity, minimizing the risk of differential sticking and eliminating additional trips to condition the well and run casing.

To address these challenges and improve efficiency, Weatherford implemented an integrated approach that combined the UBD and DwC techniques. Before drilling, the engineering team performed a comprehensive evaluation based on offset well information and the data recorded during the initial drilling operation. The resulting pressure and flow predictions informed the selection of surface equipment, including a rotating control device, a UBD choke manifold, a four-phase separator capable of handling 1.1 MMcm/d (40 MMscf/d) and 20,000 bbl/d, a flare stack and pipework. The engineering evaluation also defined a flow-control matrix that provided clear limits for the UBD operation based on flow and pressure conditions as well as the CLD equipment ratings and capacities.

During drilling, the combined DwC and UBD methodologies effectively managed the annular pressure profile and well effluents and enabled the well to reach the target TD. Throughout the 35.5-hr operation there was zero NPT related to kicks or losses, and the average ROP was 16 m/hr (54 ft/hr), significantly higher than the average rate of 8.5 m/hr (28 ft/ hr) for conventionally drilled wells in the field. The annular pressure profile was managed throughout by adjusting the surface backpressure, effectively avoiding influxes and maintaining a steady BHP environment during static conditions.

In addition to the time savings achieved through efficient annular pressure control, the cement job was completed just 3 hours after reaching TD because of the single-trip DwC system.

Time, cost savings in South America

Weatherford also has implemented this approach to drill development and exploration wells in a basin in South America, which covers an estimated 69,685 km (43,300 miles) in Argentina and central Chile. The basin comprises multiple fields and sub-areas with diverse well designs and types. In one project, wells with type-S profiles were drilled using the combined CLD and DwC approach. The vertical intervals were executed with a 61⁄2-in. PDC bit and 5-in. casing.

On these wells after the casing was landed at TD there was no well kill operation performed prior to the cement job. Having a good knowledge of the BHP conditions, managed-pressure cementing operations were carried out. During these operations the circulating BHP is adjusted by means of the choke manifold to be within the limits provided by the pore and fracture pressure limits throughout the pumping operation.

Broadening drilling horizons

The cases described above demonstrate that coupling two different innovative drilling techniques can, with proper planning and implementation, produce substantial benefits. The key to successfully combining multiple disciplines to execute challenging drilling operations is increased time and effort during the planning phase. During this stage, comprehensive engineering analyses, including tubular mechanics evaluation, hydraulics modeling, and kick tolerance and risk analysis, should be performed to assess the feasibility of the integrated operation and to verify the compatibility of all systems involved.

Considering the preliminary results of combining CLD and DwC technologies to efficiently drill complex well profiles, the potential of integrating multiple methodologies is clearly an area that deserves further exploration. Service companies must continue to develop innovative, integrated drilling techniques as the needs of operators become more complex, especially at a time when reducing drilling costs is necessary.

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