Well conditions vary greatly, and in reservoir zones where minerals such as calcium or barium scale precipitate downhole under the right conditions (e.g., water production), flow can often be restricted or completely blocked. In 2007, Welltec introduced the Well Miller RCB (reverse circulating bit), which is able to reestablish full flow by drilling through even the hardest materials and do it on electric wireline.

When wells display loss of flow conditions due to mineral deposits, the RCB is designed to both mill and recover the debris in a bailer system. This is also the ideal solution to remove temporary cement plugs and stringers left after remedial cementing work. Unique in the world of wireline/rigless interventions is this ability to both drill out and remove the cuttings to surface in one run. This is possible because the tool actually features similar downhole fluid circulating parameters as a full-size drilling rig.

The system provides two operational benefits at the same time: optimizing well intervention time and minimizing cost requirements so that more production-enhancing interventions can be done more often.

Operation

The RCB is an electric-line intervention tool operated in conjunction with Welltec’s Well Tractor that provides weight on bit and controls the reactive torque of the drilling process. The tool consists of a basic rotational unit (BRU) that includes electronics, pressure compensator and electric motor sections. The BRU drives an impeller, which creates the vacuum flow to circulate through the liquid environment tool, “dragging” debris into the 0.5-in. intake holes where it will deposit in the sand trap bailers. Depending on the configuration of the tool, a different number of bailer sections can be mounted. Each bailer section holds up to 1.5 gal (6 l).

The amount of cutting debris collected is dependent on operational requirements; the tool can be configured to fit any given operation size, amount and type of material. As the milled-out cuttings are collected and retrieved to surface in bailer sections, operators avoid later issues from debris cuttings being left in the well.

Improved efficiency

With the ability to mill, drill and collect debris in one run, substantial work hours are saved. The tool can be mobilized in a matter of hours together with the conveyance solution and required field personnel. This rapid mobilization makes proactive maintenance of the well site attractive and avoids the cost of mobilizing a full coiled-tubing (CT) spread with bottomhole assembly drill bit.

For further time and cost efficiency, interventions are performed in a rigless environment, and with the tool’s ability to operate localized with electric-line depth control, it eliminates a bigger and more comprehensive intervention that might expose the well to risks as a larger area of the well is affected. By applying large amounts of precision-controlled force in small parts of the well via remote control, operations are significantly more reliable and effective than conventional methods and ensure that the well is quickly returned to production.

Returns on investment

Ultimately, the tool reduces the time and manpower required for performing the obstruction removal service. This has an influence on associated costs which, combined with quick returns on investment and reliable interventions, makes the tool a highly value-creating solution. Compared to CT operations, cost can be reduced by more than two-thirds due to less personnel required and time-efficient operations. In fact, the RCB is able to mill out hard materials and retrieve them to surface before CT has even been mobilized to the site.

HSE

The tool exposes personnel to fewer health, safety and environment (HSE) risks and eliminates a larger intervention in the well.

Traditional intervention techniques involve many tons of equipment and many people on deck; however, by applying the RCB these requirements are greatly reduced. The tool can be operated by a crew of two people flown to the platform in a small helicopter, and the intervention can be performed in a rigless environment; a crane or mast unit is all that is required. This reduces fuel consumption, emissions and discharges caused by transporting and operating traditional intervention methods such as coiled tubing. Additionally, heavy lifts are eliminated for the offshore environment.

Case study

The tool is new to the market and has been thoroughly tested by Welltec in real-life simulated jobs. It has been successfully used to drill through different crystalline materials including scale, salt bridges and cement plugs.

In August 2007 a temporary cement plug was removed during a test job. Formerly, the removal of cement plugs required a full CT spread. However, the RCB was run in conjunction with the Well Tractor where the RCB provided drilling action and cuttings pick-up while the wireline tractor provided weight on bit and control of reactive torque.

The wireline tractor was a standard 3 1/8-in. outside diameter (OD), while the Well Miller RCB was configured with 3 ½-in. OD bailer sections and a 4-in. circulating bit. Furthermore, the bailers were configured to pick up debris down to 0.02 in.

After running in hole to the plug depth, the tool string tagged the cement and was retracted about 40 in. The RCB was activated, and the tool string began to slowly run in until the bit engaged the plug. The field personnel monitored the milling progress while evaluating tool string performance.

At one hour into the operation the RCB had drilled 41 in. through the high-strength cement. The total length milled was 60 in. When the tool string was returned to surface, the bailers were found to be packed with the cement cuttings.

It was concluded that the RCB was able to drill through the high compressive-strength cement material without encountering any problems and was also able to retrieve the milled-out material to surface. This prevented a subsequent cleaning trip, which ensured a time- and cost-efficient intervention.

Maximum rate of penetration achieved by the RCB during scale milling operations has been measured to up to 30 ft/min (10 m/min), which by far surpasses other intervention technologies. The tool also can be configured to fit different wells and purposes, which means that bailing capacity and speed can be adjusted.