Nearly every operator has faced the frustration of losing production because something in the well bore gets in the way. Most of the solutions for getting rid of that frustration are expensive, but more affordable solutions exist.

The Welltec Well Miller with the Well Tractor could be one of those solutions. For example, one of the company’s early milling jobs was the result of a failed downhole flapper valve during a batch completion program.

The operator released the drilling rig to perform other work, and when the downhole valve was cycled, it failed to open. The operator had contacted the service company requesting a solution that could be mobilized from a crane using an electric wireline unit because all of the surface design and testing was complete.

The 20-minute milling program to open obstruction from a stubborn isolation valve raised production from the well more than four times. (Photo courtesy of Welltec)
The test, which consisted of milling a downhole valve identical to those used in the completion, took 35 minutes. The actual time to mill the valve that failed to open in the completion took 7 minutes. The downhole conditions actually improved the milling efficiency.

The resulting hole, which was 3.15 in. in diameter and milled through a .79-in. isolation valve, enabled the well operator to meet its first oil requirements. The total time from rig up to rig down was 12 hours. The operator estimated that had this method not been available it would have been delayed 3 to 4 days.


Operations

The frustrations aren’t always on the production side. In some cases, blockage in the hole may impede the completion of the well. Look at another example. Recently, an operator was unable to establish an injection rate during squeeze operations. After careful evaluation, the operator concluded the isolation valve was closing when the service team attempted to establish an injection rate. Upon further testing (conducted on surface), the group discovered that, when the valve orientation was greater than 30° with reference to the bottom of the hole, it was possible that it would close.

During the milling job a positive pressure was held on the valve to ensure it stayed closed. The miller was used to mill a 1-in. hole in the isolation valve to permit fluid injection. Total milling time for the operation was 20 minutes. The resulting flow rate increased from 1.8 bbl/min (across equalizing section) at 4,000 psi to 8 bbl/min at 800 psi with an estimated time saving of 4 days when compared to a similar solution provided by a coiled tubing unit.
The milling tool was designed to engage and eliminate a wide variety of downhole obstructions. It allows the operator to remove downhole isolation valves and most other wellbore barriers by milling or drilling on wireline.

Part of the success for the operation belongs to the tractor, which sustains weight on bit and provides the torque control needed for a successful milling operation. By changing the milling bit, the operator can deal with a variety of downhole obstructions, including flapper valves and ball valves that have failed to open. In similar fashion, the milling system can help clear the well bore of cement barriers and scale.

Scale is a common problem throughout the world. In one situation, camera investigations showed barium sulfate scale bridges in the tailpipe of the tubing were restricting production in the operator’s wells.

The well operator looked at alternatives, including the use of coiled tubing, and approached the milling company for an alternate solution. The well bores in question were almost completely blocked by scale bridges estimated to be up to 12 ft (3.6 m) long.

After extensive testing, the service company and the operator chose two mill sizes to remain within the torque envelope of the tractor system.

The first mill was 3.8-in. outside diameter (OD) which was used to open the initial hole, followed by the larger 4.2-in. OD mill to achieve the final inside diameter for the well bore. The operation was completed ahead of the planned coiled tubing time frame while staying within platform restrictions on the number of people that could occupy the site at one time.
The resulting production was increased by more than 3 MMcf/d in one well and more than 1 MMcf/d in a second well. The operation was completed in 12 days versus the 35 days for coiled tubing at a cost savings of US $1 million (62% savings). In addition, the crew required was reduced from 35 to 12 with an added health, safety and environment (HSE) benefit of eliminating heavy lift requirements.

Before the introduction of the tractor milling technology, the milling technology available was related to conventional drilling operations which employs substantial weight on bit and rotation forces along with the requirement to circulate the debris out of the well bore for hole cleaning. All of these provided significant challenges that had to be overcome in order for milling on wireline to be successful.

Weight on bit. The weight on bit was managed by modifications to the tractor hydraulics, wheels and bit design. An “aggressive” wheel was developed to reduce the chance of tractor slippage, which assisted the “rotational” system within the tractor.

Torque control. Anyone who has worked with wireline will realize the importance of torque control. Here too, the aggressive wheels complement the “normal” system (the second hydraulic system within the tractor), establishing a firm grip on the casing or tubing. The bit design is again key in order to keep within the “torque” envelope. The final precaution was the addition of a swivel in the event that the torque envelope is exceeded.

Cuttings removal. With wireline, the ability to circulate bottoms-up is not an option. This was taken into consideration in the bit design and the optimal rotation speed to minimize the cuttings generated. For a flapper or ball valve, the debris is drawn into the bit and retained behind the lid (portion of the valve that is removed) when the miller is retrieved. For scale or cement milling the miller has the ability to pick up the cuttings that are generated during the milling process, thereby minimizing the cuttings left down hole.

New applications for the wireline milling system are continuing to emerge. Specialized designs have worked to regain a flow path in other single-face isolation valves and ball valves and to pressure-release plugs, ranging from disappearing plug to Inconel and Superchrome-type isolation valves. Bit designs have also been made for drilling out cement plugs and, as in one of the cases above, performing a successful scale milling and removal operation at a substantial reduction in cost.

In the system’s short history the tool — which was conceived as an efficient way to mill a stuck isolation valve — is finding new applications. The advantages of being able to mobilize without the need for a rig, reducing the number of personnel required for the operation and the added HSE benefit of reducing heavy lifts when compared to coil is resulting in more efficient interventions at reduced costs.