Milling on wireline – a wealth of applications

By Brian Schwanitz and Kristine Henriques, Welltec

A blocked wellbore with defective down hole valves is a challenge that is every oil and gas operator’s nightmare. With the enormous costs that come from missing production, it is imperative to open the faulty valves and remove obstruction as quick as possible.

Before the introduction of the electric line powered Well Miller ®, the available milling and drilling technologies have been conveyed by means of conventional drilling operations, which employed a rig or coiled tubing (CT) unit with substantial weight on bit and rotation forces, along with the requirement to circulate the debris out of the wellbore. For disposing of a small obstruction, this seems like overkill in terms of equipment, people and needless to say, costs.

These traditional intervention techniques also involve very expensive and heavy technologies that also have been dependant upon drilling rigs for deployment. All of these factors provided significant challenges that had to be overcome in order for milling on wireline to be successful.

This was accomplished with the introduction of the Well Miller ® , resulting in a technological revolution in down hole interventions. By providing the ability to mill and drill on wireline, this new desirable service has impacted the market and has been in high demand among the operating community since its introduction. This powerful wireline drilling solution allows operators to remove down hole defective isolation valves and other wellbore barriers with a much lighter intervention technique than drilling rigs or coiled tubing services.

During the milling operations, the Well Miller ® is operated along with the Well Tractor ®, which sustains weight on bit and provides reactive torque control to ensure a successful milling operation. An automatic feed control stabilizes the milling rate as the bit is pushed slowly through a milled out valve. The bit design is fundamental to the success in order to minimize cuttings and to keep within the “torque” envelope. For valve milling, the debris is drawn into the bit and retained behind the cut out “lid” until the Well Miller ® is retrieved and returned to surface. Purpose built milling bits are created on demand, which means that the Well Miller ® application can be designed to engage and remove any obstruction down hole. For scale milling applications, specialized bits have also been designed by Welltec ®.

Wireline Milling Advantages
Utilizing the Well Miller ® to remove small but difficult obstructions has made a huge impact upon the cost of such operations because of eliminating the need for a rig or CT unit. Actual job experience has been that a valve can be milled out on electric line in a matter of hours and be rigged down before a CT unit with BHA (Bottom Hole Assembly) and pumping fluids could have been mobilized. Additionally, the Well Miller ® system brings a high degree of milling precision and depth control reliability to an intervention, which further increases operational efficiency.

The Well Miller ® service reduces risks in the form of enhanced operational safety, and offers benefits which are synonymous to a medical procedure in the form of “key hole surgery”. It has the ability to operate with exceptionally localized precision on electric line monitor and control, even though “the surgeon” is several kilometers away. In doing this, a broader and more comprehensive intervention is eliminated that might expose the well to risks connected with affecting a larger segment of the well than necessary. The Well Miller ® avoids the need for using brute force by applying large amounts of precision-controlled force to select localized areas in the well via remote control, resulting in a more reliable and effective intervention.

This powerful Well Miller ® technology falls under the category of precision down hole robotics and it has performed a wide range of successful wireline milling operations – all executed with extreme precision and with financial benefits to the operator, which is illustrated in the case stories below.

Case Story 1: Milling Flapper Valve
The Well Miller ® performed a successful milling operation in a deviated oil well in Angola, Africa. The operation was to mill out of a 3.86” diameter hole in a flapper valve and to recover the milled section. The operator chose Welltec’s technological solution because the Well Miller ® configuration uses an outer sleeve which covers the pilot bit and rests on the flapper while drilling with the weight of the tool string above to prevent spinning. This technique proves a major advantage over coiled tubing milling with a mud motor.

The operation required three (3) trips in the well. First, the Well Tractor ® and Well Miller ® ran in hole with a 1” pilot mill bit and tagged the top of the flapper. After 15 minutes, a successful cut was indicated at surface. An additional 43 minutes of milling time were allocated in order to ensure a complete pilot cut. In run two, the same tools were run in hole with a 3.77” cup mill bit. A cut was again registered at surface after 15 minutes; however, the center cut was not recovered when the tools were returned to the surface. A third run was made and an additional 53 minutes of milling was approved to ensure the final cut was made and the core retrieved. After returning to surface, the centre core was still not recovered (an example of a milled out valve can be seen in Figure 1).

Figure 1 – Milled Valve

It was decided to mobilize coiled tubing with a mill. It quickly became evident that the flapper was not in place. The operator theorized that the first cut had been successful and that the center core had been left resting on top of the flapper. The second mill run likely pushed the center core through and then made a second offset cut which broke the flapper in two. This was a fortunate occurrence as the pieces fell to the top of an isolation valve and cleared the valve assembly of any restriction.

The operator concluded that flapper and ball valve milling with electric line is far more precise and less risky than coiled tubing milling.

Case Story 2: Increasing Production
The Well Miller ® also features the ability to clear the wellbore of scale and cement barriers, which is a common problem throughout the world. In a dry gas well offshore UK, camera investigations showed that Barium Sulfate scale bridges in the tailpipe of the tubing were severely restricting production. The operator approached Welltec ® for a wireline alternative to clear the two wellbores which were nearly 100% blocked by scale bridges (see Figure 2).

Figure 2 – Barium Sulfate Blockage

After extensive testing, two different mill sizes were chosen. The first mill was 3.8” outside diameter, which was used as an initial hole opener, followed by a 4.2” outside diameter mill to reach the inside diameter of the wellbore. The operation was completed well ahead of the scheduled coiled tubing time frame and stayed within platform restrictions for people on deck.

A total of 42 ft of Barium Sulfate scale was successfully removed from the well. The resulting production increased by more than 4 MMcf/d while full access to the reservoir was re-established for future operations. Time-wise, the operation was completed in 12 days versus an estimated 35 days planned for coiled tubing to do the same job, and a cost saving of $620K was achieved. Furthermore, the crew required was reduced from 35 to 12 people, providing a positive HSE impact, along with the benefit of eliminating heavy lifts associated with coiled tubing.

Case Story 3: Scale Milling
The buildup of scale deposits is an operational challenge when trying to maximize production. Scale can be located in many areas of the reservoir; over perforated intervals, in the production tubing, within the SSV (Subsurface Safety Valve), or even in the surface processing system. Regardless of the location, the problem with scale buildup is a costly and an energy-consuming process to remove.

In the North Sea, scales consist mainly of either sulfates, carbonates or a mixture of the two. In a field in the Norwegian sector, one well had scale buildup over a SSV located at 1,715 ft. The operator performed several traditional slickline and pump down attempts without being able to pull the insert valve or make the valve stay open during production. The contingency method would have been to utilize coiled tubing, but in this case the mobilization costs were excessive.

As an alternative, the operator chose the Well Tractor ® and the Well Miller ® to remove the scale deposits. A custom-designed scale milling bit was developed for this operation, which enabled the Well Miller ® to mill from 910 ft to 1,715 ft in one day, which was a full day ahead of the predicted timeframe. Due to increased operational efficiency, short rig-up time and reduced personnel requirements, total operation cost was reduced by US$3 Million as compared to the predicted coiled tubing costs. Additionally, the operation eliminated heavy lifts and reduced operational hazards because of the lightweight, remote-controlled precision technology that was deployed. The well was quickly placed back on production after the scale had been milled out.

Future Applications
Due to customer demand, new applications for the Well Miller ® are emerging. Specialized designs have worked to regain a flow path in single-face isolation valves and ball valve applications. Also, milling to provide pressure release by plugs and in milling Inconel and Superchrome isolation valves has been tested. Another application under development is reamer milling, which is a tool application to increase the inside diameter of the selected profile by milling out excess material.

Milling and Retrieving Cuttings to Surface
Recently, Welltec ® introduced the Well Miller ® RCB (Reverse Circulating Bit) to the market. The tool is able to establish full flow conditions by drilling through the hardest materials and has the ability to both drill out and remove the cuttings to the surface. This is a first in the industry for wireline interventions. Just as we see with the “original” Well Miller ®, the Well Miller ® RCB can be operated in the same manner, in a rig-less environment and can be mobilized in a matter of hours while also providing a positive HSE impact. All of this reduces cost and optimizes well interventions.

With the existing and future Well Miller ® applications, Welltec ® has taken technology further by inventing profitable and efficient alternatives to existing approaches. What will be next?

Company contact details:
Brian Schwanitz
VP Global Sales and Marketing
T: +45 48 14 35 14
E: bschwanitz@welltec.com
www.welltec.com