Issues can develop in a producing well that can negatively affect operations, production, and ultimately, revenue generated. Oilfield services provider Aker Solutions has recently completed trials of a new well intervention system designed to track coiled tubing operations in real time. Donald Kellock explains.
Although technology has advanced in recent years, the main method of determining depth during coiled tubing (CT) operations still relies on a mechanical wheel in contact with the pipe, which effectively measures the length of pipe that has passed.
The main discrepancy in this system comes from slippage of the wheel against the surface of the pipe, due to a build-up of rust, wax, oil or other substances. In addition, it cannot take into account downhole stretch of the CT itself, which can vary on each job due to temperature, pressure, or deviation.
Depth control is more critical during complex well intervention operations, so it is important that accurate measurements can be taken to determine the position of the bottom-hole assembly (BHA) in the well.
Logging a well or correlating depth on CT in real time is not a new innovation. It has traditionally been carried out using a standard e-line cable (5/16-in. or 7/16-in.) inside the CT. This system of “stiff wire” logging has several drawbacks: it limits the fluid rates and types of fluids that can be pumped, it does not allow balls to be pumped to activate ball drop tools and adds a significant amount of weight to the coiled tubing reel when installed.
Aker Solutions designed and developed a real time coil (RTC) system with a view to overcoming the problems associated with the traditional method. It captures all of the necessary downhole data and transmits it in real time back to the coiled tubing cabin. This is all achieved by using a 1/8-in.- diameter wire inside the CT.
System overview
The RTC system consists of three main components: the downhole BHA/ sensor tools, tubewire conductor and the data acquisition system (DAS).
The first component in the BHA is a cablehead assembly, where the electrical wire terminates into the downhole tools and is sealed-off from any fluid or pressure. The cablehead also includes check valves and a ball-activated disconnect function, which releases the tool and the cable in the event of being stuck downhole.
The cablehead is a simpler design than previous “stiff wire” cableheads and can be installed in the field in about 45 minutes.
Attached below the cablehead is a gauge carrier to house the logging tools. The cablehead is connected to the gauge carrier by a castellated quickconnect sub, which means there are no additional electrical connections to make up.
We chose to run the logging tools in a gauge carrier to offer protection to the tools and make it simpler to swap out tools during operations.
An optional lower disconnect sub can be installed, should the need arise to run additional logging tools or perforating guns below the gauge carrier. This allows electrical connection through the entire BHA but provides a disconnect function so the whole BHA is not left downhole in the event of getting stuck.
The entire BHA has a 20mm thrubore, which allows for pumping of fluid and also allows pumping of balls to activate any ball drop tools. It was developed as a modular system to allow flexibility to run various combinations of tools depending on the needs of the customer.
The tubewire conductor is a 1/8- in. incoloy alloy-encapsulated wire, which is installed in the bore of the CT. This allows power and signals to be transmitted between the BHA and the DAS on surface. The incoloy outer tube protects the wire from abrasive or corrosive fluids being pumped through the CT and, due to its small diameter, it has no effect on pump rates and still allows the capability to pump balls for activating downhole tools.
The surface logging DAS allows the CT operator to easily see all of the downhole parameters in the control cabin, and the data can be downloaded to a memory stick to be viewed on a laptop.
The logging graph can be printed off in API format, so that checks can be made on exact locations for perforating or setting plugs, etc. The DAS is a small, portable, Zone-1-rated enclosure, with push button controls, which can easily be moved between CT units.
Capabilities
The downhole logging tools in the RTC system consist of a casing collar locator (CCL), scintillation crystal gamma ray sub, dual-pressure sensor (inside CT and in wellbore), and a fast-response fluid temperature sensor. The system is rated to 175°C and 10,000psi. The logging tools also have a built-in memory, with a 26-hr lithium battery back-up.
When the cablehead and gauge carrier are installed on the CT, it results in a BHA length of about 14.5ft. With a standard 18,500ft CT string, the additional weight of having the tubewire installed is about 250kg, which can be up to 10 times heavier in a conventional e-line cable.
The system is also capable of running a downhole camera and is compatible with different types of firing heads and firing panels.
Development
The biggest challenges during design and manufacturing centered on the logging tools and gauge carrier. We required as large a through-bore as possible in the tools, while keeping the tools to a maximum OD of 2-7/8in., ensuring length was kept to a minimum and still meeting the 10,000psi pressure rating requirement.
In conjunction with Probe (a measurement tools and systems manufacturer), Welltools Ltd., and Wellvention Ltd., we were able to design a tool to meet all of these requirements.
Aker recently completed testing the system in a test well, which involved running different downhole tools, including motors, burning shoes, Venturi junk baskets, and fishing tools.
This was done with the logging tools installed and allowed Aker to correlate against known tubing connections in the well. It also provided downhole pressure and temperature data.
The results proved that the sensors downhole do provide different data trends from those traditionally observed at surface and allowed Aker to operate the tools in a different manner than it would normally.
Several tests have also been carried out to prove the capability to pump balls through the CT string and applicable tools. The emergency downhole disconnect systems were also tested.
The system has been designed to run in conjunction with a wide range of standard CT tools with a view to making it a standard deployment on all CT operations in the future.
The RTC system will allow us to more accurately place tools in the wellbore and gives us downhole data to make the correct decisions during operations. OE
 Donald Kellock joined Aker Solutions in 2011 and has been involved in developing the Real Time Coil (RtC) system. He has a Masters degree in mechanical engineering from Strathclyde University. |
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