Fill cleanouts, matrix stimulation, cementing, and downhole milling are some common coiled tubing (CT) operations. The difference between the success and failure of these jobs relies heavily on the knowledge and prediction of the behavior of downhole parameters such as temperature and pressure. Another significant percentage of CT operations depends on highly accurate depth control to ensure the intended result of the operation. These depth-critical operations include setting packers, tubing patches, CT-conveyed perforating, and zonal isolation. And in virtually all CT operations, including fill cleanouts, stimulations, and cementing, the knowledge of an actual tied-in depth is advantageous to operations.
The purpose of this paper is to present a simple and reliable system that allows real-time monitoring of downhole pressure and temperature, and provides depth correlation using a casing collar locator (CCL). In this system, the downhole parameters are recorded in real time without the limitations of conventional wireline-enabled coiled tubing units.
The information presented in this paper summarizes the operations performed during the field-testing of the system at Alaska's North Slope. Twenty-seven CT operations were successfully performed using the system; this demonstrated its reliability and provided the crew with the information needed to improve the efficiency of the operations being performed. The system comprises three main components:
Downhole tools capable of measuring and transmitting bottomhole pressure and temperature as well as identifying casing collars with high accuracy.
Fiber optic and fiber optic carrier, which provides real-time transmission of downhole data to surface.
Wireless bulkhead and surface interface modules to provide two-way communication between the tools and the monitoring system.
Most CT operations have a common challenge: to evaluate and control each stage of the job through educated guesses of what is happening downhole based on surface data and feedback. Downhole pressure is estimated from pressure readings at the pump, wellhead, or both. Actual tool depth is inferred from the amount of coiled tubing going in the hole, with errors as high as 0.3% being accepted as common. Different methods are used to determine the actual depth depending on the depth accuracy required for any individual job. These methods include tagging a known bottom or restriction, use of tubing tail locators, running a memory logging tie-in and flagging the CT, and running mud pulse telemetry logging tools. These techniques can be time consuming, expensive, or add complications to the operations.
The effect of actual downhole temperature on day-to-day coiled tubing operations is not completely understood but is believed to be negligible, with the marked exemption of cementing operations, mainly because there is no practical way of monitoring it to study actual values and changes while operations are underway.
The use of an e-line enabled Coiled Tubing unit can overcome these challenges1, but it introduces limitations, such as significantly higher cost and maintenance requirements, limited fluid compatibility, and flow area restriction within the CT. These limitations effectively reduce the range of possible CT operations that could benefit from real-time downhole readings.
Although there have been significant technology advancements since loggin operations with a CT unit started in the early eighties---notably in the connections, weak points, and understanding of "slack wire management"---the general principle has not changed. Information is acquired using downhole tools. The information is then transmitted uphole via an electric cable that also doubles as a power conduit for the tools. A collector ring assembly is then used to transfer the information out of the coiled tubing reel and into the logging unit, without restricting CT movement. The final link is a depth encoder signal sent to the logging unit to enable information to be recorded in a conventional method.
New fiber optic manufacturing processes using advanced alloys and advances in wireless communication capabilities present the oilfield with new opportunities and novel ways of tackling existing challenges. Many of these advances have been incorporated in the development of the acquisition system presented in this paper.