An innovative logging while tractoring (LwT) solution has been evolved by Aker Solutions to gather more accurate real time downhole data using motor driven tractor wheels to convey logging tools in steeply angled borehole.
The traditional and currently widespread method of introducing logging tools into the wellbore to depths of many thousands of feet suspended by a wireline in order to record a broad spectrum of formation and flow parameters as an array of sensors record data during the process of retrieval continues to be effective in vertical and low angle well tracks. The global advent of horizontal wells has challenged this technique, necessitating new technologies.
One approach has been small bore flexible coiled tubing fed into a well with logging tools attached. However, the surface equipment required is bulky and its operation time consuming with the added drawback of restricted well flow, reducing measurement accuracy of production logging tools.
Enter wireline tractoring, an innovative alternative consisting of a slimline tubular apparatus fitted with powered traction wheels able to crawl within horizontal wells, carrying tools ahead of it. Introduced in the North Sea in the mid-1990s, wireline tractors have proved effective in gathering downhole data from horizontal and deviated wells.
They have been continuously developed such that today's tractors are capable of a very wide range of tasks, with performance and reliability that delivers clear time and cost saving benefits to the operators', comments Lasse Haugland, engineering and R&D manager for Aker Solutions' well service business.
Aker Solutions has been at the forefront of tractor development, its PowerTrac Advance tractors able to perform production logging as well as a wide range of other downhole tasks, including perforating and debris and scale removal.
The tractor at the heart of PowerTracAdvance is a slimline, stainless steel tubular device, typically around 2-4in (50-100mm) in diameter and some 13ft (4m) long and sized to meet well conditions. Housed in the body of the tube are a set of wheels on spring-loaded, retractable arms to contact the wellbore. The wheels, usually eight in number, are driven by small electrohydraulic motors, powered by direct current sent from the surface though a single cable attached to the tractor. Wheel rotation drives the tools forward in horizontal or inclined wells to the precise logging objective, or for other operations.
The tractors are compact and easily transported, including by helicopter, and able to interface with other Aker Solutions tools and almost all industry-standard logging tools. Furthermore, the tractors are connected to the surface by a readily available common sheathed copper core monocable.
The LwT tools come in three principal diameters, 21/8in, 31/8in and 43/4in, applied on the basis of well geometry and force needed to reach the maximum target depth.
Aker Solutions set out two years ago to develop a reliable logging while tractoring system. The result is a new sub, a short tubular section attached to the top end of the tractor and packed with an array of electronics selected and configured to withstand wellbore conditions, operable at up to 180°C (356°F) and 15,000psi.
The technology in the sub separates data and power signals as they pass simultaneously through the monocable, which are then processed further at the surface in a 'black box'. The net result is that data can now be collected as the tractor moves down into undisturbed wellfluids thus providing a more representative downhole picture, and in real time. In addition to 'logging down', another data set can be gathered on the way out of the well, facilitating the acquisition of two sets of data as opposed to one when using the traditional 'logging up' approach.
This world first wheel-based monocable LwT tool, since being trialled in the Siri field offshore Denmark at the end of 2008, has been successfully implemented farther afield. In Saudi Arabia, six jobs were accomplished for Saudi Aramco. Four of these were in open hole, in one of which logging down data was acquired in 6395ft (1949m) of horizontal well section in one pass, reaching a maximum target depth of 15,795ft (4814 m). In another Saudi Aramco well a maximum LwT tractor distance of 16,604ft (5061 m) was completed over multiple production logging intervals in one run. Also notable was a maximum well flow of 13,500b/d recorded while tractoring down in one of these wells.
The Aker Solutions LwT methodology overcomes two major difficulties encountered with wireline tractors that must first be positioned before logging up. The monocable and sub electronics remove the need for multiple cables connected to the tractor to separate data and power signals, permitting logging while tractoring down, formerly impossible because of electrical noise resulting from these simultaneous signals. Also, LwT upwards is now more efficient as the tool does not have to be periodically powered down between formation intervals.
An added benefit of the monocable and electronics, enabling a continuous stream of real time LwT data to surface, is the downward passage of logging tools through as yet undisturbed wellbore fluids, consequently furnishing a record that can be compared to a subsequent LwT pass logging out.
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