Exploring, drilling and completing hydrocarbon and other wells are generally complicated, time consuming and ultimately very expensive endeavors. As a result, over the years, well architecture has become more sophisticated where appropriate in order to help enhance access to underground hydrocarbon reserves. For example, as opposed to wells of limited depth, it is not uncommon to find hydrocarbon wells exceeding 30,000 feet in depth. Furthermore, as opposed to remaining entirely vertical, today's hydrocarbon wells often include deviated or horizontal sections aimed at targeting particular underground reserves. Indeed, it is not uncommon for a well to include a main vertical borehole with a variety of lateral legs stemming therefrom into a given formation.
While more sophisticated well architecture may increase the likelihood of accessing underground hydrocarbons, the nature of such wells presents particular challenges in terms of well access and management. For example, during the life of a well, a variety of well access applications may be performed within the well with a host of different tools or measurement devices. However, providing downhole access to wells of such challenging architecture may require more than simply dropping a wireline into the well with the applicable tool located at the end thereof. Thus, coiled tubing is frequently employed to provide access to wells of more sophisticated architecture.
Coiled tubing operations are particularly adept at providing access to highly deviated or tortuous wells where gravity alone fails to provide access to all regions of the wells. During a coiled tubing operation, a spool of pipe (i.e., a coiled tubing) with a downhole tool at the end thereof is slowly straightened and forcibly pushed into the well. This may be achieved by running coiled tubing from the spool and through a gooseneck guide arm and injector which are positioned over the well at the oilfield. In this manner, forces necessary to drive the coiled tubing through the deviated well may be employed, thereby delivering the tool to a desired downhole location.
As the coiled tubing is driven into the well as described, a degree of fluid pressure may be provided within the coiled tubing. At a minimum, this pressure may be enough to ensure that the coiled tubing maintains integrity and does not collapse. However, in many cases, the downhole application and tool may require pressurization that substantially exceeds the amount of pressure required to merely ensure coiled tubing integrity. As a result, measures may be taken to prevent fluid leakage from the coiled tubing and into the well. As described below, the importance of these measures may increase as the disparity between the pressure in the coiled tubing and that of the surrounding well environment also increases.
For example, it would not be uncommon for a low pressure well of about 2,000 PSI or so to accommodate coiled tubing at a vertical depth of over 10,000 feet. Due to the depth, if the coiled tubing is filled with a fluid such as water, hydrostatic pressure upwards of 5,000 PSI would be found at the downhole end of the coiled tubing. That is, even without any added pressurization, the column of water within the coiled tubing will display pressure at the end of the coiled tubing that exceeds the surrounding pressure of the well by over 3,000 PSI. Therefore, in order to prevent uncontrolled leakage of fluid into the well from the coiled tubing, a backpressure valve may be located at the terminal end of the coiled tubing. In this manner, uncontrolled leakage may be avoided, for example, to avoid collapse of the coiled tubing as noted above, and for a host of other purposes.
In many circumstances, downhole tools may be provided downhole of the backpressure valve. For example, a clean-out tool for cleaning debris from a lateral leg as described above may be disposed at the terminal end of the downhole assembly. Theoretically, a locating tool configured for locating a lateral leg stemming from the main borehole as described above may similarly be coupled to the backpressure valve above the clean-out tool. For such an application, an uninterrupted fluid path would be maintained between surface equipment and the locating tool. In this manner, the locating tool could communicate with surface equipment via pulse telemetry. That is, upon locating of a lateral leg, the tool may be configured to effect a temporary but discrete pressure change through the coiled tubing flow that may be detected by the surface equipment.
In an attempt to allow the pulse telemetry to be effectively employed, the backpressure valve above the locating tool may be opened when the tool is positioned downhole near the sought lateral leg. In theory, this would allow any pulse generated by the tool to make its way uphole through the coiled tubing and to the surface equipment. So, for example, where a surface equipment is employed to pump about 1 BPM of fluid through the coiled tubing to achieve a detectable pressure of about 5,000 PSI, the locating tool may be configured with an expandable flow-restrictor to effect a detectable pressure drop to about 4,500 PSI. That is, upon encountering the lateral leg, the flow-restrictor of the locating tool may expand in order to generate the detected pressure drop. With the lateral leg located, the clean-out tool would then be advanced thereinto for clean out of debris.
Unfortunately, the described technique of employing a pulse generating tool, such as the indicated locating tool, downhole of a backpressure valve, remains impractical. This is due to the fact that a conventional backpressure valve is subject to periodic throttling of the valve between open and closed positions with the closed position killing any signal from the locating tool. That is, once uphole pressure cracks open the backpressure valve, an equilibrium between pressure at either side of the valve is naturally sought, allowing the valve and seat to periodically open and close relative to one another in an uncontrolled manner. Thus, as a practical matter, where a pressure differential between the well and coiled tubing is significant enough to require use of a backpressure valve, hydraulic pulse communication from below the valve remains an unavailable option.