RIASSUNTO
Abstract
Jarring techniques to free stuck drill strings have been used for decades in the drilling industry. However, jarring in directional and horizontal wells often remains without success. One of the reasons for insufficient jarring success in wells with extreme drag, is the inability to get sufficient pull at the jar. The jar impact forces are insufficient to free the string.
A drill string tool has been developed to enable jarring at high firing loads: the ‘down-hole clutch'. The down-hole clutch enables drill string rotation in the event of getting stuck. Deployment of a down-hole clutch will significantly increase the jarring effectiveness.
The benefits of the down-hole clutch tool in stuck pipe scenarios tool are quantified via torque and drag, and jarring calculations. The calculations to predict the jarring effectiveness are performed using a recently developed jarring simulation program. This program allows modelling of clutch-aided jarring. Other important clutch benefits are the improved hole cleaning and the operational advantage of exactly knowing the jar firing force.
A 6-3/4 inch prototype of the clutch tool has been built in 1999. Based on the calculations and the clutch design, the operational implications of running a clutch tool are reviewed: the placement in the string, engagement and disengagement of the clutch, the loading of the string during jarring and the elimination of the need for a second jar. Future alternative applications for the tool are for instance casing liner running and the down-hole expansion of casing.
Introduction
The idea to develop the concept of the down-hole clutch originated from difficulties that were encountered while drilling wells. In such wells it is often a problem to achieve sufficient margin of overpull at the jar. Jars are fired at very low tripping loads and often the driller has great difficulty in cocking the jar again.
the problem of too little available overpull at the jar, is not solely due to the magnitude of the drag that is also counteracting the string while trying to trip out of hole. On top of that, there is extra drag induced by a ‘string wrapping effect'. The left picture of Fig. 1 illustrates what happens while applying an overpull from surface to fire a jar that is located in the bottom hole assembly. The tension in the string will increase and at the same time the contact forces between the string and the borehole wall will increase as the string is being wrapped against the wall in curved well sections. In other words, the counteracting drag will further increase while the pull is being applied. The net overpull that is seen by the jar is therefore lower than the upward pull applied in excess of the string weight-up (i.e. buoyant string weight plus drag). This effect occurs in any directional well with a build-up section. The negative effect can become considerable, especially in wells that have a couple of doglegs in the well trajectory, or even more importantly: wells that have azimuth turns.
The right side of Fig. 1 shows what happens when the clutch allows string rotation. The drag forces that act along the drill string, will be redirected in the direction opposite of the relative movement between string and borehole wall. The drag thus changes direction from axial to tangential (circumferential), and will now contribute to a torque build-up. With the same hookload, much more pull becomes available at the clutch and bottom hole assembly. Since the maximum surface pull is limited by either rig capacity or the strength of the string at surface, it is obvious that the maximum pull that is achievable down-hole has increased for the same string and rig properties. The reduced axial drag will also greatly increase the achievable set-down weight.
It must be noted, that the loading of the string now consists of two separate contributions:
tension as a result of the buoyant string weight
torsion induced by the drag forces along the rotating part of the string
The extra torsional loading of the drill string by the drag forces, requires attention to ensure that the operational safety margins are maintained.