When a borehole is drilled for hydrocarbon exploration, it is conventional to insert a casing string into the borehole to protect the borehole formation. A liner string can then be suspended within the casing string and can be connected to the top side by a drill string. Normally, cement is injected into the annulus between the outer surface of the casing string and the inner surface of the borehole in order to secure the casing string.
There are devices available that permit pressure testing of the casing string, or if present also permit pressure testing of the liner string, or which permit activation of pressure activated tools in the casing or liner string, the majority of these devices permitting pressure testing, or pressure activation respectively, after the cement has been inserted into the annulus.
However, in order to be able to pressure test the casing string after cementing, it is known to run in a packer tool. The packer tool comprises an outer expandable seal that when expanded seals against the inside of the casing string, which then permits pressure testing above the site of the seal. However, using such a packer tool can be detrimental to the casing string, as the packer tool exerts very high loads on casing when pressure testing, typically in the region of 10,000-15,000 psi. Further, the packer tool must be retrieved from the well after the testing operation has been completed.
Alternatively, a seat is provided at a suitable point on the inside of the casing or liner string so that when a plug is released, it travels down the casing or liner string and will hopefully land on the seat, thereby forming a seal so that pressure testing can occur above the plug. However, the plug and seat arrangement has the disadvantage that it is not certain that the plug will correctly land on the seat. The plug is normally released during the cementing operation and often does not land correctly on the seat, making it impossible to perform the pressure test. Further, the plug and seat pressure testing arrangement has the disadvantage that the cement is usually in position and has set or hardened by the time the pressure test is conducted. Therefore, if there is a leak in the system, the casing cannot be retrieved, resulting in expensive and time consuming remedial work to ensure pressure integrity.
Further, it has been known for the plug and seat pressure testing arrangement to fail during a pressure test. If this occurs, then the build up of high fluid pressure that precedes the failure can expel the cement from its intended location, and thus causes a poor cement job that requires remedial work.
If a packer, or the plug and seat arrangement is used after the cement has set, the high fluid pressure that is exerted on the casing can cause the metal casing to be bowed outward or expanded. This causes the cement to be displaced. Therefore, after the fluid pressure has been removed, the profile of relatively elastic metal casing will return to its original pre-pressure test state, but the cement may have set or hardened, and thus will not return to its original pre-pressure test state. Therefore, a micro-annulus may be formed between the outer diameter of the casing and the cement bond in the borehole, which may lead to gas migration up the borehole, and/or loss of zonal isolation.
In order to activate pressure operated tools located downhole, such as a conventional liner hanger system, or a conventional running tool for running a liner hanger system downhole, it is known to drop a ball down the casing string in the fluid path. The ball eventually lands on a ball seat located below the tool to be activated. Thus, the fluid path is blocked. This results in an increase in the fluid pressure, which can then activate the pressure operated tool. Then, when the pressure operated tool has been activated, the fluid pressure is increased such that the ball and/or the ball seat is sheared out of position, down the string. Fluid circulation can then continue in the same manner as before the ball was dropped.
However, there are various problems associated with this conventional apparatus and method for activating a pressure operated tool. The time taken for the ball to reach the ball seat can be considerable, and there can be problems with getting the ball to land on the ball seat, particularly in highly deviated wells such as horizontal wells, where the ball may have to travel a relatively long distance through a horizontal section of the well. Also, when the ball and/or the ball seat has been sheared out of position, the formation receives a hydraulic shock, which can lead to a loss of circulation of the fluid.