Paper: CostTime Saving Benefits Of Using A Gyro MWD Tool For Top Hole Navigation On Multiwell Structures

Historically, multi well platform directional drilling has been challenging especially in the initial drilling phase. Wells have to be carefully navigated through a maze of other wells and eventually steered clear of all interfering wells to the desired target. (ref #1) To avoid drilling into another well, ellipses of uncertainty and well proximities are calculated as part of the well planning program so that each well can be monitored relative to the others wells around it and collisions avoided. (Ref # 2) As well as an appropriate well plan, the survey tools used for the drilling phase must be modeled and any errors taken into account when the drilling phase is underway. All navigation orientation systems used on modem multi well structures will be either gyroscopic for the top hole, or magnetic systems, once the interference section of the hole has been overcome. Because of the proximity of other wells it is impossible to use any magnetic based survey system in the initial part of the well so gyro survey tools are normally used to give the directional driller the required azimuth and toolface data he requires to orient and steer the motor assembly through this hole section. A typical bottom hole assembly for this initial top hole phase consists of a bit, motor, bent sub or a bent housing motor, orienting sub, non magnetic drill collar, heavyweight drillpipe and drill collars. The orientation of the bent sub and motor is facilitated by the alignment of a key in the orienting sub with the highside scribeline of the bent sub (Ref # 3). The bottom of the gyro tool is dressed with a muleshoe which will locate the key in the sub and force the gyro tool to seat with the muleshoe properly aligned in the keyway. (Ref # 4). This means that any toolface reading from the gyro will indicate the direction in which the motorbent sub is "pointing". By placing weight to this assembly, the bent sub becomes the fulcrum and the bit is forced to the highside of the hole thereby initiating a building of hole angle. The severity of the bent sub angle will determine the rate of build.