Controlling Mud Losses into Caves with Bridging Techniques in Ultra-Deep Long Open Hole

The well Wh-1X has experienced whole mud losses into induced fractures formed when killing the well at 6828 m (φ215.9 mm, open hole length was 1178 m) and mud losses into caves at 7114.65 m in a sidetracked section (open hole length was 1464.65 m). The mud losses at the bottoms of the hole had to be stopped. It was believed that, to minimize the risk of pipe sticking caused by fracturing the upper weak formations when controlling mud losses into interbedded sandstone and mudstone at the bottom of the well with bridging method, the whole open hole section should be strengthened to bear pressures that may fracture the open hole. First, the drill string was run inside the technical casing to improve the pressure-bearing capacity of the weak formations above the hole bottom with bridging. Second, run the drill string into the open hole and control mud losses taking place at the bottom of the hole. By rotating the control head during mud loss control, the risk of pipe sticking was minimized. It was further believed that, failure in previous mud loss control was because of the serious defects in sizing and concentration of the bridging materials used. To succeed in controlling mud losses into fractures and caves with bridging method, three prerequisites should be satisfied: Large size and high concentration of firstclass size bridging agents, gradient matching and reasonable concentration of various sub-size bridging agents, high total concentration of the bridging agents. Unobstructed wellbore is important for the bridging slurry to reach the depth at which mud losses are taking place. Using the technology and principle aforementioned, three mud loss zones, 5650-5668 m, 5830-5842 m and 6279-6288 m in the sidetracked hole, were bridged successfully by running the drill string inside the casing twice, and two cave loss zones, at 6644-6649 m and 7114.54-7114.65 m respectively, were bridged successfully by running the drill string once into the open hole. The five loss zones were all at the bottom of the well when mud losses were taking place. Success in mud loss control helped drill the well successfully.