A Bulk Expansion Pressure-bearing Lost Circulation Process Suitable for Use in the West Wing of Tianhuan Collapse in Ordos Basin

In the west wing of the Tianhuan Collapse in Ordos Basin, Changqing Oilfield, mud losses have often been encountered when drilling to the Luohe Formation and Liujiagou group, in which fractures (both vertical and horizontal) are very developed and formation water is active. Rates of mud losses resulting from pressure differential are in a range of 20-100 m³/h. Conventional methods seldom succeeded because of the difficulties in controlling this kind of mud losses. Many kinds of lost circulation material were tested in laboratory and a bulk expansion lost circulation material, HSW-1, was selected. In controlling mud losses, HSW-1 slurry was first spotted into the thief zone to form a pressure bearing barrier to protect the thief zone from being eroded by mud flow. Cement plug was then spotted into the thief zone protected by HSW-1 slurry to completely stop mud losses. HSW-1 when in contact with water will expand to 500-800 times of its original volume, and the pressure bearing capacity of the set HSW-1 slurry is 11 MPa at test pressure of 7 MPa. HSW-1 works normally at 120℃, and time for the HSW-1 to set is 2-6 min. A special tool has been developed for the targeted delivery of HSW-1 to the place where HSW-1 is required because of its short set time, and success rate of mud loss control with HSW-1 can thus be increased.