Lost Circulation Control with Idea Packing on Well Wenchu-6

The well Wenchu-6 is a pilot test well for the XX gas storage in Tuha oilfield. This well penetrated in its 3rd interval (reservoir section) the Xishanyao Formation, an ultra-low pressure depleted formation with formation pressure coefficient of only 0.25-0.27. This low formation pressure plus fractures generated in the sandstones by fracturing job in wells nearby easily led to lost return of drilling fluids during drilling. To ensure the success of coring in the reservoir section and well cementing, a mud loss control technique based on ideal packing was adopted involving the optimization of bridging particle size distribution and the use of several special lost circulation additives such as delayed hydration and swelling particles, elastic graphite particles and high-efficiency rigid bridging particles. Several times of mud losses during coring and well cementing in the 3rd interval have been cured with this technique. Four whole drums of cores with length of 32.2 m were taken in the sections of mud losses. Before cementing the 3rd interval, the well was treated with drilling fluid containing the lost circulation additives as said above. The lost circulation additives were then screened out of the mud and the borehole formation was tested for its pressure bearing capacity; it was 5.8 MPa (maximum). No losses were ever encountered during well cementing with cement slurries of normal densities. The successful practice of the lost circulation control technique based on ideal packing theory provides a new clue for lost circulation control in similar sandstones of ultra-low pressures and a technical support for efficient development of the gas storage in subsequent operations.