Owing to the economic, well control and drilling efficiency problems it propagates, preventing or curing lost circulation has evolved into a critical issue for the industry. Accordingly, a less expensive drilling fluid system often is selected for a particular interval because it will mitigate the economic ramifications of downhole losses. Conversely, fluid systems are also chosen solely for their capacity to minimize losses downhole. Regardless of the application, special analyses are required to not only determine specific preventative measures for drilling fluid loss, but also for devising an appropriate treatment program to cure losses should they occur. Recently, specialized and chemically activated cross-linked pills (CACP) designed to stop whole mud loss have been developed and have shown significant advantages over conventional loss circulation methods.

This paper describes the development and application of the specialized cross-linked pills for controlling downhole mud loss. The authors will detail the laboratory methods used to formulated appropriate formulations, the placement procedures, and the successful field applications of this latest generation lost circulation control mechanism.

As detailed in the paper, formulations of the new cross-linked pills normally are tailored to specific downhole conditions of temperature and pressure to create a barrier between the loss zone and the wellbore. Further, they have been specially formulated to enable pumping through narrow drill pipe, coil, or mud motor dimensions and to "set" in a specific time frame. Chemical activators or retarders to control setting times achieve this.

These pills also contain controlled concentrations of particulate sealing agents that can be used selectively to either to bridge the loss zone at the formation face or allow deeper penetration into the fracture.

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