This paper describes a fluid system developed to build integrity continuously to prevent lost returns while drilling. The primary attributes of the fluid that enable this are high solids content and extremely high filtration rates, as reflected in API fluid loss tests. It is referred to here as a drill and stress fluid (DSF). In field applications, DSF water-based systems appear to be effective over a wide range of conditions. Circulating pressures have been sustained that exceed integrity at the bit by 1.0 to 3.0 ppg without detectable losses in depleted formations with permeability ranging from 1 to 1,000 md and pay zones of 50 to 700 ft (165 to 2,300 m) in length. The mechanism through which the DSF is believed to arrest the growth of lost returns fractures and build near-wellbore stress is described. Generalized design criteria for a DSF system are presented and the assumed relationship between the design parameters and fluid performance is discussed. The results of the application of DSF are presented for eight wells, including post treatment evaluation logs of the drilling-induced fractures created while building stress. Operational practices that facilitate the safe use of an extremely high fluid loss system with overbalance exceeding 2,000 psi are also discussed. Introduction In the mid 1990's, the operator developed a family of practices for managing lost returns that are referred to as the Fracture Closure Stress (FCS) Operational Practices (Dupriest 2005). These practices are now applied uniformly worldwide by operator's affiliates, and the success rate in permeable formations is very high (Dupriest 2005). However, the FCS Practices are discrete treatments as are the majority of industry concepts. Drilling operations must be interrupted to position a discrete pill at the loss zone and non-productive rig time accumulates as the operation is performed. The drill and stress fluid concept is built on many of the basic principles proven in the development of the discrete FCS Practices. However, in the DSF process, the integrity and increase in near-wellbore stress are built continuously through specific attributes of the drilling fluid so that rig time is not lost. The rig time cost savings from continuous treatment is significant. Even greater costs may be incurred if it is necessary to run a string of casing immediately above a low integrity formation to prevent underground flow or borehole collapse following lost returns. When losses occur, the bottomhole pressure immediately declines to equal the fracture closure stress (FCS) of the induced lost returns fracture, which approximately equals the far field stress. The fracture essentially acts as a pressure relief valve and it is not possible to elevate the bottomhole pressure by filling the annulus continuously, regardless of fluid density used. If the FCS in a depleted reservoir is less than the pore pressure in shallower formations, underground flow occurs downward to the loss zone. If the FCS is less than the pressure required to stabilize the exposed shale intervals, the borehole will collapse and the string becomes stuck. Continuous treatment is also needed if long intervals of low stress formations must be penetrated and equivalent mud weight (EMW) cannot be cut below the FCS so that losses occur in every increment of new hole drilled. The operator's discrete practices have been effective in stopping these losses, but they return when drilling resumes and new, low stress, formation is exposed. This is a common issue in high angle, extended reach wells due to the long measured depth required to traverse even relatively thin sands.

This paper describes how intentionally increasing the permeability of a non-aqueous fluid (NAF) filter cake can enhance treatments to free differentially stuck pipe and recover from lost returns. NAFs are well known for their

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