ABSTRACT:

With increasing wellbore complexity, drilling into more technically demanding areas and increasing pressure to improve drilling economics, the requirements to minimize drilling fluid losses and optimize wellbore stability have become a necessity. In areas of hard and fractured shales these concerns become compounded especially as wells of higher inclination and drilling into depleted reservoirs are becoming more commonplace. In such formations natural fractures can readily extend and drilling-induced fractures occur, resulting in high risk of wellbore failure and/or lost circulation. The development of unique drilling fluid additives which can aid in increasing wellbore stability and minimizing fluid losses, coupled with the development and correct use of drilling fluids designed to optimize stability of the wellbore have been a key in achieving the desired operational goals. Developing efficient treatments and engineering techniques tailored to the formation requirements have been equally important. The net result of the correct application of these unique systems and additives has been to minimize the incidence of drilling induced lost circulation, to minimize fluid lost to formation, and to minimize drilling down-time required for treatment of losses and wellbore instability. The development and application of a uniquely engineered water-based drilling fluid is described that demonstrates enhanced shale-stabilization properties across an extensive variety of shale types, thus allowing water-based fluids to be used in applications previously reserved for invert emulsion systems. In addition, the development and use of a unique class of sealing additives is outlined, showing how further stabilization of the wellbore and minimization of losses can be accomplished. Unique laboratory test data on both the fluid system and sealing additives is shown along with initial data from the first field wells using the new system. The authors discuss the design of these unique drilling fluid additives, reviewing all aspects of additive selection. The treatment techniques required to ensure optimal wellbore stability and minimal induced fracture losses are also reviewed along with the engineering tricks required to ensure successful application of such treatments. The authors also review the results of field trials

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