Existing borehole stability analysis tools are based on an assumed, fixed, circular borehole geometry. In a strict sense, these tools are valid only as long as the borehole remains circular. They cease to be valid and their applicability becomes limited when borehole breakouts or fractures are present at the borehole wall. Reliance on these methods can limit the ability to evaluate borehole stability and determine the safe mud weight window to the geometry or geometries on which the model being used is based-a circular borehole. When breakouts or fractures have formed, a different model is required to analyze the conditions around the borehole, to determine required mud weights, and to evaluate methods that can be employed to restabilize the borehole. The authors demonstrate this effect by analyzing and evaluating two specific borehole geometries-the circular borehole and a borehole intersected by an induced or natural fracture. Using existing borehole stability theories and basic hydraulic fracturing analysis, the authors review and evaluate various means available to alter the size of the mud weight window. Of particular interest are the two limiting cases: a circular borehole with no defects and a drilling-induced or natural fracture intersecting the borehole. Mechanisms reviewed include chemically altering the rock's mechanical properties or altering the borehole's surface characteristics, creating impermeable bridges within existing fractures, plugging existing or induced fractures with high-viscosity deformable or undeformable materials, and permanently sealing fractures with "rigid" cement. Calculations presented demonstrate the "borehole strengthening" that can be achieved by strengthening the rock matrix or by plugging an existing natural or induced fracture using materials that are deformable and do not rigidly adhere to the fracture walls (such as extremely viscous gel-like materials), materials that are not deformable (such as cement) and require time to cure, and materials that glue the fracture walls together.
1. INTRODUCTION
Lost circulation during drilling operations continues to be a significant problem in the oil and gas industry. Control and remediation of lost circulation can require large expenditures and, if impossible, can result in the loss of the well. Lost-circulation problems will continue to plague the industry as it drills for oil and gas in reservoirs or fields with partially depleted sands that have to be penetrated to reach deeper productive intervals. Effective means to assess lost-circulation technologies are required.
Lost circulation can occur in a number of ways. It can occur gradually through leakoff; abruptly as we penetrate into subsurface voids, rubbelized zones, and high-permeability intervals; or through the sudden initiation and continued propagation of a fracture. The remainder of this discussion focuses on the last of these lost-circulation issues-lost circulation through fracturing. Using existing borehole stability theories and basic hydraulic fracturing analysis, we review and evaluate various means available to help eliminate or remediate fracturing-related lost circulation. Of particular interest are two limiting cases:
Avoidance of fracture initiation in a circular borehole with no defects
?Arresting or mitigating drilling-induced or natural fractures intersecting the borehole
Mechanisms reviewed include chemically altering the rock's mechanical properties, altering the borehole's surface characteristics, creation of impermeable bridges within existing fractures, plugging existing or induced fractures with highviscosity deformable or undeformable materials, and permanently sealing fractures
