Lost Circulation Material Design for Engineered Fracture Gradient of Drilling

Lost circulation material (LCM) is known to enhance the breakdown limit of wellbores. However, the related gain in the wellbore circulation density remains a weak link in the literature. A design scheme for the LCM blend is developed in this work that allows for quantification of the maximum enhancement in fracture gradient of wellbores. An analytical solution is developed for this purpose that accounts for wellbore inclination and azimuth. The LCM blend is selected by applying appropriate criteria for the particle size distribution (PSD) to the estimated width of near-wellbore fractures. A constitutive model for stress-strain behavior of the LCM is developed from in-house laboratory data of axial compression test on LCMs. The test is intended to simulate compression of the LCM plug upon partial closure of the fracture faces. The obtained constitutive model is used along with the presented analytical solution to estimate the fracture width, tip stress intensity and the wellbore stress redistribution after fluid filtrate leakoff and closure of fracture faces on the LCM agglomerate. The LCM blend composition is selected by optimizing the resulting variations in the fracture tip stress intensity and wellbore stress enhancement between the pre-leakoff (suspended particle) and post-leakoff (solid agglomerate) states of the deposited particles inside of the fracture. The compromise between fracture tip stress intensity factor and compressional stress redistribution around wellbore wall is shown to determine the extent of enhancement in the wellbore fracture gradient, i.e., the maximum circulation density that would not cause wellbore breakdown. Case studies with fracture gradient gains as large as one lbm/gal are presented.