Lost Circulation Material Size Selection for Naturally Fractured Rocks

Abstract

This paper presents a scheme for designing lost Circulation Material (LCM) blends that are commonly used to mitigate loss of wellbore drilling fluid through natural fractures of rock formations. The method involves using the history of drilling fluid loss rates to estimate the mean aperture size of the formation natural fractures. This estimate is enabled via an inverse solution based on a nonlinear optimization algorithm to obtain the most likely value of the effective permeability of the natural fractures network. The inverse solution embeds a forward semi-analytical solution that models the displacement of formation rock fluid by the drilling fluid as the losses occur. The forward solution accounts for yield-power law rheology of drilling fluid and considers the formation fluid displacement occurring through a network of connected natural fractures, as opposed to a single isolated fracture. Once the mean aperture size is determined, a blend of selected LCMs may be recommended via a particle size criterion that optimizes the LCM particle size distribution for effective plugging of fractures. The overall LCM design scheme is discussed via a case study.