Coupled solid and fluid mechanics modeling of formation damage near wellbore

The infiltration of drilling mud below the bit and into the wellbore wall causes pressure gradients that significantly degrade drilling performance, wellbore stability and production. Due to heterogeneity, standard constitutive relationships and models yield poor predictions for flow (e.g. permeability) and rock properties (e.g. elastic moduli) of the invaded (damaged) formations. This severely reduces our ability to, for instance, estimate pressure build-up, optimize the mud cake properties or predict rock mechanical behavior. We propose a numerical model for permeability estimation in damaged formations near wellbore (e.g. sediments invaded by fines or sand crushing remnants). Grains of two length scales are present, but only larger ones are load-bearing. Detailed cemented granular packs were modeled using a discrete element method software, and ensuring mechanical stability. The particle positions and arrangement were available for subsequent pore throat network analysis. The standard network modeling approach for analysis of packing of nearly equal grains (Delaunay tessellation) cannot be used since grains of two different length scales create a high fraction of distorted pores. The main novelty of this work is adapting the network flow model to work with two length scales, and we present both the network creation and flow model in the multi-scale case. The effects of particle size and initial formation porosity on formation damage are studied in detail. Our study confirms that large particles tend to occupy the formation face, while small particles invade deep into the formation. Moreover, particles which are smaller than pore throats (entrances) impair permeability more than those larger than pore throats. Our study also indicates that a higher initial formation porosity leads to more particle invasion and permeability impairment Thus in order to reduce formation damage, mud particle size distributions should be carefully selected according to given formation properties. (C) 2013 Elsevier B.V. All rights reserved.