Permeability Estimation of Damaged Formations Near Wellbore

Abstract

AbstractWe propose a numerical model for permeability estimation in damaged formations (e.g. sediments invaded by fines or sand crushing remnants). Grains of two length scales are present, but only larger ones are load-bearing. Porous media were modeled in Itasca's PFC3D, ensuring mechanical stability and making the particle positions and arrangement available for 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 portion 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.Despite some simplifications in our initial modeling, it may nevertheless serve as a tool to predict formation damage according to given parameters, and to understand the mechanism of formation damage from a microscopic point of view. Further, while our motivation is formation damage during drilling, the modeling presented is applicable to any situation where migration of finer particles impairs a granular or sandstone medium.