ABSTRACT:

It is believed that coupled fluid-mechanical-erosional phenomena associated to sand production derived from microscale based mechanisms as the disaggregation of particles from the rock matrix and their transport through the pores are best described at the particle and pore level. The present work presents results obtained from a numerical simulation study of these phenomena at the pore scale level. This study was carried out by coupling the discrete element method for the analysis of the mechanical behavior of the rock and lattice-Boltzman method for the simulation of fluid flow at the pore level. Computer codes were written to simulate both processes using the procedures described. One objective of the work was an improvement in the knowledge of micromechanical processes leading to solids production. It is believed that such understanding will improve the constitutive relationships necessary for sand prediction and used in continuum based methods of prediction, more amenable for applications in real case situations. Rock properties and texture, boundary conditions (stresses and drawdown) in the disaggregation and particle transport were considered. Preliminary results of the DEM-two phase LBM coupling are also presented. Advantages and limitations of this micromechanical approach are also discussed.

1. INTRODUCTION

The different processes involved in sand production are generally associated to deformation and failure of the rock, rock-fluid interaction, and solids transport. Sand production can be described by two mechanisms [1]:

1. Mechanical instabilities and localized failure of the rock in the vicinity of the well due to stress concentrations;

2. Hydromechanical instabilities due to erosion which manifests in the disaggregation and particle transport caused by seepage forces.

These two mechanisms are coupled as stress concentrations lead to localized failure which in turn leads to amounts of displaced particles transported by the fluid. The displacement of particles leads to an increase of rock porosity causing a readjustment of the interparticle forces leading to a further increase in the failed zone of the rock mass. Models that try to predict sand production rates and produced volumes are generally continuum based, incorporating fluid-mechanical coupling and erosion phenomena [1, 2, 3, 4]. For the solution of these problems it is necessary that a constitutive relation for the eroded mass of solids is established. Such relations are inspired in the filtration theories for fines through a solid matrix. Besides the constitutive relation for erosion (relating the erosion process to pore flow), relationships between mechanical processes (deformation and failure) with erosion have also to be established. All the mechanisms that couple mechanical, flow and erosion happen at the microscopic scale as disaggregation of a particle from the rock mass and its transport through the pore structure is best described at the particle and pore scale. A better understanding of such basic mechanisms may help substantially in the improvement of the constitutive relations necessary for sand production prediction. Numerical modeling at the particle and pore scale is a tool that may be useful in the study of the fundamental mechanisms as described for sand production.

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