Abstract

Recently conducted small (laboratory) scale experiments with fluid injection in unconsolidated sand packs revealed various patterns of fluid flow. Some of these patterns depart from that commonly discussed in literature. The experiments have been conducted in a pressure chamber under true-triaxial confining stresses with two types of sand packs: (a) homogeneous and isotropic; and (b) a layered, transverse-isotropic sand pack with fluid injection into a thin sand layer between two weakly permeable or impermeable confining layers. Tests were designed to investigate how fluid injections below and above the minimum confining stress affect the flow behavior in the sand pack and injectivity. Under matrix injection conditions (i.e., injection pressure below minimum confining stress), the initial sand pack structure is practically undisturbed. Under fracture injection conditions (i.e., injection pressure above the minimum confining stress) using a clear low viscosity fluid a heterogeneous and/or anisotropic fluid flow pattern is observed in both types of sand pack. Heterogeneous fluid flow in such cases is associated with the formation of large scale perturbations of the initially homogeneous sand packing in the regions adjacent to the wellbore. Under fracture injection conditions using a solids-laden fluid, a very different fluid flow pattern results. Specifically, highly localized fluid flow pattern through narrow crack-like openings is created in the sand pack, where the ‘cracks’ have a clear filter cake on their surface. This paper briefly reports various test conditions and their corresponding fluid flow patterns, discusses the underlying physics, and outlines a mathematical model of the observed fluid flow through unconsolidated sand under matrix and fracturing conditions.

1. INTRODUCTION

Water injection is widely used for enhancing the transport of hydrocarbons in waterflooding operations, for restoring depleted reservoir pressure, as well as disposing of waste water. A high rate injection is usually required to achieve the objectives of waterflooding. Loss of injectivity can be a significant problem in some unconsolidated sand formations. Understanding the root cause(s) and the development of a predictive model of such phenomena are required for solution of the problem.

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