Abstract

Earlier laboratory experiments conducted with perforated, gravel-packed core samples (low compressive strength, high permeability Castlegate) had shown the importance of clean perforation tunnels in maintaining gravel-pack productivity. Flow data for these tests indicated that the debris and loose sand in the tunnel was responsible for the low post-gravel-pack productivities, in this paper a theoretical investigation of the impairment mechanism is presented. Sectional photography and detailed scanning electron microscope (SEM) analyses were conducted on the gravel-packed core samples consolidated in-situ with clear resin. Numerical simulations of the experiments were conducted to quantify the extent of damage and permeability reduction induced by the perforation generated debris.

SEM analyses showed a high degree of invasion of smaller particles into the gravel-pack (inside the perforation tunnel), resulting in permeability reduction in perforations that were not cleaned out prior to gravel packing. Size analyses showed the invading particles were smaller than the virgin rock matrix particles. Experimental observations indicated a time, rate and flow dependent invasion that was temporarily reversible with injection, indicating a migration type impairment phenomenon. Numerical simulation of the gravelpacked perforations showed small but very low permeability regions around the perforation tunnel. The fines invasion of the gravel-packs resulted in large frictional pressure drops in the narrow casing-cement entrance holes. Comparison of numerical and experimental flow results showed higher turbulence factors (Forchheimer's equation) for the impaired gravel-pack in the perforations.

Introduction

This paper follows up the work presented previously involving laboratory experiments to investigate perforation-induced gravel-pack impairment. Ideally, gravel-packing should not result in any decline in productivity, but field observations have shown otherwise, especially in the case of cased hole or internally gravel-packed wells. Controlled field studies have also indicated that perforations could be responsible for the poor gravel-pack performance. Tests were conducted in the laboratory to study the flow performance of perforated (single shot), gravel-packed outcrop rock samples. Visual observations showed perforations filled with rock debris that was not removed by drawdown-induced flow. Comparison of post-gravel-pack productivities revealed lower values for realistic (debris and loose sand in the tunnel removed by drawdown-induced flow) perforations than ideal (debris and loose sand removed by artificial means) perforations. Flow and pressure drop measurements revealed a rate dependent steady decline of productivity for the realistic perforations, leading to the conclusion that clean perforations are essential for minimum gravel-pack impairment.

The current study focuses on strengthening the conclusions from the flow data analysis and also on exploring further the mechanism of perforation-induced gravel-pack impairment. Experimental results from four types of perforations are considered here: ideal, realistic, abrasi-jetted (perforation formed by an abrasive sand slurry) and drilled hole. Prior computer aided tomography (CT) scans and thin section work on perforations have revealed a small region around the perforation tunnel with highly fractured grains with a size distribution smaller than the virgin rock, usually referred to as the crushed zone. Detailed SEM analysis was conducted on sections from the core samples. Results are provided for realistic, ideal and abrasi-jetted perforations to compare and contrast the extent of damage at the gravel-formation interface.

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