Abstract

Bauxite ceramics are a high strength proppant, which is often used for fracturing stimulation in deep and hot wells. Due to its higher specific gravity (3.5 - 3.6), it is very challenging to design and execute the post-fracturing proppant cleanout with coiled tubing (CT) successfully in the highly deviated large wellbore.

In a sand cleanout with CT the solids are moved by two modes of transport: circulation and wiper trip, in which the localized fluidization is caused by jet turbulence. The jetting turbulence is produced by the down hole wash tool, and as the CT is pulled from the hole it fluidizes and propels the solids in the direction of the wellhead. Some of these solids may stay in suspension and be circulated out of the well depending on the flow velocities, hole geometry and fluid choice. The remainder will tend to fall out of the flow channel downstream from the wash tool. These solids will be continually refluidised by the movement of the wash tool. The speed with which the wash tool can be pulled out of the hole is a complex function of the choice of fluid, flow velocities, hole geometry, hole deviation and the physical properties of the particles. When conditions result in complete removal of the solids, the corresponding maximum value of the CT speed is defined as the optimum wiper trip speed.

In this study, the solids transport test results with bauxite in a full scale flow loop are summarized and four case histories performed on both subsea and land wells are presented. The engineering, implementation and challenge of CT post-fracturing sand cleanout for each individual field case are discussed. The paper also describes how to use a customized down hole switchable wash nozzle and a sophisticated solids transport computer modeling software to optimize the post-fracturing sand cleanout process with 100 % removal efficiency.

Introduction

Many materials have been used for the fracturing treatments since five decades ago. Today's most commonly used fracture proppants include various sands, resin-coated sands, intermediate strength ceramics and bauxite. After the fracture treatment, some of the proppant is left behind in the well and has to be cleaned out before the well is handed back to production. The specific gravity of proppant could vary from 1.25 to 3.6. Therefore, it is very challenging to design and execute the sand cleanout successfully for such a wide density range of proppants, especially for a highly deviated wellbore.

Based on comprehensive research1–6, an effective CT sand cleanout methodology has been developed, patented7, and proved by the field operation8–12. The developed sand cleanout process includes a down hole wash tool and the methodology for CT in vertical, deviated and horizontal wells. The preferred down hole tool includes the jetting nozzles with switchable forward and backward facing jets. By selecting the backward facing jets and controlling pull-out-of-hole (POOH) speed (at rates determined by the associated particle transport software), the settled sand bed can be "swept" out of the hole with near 100% efficiency.

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