Abstract

Laboratory experiments were conducted to study the factors that influence the successful placement of cement and resin plugs using thru-tubing dump bailers. The five-phase study included:

  1. cement slurry design,

  2. full-scale visual tests,

  3. full-scale cured plug tests,

  4. shear bond tests, and

  5. gravel pack penetration tests.

This paper presents results from the test program and provides data on commonly used dump bailer practices and equipment. Recommendations are made for placing cement and resin plugs utilizing dump bailers. This information should be useful to operators in planning and completing successful thru-tubing plugbacks.

Introduction

Oilfield operations often require that cement or resin plugs be placed downhole. Reasons for this operation vary, ranging from isolation of zones to water shutoff to well abandonment. Frequently, operators only need to place small volumes of cement or resin without pulling the existing well completion; this can be accomplished by using thru-tubing dump bailers to convey and dump the plugging material downhole.

Dump bailers are relatively simple pieces of equipment and are generally classified into two types: gravity and positive displacement. Figure 1 schematically shows these two types. Gravity bailers dump their contents by using an explosive blasting cap to shatter a frangible nose at the bottom of the bailer. The material then dumps by gravity, provided the density of the plugging material is greater than the density of the surrounding fluid.

Positive displacement bailers release their contents under the urging of a spring-assisted weight. The initial movement of the weight causes a pressure rise inside the bailer which opens ports at the bailer bottom. The propelled weight then displaces the bailer contents through the open ports as it travels through the inner diameter of the bailer.

Two general situations exist when performing thru-tubing dump bailer operations:

  1. the plugging material may be dumped inside casing, or

  2. the plugging material may be dumped inside a gravel pack screen and allowed to migrate into the sand pack outside the screen.

These are referred to as conventional and gravel pack plugbacks, respectively. In both cases the plugging material is often dumped on top of a thru-tubing bridge plug which was previously set. Figure 2 shows a schematic representation of these two situations.

Problems have been encountered in the field when trying to place plugs using dump bailers. These include failure to dump, incorrect fillup heights, and inability to hold pressure. In recognition of these problems, a laboratory study was undertaken to investigate the factors that influence the successful placement of cement and resin plugs using dump bailers. The test program encompassed five areas:

  1. cement slurry design,

  2. full-scale visual tests,

  3. full-scale cured plug tests,

  4. shear bond tests, and

  5. gravel pack penetration tests.

The information presented in this paper will include data on commonly used dump bailer practices and equipment, results of the test program, and recommendations for conducting plugback operations.

CEMENT SLURRY DESIGN

One of the major problems in designing a dump bailer slurry is the development of gel strength in the cement slurry.

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