THE EFFECT OF GAS MIGRATION ON CEMENT SHEATH

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THE EFFECT OF GAS MIGRATION ON CEMENT SHEATH

CHAPTER ONE

INTRODUCTION

Background of the study

Gas migration can occur in the earth’s crust as either a continuous or discontinuous phase through porous, water-filled media to the surface. The direction of gas migration in water-saturated formations is generally vertical (upwards or toward the surface) as the primary force moving the gas is the difference between the specific weights of gas and water (buoyancy). There are many examples around the world where gas migration to the surface, from oil- and gas-bearing formations, has created serious environmental hazards (i.e., explosions, fires, noxious odors, and potential emission of carcinogenic chemicals). In highly populated areas such as Southern California, previously undeveloped surface areas overlying oil fields have become more valuable and are currently being developed. As a result, environmental problems often occur today where there were few in the past. This migrating gas may be entrapped under paved streets, parking lots, and structures leading to severe environmental problems. An example of this type of problem occurred on March 24, 1985, when the Ross Dress for Less Department Store in the Fairfax area of Los Angeles, California, exploded, causing injury to over 23 people and the destruction of the store. The migrating gas accumulated in the ground under the paved areas and commercial structures. The entrapped gas was able to migrate into a confined space within the structure and was ignited by a spark generated by a time clock when an employee was checking out. The fire burned for days through cracks in the sidewalks and around the foundations of structures. It was difficult to extinguish the fire because the migrating gas continuously reignited the flames.

A serious environmental hazard can also exist because natural gas migration (primarily methane) can serve as a carrier for carcinogenic chemicals such as benzene. These problems also relate to the development of sites overlying gas storage reservoirs. The underground storage of natural gas in depleted oil fields (reservoirs) has now become a common practice in many urban areas. The reason for increased use of gas storage reservoirs is that peak demand deliverability (e.g., during cold weather) can be satisfied without dramatically increasing the size of interstate transmission lines. Underground gas storage facilities in abandoned oil fields, however, have demonstrated a long history of gas migration problems. There are many examples of environmental hazards associated with the escape of gas from these reservoirs. Wellbores, faults and fractures, as well as leaking (fractured) reservoir caprocks can contribute to the migration of gas. The risks of gas leakage must be continuously examined over the entire lifetime of a storage project. Wellbores can develop leaks as a result of corrosion of the casing, deterioration of the cement seals, and mechanical damage resulting from seismic activity.

In summary, it is imperative to understand the mechanisms and processes involved in gas migration from underground storage to the earth’s surface in order to reduce the risk of injury and property damage in urban areas. Furthermore, changes in the rate of gas migration, as discussed previously, give important clues as to local tectonic activity.

Statement of the problem

There may have been previous researches in this subject. This work gives further explanations and analysis in the effect of gas migration on cement sheath