Environmental Hazards and Mitigation Measures for Oil and Gas Field Operations Located in Urban Settings
Authors: Chilingarian G.V. , Endres B. , Pokinwong P. 
Published: July 2010

This paper presents a methodology for evaluating the Environmental hazards posed by gas migration from oil and gas reservoirs, or underground natural gas storage facilities, into the near-surface environment. Faults, fractured zones, and improperly completed or abandoned wellbores (e.g., due to poor cementing practices) are described as the primary pathways by which the gas can reach the surface. Furthermore, the gas migration problem com be exacerbated by subsidence, earthquake activity and well corrosion.

Soil gas monitoring, geochemical gas fingerprinting, and geological profiling are used in order to identify the magnitude and location of the environmental risks. Shallow and deep soil probes are used in order to define the near-surface hydrology, and to identify possible collector zones where gas concentration can build up to dangerous levels.

These techniques have proven to be important in the planning for and design of mitigation systems necessary to protect residential and commercial properties from the migrating gases. For example, some jurisdictions have imposed regulatory controls and design requirements regarding the insulation of gas mitigation systems. Safe operating procedures must be established for the operation of oil and gas fields, and underground natural gas storage facilities.

A number of case histories are discussed here that have been used by the authors to validate the methodology, and to illustrate the seriousness of the problem. A clear case is made for the need to perform ongoing monitoring for these conditions, especially in an urban selling

Introduction

The major paths for vertical migration of gas are provided by faults and fractures in the rock formations that overlie the reservoir which occur as a result of tectonic activities. Figure I illustrates some potential migration paths. Secondary collector zones for migrating gases are shown in Zone III of Figure I. In many geological settings, however, a fault can be discontinuous, but still allow the gas to literally hopscotch from one fault to another, or to act in conjunction with leaking wellbores in the same manner.

Wellbores of operll1ional idle, or abandoned wells often act as a pipeline for the flow of large volumes of gas to the surface. This is an especially serious problem where the annular space between the drillhole and the casing was not properly scaled with cement. Fractures (usual1y vertical) can also form during cementing operations hydraulic fracturing and acidizing. Vertical fractures may extend for many feet from the wellbore depending upon the lithology and the injection pressures used for placement of the cement. The cement will fill some of the larger fracture surrounding the casing but may not enter the smaller fractures away from the wellbore.

Sources of Gas for Migration

During the course of oilfield production, fluid is produced from the reservoir causing a drop in pressure. This liberates the gas held in solution, and allows the gas to migrate. The free gas can migrate upward due to differences in the specific weight of the gas and the surrounding fluids (viz.. upward buoyancy forces).