A Risk Based Management Approach to the Problem of Gas Migration
Authors: Schmitz R.A. , Cook T.E. , Ericson G.M.J. , Klebek M.M. , Robinson R.S. , Van Stempvoort D.R. 
Published: December 2007

R.A. Schmitz, Husky Oil Operations Ltd., T.E. Cook, Petroleum Industry Training Service, G.M.J. Ericson, Saskatchewan Energy and Mines, M.M. Klebek, Alberta Environmental Protection, R.S. Robinson, Saskatchewan Environment and Resource Management, and D.R. Van Stempvoort, Saskatchewan Research Council

Gas migration, i.e. the leakage of gas outside the wellbores of oil and gas wells, has been recognized for many years as a problem but is still not clearly understood. It can exhibit itself as gas pressure on the surface casing, gas migrating through the soil around the wellbore, or both. While it is probable that it occurs in other regions, the phenomenon is particularly visible in the Lloydminster area of Alberta and Saskatchewan, in part because of the high well density, as well as the fact that much of the land is under cultivation, which makes the effects on vegetation more evident. There are, as of yet, no technologies which can consistently demonstrate success in meeting the existing regulatory standards, which dictate "zero tolerance". Based on the available data it is estimated that it could cost hundreds of millions of dollars to eliminate the problem.

Research and development work is underway to assess the risks and impacts of gas migration on soil, the atmosphere and groundwater, as well as to improve technologies to eliminate, reduce or mitigate the problem with the ultimate goal of developing a risk-based management approach. While the problem is technically complex and the costs of meeting zero tolerance standards may be extremely high, initial research results indicate that the risks and impacts associated with gas migration, generally speaking, appear to be low.

This paper will give an overview of the problem and an update of the work which has been done, with particular emphasis on on-going work to assess gas migration through groundwater aquifers. The results of measurements of methane around leaking wellbores and the conclusions which may be drawn from the data will be described.

Introduction

The Lloydminster heavy oil area of Western Canada is centered around the border between the provinces of Alberta and Saskatchewan (see Figure 1). To date, roughly 20000 wells have been drilled to develop the heavy oil and shallow gas reserves located here, and, with the advent of new pumping technology capable of improving heavy oil well productivity, development activity is likely to continue in the area for many years to come.

Naturally, there are a number of environmental issues which have accompanied this development. For example, the viscous nature of the heavy oil which is produced necessitates close well spacing, resulting in land use concerns. Where feasible, the industry has responded to these concerns through the use of deviated and horizontal drilling, which decrease land use by allowing a number of wells to be drilled from one location, or by replacing a number of vertical wells with a more productive horizontal well. Increasing sand production which has come about as the result of new pumping techniques is also presenting a challenge to the industry; a challenge which is being met with innovative technologies such as sand washing or disposal in salt caverns. among others.

Perhaps one of the most challenging environmental issues is that of gas migration, i.e. the leakage of gas on the outside of the wellbores of oil and gas wells. This problem can exhibit itself as gas pressure in the surface casing annul us, gas migrating through the soil around the wellbore or both. Figure 2 gives a simplified schematic of the problem. Based on surveys done by Husky Oil, roughly 45% of the wells in the area show indications of gas migration in the surface casing, in the soil, or both. Extrapolating from these surveys indicates that some 9000 area wells would be affected

It is obvious from these statistics that the problem of gas migration is a significant one because of the number of wells affected. However, its importance is further magnified when the technical complexities and the costs of addressing the problem are considered.

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