Abstract

Methane gas is emitted from surface casing annuli and surrounding soil at many oil and gas wells in the Lloydminster area. Attempts to stop this leakage have been mostly unsuccessful, and environmental and safety concerns have been unresolved partly because of the lack of any quantitative assessment of the magnitude of this leakage. This project is aimed primarily at providing a quantitative estimate of the magnitude of methane emissions from these oil and gas wells.

Leakage rates from surface casing vents have been relatively easy to measure, using conventional flowmeters, and estimating this contribution to the total leakage has consisted primarily of data gathering and statistical evaluation. Measured surface casing vent leakage rates vary from 0.01 to about 200 m3/d. There does not appear to be any consistent variation in flow throughout the year; some surface casing vents vary by an order of magnitude in a random way with time, others are relatively constant.

Soil gas migration, the leakage of gas through soil in the vicinity of oil and gas wells, is more challenging to measure. Following a review of relevant literature, a direct measurement technique using a surface emission flux chamber was selected as the primary approach for this project. Measurements have shown that soil gas leakage rates are mostly less than 0.1 m3/d, and no wells have been found with methane leakage greater than 60m3/d.

A simple numerical model has been used to help understand the relationship between soil gas composition and leakage rate. Lab experimentation has confirmed that bacterial oxidation reduces methane leakage rates in the soil around wells. Results from this work help to assess the importance of these methane emissions relative to other natural and man-made sources.

Introduction
Background

Whenever a hole is drilled into the earth's surface, there arises the possibility that fluids previously trapped by impermeable layersmay now migrate to shallower zones or to the surface. Minimizing such uncontrolled fluid flows has been of concern since the beginning of oil and gas drilling, and much effort has been expended to improve well completions to meet this goal. Although major leakage is now rare, preventing all flow behind casing has continued to be an elusive target.

Specific problems with shallower wells, particularly heavy oil and gas wells in the area around L1oydminster, began to be identified in the 1960's. As more and more wells were completed, two types of leakage were noted to be common. For wells completed with surface casing, gas leakage was often detected from the vent on the annulus between the production and surface casings (see Figure 1). Leakage of gas was also found outside the outer casing (either surface or production) in some wells; this latter kind of leakage has been defined as "gas migration."

Although the regulations governing completion, operation and abandonment of oil and gas wells differ in some details between Saskatchewan and Alberta, the net results of these regulations are similar in the two provinces.

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