Abstract

The uncemented annulus of an oil and gas well is a possible pathway for migration of thermogenic methane into the shallow subsurface. Poorly cemented wells allow for methane to migrate upwards from the producing formation and accumulate in the annulus between the production casing and the surrounding rock matrix. Additionally, methane from intermediate formations above the target formation may also leak methane into the annulus if they are not sealed off by cement. In the United States., the annulus of the well is capped at the surface by the bradenhead valve to prevent the venting of methane to the atmosphere. However, this valve allows gas to build and increase the pressure above the fluid in the annulus. In this work, we investigate how the buildup of bradenhead pressure influences methane migration in the subsurface. We present a two-dimensional model that simulates single phase flow of dissolved methane away from the wellbore. We consider best and worst case scenarios and we vary the boundary conditions to represent both closed and open wellbores. Our results show that the buildup of pressure in the annulus of a wellbore has a very small impact on methane migration. Of the parameters we consider, the density of the fluid in the wellbore has the greatest influence on methane migration.

1. INTRODUCTION

Over the last ten years, the extraction of resources from unconventional formations has fueled a significant rise in United States oil and natural gas production [1, 2]. Public concern with the potential environmental impacts of the extraction process has increased as well. In particular, debates have arisen about the fate of the chemicals used in the high volume slick water hydraulic fracturing process [3]. As a result, a number of studies have begun to investigate the quality of groundwater in regions with heavy unconventional oil and gas development [4, 5, 6, 7, 8].

These studies have found varying degrees of methane in the shallow subsurface in the vicinity of oil and gas wells [4, 5, 9]. Although methane itself is not hazardous to human health, unless present in explosive quantities, its isotopic signature provides insight into the origin of the gas. Methane formed through intense heat and pressure (thermogenic) and methane formed by microbes (biogenic) can be distinguished from one another through analysis of the carbon and hydrogen isotopes of the methane molecules [10]. These properties make methane an attractive tracer that could indicate contamination from oil and gas drilling operations. If thermogenic methane is found in a drinking water aquifer, its presence suggests that a flaw in the oil and gas well may exist or a permeable pathway may be present between the producing formation and the shallow subsurface [4, 11].

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