Many abandoned gas wells in western Canada leak gas to the surface. This paper presents the results of an experimental study to determine the possibility of using an asphalt-in-water emulsion as sealant for these abandoned leaky wells as a more efficient and cost-effective alternative to the conventional cement grout barrier. A second application of this process would be to hinder the water flow from water producing formations.
The asphalt-in-water emulsion used in the experiments had asphalt particles with an average particle size of around 4 µ m and a 60 percent internal phase concentration. The emulsion exhibited a pseudo-plastic behaviour. Its viscosity ranged from 60 cp at a shear rate of 60 rpm to 145 cp at a shear rate of 6 rpm. The emulsion was found to be stable, even though it settled with time. Its original properties were retained after agitation and settling had occurred.
Core flow experiments were performed in order to simulate injection in the near-wellbore region. The cores were prepared using silica sand with a 60- to 80-mesh size. The resulting cores had a porosity range of 25 to 32 percent and a permeability range of 2.5 to 9 Darcy.
Experiments were performed under varying injection pressure conditions. In each case, solid plugs were formed and their length in the porous medium increased as a function of the injection pressure. In order to extend the length of the plugs, several anionic surfactants were tested as pre-flush solutions for conditioning the surface of the sand grains. This resulted in increasing the emulsion penetration into the cores.
The basic mechanism of the process is to pressure up the emulsion next to a formation, forcing the emulsified asphalt to enter the porous medium and to form a flexible sealing plug. We surmise that the plug is formed through a combination of two pore blocking mechanism: one by blocking the pores by the larger-than-pore droplets, and the other by adsorbing onto the sand grain surfaces.
The experimental results have shown that the asphalt-in-water emulsion can be successfully used to seal a gas or water producing formation. The study concludes with recommendations for field application and suggests ways for further research to improve the process.
Proper well abandonment is the final and critical step in the process of natural gas and oil exploitation. Depleted oil and gas wells produce small non-economic amounts of gas, often the result of gas migration either through the cracks, fractures and voids in the formation along the outer wall of the casing, or between the original cement and the formation. The gas may escape to the atmosphere by flowing around the surface casing or migrate into shallow aquifers.
The current procedures for well abandonment involve placing a cement plug over the production zone and maintaining a column of water from the top of the plug to the surface. Sometimes, these procedures may not seal off all of the producing zones and remedial well abandonment treatments are needed.
