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As the natural degradation of organic matter, methane can be produced naturally in soil and other areas underground. Gas migration is the movement of methane (main gas) or other gases from a subsurface zone to other places through a barrier. This barrier is either natural, such as overburden, shale or artificial such as cement sheath, external casing packed, pipe joints, plug, etc .Gas migration has long been recognized as one of the most troublesome problems in the oil& gas industry due to the potential risks to health, safety and the environment. Gas migration may lead to an explosive atmosphere in limited spaces. Back in 2007, a Virginia dairy farm's manure pit killed five people, including a Mennonite family, in tragic sequence as each tried to save the other. Needleless to say, gas migration is also a source of greenhouse gas emissions.
Figure 1 Examples of gas migration
In oil & gas industry, gas migration refers to the invasion of formation gas into the cemented casing/borehole annulus due to a pressure imbalance at the formation face, where the gas may migrate to a lower pressure zone or possibly to the surface, creating permanent channels and weakening cement compressive strength as shown in Figure 2[2,3]. There are two major types of gas migration: short-term and long-term. Short-term gas migration occurs before the cement sets, and long-term gas migration develops after the cement has set .
Figure 2 Gas Channel Migration
Successfully cementing a well that has potential for gas migration involves a wide range of parameters: fluid density, mud removal strategy, cement slurry design (including fluid-loss control and slurry free water), cement hydration processes, cement-casing-formation bonding and set cement mechanical properties . Gas migration through hydrating cement slurry is a major reason for well completion failures. From a liquid cement slurry to a solid cement sheath, cement will go through a gel state. During this state, the cement column can lose the ability to transmit hydrostatic pressure effectively against formation pore pressure and some formation fluids may flow into the gelled system that has the potential of destroying barrier. So some additives are also used in cement to shorten gel state time.
In order to avoid the potential risks to health, safety and the environment of gas migration, it is necessary to test gas migration of the oil and gas well regularly. Alfa Chemistry has established program and advanced equipment which can accurately test any gas migration. We also have experienced technical teams in the aspect of gas migration analysis. No matter what your concern is, we are always ready to help. If you have any problems regarding gas migration, please contact us without hesitation.
Author unknown (2013) Gas migration preliminary investigation report.1-17.
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Nikiforuk A, (2013) Shale Gas: How Often Do Fracked Wells Leak? TheTyee.ca
Parcevaux P, Rae P, Drec P (1990) 8 Prevention of Annular Gas Migration. Developments in Petroleum Sciene Vol 28, 1-3.