Migration of Gas from Oil/Gas Fields by J. O. Robertson, G. V. Chilingar, L. F. Khilyuk & B. Endres, Available Online June 7, 2012, Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 34:15, 1436-1447, DOI:10.1080/15567030903077899
Oilfields have demonstrated a long history of gas migration problems. Experience has shown that the migration of gas to the surface can create a serious potential risk of explosion, fires, noxious odors, and potential emissions of carcinogenic chemicals, such as BTEX.
….a bubble of gas can float upwards in a fracture even in a downward moving stream of water that has a velocity <0.22 cm/s. …
Actual rates of the upward gas migration depend on the permeability along the path of migration. Gas flow through fractures can be thought of as a “pipeline” type flow. In the case of an oilfield, large fractures, faults, or man-made channels (wellbores) can serve as gasfilled paths of migration, or “pipelines.” This type of gas flow occurs in the gas-filled zone between the surface and the top of the water table. The force moving the gas is the potential drop between the point of origin and the end of the path or channel. The volume of migrating free gas along continuous gas-filled paths is dependent upon the width of the gas-filled fracture. In general, gas migration through gas-filled paths is capable of producing significant quantities of gas. The volume of gas found at the surface is dependent upon the mechanical aspects of the path traveled by the gas and the pressure gradient. Physical features of the path, such as fracture width, length, saturation with water, tortuosity, absolute and relative roughness of the fracture walls, and so on, have a great effect upon the quantities of gas that can move along the path. … As discussed under “Paths of Gas Migration,” man-made channels (wells) and fault zones are the most likely zones where large volumes of migrating gas flow will occur. … The major paths for vertical migration of gas are faults and fractures. These breaks or channels in the rock may be formed by natural lithification processes and tectonic activities. Additional avenues of migration may be created during drilling operations, which not only create a hole (or wellbore) through the strata but also cause fracturing of the formations around the wellbore. This problem is often compounded by inadequately sealing the space between the wellbore and the well casing with cement or improperly filling the fractures around the wellbore itself in an attempt to prevent future gas and liquid migration (poor cement bonding). Also, production of petroleum can cause subsidence that can generate faults and vertical fractures along which gas migration can occur. Fractures are created in the layered rock as a direct consequence of many oilfield operations, such as drilling, cementing, acidizing, and repressurizing of wellbores and depleted reservoirs. These fractures are potential paths for the migration of gas. Wellbores of idle or improperly abandoned wells often result in literal pipeline flow of large volumes of gas to the surface. This is particularly important where the abandoned wells were not properly sealed to prevent gas to migrate along the old wellbore. At the time of drilling the well, many fractures are formed owing to the percussion activity of the drill bit. In addition, cementing operations pressurize the wellbore further with the potential of creating additional fractures. During the cement squeezing operations, the wellbore may be even further hydraulically fractured. These vertical fractures may extend for tens of feet from the wellbore depending upon the characteristics of the formation and the injection pressures used for placement of the cement. The cement will fill some of the larger fractures surrounding the casing, but the cement particles cannot enter the smaller fractures away from the wellbore. The end result is that a fracture system is created parallel to the wellbore forming a potential path for the migration of gas.
Acidizing, a common practice in revitalizing oilwells, causes acid to enter the pore channels and fractures of the reservoir, usually under high pressure, fracturing and etching (widening) of pre-existing fractures. Over time, the ring of cement around the well casing is damaged by deformation (caused by tectonic and seismic movements and subsidence owing to oilfield operations) and by chemical decomposition. Formation fluids usually corrode both the cement and casing, generating additional paths for future migration of gas and fluids by breaking down the intended cement seal. The breakdown of the cement can open paths for the flow of gas and liquid behind the casing. Corrosion also results in casing leaks (holes). Idle and/or abandoned wells are likely candidates for channels of communication to the surface of gas because of their deterioration with age. … Any holes in the casing and the surrounding cement sheath in the annular space between the casing and the wellbore, within the calculated “zone of escape,” will result in gas leaking out of the wellbore and then toward the surface. … Initially, if the gas is held in solution in an oil reservoir, it is not free to migrate. During the course of oilfield production operations, as fluids are produced, the drop in pressure liberates the gas held in solution. This freed gas replaces the produced fluid. The free gas can migrate upward (due to differences in the specific weight between the gas and the surrounding fluids) to the top of the reservoir…gathering under the caprock where it forms a free gas zone.
The lower explosive limit of the oilfield gases (composed primarily of methane) is approximately 5% by volume when mixed with 95% by volume of air. This mixture of gases is a serious explosion and/or fire hazard, especially where this gas is capable of migrating into a confined space, such as a room or an electrical vault. A clear recognition of a very serious problem with oilfield gases migrating to the surface and causing an explosion was the incident in the Fairfax area of Los Angeles on March 24, 1985, which demolished the Ross Department Store and injured over 23 people (Cobarrubias, 1985). Escaping oilfield gases along the fault zone burned for days through cracks in the sidewalks and within the parking lot surrounding the store. [Emphasis added]