Subterranean hydrocarbon deposits often contain Low-molecular-weight gaseous hydrocarbons at various pressures, temperatures and depths. Failure to prevent the invasion of this gas into a cement slurry during completion can result in numerous problems. A few of the known manifestations of gas channeling are poor zonal isolation, annular pressure buildup, loss of hydrocarbons to non-recoverable zones, required squeeze cementing and flow of gas to the surface.
The invasion of gas into cement slurries has been studied using a simulated well bore model. Results indicate that a cement slurry loses its ability to transmit pressure with time. This loss is caused by gel structure development due, in part, to cement hydration and fluid loss. Gas flow can be initiated when the pressure transmitted by the fluid column becomes less than the gas pressure. The relationship between gas flow and this pressure differential has been determined for several cement systems. This relationship has been termed "gas conductivity" and is a measure of gas permeability of cement slurries prior to the development of compressive strength.
Two different design approaches were investigated in order to reduce gas conductivities in cement slurries. One involves inhibition of gas flow by the deposition of an impermeable cement filter cake against the formation. The second design incorporates a modified cement slurry which interacts with incoming gas to form an impermeable barrier in the cement pore spaces, thereby inhibiting further gas flow. The use of this "gas induced" cement barrier to prevent gas flow has been successfully applied in Canada, Europe and the United States. Several case histories are discussed.
References and illustrations at end of paper.
In most cementing operations, gas pressure is controlled by pressure exerted by the fluid column. The pressure differentials across zones or to the surface in these wells are low enough that the cement slurry contains the gas within the formation until the cement is completely set.
In cases where gas pressure is near the designed hydrostatic pressure exerted by the total fluid column, even a small loss of annular pressure can lead to gas migration. If the cement does not develop enough strength adjacent to the gas-bearing zone before static pressure drops below the gas pressure, channeling will occur. The channel will propagate through the semi gelled cement column and will not close, due to the lack of cement fluidity and flow rate of gas. Several approaches have been suggested to prevent gas migration after cement placement. An excellent literature review has been made by Beirute and Cheung.1 This study deals with the prevention of gas migration into a cement slurry as it changes from a fluid to a "semisolid" mass. This transition occurs during cement hydration and can be accompanied by fluid loss to the formation. Both of these processes are responsible for the loss of pressure transmitted by the cement column, which is the primary cause of gas migration.
A schematic of the annular gas flow model is shown in Figure 1.