Gas migration through the cement column is considered as one of the most sever encountered phenomena during drilling or completion procedures, and has been recognized as one of the most troublesome damaging effects of the petroleum industry on the environment. There are many theories explaining the migration of gas through the cement which mainly considered the following as major factors contributing to gas migration: fluid density, mud removal strategy, cement slurry design, cement hydration processes, cement-formation-casing bonding and cement mechanical properties. Considering cement slurry design, the permeability of the slurry during transition state is explained as one of the main controlling parameters. Therefore low permeability is defined as one criterion for good cement formulation.
In this paper, an approach has been presented which relates permeability, k, to SGS, fluid loss value and pore size distribution of the cement. Modeling k goes through the consideration of the cement as a two-phase porous-elastic material with a changing structure represented by variation of SGS and fluid loss value with time.
The pore sizes of the particles are introduced in the model by using Carman-Kozeny equation, in which the permeability at the beginning of transition state (t = 0) is calculated. The Sabins et al. equation, 1982, which is originally developed to compute pressure drop due to gelation, is used as the basic equation to relate SGS and fluid loss value to k (along with Darcy flow equation). Maximum Darcy flow rate, qmax, which is, according to Sabins et al, simply measurable by using static gel strength and slurry permeability tester, is used to reflect the effect of fluid loss values on k values. Finally the predicted k-values by the equation is compared with measured data.
The approach can be used to optimize cement formulation and simulate gas migration through the cement column.
Annular fluid migration may occur during drilling or well completion procedures, and has long been recognized as one of the most troublesome problems of the petroleum industry. It consists of the invasion of formation fluids into the annulus, because of a pressure imbalance at the formation face. The fluids may migrate to a lower pressure zone, or possibly to the surface. Within this category of problems, gas migration is the most frequent, and no doubt the most critical and dangerous.
Gas migration is a complex phenomenon involving fluid density control, mud removal strategy, cement slurry properties, cement hydration, cement to casing and cement to formation bonding, and cement mechanical properties (1), (2). Since the problem was recognized, considerable effort has been exerted to explain the problem and find solutions.
Although gas may enter the annulus by a number of distinct mechanisms, the prerequisites for gas entry are similar. There must be a driving force to initiate the flow of gas, and space within the cemented annulus for the gas to occupy. The driving force comes when pressure in the annulus adjacent to a gas zone falls below the formation gas pressure. Space for the gas to occupy may be within the cement medium or adjacent to it. It is worth now to mention different parameters that theories have considered to be important in gas migration severity.
Fluid density: gas can invade and migrate within the cement sheath only if formation pressure is higher than hydrostatic at the bore hole wall. Therefore, as a primary requirement, slurry density must be correctly designed to prevent gas flow during cement placement. However, there is a danger of losing circulation or fracturing an interval if fluid densities are too high. Cement slurry will not transmit hydrostatic pressure forever. The transition from a liquid that controls formation pressure to an impermeable solid is not instantaneous. No matter how carefully slurry has been designed to counterbalance formation pressure, it will not necessarily resist gas invasion through the hydration process.
Mud removal strategy: If channels of mud remain in the annulus, the lower yield stresses of drilling fluids may offer a preferential route for gas migration. Furthermore, water may be drawn from the mud channels when they come into contact with cement. This can lead to shrinkage induced cracking of the mud, which also provides a route for gas to flow. If the mud filter cake dehydrates after the cement sets, an annulus may form at the formation cement interface, thus providing another path for gas to migrate.