Annular gas flow following cement has been a problem in many areas throughout South-East Asia. In some offshore fields, gas migration occurs on surface, intermediate and production casings and liners. This phenomenon gives rise to gas pressure on the annulus between casings or phenomenon gives rise to gas pressure on the annulus between casings or even interzone communications. Both can cause costly remedial repair work and unpredicted high volume flow to the surface has on occasions caused catastrophic disasters.
Laboratory testing and field applications in many areas of the world have shown compressible cement slurries to possibly be the most effective means of helping to prevent annular gas flow developed thus far. This paper will review the development and application of compressible cement slurries.
Over 800 compressible cement slurry applications have been run throughout the world and the success ratio is greater than 90%. Applying this technique on a more wide-spread basis may possibly reduce the many problems associated with annular gas flow in South-East Asia.
Initial application of compressible cement slurries in South-East Asia has been confined to offshore Bomeo. Here, annular gas flow has been encountered on surface and intermediate casing. Some wells have gas flow back to surface within ½ to 1-½ hours after the primary cementing job has been completed. This occurs on casings which are cemented back to surface as well as those where cement is only brought back just into the last cemented casing.
In this particular area, weak formations with natural fractures extend irregularly to a significant depth below the sea bed. Lost circulation is a common occurrence even at the minimum mud density required to control the gas pressure. When annular gas flow is not controlled, lost circulation and interzone gas migration into the fractured zone apparently can result in gas percolation from the gas zone to the ocean floor some distance from the well bore even with no gas flow from the conductor pipe-surface pipe anulus. Conventional techniques for improving mud displacement, fluid loss control and cement bonding have met with very limited success. Annular gas flow may still occur and, at the least, monitoring and disposal of the gas at the surface is required.
Annular gas flow can occur in two distinct forms within a wellbore:
interzone communication; and
direct gas flow to the surface.
Both systems waste valuable natural resource, are dangerous and are costly to repair.
Up until recently well operators have been more or less unsuccessful in solving the problem of annular gas flow. Early studies using a large scale laboratory model evaluated different drilling fluids, spacers, flushes and cementing compositions with respect to density, viscosity and flow characteristics. Fluid loss, pipe configurations, and mud displacement were stated as factors which should be given attention. These studies demonstrated more consideration should be given to the planning of primary cementing operations in a gas well.
To help control annular gas migration, a positive hydrostatic pressure greater than the gas formation pressure must be maintained. Incompleted drilling fluid displacement can create a channel through which formation gas can migrate. The problems of annular gas flow cannot be minimized unless the mud displacement efficiency of the cement is high.
A later study indicated cement slurries without fluid loss control may fail to transmit full hydrostatic pressure should a restriction or bridge occur as a result of water loss from the cement slurry into a permeable sector. This would allow the pressure below the restriction to fall below the gas zone pressure and result in gas entry into the wellbore. By minimizing the fluid loss of the cement slurry, some annular gas flow problems were eliminated.
Further research evaluated the influence of common slurry properties and miscellaneous techniques. These included applying surface pressure, increasing the slurry and/or increasing the mud density to increase the initial hydrostatic pressure; using multiple stage cementing to decrease the height of the cement column (for mostly intuitive reasons); adjusting the thickening time in an attempt to decrease transition time; eliminating free water on the theory annular gas flow was promoted by free water channels; increasing the mixing water density in an attempt to maintain annular pressure through the permeability of the partially set of gelled cement column.
To this point, both research and field experience showed nearly all good cementing practices had on occasions been credited for helping control annular gas flow. However, no single method existed which was consistently successful for controlling annular gas flow.
An extensive research program performed by Tinsley et al, developed a method which proved very effective in preventing or minimizing annular gas flow where other methods have failed. Briefly, this new method used a compressible cement slurry to provide a means of effective annular pressure maintenance.
A cement slurry, before hydration begins, behaves like a true fluid in that it can transmit hydrostatic pressure based on fluid density and depth. In the early stages of the hydration process, a cement slurry begins to develop measurable static gel strength and, under certain conditions, loses its ability to transmit pressure. When this stage occurs, the slurry is neither a liquid nor a solid, but a plastic-like mass of weekly associated particles containing water in the pore spaces. However, prior to reaching particles containing water in the pore spaces. However, prior to reaching this stage, the initial hydrostatic pressure is equal to the summation of the annular fluid gradients times the vertical intervals covered. As static gel strength develops, a reduction in the volume of water within the pore structure of the cement results in a pressure decrease. pore structure of the cement results in a pressure decrease. Two mechanisms cause this volume decrease. First, even with the best fluid loss control additive, the fluid leak-off rate is still significant and filtrate may be lost to permeable formations.