One major problem for Operators when cementing casing through gas zones has been the effect of gas channeling, or migration, in the cemented annulus. This problem has been found to be definitely more difficult to solve in shallow, low temperature wells, even though the cement slurries were designed with sufficiently high density and very good fluid loss control. This paper analyses the problem of gas migration, presenting the mechanisms for its occurrence within the cement matrix or at the cement to formation interface. A description is then given of a successful technique of combining a special cement system with good cementing practices, so as to provide excellent long term zonal isolation across problematic gas zones. This technique is based firstly on the proper displacement of the drilling mud through good cementing practices, and then secondly on the placement in the annulus of a well dispersed, non-gelling impermeable cement. The key of this system is a specially stabilized latex additive which, through its film forming properties, reduces to zero the gas permeability in the cement slurry when the gas attempts to permeate under differential pressure. Field case histories of this technique are presented, illustrating the versatility of the system in solving gas migration problems in extreme situations.
During and after primary cementing, by far the most critical problem is to prevent the percolation of gas through the cemented annulus that leads to the development or wide and very conductive channels in the cement sheath. This gas channeling often occurs soon after the cement is in place and, once established, has proved to be extremely difficult and costly to repair.
This paper is then intended to identify the causes of such a phenomenon and to present a cementing technique which has proved to be very efficient in fields where gas migration was recognized lo be a major problem.
If there is, after a given period a differential pressure favoring gas migration, it is dear that gas may follow one or both of two basically different paths:
within the cement structure (called matrix channeling) occurring anytime when the cement behaves as a liquid or a permeable solid:
at the cement and formation or/and casing interface (called interfacial channeling) occurring once the cement has hardened and becomes an impermeable solid.
Density control; As long as the cement behaves as a true liquid, gas is able to channel in the annulus only if gas pressure is higher than the full (mud. washes, spacers and cement) hydrostatic pressure exerted on the formation Then, first, the density of fluids placed in the gas well must be properly designed by hand or with a free fall (U tubing) computer simulator so that, at any time and at any point in the well, the hydrostatic pressure exerted by the column of fluids (in static condition) plus friction pressure (during placement) stays between the pore and the fracturing pressure of the formation.
