A major concern in drilling operations is the problem of carrying out cementing jobs for production casings where no fracture margin during the slurry displacement exists. In general in order to resolve this problem, a double stage cement job, the use of a liner with a subsequent tie-back or one more casing string is required.
In the Adriatic Offshore we have experimented with the use of light weight high mechanical resistance slurry (SG = 1.6 /1.7 kg/l) in 7" production casing cement jobs in order to complete the operation in a single stage.
These cement jobs have been carried out, with positive results, in six of the fourteen Angela platform wells, in three of the four Antonella platform wells, in one Anemone cluster well (double stage cementing to avoid one more casing string), in the Antares 1 s.t. and the Agostino 11 s.t. wells.
The wells, where the 1.6/1.7 Kg/l light weight slurry has been used, have an average total depth of 3600 m VD, their intermediate casing shoes are set at an average depth of 1800m VD.
The use of light weight slurry, composed of 10 / 30 % Microsilica by cement, has led to a reduction in the hydrostatic load, an optimisation of rheology and filtration control (maximum differential pressure of 400 atm exist at the end of displacement) while guaranteeing the necessary high mechanical resistance. According to field data, we can conclude that the use of light weight slurry has the following advantages:
a single stage cement job can be used instead of the more complex double stage one with the consequent improvement in well and rig safety; in particular cases, a light weight slurry can avoid setting one more casing string;
improved quality of the cement job in comparison with the not always reliable results of a double stage cementing;
operational time and cost reduction.
The Adriatic Offshore is a mature gasfield; the development of the area by drilling new wells must face problems related to the depleted levels crossing (that's to say fracture gradients reduction versus the original values). The fracture gradients are calculated by the Poisson's formula:
FrG =PG+ /(l-)(OvG-PG)
= Poisson factor
FrG = fracture gradient
PG = pore gradient
OvG = overburden gradient
We use, based on our experiences in the area, /(1 -) = 2/3.
Therefore, besides the necessity to get a quick and economic drilling, it is fundamental the realization of a good cement job for the production casing as the static fracture margin is too low to realize a single stage cementing. To avoid either the extra costs involved in a double stage cement job or the problems we experienced in the past during this operation, we decided to achieve the result by a lighter high mechanical resistance slurry.
After the analysis of the Agip's laboratory data, we tested the light weight slurry (1.7 Kg/l - 40% Microsilica by volume) for one 9 5/8" intermediate casing cementing in the "Cervia" field; the operation was successful.
Nowadays we use light weight Microsilica slurry (range 1.6 – 1.7 Kg/l), when it needs, as a regular operation.
Light weight slurry is composed by a variable Microsilica percentage that, added to the cement, increases proportionally by volume the slurry compressional resistance. The typical example of a light weight slurry composition (1.7 Kg/l - 10%), we used during the cementing operation of the 7" production csg in the Angela field, is: