Very little quantitative data are available on how to cement slimhole wells while still adhering to good cementing practices. Therefore, a combined theoretical and experimental study has been carried out. from which a number of slimhole cementing guidelines have been derived. The analysed example case considered the cementation of a 3-1/2" liner in a 4-1/8" hole, in a relatively deep, high pressure, high temperature well drilled in a formation with minimal difference between pore and fracture pressure gradients.
It was found that a conventional (solids laden) high pressure, high temperature (HPHT) water-based drilling fluid cannot be fully displaced by the cement slurry, without exceeding the pressure limits of the formation. On the other hand, a potassium formate brine based HPHT drilling fluid can be displaced with close to 100% efficiency, even with sub-optimum cementation practices (i.e. no pipe movement, low annular velocity of the mud in the initial stage of the cementation). The low, flat gels and thin, tough filtercake greatly facilitate mud removal during cementing.
Slimming down wells to cut overall drilling costs has been a subject of study within the Shell Group for several years. The broader framework of the Slim Hole Drilling, Evaluation and Completion project has been outlined in a number of recent Shell publications (see e.g. Refs. [1–3]). The objective, as described in Ref. , is to install a 3-1/2" liner in 4-1/8" hole in the reservoir section of the well. On the mechanical side there is already equipment available, e.g. 5" × 3-1/2" rotating liner hangers, floppy cement plugs that wipe the drillpipe as well as the liner, and even centralisers for 3-1/2" casing in 4-1/8" hole.
One of the major concerns during cementing of slimhole wells is that very high frictional pressure losses will be induced when the cement slurry is pumped into the narrow annulus at the high displacement rate needed for optimum mud displacement. Fig. 1 illustrates this. However, very little quantitative information is available about the rheological properties required for a cement slurry that will make it possible to cement slim hole annuli using good cementation practices, i.e. application of high displacement rates, pipe movement, the adequate type and volume of spacer, etc. This paper describes a research project carried out to determine the optimum fluid properties and cementing procedures for slimhole wells.
The first stage of the project consisted of identifying the limits of present day cementing technology (i.e. which hole/casing configurations can still be cemented using available cement slurry designs) by running computer simulations of a cementation with very strict boundary conditions. Rather than using a number of area specific well data sets, the cementation of a 3-1/2" liner in a 4-1/8" hole in relatively deep. high pressure. high temperature (HPHT) well was analysed as an example case. In the second phase of the project, the conclusions and recommendations, but also the uncertainties, obtained from the cementing simulation analysis were verified by a large scale experimental displacement study. From the experimental work a number of guidelines were derived on how to cement slimhole wells.
The specific aim of the analysis was to determine the influence of parameters like drilling fluid rheology. cement slurry rheology, and the length of the liner to be cemented, on the displacement velocity of the cement in the annulus that can be achieved without fracturing weak zones.