Designing Fluid Velocity Profiles for Optimal Primary Cementing
Authors: SILVA M.G.P. , Martins A.L. , BARBOSA B.C. , Garcia H. 
Published: December 2007

With the increased drilling of highly deviated wells in regions of high temperature and pressure, frequently crossing weak formations, a perfect adjustment between the parameters of the drilling fluids and of the primary cementing slurries becomes imperative. With this aim, and using as a guide the success factors determined from the analysis of successful and unsuccessful primary cementing operations performed in the Campos Basin, offshore Brazil, tests were conducted in a physical simulator of downhole conditions to document the quality of zone isolations obtained with variation of the rheological parameters of the drilling fluids, the primary cement slurries, the inclination and caliper of the well, the casing eccentricity and the annular displacement velocity. In parallel, starting from accurate determinations of the rheological behavior of the cement slurries and drilling fluids and from the casing eccentricity, the velocity profiles in the annulus were determined from theoretical models. The shape of these profiles was one of the parameters used to justify the success or failure of the simulated cementing job. A three parameter rheological model was used.

Finally, based on success factors determined from data base analysis and from experimental validation of the theoretical model used for designing velocity profiles, general directives were established for the rheological parameters of the fluids to be used before and during cementing, for the shape of the velocity profile, as well as for the optimal annular velocity range, in order to guarantee the flow of the slurry to all parts of an eccentric annulus and thus to achieve an excellent success ratio for primary cementing operations.

Introduction

Activities in the Brazilian petroleum industry have lead to drilling under ever-more-challenging conditions, characterized by locations in remote areas, depleted reservoirs, high temperature and pressure fields and water depths as great as 2000 m.

For the exploitation of these fields to be viable, it became necessary not only to significantly reduce the costs of the wells, but to refine the conventional drilling and completion procedures and to introduce of new techniques whenever necessary. This refinement aims at minimizing the operational risks which could prevent the continuation of the well or reduce its productive life. Following a worldwide trend, one of the strategies adopted to reduce the risks inherent in oil exploration in hostile conditions has been that of drilling more highly deviated and horizontal wells.

Highly deviated wells, drilled under high temperature conditions, mean that the formulations of the drilling muds and cement slurries must be much more complex than those used in vertical wells, to minimize the formation of a solid bed formed of cuttings which have fallen out of the drilling fluids and to minimize the early setting of the cement due to the loss of stability of the slurry. Such formulations make it much more difficult to adapt other physical properties of the drilling fluids and cement slurries to these two operational requirements with a view of making possible the complete substitution of the first fluid by the second and thus increase the chance of successful cementing operation of the annulus. Another difficulty is the risk of chemical incompatibility between the drilling fluid and the cement slurry. It is therefore desirable to minimize the area of contact between the two during the displacement.

Highly deviated wells, crossing zones with low fracture gradients, require fine adjustments to be made to the annular pressure during drilling, well conditioning and cementing, in order to avoid causing mechanical damage to the hole walls, which would cause undesirable washouts, which in turn would greatly increase the chance of not obtaining satisfactory zone isolation.

As can be seen, the optimization of the hydraulic design of the fluids used in the cementing operations is imperative to the obtention of high-quality sealing of well annuli, especially those which are highly deviated, where there are many other competing factors. Generally, when insufficient attention is given at this stage, the results shown by the acoustic logs are poor and successive corrections are required across the productive intervals. It should also be noted that if the well has a high temperature, as well as being deviated, the chances of sticking the string when squeeze cementing are high and should be considered among the risk factors which could cause the loss of the well.

This paper was written with the aim of maximizing the chances of getting successful cementing operations of highly deviated and horizontal wells, using as a basis the success factors obtained by Silva from an analysis of the data from successful and unsuccessful cementing operations in the Campos Basin.

P. 605