Today there are a number of oil fields that have been developed with horizontal wells with open hole completions.

The performance of potentially high productivity wells gives the greatest concern. The economy of the whole development project may be jeopardised by any failure in obtaining the targeted production. This is particularly true for turbiditic deep water reservoirs where the economy relies on a limited number of highly productive wells.

In fact, what is the problem?

Generally these wells are completed with barefoot screens and run with the mud and the mud cake still in place in the open hole section. The section may be 1000 to 2000 meters long or even more. To our knowledge, there is no way to perfectly clean the entire annulus nor remove the entire cake during the completion phase.

The well is put on stream hoping that Nature will complete the clean-up. In fact, as the reservoir is heterogeneous along the drain, and that these highly productive wells produce under limited drawdown (high P.I), the prospect is that only a limited amount of the drain length will clean up and produce.

One may get the desired oil rate at the wellhead, but what is happening down hole ?

- the reservoir flows through a limited length resulting in high local fluid velocity

- the high local velocity may induce borehole weakening and sand production

- high velocity and sand production may induce screen wash out and well plugging (already experienced in the North Sea and the Gulf of Mexico)

- limited area opened to flow induces a P.I reduction and a greater pressure gradient for radial flow which can cause early gas or water coning if the contact level is close.

The challenge is to find the right combination of actions that will lead to (i) an easy and readily removable cake (external AND internal) and (ii) without creating additional damage.

This paper describes the necessary laboratory approach prior to designing the drilling fluid :

- the necessity to have a reproducible core plug preparation (Swi),

- the importance of mastering solids size versus sandstone permeability,

- the necessity to adapt linear flow (core plug) to radial flow conditions (borehole),

- the type of damaging and cleaning-up laboratory procedures leading to a representative comparison between fluids,

- the relative inadequacy of the return permeability (RP) concept in the case of open hole if it is misused,

- the necessity of analysing parameters like Flow Initiation Pressure, and the difficulty in analysing the Lift Off Pressure.

The above considerations are illustrated by the analysis of an extensive laboratory testing on OBM . Guidelines are drawn for the best jamming ratio compromise and a closer analysis of the drainage pressures is recommended.

The necessary synergy between the drilling phase and the completion phase is discussed as the best performance of a properly designed Drill In Fluid can always be ruined by an inadequate completion procedure. For instance, do we have to clean open hole drains drilled with an OBM ?

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