The impairment of permeability in the near well bore region is dependent upon fluid loss to the formation. The dynamic filtration properties of KCl-Polymer and Gypsum-lignosulphonate muds have been individually investigated and the role of individual components has been analysed. The associated permeability impairment due to particle invasion and the response of cores to backflushing has also been investigated. The scanning electron microscope has been employed to qualitatively identify particulate plugging.


The location of a zone of impaired permeability around a drilled wellbore, has substantial repercussions on the productivity of such wells. The significance of this impairment is particularly great to offshore locations where in view of current rig costs, remedial treatments can be very expensive. Also, the alternative of drilling additional wells is not always feasible conductors because of the limitation on the number of conductors on a platform. It is therefore desirable to take all reasonable steps to limit permeability impairment.

Formation damage, by whatever mechanism, is dependent upon the loss of mud filtrate through the borehole wall and its subsequent invasion of the formation. To circumvent the necessity of investigating a large number of complex damage mechanisms, an alternative approach for the control of formation damage is to investigate borehole filtration and its dependance upon physical parameters such as annular velocity, the constituents of the drilling fluid and reservoir rock properties. Numerous investigators have studied the static and dynamic filtration of drilling fluids (1–11) but little attention has been given to how individual mud components affect the filtration properties.

Since it is unrealistic to expect that fluid loss can be eliminated, any investigation of the contribution that each mud component makes to fluid loss control should also consider the severity and permanency of impairment that can result by plugging of the porous media by these mud particulates. The subject of mud particulate plugging has been given some attention by previous investigations (12–16) but the response of such damage to backflushing has not been given the same attention (13, 15).

This paper presents some of the initial findings of a research programme aimed at investigating:

  1. the dependence of borehole filtration upon

    • individual components of a drilling mud

    • mud circulation rate in a simulated borehole

    • rock properties such as permeability and mean pore size

    • the degree to which particulates in a mud system can invade a porous media and impair permeability, and the response of this damage to backflushing.

  2. the effect of backflush flowrate upon the removal of damage caused by mud particles.

The filtration experiments although concentrating upon dynamic filtration, since this accounts for most of the filtrate invasion around the borehole (1), have also been conducted with static filtration on cores and filter paper.

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