The popularity and breadth of application of horizontal well technologycontinues to expand in some cases, however, horizontal well performance isdisappointing - often as a result of drilling-induced formation damage. Thisdamage may occur over the entire pay interval, or may occur in discreetintervals in varying degrees, depending on drilling conditions and charges inlithology along the wellbore. The resultant inflow may occur over one or moreshort segments rather than the entire horizontal wellbore. This limiteddrainage profile can lead to premature coning, and poorer than expectedperformance.

Formation damage is normally modeled using a skin factor, s, in the Darcyflow equation, assumed to be a constant. The skin value used, if not measureddirectly from pressure transient analysis, can be based oil laboratory regainpermeability tests conducted on core samples. These tests involve comparing thefinal (regain) permeability, after mud damage during leakoff, to the initialundamaged permeability. The final regain permeability is typically measured atsome arbitrary flowrate, (and therefore, sort arbitrary drawdown). Our researchindicates that this test period is over-simplified because it does notrecognize the dynamic nature in which wellbore cleanup occurs.

A more comprehensive test procedure has been developed for mud leakoff andregain permeability testing, whereby the regain permeability is measured atincrementally increasing pressure differentials across the core. Imposing apressure differential and measuring the resultant flowrate is consistent withapplying a drawdown in the actual well. Application of the new test procedurewith different drilling muds on several core plugs of varying permeabilityindicates that fluid inflow will not occur until a minimum "threshold pressure"is achieved. Lab results show distinct differences in threshold pressuremagnitude between different mud systems and rock permeability. Regainpermeability improves with increasing pressure drop and with increasingvolumetric throughput. In some cases it returns to its' non-damaged value.

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