A recent major re-analysis of the results of the 1996-1997 mud testing program has provided a lot of insight into the factors driving mud performance in the field. To end 1997 we had carried out 22 core tests involving WBMs and 10 involving SBMs/OBMs on samples of varying rock quality. Figure 1 is the traditional manner of presenting the results. Generally the better muds have higher return permeability at both initial and final cleanup (10 psi and 500 psi). From this work and other experiments we had identified water block as the major cause of formation damage, however many questions remained unanswered:- - How much of the variation in mud performance is due to variation in rock properties? - Will all invaded mud filtrate flow back over time or will a permanent block result? - Is gas breakthrough instantaneous or is there a threshold pressure to overcome? - If the latter then how big is this threshold pressure? - What factors control this threshold pressure? - How important is depth of invasion? - What role does filter cake play in preventing both invasion and flowback? - Is drawdown applied purely at the sandface or is there a pressure gradient over the invaded zone? - What happens if only limited drawdown is applied across the invaded rock? - What is the role of emulsions as observed in oil base mud applications?