RE:RE:RE:A question to the wise re: no flow rates.Good question - and the science behind drilling fluid (mud) is extremely technical (regardless of how much flack mud engineers take on site). When it comes to logging, it's better to use water based fluid because any fluid that seeps into the formation will be water, however, when you are using oil based mud (OBM or invert - called that because it is an inverted emulsion with little water droplets acting as solids within the oil fluid to give it viscosity and solids carrying capacity), any fluid seepage into the formation will be oil, in which case it can influence resistivity logs which is one of the majors logs used to identify hydrocarbon presence. The other way that water based muds can "damage" the well is that clays and shale (which can be hydrophilic and swell in the presence of water) can cause driling problems, and hole damage by the swelling formation in the form of cleaving or sloughing - OBM doesn't cause any swelling and has almost no negative reactions to formations. The other thing to keep in mind is that when drilling with water based mud the filter cake is usually thicker to prevent water loss to the formation, OBM has very thin and tight filter cake, and seepage is much less of a problem. The filter cake needs to be removed in order to get an accurate flow test or logging readings, and this can become a problem with WBM in the hole. OBM is very easy to work with, in that there are not many additives needed (a bit of a simplification), while WBM requires a lot of additives, and they interact with each other (sometimes negatively).
Mud weights, the ability to increase the hydrostatic pressure exerted by the mud, in either OBM or WBM can be adjusted... to a point. The weighting material in both cases is usually barrite, but of course the max density of the mud can only be to a certain point, before you have to start using some very exotic and expensive weighting materials (but that's doens't start until around 1600 kg/m3 densities, which is one heck of an overpressured formation). To weight up both muds you need viscosity (polymers or bentonite in WBM, and generally a OBM compatible bentonite), in which case WBM usually needs more additives to become capable of carrying the high densities.
Hope this helps