These two types of drilling fluids have very different behavior. In recent years, the oil-based drilling fluid has gained popularity, but the present environmental movement brings water-based drilling fluids back. They both have their advantages and disadvantages as briefly discussed here.
Water-based drilling fluids use water as the continuous phase. Additives are used to change density and viscosity and to control filtrate losses. Also, chemical compatibility with the rock is important. Water-based drilling fluids are cheap. They also are good for curing mud losses. By pumping coarse bridging materials (called lostcirculation material), mud losses often are cured. It is believed that the main mechanism is that in water-wet formations, filtrate losses occur, leaving dense particles in the mud in the fracture. The main disadvantage with water-based drilling fluids is that they are reactive to clays and lead to time-dependent borehole problems. The hole size often increases with time in shales.
Oil-based drilling fluids use oil as the continuous phase. Lower well friction is one of the advantages with oilbased drilling fluids. They are therefore often used in long-reach wells where friction is a critical parameter. Usually the borehole does not show time-dependent deterioration as with water-based fluids. The capillary pressure prevents oil from invading a water-wet rock.
There is, however, one disadvantage with oil-based drilling fluids. If circulation losses arise during drilling, it is often difficult to stop the losses. A severe loss situation can be quite time-consuming to cure. One main mechanism is that there is little or no filtrate loss toward the water-wet rock, such that the drilling-fluid viscosity will not increase and the oil-based mud will continue opening the fracture.