Water-based drilling fluids are increasingly being used for oil and gas exploration. Unfortunately, their use leads to clay swelling which occurs in exposed sedimentary rocks found particularly in the cap of the oil/gas reservoir. This has an adverse effect on drilling operations and leads to wellbore instability, for which the loss of production costs have been estimated as greater than $500 M per annum. The mechanisms and time scales of osmotic capillary swelling, which occurs in the interparticle pores of the shale and has a far greater impact on wellbore instability, is still a mystery. SE-SANS will allow us to conduct real space measurement of the change in pore size and distribution in a natural shale sample as a function of the supernatant fluid salinity. The method will also allow investigation of larger pores, up to a micron in size.