Gravel packing a region of a wellbore is carried out using one or more hydrophobically-surfaced particulate materials as the gravel. Placing the gravel pack is carried out using an aqueous carrier liquid, with hydrophobic particulate material suspended in the liquid and a gas to wet the surface of the particles and bind them together as agglomerates. The presence of gas lowers the density of the agglomerates relative to the particulate material in them, so that transport of the particulates is improved. The invention enables the placing of longer, uniformly packed, gravel packs, especially in horizontal and near horizontal wellbores.