Abstract: Drag-enhancing structures for downhole operations are included in a downhole assembly that further includes a tool string, extends past a maximum transverse perimeter of the tool string, and increases resistance to fluid flow past the downhole assembly when the downhole assembly is pumped in a downhole direction within a wellbore conduit. The systems and methods include conveying the downhole assembly in the downhole direction within the wellbore conduit. The systems and methods further may include decreasing the resistance to fluid flow past the downhole assembly after the downhole assembly is located within a target region of the wellbore conduit and/or flowing a sealing material past the downhole assembly while the downhole assembly is located within the wellbore conduit.

Abstract: One embodiment of the invention is a variable density drilling mud comprising compressible particulate material in the drilling mud wherein the density of the drilling mud changes in response to pressure changes at depth. A second embodiment is a method for varying drilling mud density. The method comprises estimating the pore pressure and fracture gradient, and choosing a drilling mud with compressible materials wherein the effective mud weight of the drilling mud remains between the pore pressure and the fracture gradient in at least one interval of the wellbore. A third embodiment is an apparatus for drilling a wellbore.