The Sealing Capability of Compacted Drilling Fluids Solids in Oil and Gas Wells

Abstract Plugging and abandonment of oil and gas wells are complex and expensive operations, requiring among other things proper sealing of the annulus between the casing and the rock formation. Such sealing is usually obtained using ordinary Portland cement. However, if settled and compacted drilling fluid solids that are present in the annulus provide sealing comparable to that of cement, the plugging operations may be significantly simplified, with a corresponding reduction in costs. To this end, knowledge is needed of both the hydraulic and mechanical properties of the sedimented and compacted drilling-fluid solids. This paper presents a study on two model water-based drilling fluids with (1) barite as the only solid material and (2) both barite and bentonite clay. The drilling fluid solids were compacted in a compaction cell that prevents radial deformation under increasing axial stress. The stress was applied stepwise to 20 MPa while keeping a pore pressure constant at 2 MPa. At each pressure step, the permeability of the compacted mud was measured. The homogeneity of the samples was verified based on post-test X-ray tomography images, tensile strength was measured with the Brazilian method, and wave velocities were measured with the ultrasonic through-transmission method. The permeability of the compacted drilling fluid solids was found to decrease with compaction pressure. The bentonite additive also reduced the permeability significantly. Comparison to published data on Portland cement at similar conditions showed that the permeability of the barite-bentonite mixture may be lower than that of cement. This suggests that the compacted barite-bentonite mixtures can be similar or even superior to neat Portland cement with respect to permeability. The larger ductility of barite-bentonite compacted material further indicates that the material is a good candidate for a well barrier material that could support the sealing abilities of brittle cement.