Slightly Amphiphilic Polymer Microspheres As an HTHP Fluid Loss Control Additive for Oil-Based Drilling Fluid

The application of oil-based drilling fluids in high temperature deep well drilling and the other complex well drilling process has been severely restricted due to the limited types and poor high-temperature high-pressure (HTHP) fluid loss (FL) control capability of fluid loss control additives (FLCAs). Aiming at the issue, a kind of slightly amphiphilic polymer microspheres, i.e., poly (acrylamide-co-divinylbenzene) (PACD) microspheres, with mean diameter 3.2 μm and polydispersity 1.82, were selected as the FLCA for oil-based drilling fluids in the study. The performance of PACD microspheres in terms of FL control and the structure of filter cake was compared with the traditional additives including oxidized asphalt and nano-CaCO3. The results showed that PACD microspheres have superior HTHP FL control capability. The HTHP FL of oil-based drilling fluids at 180 °C can be controlled within 10 mL as the addition of PACD microspheres was kept at 5 kg/m3. The FL control mechanism of PACD microspheres was analyzed based on the physical properties of PACD microspheres, the rheological properties of oil-based drilling fluids and the macro-appearance and microstructure of the filter cake. It was concluded that the superior HTHP FL control capability of PACD microspheres is due to the finite deformation property and the slight amphipathy of the microspheres.