Formulation and evaluation of β-cyclodextrin polymer microspheres for improved HTHP filtration control in water-based drilling fluids

β-cyclodextrin polymer (β-CDP) microspheres were investigated as high temperature resistant filtration reducers for water-based drilling fluid in this study. The microspheres were synthesized by crosslinking reaction between β-CD and epichlorohydrin with inverse emulsion polymerization. The structural and morphological characteristics of the prepared microspheres were examined by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), particle size distribution measurement, and thermogravimetry analysis (TGA). The filtration control properties of the microsphere were evaluated via American Petroleum Institute (API) filtration test and high temperature and high pressure (HTHP) filtration test. The results indicated that β-CDP microspheres could effectively reduce the filtration of bentonite suspension both before and after thermal aging at 200 °C without significant influence on drilling fluid's rheology. A substantially enhanced filtration control behavior was observed after thermal aging, which was contrary with conventional filtration reducers. The β-CDP microspheres could absorb a large number of water and became swellable and deformable, which enhanced the compressibility of filter cake. What is more, hydrothermal reaction occurred for part of β-CDP microspheres under high temperature environment, resulting in the formation of numerous carbon spheres ranging from hundreds of nanometers to several micrometers. The β-CDP microspheres, bentonite particles and small carbon spheres in combination contributed to form a dense and compact filter cake with low permeability. β-CDP microspheres were potential candidates as environmental friendly and high temperature filtration reducers in deep well drilling.