Mechanisms of Rheological Deterioration in Spent-Oil-Based Drilling Fluids and Viscosity Reduction Using an Amide-Carboxylic Acid Additive

Abstract

During drilling operations, the continuous accumulation of low-gravity solids (LGSs) may progressively deteriorate the rheological properties of oil-based drilling fluids (OBDFs), thereby limiting fluid reuse. Centrifugation or dilution provides only limited rheological improvement. To identify the factors contributing to viscosity increase and develop an effective treatment strategy, we evaluated two spent OBDFs from a South China Sea drilling site in terms of rheology, electrical stability (ES), oil/water ratio, and solid content before and after centrifugation. The extracted LGSs were characterized by scanning electron microscopy, X-ray diffraction (XRD), and contact-angle measurements. Bentonite, Rev Dust, and barite were used as representative solids to assess their effects on model-system viscoelasticity. The estimated LGS contents of the two spent OBDFs were 16.41% and 12.04%, respectively. Centrifugation reduced these values by only 2.09 and 1.47 percentage points, respectively, accompanied by modest rheological improvement and a decrease in ES. Extracted LGSs contained 51.86% clay minerals and exhibited irregular and lamellar morphologies. The compressed LGS pellets were readily wetted by water, and white oil rapidly spread over their surfaces. These characteristics may increase particle/particle and particle/liquid interactions, thereby increasing flow resistance. Adding bentonite, Rev Dust, or barite increased both the storage modulus (G′) and loss modulus (G″) in the model systems. To address these issues, a viscosity reducer for OBDFs, designated as OCD, was prepared and characterized using Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). Its performance was evaluated through rheological and ES measurements, thermal-aging tests, and combined treatment with centrifugation. At an OCD concentration of 2.0 wt%, both G′ and G″ decreased, while the loss factor generally increased in drilling fluids containing different solid phases. For the investigated spent OBDFs, the apparent viscosity (AV) and plastic viscosity (PV) decreased by 19.98% and 16.35%, respectively, accompanied by a slight increase in ES, while a certain viscosity-reducing effect was still maintained at 160°C. The combined application of OCD and centrifugal solids control provided a greater improvement in the rheological properties of spent OBDFs than either treatment alone. The FTIR spectra of the treated solids showed additional absorption features associated with OCD, while focused beam reflectance measurement (FBRM) revealed changes in chord-length distributions and chord counts during cumulative OCD addition. These observations are consistent with possible OCD/solid interactions and changes in the particle-association state. Overall, this study provides a potential strategy for rheological control and reuse of spent OBDFs.