Rheological Properties of Clay-Based Drilling Fluids and Evaluation of Their Hole-Cleaning Performances in Horizontal Directional Drilling

Hole cleaning is a significant issue in well drilling, and this problem becomes more severe in horizontal directional drilling (HDD). One of the main functions of the drilling fluid is to transport cuttings from the borehole to the ground surface. A better understanding of the rheological properties of drilling fluid is essential to predict its hydraulic behavior in the annulus and choosing the optimal drilling fluid for hole cleaning in HDD. In this paper, the authors explore the rheological properties of drilling fluids with various bentonite concentrations through laboratory tests. A six-speed rotational viscometer was used to obtain the shear stress and shear rate relationships of drilling fluids, and both the Bingham plastic (BP) model and the Herschel-Bulkley (H-B) model were adopted to describe the rheological properties. The effect of increasing bentonite concentration on the conventional hole-cleaning capacity index, yield point to plastic viscosity ratio (YP/PV), was calculated using the YP and PV values obtained from the BP model. Based on the cuttings transport mechanism and flow patterns, the hole-cleaning capacity of drilling fluid was divided into two components: the carrying (suspension) capacity and the sweeping capacity. The rheological parameters and the hydraulic parameters, such as the width of the flattened velocity profile (plug width, $h$), the frictional pressure loss ($\mathrm{\Delta }P$), and wall shear stress (${\tau }_w$), are used together to reflect the hole-cleaning performance of drilling fluid from the two aspects. The results showed that as the concentration of bentonite increased, YP, PV, and H-B yield stress (${\tau }_y$) increased significantly, while the $k$ and $n$ values (H-B model) varied in a narrow range. The YP/PV, $\mathrm{\Delta }P$, and ${\tau }_w$ values increased as more bentonite was added, while $h$ increased first and then plateaued after 3.5% concentration. Although the high values of YP/PV, $h$, ${\tau }_y$, and ${\tau }_w$ will theoretically benefit the cuttings transport, the accompanying high frictional pressure losses may raise concerns about borehole stability.