8 International Journal of Petroleum Technology, 2014, 1, 8-13 E-ISSN: 2409-787X/14 © 2014 Avanti Publishers Improved Determination of Apparent and Plastic Viscosity for Aqueous Solution of Drilling Fluid Additives Dilip K. Rajak, Akhilendra K. Pathak and Chandan Guria * Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826 004, India Abstract: Rheological behavior of drilling fluids is very complex. Exact determination of shear rates helps to predict apparent and plastic viscosity very accurately, which will help to monitor drilling operation efficiently. This paper deals with the improved estimation of shear rates of drilling fluids with varying rotor rotation using coaxial-cylinder Fann viscometer, which is based on generalized difference equation under purely steady, laminar and isothermal tangential fluid flow condition. Rotor rotating speeds and bob dial readings are the input variables for shear rate prediction. The proposed equation was used to calculate shear rates accurately (hence apparent, plastic viscosity and yield point) for several non-Newtonian fluids, mainly, aqueous suspension of bentonite, xanthan gum, poly anionic cellulose and carbomethoxy cellulose solution. Finally, the predicted consistency plots were compared with those which are obtained from the conventional method of estimating the rate of shear for drilling fluids. Keywords: Shear rate, rheology, rotational viscometer, difference equation, plastic viscosity, apparent viscosity. 1. INTRODUCTION Rheological behavior of drilling fluid additives (e.g., bentonite, xanthan gum, poly anionic cellulose and carbomethoxy cellulose and etc) is very complex due to non-Newtonian nature of the materials. The presence of fixed negative charges on bentonite (i.e., montmorillonite) surface yield a true colloidal suspension of aqueous bentonite solution. Similarly, water soluble polymeric drilling fluid additives behave as pseudo-plastic fluid behavior due to high degree of hydrophilic functional group substitution and degree of polymerization. The performance of oil well drilling was significantly affected by various rheological properties e.g., yield point, apparent viscosity, plastic viscosity and gel points [1-4]. Moreover, safety of the well was also restored while drilling by manipulating the exact values of these rheological properties. Fann viscometer is commonly used to measure the rheological properties of drilling fluid in oil fields as well as laboratories. It is a rotational coaxial-cylinder viscometer where drilling fluid is confined into the annular space between two cylinders (i.e., rotor: outer cylinder, and bob: inner cylinder), one of which is in motion (usually rotor) and other (usually bob) remains stationary after deflection while operation. Rotor rotations and bob deflections are the two measured readings which are directly used for rheological analysis of the test fluid. The torque exerted on the inner bob wall is measured directly from dial reading (i.e., bob deflection, θ) for given rotor rotation (i.e., rotor *Address correspondence to this author at the Department of Petroleum Engineering, Indian School of Mines, Dhanbad 826 004, India; Tel: +91-326-2235411; Fax: +91-326-2296632; E-mail: cguria.che@gmail.com rpm), and it is converted easily into shear stress by neglecting end effect for known viscometer dimensions [3]. However, the major difficulty arises to predict the wall shear rate, which is mainly due to the non-uniform distribution of fluid flow in the concentric cylindrical annulus. Moreover, there is no exact method of calculating shear rate distribution using viscometer readings, unless the fluid model is assumed a-priori. Details of these techniques for shear rate estimation have been reviewed by [4-9]. To determine the rheological properties of drilling fluid, simple Newtonian approximation was assumed and the predicted shear rate is dependent on rotor rotation only [2-3]. This procedure is quite common and it is mostly practiced to determine the plastic viscosity and apparent viscosity of drilling fluids in oil fields and laboratories [1-3]. The major drawback of using this procedure is that the predicted shear rates are independent on the nature of the drilling fluids. In this regard, several studies have been made to predict the wall shear stress without assuming rheological model a-priori [9-10]. In this study, the work of Kumar and Guria [9] and Kumar et al. [10] was used to predict apparent viscosity and plastic viscosity of drilling fluid additives particularly aqueous suspension of bentonite, xanthan gum, poly anionic cellulose and carbomethoxy cellulose using Fann viscometer readings. 2. EXPERIMENTAL 2.1. Materials In the present study, Fann 35 Viscometer (API RP 13B: Model 35) was used for rheological analysis of aqueous suspension of xanthan gum (XG), poly anionic cellulose (PAC), carboxy methyl cellulose (CMC) and