Determination of drilling fluid rheology under downhole conditions by using real-time distributed pressure data

Abstract One the most important tasks during any drilling operation is to measure rheological properties of drilling fluids to allow for optimum maintenance and wellbore hydraulics management. In current drilling practice, such measurement is routinely carried out by a mud engineer at the rig site using test protocols and equipment that quantify the relationship between the fluid's shear stress and shear rate, as standardized by the American Petroleum Institute (API). Usually, measurements are conducted at atmospheric pressure and standardized temperature and are therefore generally not representative of actual downhole pressure and temperature conditions. High Pressure High Temperature (HPHT) viscometers can of course be used at the drilling fluid design stage to determine its rheological properties under downhole condition. However, these properties are subjected to change during drilling operations due to variation in mud maintenance and the introduction of drilling solids and contaminations. In addition, installation of a HPHT viscometer at the rig site is highly impractical due to its cost, maintenance, calibration difficulties, and lack of dedicated personnel to run the equipment. This paper presents a novel method to determine mud rheological parameters in real-time by using downhole sensor data. The well itself is used as the equivalent of a large pipe viscometer, with pressure measurements conducted along its length using multiple sensors placed at strategic positions in the drillstring. Contrary to traditional methods, rheology determination is performed at actual downhole pressure and temperature, without any human interaction and without the need for any rheology measurement equipment on surface. Frictional pressure loss is recorded at several flow rates, preferably while ramping up pump rates after making connections, and parameters associated with two- and three-parameter rheological models (e.g. the n, K and τ y parameters of the Yield Power Law (YPL) also known as Herschel-Bulkley model, which most accurately describes the majority of drilling fluids) are derived. In addition, time-dependent fluid characteristics such as gel strength can also be quantified using this method. Pressure data sets from wired drillpipe during field trials with a 12.6 ppg and 13.1 ppg synthetic based muds were used to validate the applicability of the new method in the field. These validations show that the method can be applied successfully in the field when downhole distributed pressure data is available. This novel approach represents a significant step in the direction of achieving full automation of drilling fluid property monitoring and maintenance with an elegant method that requires no human interaction, eliminating the need for surface measurement equipment, while at the same time representing more accurately the downhole pressure and temperature environment. Given the importance of accurate rheology characterization, particularly in the new era of active annular pressure management using Managed Pressure Drilling (MPD) and Dual Gradient Drilling (DGD) techniques that rely heavily on accurate hydraulics modeling, the benefits are self-evident.