(468d) On Synthetic Drilling Fluid Rheological Modelling

Although different studies have been proposed to characterize SBF rheology, yield stress and time dependency proved difficult to determine. This study explains how most of the difficulties disappear when taking into account thixotropy by considering internal microstructure suspensions. The bahavior of structural units and effective volume fraction, as a function of shear rate, has been analyzed by Quemada (1997) to develop a rheological model for particle suspensions. We used and modified this rheological model to describe the behavior of Synthetic Drilling Fluids. For validation purpose, an industrial SBF, part of a batch used for drilling operation in a shale reservoir, is analyzed. A novel experimental procedure is proposed to estimate the parameters of the model, which makes estensive use of rotational rheometers. Different set of temperatures and resting times have been tested to develope semi-empirical correlation for the model parameters dependence on the aforementioned properties of the system. Moreover, a covariance analysis is also performed to demonstrate the statistical robustness of the semi-empirical model. In addition, a practical use of the model is demonstrated by accurately estimating the settling of retrieved shale cuttings through the analyzed synthetic fluid.

The proposed model displays the ability to evaluate SBF rheological parameters under the effect of temperature, shear rate and resting time. Results show how the driving force for microstructural change is the result of competition between breakdown due to flow stresses, build-up due to no-flow collisions, and Brownian motion (random thermal agitation of molecules). Both the experimental observations and the model indicate that changes in viscosity and yield point occur simultaneously. They derive from the same physical mechanism, the existence of micro-structural units in the fluid. The magnitude of the change is affected by temperature, as well as shear rate and resting time, and it is driven by the response of the microstructure’s parameters. From this analysis, it can be concluded that the most important microstructure’s parameters are the packing factor, the effective volume fraction (EVF) and the asymptotic viscosities (at zero and infinite shear rate). The most relevant rheological parameters of SBF fluids can also be determined by the introduced protocol, under conditions that are usually met in the oil and gas field.

The experimental protocol represents a design tool for onshore and offshore drilling operations that involve SBFs, with consequent improvement in the optimization of flow systems. A better characterization of SBFs will also increase their cuttings transport efficiency. Moreover, the accuracy in estimating cuttings’ settling and slip velocities will also be enhanced. The latter will improve the estimation of cuttings concentration profile, improving control on the pressure profile along the well, for increased wellbore stability.