Rheological Characterization of Water-Based Drilling Fluids - A Comparative analysis of Manual and Automated Measurements
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Drilling fluids play a vital role in the drilling of exploration and production wells and adequate understanding of the impact of drilling fluid rheology is a pre-requisite, so as to selectively design fluids that could address difficulties encountered in oilfield drilling operations. Many polymeric drilling fluids are extensively used in the industry today due to the performance deficiencies of conventional aqueous systems that limit their application in technically demanding operations (Young, et al, 2001). Poly-anionic cellulose (PAC) is a viscosifying agent, that is being increasingly used as a main additive to water based muds for multifold property enhancements like filtrate loss control, reduction of mud cake thickness and shale inhibition (Fereydouni, 2012). Rheological characterization of such fluids will further investigate their wider rheological properties and other implications that could unveil their applications with diverse portfolio. This master thesis project presents experimental rheological characterization of different concentrations of PAC along with selected commercial compositions of Water base mud (WBM) additives like Barite, sand particles and Xanthan gum in order to replicate wide drilling fluids portfolio. In addition to the primary rheology properties like density, viscosity, dynamic rheological properties like shear-thinning, visco-elasticity, thixotropy and yield stress of above said compositions are investigated. Currently mud characterization tests are predominantly carried out manually which varies from choice of rheometrical instruments and personnel competence. However, high dependency on manual processes often results in low consistency and repeatability of tests which in turn brings out uncertainties in drilling operations. Utilizing differential pressure for measuring two variable i.e flow rates and effective viscosity has been used in other industries for decades due to automation requirements. By placing differential pressure (DP) sensors alongside the circulation path from the mud pump to the connection leading to the drill string, the fluid rheological properties can be examined more thoroughly at various fluid flow rates, localized fluid velocities, temperatures and compositions (Carlsen, Nygaard, Time, 2012). As drilling operations now are becoming more automated, this concept has emerged as a solution for automating the drilling process (Nygaard, 2011). Dual DP is the terminology that refers to pressure measurements between two differential pressure sensors. Hence, the second scope of this project included flow loop experiments conducted with selective compositions of PAC and additives, at the simplified rheology measurement (test rig) facility of Statoil Bergen Sandsli, where DP transducers and Coriolis sensors installed on the flow loop estimates true rheological properties of test compositions based on frictional pressure difference measurements on pipe and annuli. A comparative analysis was performed between laboratory scale experimental rheological characterization from manual measurement techniques using Fann 35 viscometer, Offite 900 viscometer, mud balance and Anton Paar rheometer and real-time measurements using the dual DP technology performed at test rig scale. The aim was to compare any critical findings between the results attained from the manual methods and the dual DP methodology. The dual DP cell may provide a means of attaining more consistent data regarding rheological properties, improve understanding of drilling fluid properties and invalidate uncertainties associated while undertaking existing manual experimental exercises. Hence, the present scope of this master thesis project represents the first step of providing in-depth insight for evaluation of the rheological properties of selected compositions of water-based drilling fluids in an effective manner, which will help filling the gap of reliable data on drilling fluid properties, that is of great importance in automated drilling operations. The experimental results proved that the inline estimation of the mud rheological properties from dual DP technology proves to produce more accurate results, but matches to some extent with the manual tests performed at the laboratory, where in-line (or in-situ) refers to continuous and constant measurements of parameters (pressure and density in this case) by sensors situated in a flow through system (Coastal Wiki, 2016). Factors like subsequent calibration of laboratory rheometric instruments, sample mixing procedures, sample resting times and good understanding of shear and temperature history of a sample are crucial in adding accuracy to results. It was also found that the lab tests results for the same test sample varied to a certain degree when different rheometric test instruments were used for measurements, which again makes it critical in consideration of correct results.