The Influence of Drilling-Fluid Rheology on Cuttings-Bed Behavior
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Transporting cuttings out of the wellbore is an important part of every drilling operation to ensure efficiency and the reduction of non-productive time. The drilling fluids used for this task are complex fluid systems, generally with either oil or water as a base substance. The hole-cleaning performance of these two fluid systems is reportedly different from each other. Industry experience has shown that oil-based drilling fluids are performing better than water-based drilling fluids, even when the viscosity is similar. Additionally, research results reported in the literature show diverse conclusions with superior behavior for either water-based or oil-based fluids, or findings where neither of the types excelled. A general conclusion has not been made and the reasons for the different behavior are not entirely understood. The present thesis investigates the influence of the viscoelastic properties of drilling fluids on cuttings transport and hole cleaning. For this purpose an extensive rheometer study was conducted to measure the rheological properties of drilling fluids. Flow-loop experiments compare the performance of these fluids regarding their hole cleaning and cuttings-transport abilities in a controlled experimental setup. The tested fluids were three oil based and one water-based drilling fluid. The oil-based fluids were water-in-oil emulsions with a yield stress (named OBM A, B, and C), containing barite, CaCl2, bentonite, lime, emulsifier, and a fluid loss agent. The water-based fluid was a KCl fluid, viscosified with xanthan gum. Other additives were glycol, polyanionic cellulose, starch, soda ash and barite. OBM B and the KCl fluid were adjusted to have a similar viscosity profile in the relevant shear-rate range of the flow-loop experiments. The fluid composition and description is presented in chapter 2.2. The rheological investigation involved a preconditioning study, measurement of high shear rate and low shear-rate flow curves, measurement of temperature dependence, amplitude-sweep tests, shear-stress sweep tests and the measurement of thixotropy. A detailed description of the measurement parameters is given in chapter 2.1. The experiments were carried out at the shared fluid laboratories of SINTEF Petroleum and the Department of Geoloscience and Petroleum at NTNU in Trondheim, and partly at SINTEF Petroleum in Bergen. The flow-loop experimental rig had a 10 m long test section with a fully eccentric rotating drill string. Experiments were performed at varying fluid velocities and with or without drill-string rotation. Chapter 2.3 presents the main results of the rheological analysis and the flow-loop experiments. In a direct comparison between OBM B and the KCl fluid, the OBM B performed better and removed more sand out of the test section than the KCl fluid, for experiments without drill-string rotation. Drill-string rotation demonstrated dominating behavior over the flow related properties regardless of the rotational velocity. The viscoelastic properties were found to only have a small impact on the cuttings transport, but rather to influence the cutting beds resistance to erosion. An about 100 times higher strain tolerance was found for the KCl fluid in amplitude-sweep tests, compared to OBM B. This higher elasticity increased the cuttings-bed resistance to erosion and created a stronger connection between the cutting particles. Comparably, the yield stress in the OBM B was broken more easily than the elasticity in the KCl fluid, leading to more efficient hole cleaning for OBM B compared to the KCl fluid. Other rheological results indicated a strong time dependence of the fluid properties. The thixotropy measurements displayed a structural recovery, which is exceeding the structure of the initial rest interval by up to 160 %. The time dependent structural changes of the OBMs were also investigated in a preconditioning study to quantify the effect of pre-shearing and waiting time on flow curves and amplitude sweeps. The effects of pretreatment are influencing the reproducibility of the results. Establishing a strict measuring routine is therefore recommended as the results are expected to be applicable to most oil-based drilling fluids. The main results of the present PhD-thesis are presented and discussed as follows: ▪ Rheological results in chapter 2.3 ▪ Flow-loop results in chapter 2.4 Detailed experimental studies and results are presented in five articles in the appendix.