Abstract Thixotropy is the reversible breakdown of fluid microstructure when sheared. The microstructure achieves a new steady-state once shearing stops. Drilling fluids are thixotropic due to their rheological makeup which means that fluid microstructure has a time-dependent response to changes in applied shear rate. In contrast to the literature focusing on the time-independent nature of drilling fluid, few studies have focused on the time-dependent response and even fewer have considered cuttings transport while doing so. The purpose of this study is to investigate the fundamental relationship between thixotropy and drilling hydraulics. An algorithm is developed to model thixotropy using flow history. The results show that the addition of drill cuttings does not directly affect the thixotropic behavior, rather the steady-state response is impacted which consequently changes the thixotropic response. Since the fluid microstructure takes time to respond to shear rate variations, viscosity lags behind shear rate variations causing annular pressure loss to fluctuate. The magnitude of pressure fluctuations is inversely proportional to characteristic time and directly proportional to stretching exponent. At smaller characteristic time coupled with smaller stretching exponent, high yield stress deteriorates cuttings transport. For larger values of characteristic time and stretching exponent, a clear trend is not observed, and further investigation is recommended. Nevertheless, when the thixotropic behavior of drilling fluid is considered, the results show that high flow rates and yield stresses do not guarantee efficient hole cleaning. Out of the two industrial fluid samples discussed, WBM yields higher pressure fluctuations and better cuttings transport compared to OBM. Since the proposed algorithm does not differentiate between the types of drilling fluids, this is due to WBM's smaller characteristic time and larger stretching exponent. It is suggested that a fluid exhibiting a slower response to shear rate changes causes higher pressure fluctuations and better cuttings transport.

中文翻译：

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触变性对钻井水力学的影响

摘要 触变性是流体微观结构在剪切时发生可逆破坏。一旦剪切停止，微观结构就会达到新的稳态。钻井液由于其流变组成而具有触变性，这意味着流体微观结构对施加的剪切速率的变化具有时间依赖性响应。与关注钻井液的时间无关性的文献相比，很少有研究关注时间相关的响应，甚至很少有研究在这样做时考虑钻屑传输。本研究的目的是研究触变性与钻井水力学之间的基本关系。开发了一种使用流动历史模拟触变性的算法。结果表明，钻屑的加入不会直接影响触变行为，相反，稳态响应会受到影响，从而改变触变响应。由于流体微观结构需要时间来响应剪切速率变化，粘度滞后于剪切速率变化，导致环空压力损失波动。压力波动的大小与特征时间成反比，与拉伸指数成正比。在较小的特征时间加上较小的拉伸指数，高屈服应力使岩屑输运恶化。对于较大的特征时间和拉伸指数值，没有观察到明显的趋势，建议进一步调查。然而，当考虑钻井液的触变行为时，结果表明高流速和屈服应力并不能保证有效的井眼清洁。在讨论的两个工业流体样品中，与 OBM 相比，WBM 产生更高的压力波动和更好的岩屑传输。由于所提出的算法不区分钻井液的类型，这是由于 WBM 较小的特征时间和较大的拉伸指数。建议对剪切速率变化表现出较慢响应的流体会导致更高的压力波动和更好的岩屑传输。