考虑颗粒随机分布特征的水平井环空岩屑起动流速

Incipient flow rate of cuttings in horizontal well annulus considering random distribution characteristics of particles

水平井钻井技术已广泛用于页岩气开发,但水平井钻井过程中环空岩屑易沉降在井眼低边形成岩屑床,导致钻柱高摩阻、高扭矩,卡钻、埋钻事故频发。目前考虑环空岩屑颗粒随机分布特征对起动速度影响的研究较少,受力及作用规律分析尚不完善,为了明确岩屑床颗粒重新起动流速及规律,更好地保障井筒的流动安全,在引入粒子随机分布函数确定岩屑起动百分比,考虑净重力、拖拽力、举升力、附加质量力、固体颗粒黏结力、液流压耗、钻井液静压力、范德华力建立了岩屑滚动、举升起动力学模型的基础上,分析了不同影响因素的作用规律,并利用已有的实验数据验证了模型准确性。研究结果表明,(1)举升方式的起动流速大于滚动方式,当床层高度足够小时,对应的流速即为无岩屑床生成的钻井液最小流速;(2)随着岩屑粒径的增大,岩屑起动流速先增大后降低,临界粒径为1.1 mm左右;(3)岩屑颗粒的相对隐蔽度、岩屑密度、钻井液有效黏度、钻柱偏心度、岩屑起动百分比越大,所需的起动流速越大,但岩屑起动流速随着井斜角的增大而降低。结论认为,研究取得的新认识可为页岩气水平井水力参数设计和岩屑运移提供技术支撑,保证了页岩气水平井的安全快速钻进。

Horizontal well drilling has been widely used for the development of shale gas.In the process of horizontal well drilling,however,cuttings in the annulus tend to settle in the low side of the wellbore to form a cuttings bed,which leads to high drag and torque and frequent pipe sticking and burying.At present,the influences of the random distribution characteristics of cuttings particles in the annulus on the incipient flow rate are less researched,and their force and action laws are not analyzed completely.In order to understand the incipient flow rate and laws of cuttings bed particles and ensure the safety of wellbore flow,this paper introduces the random particle distribution function to determine the percentage of incipient particle motion,and establishes the starting mechanical models of cuttings rolling and lifting by considering net gravity,drag force,lift force,additional mass force,adhesive force of solid particle,fluid pressure loss,static pressure of drilling fluid and Van der Waals force.Based on this,the action laws of each influential factor are analyzed,and the model is validated using existing experimental data.And the following research results are obtained.First,an incipient flow rate is higher in the lifting mode than that in the rolling mode.When the bed is thin enough,the corresponding flow rate is the minimum flow rate of drilling fluid with no cutting bed generated.Second,the incipient flow rate of cuttings particle increases first and then decreases with the increase of its size,and the critical particle size is about 1.1 mm.Third,the required incipient flow rate of cuttings particle increases with the increase of relative hidden degree of cuttings particle,density of cuttings,effective viscosity of drilling fluid,eccentricity of drill string and percentage of incipient particle motion,but decreases with the increase of hole deviation angle.In conclusion,these research results can provide technical support for hydraulic parameter design of shale-gas horizontal wells and cuttings migration understanding and ensure the safe and fast drilling of shale-gas horizontal wells.