巨厚砾石层气体钻井井筒不规则性对井斜的影响研究

Effect of Irregular Wellbores on Well Deviation in Air Drilling Through Thick Conglomerate Formations

直井易斜和下套管困难是巨厚砾石层气体钻井存在的2个亟需解决的问题。现场测试数据表明,即使使用普遍认为具有较好控斜效果的空气锤钻井技术,井斜控制依然困难,且井筒的规则性很差,这很难用现有的控斜理论解释。现有底部钻具组合(BHA)受力模型均未考虑井筒的不规则性,都假设井筒光滑规则。为此,基于现场实测数据,利用有限元方法建立了BHA与不规则井筒相互作用的力学模型,分析了井筒不规则性对BHA受力特征的影响。模型分析表明,不规则井筒易形成附加支点,缩短有效钟摆长度,使降斜力大幅减小,甚至可能使钻头侧向力成为增斜力,造成井斜控制失败。实例分析证实,巨厚砾石层采用气体钻井时,预弯钟摆BHA所钻井筒相对较为规则,控斜效果好,下套管作业顺利。现场实测数据间接证明了井筒不规则性对井斜存在重要影响,BHA力学分析时应考虑井筒的不规则性。

Proneness to well deviation and difficulty in casing running are two bottlenecks to be solved urgently in air drilling through the thick conglomerate formation.Field test data revealed that well deviation control was still challenging even air hammer drilling with a marked control effect on deviation was adopted,and wellbore regularity was poor.This phenomenon is difficult to explain with present theories of deviation control.Existing bottom hole assembly(BHA)force models do not consider the irregularity of the wellbore,and assume that the wellbore is smooth and regular.In this paper,the mechanical model of interaction between the irregular wellbore and BHA was built with finite element method on the basis of field data,and the influence of wellbore irregularity on the mechanical characteristics of BHA was analyzed.The model results demonstrate that the irregular wellbore is easy to form additional fulcrum and shorten the swing distance of pendulum BHA,thus greatly reducing deviation-reducing force.It may even convert the lateral force on the drill bit into a deviation-increasing force,resulting in well deviation control failure.It is confirmed by case analysis that the wellbore drilled by the pre-bent pendulum BHA is regular in air drilling through the thick conglomerate formation,with effective well deviation control and smooth running of casing.Field data indirectly supports the significant impact of wellbore irregularity on well deviation,which should be considered in the BHA mechanical analysis.