| The problem of well diviation is a very important issue in the project of oil and gas exploration and production.The problem is more outstanding in steep formations,which not only leads to low rate of penetration,long drilling period and high drilling cost but also makes poor wellbore quality and sometimes we have to abandon the low-quality wells or delay the drilling procedure,and even the target we set can nont be achived.This issue is commonly agreed by the drilling supervisor and technicians.So research on the well deviation control is of great importance to both production pratice and theory study.Baka block is along an anticline with great strata dip(10°-80°),the deviation of which is hard to control.It can be seen form the datas of drilled wells in baka bock,that the wellbore quality data is superstandard,which longer the drilling cycle and makes great economic loss.There are many factors that associated to wellbore quality control:one of the most important factors is the selection of the BHA.A suitable BHA can not only increase rate of penertration and prevent well declining but also decrease hole accidents.In this way can we achive a goal of safe,fast and low cost drilling.The factors that influent the well deviation can be separated into two types:one type belongs to the formation factors;the other one belongs to the BHA factors.The constitutional factors of the formation leads to deviation are the formation anisotropy and formation angle.Because of the differentces of formation layering,joint,lamination,diagenetic state,rock composition,component,adglutinate and grain size,the drillability of the formation along different directions is not the same.Generally speaking,the drillability perpendicular to the layer is higher while paralleling to the layer is lower.The BHA factors are mainly its deformation and slope.The deformation and slope of BHA will induce two results:one is the slope of the drilling bit will cut the wellbore asymmetrically,the other is the bit will make side drilling by the side force,which makes the newly drilled borehole begin to deviate.The formation deviating force is the description of the effect that the formation makes the wellbore begin to deviate.It is impossible for human to change it,so the research aims at the optimization of the BHA.It can be seen by vestigation of baka block,that the slick assembly and pendulum bottomhole assembly with stabilizers are commonly used.The pricipal for deviation prevention is to balance the formation deviating force.The force that reduces deviation of the BHA is the bit side force.There are many BHA models,such as finite element method,energy method,beam column method.However beam columm method is of great accuracy,and it is fast on computer,so it is suitable for project application.The stress analysis of BHA is sdudied by beam column method and a stress model is established.Equation of three-moments is got.The model is combined with the pratical conditions of downhole BHA,and a model for calculating bit side force is developed under some assumptions.A model for calculating formation deviating force is also established and a program of BHA analysis is made by VB6.0.This program can compute the bit side force and formation deviating force of different BHA and drilling paramerters.It is found that the total force on the bit has close relationship with the changes of the deviation angle by program analysis.This shows that the method can be applied in the research.The used BHA of 19-6 well in baka block is assessed by the program.The angle-drop ability of the uses BHA is calculated and the bit side force can not balance the formation deviating force.The reason is the irrational selection of BHA and drilling parameters.The effect of different drilling paramerters on bit side force is analysied and the used BHA in baka block is optimized based on this.Pendulum bottomhole assembly with two stablizers and anti-pendulum Assembly are recomemded to baka block.It is found they have better angle-drop ability. |
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