Cutting of Siliceous Dolomiteby Special-Shaped PDC Cutter under Confining Pressure

Tian Wanqing1 Liu Weiji²Qin Dehong1 Liu Chenglong1Wang Haitao1Hu Hai² （1.PetroChina TarimOilfield Company;2.SchoolofMechanical Engineering，Southwest Petroleum University）

Abstract： The dolomite formation is an importantfield for oiland gas exploration inthe future.However，due to the high confining pressure environment in wellbore and thehard-to-drillcharacteristics of dolomite，traditional planar cuter PDC bit is prone to problems such as cuter breakage and loss when drillng in dolomite formations， making it dificult to meet the requirements of effcient drilling.To provideanew idea forthe cuterlayout of PDC bit thatcan eficientlydrill dolomite formations under confining pressure，basedon the independently developed rocking cutting and breaking device of true triaxial PDC cutter，taking siliceous dolomitecolected in the Tarim Basin as the test object，4 types of PDC cutters were selected to carry out indoor cuting tests under confining pressure.Moreover，the mass fraction，surface roughness，，fractal dimension and cutting force ofcuttings obtained aftera single cut were studied，andthe rock-breaking specific energyof different PDC cutters was compared.The research results show that the cuting force curves of axe-shaped cutter and multi-edged cutter are relatively smooh，with smallfluctuations，and the peak value of cutting force is also smaller.At a cutting depth of 1.0mm ，observed from the roughness index and fractal dimension ofcuttings，the axe-shaped cuter has the strongest ability tocause large volumebreakage，anditsrock breaking abilityis least affected byconfining pressure，folowed by the multi-edged cuter.The rock breaking abilityof conical cutteris most affected byconfining pressure.When theconfining pressure is not lessthan 15 MPa，the rock-breaking specific energy of conical cutter is the largest，followed by flat cutter，multi-edged cutter and axe-shaped cutterinturn.The research results providea theoretical basis for designing aggressive，impact-resistant and abrasion-resistantPDCbit cutterlayouts applicable to dolomite formations.

Keywords： PDC bit；dolomite； special-shaped cutter;confining pressure;；cutting force；rock-breaking specific energy； fractal dimension；roughness index

0 引言

据统计，世界上大约1/3的油气资源蕴藏在超深层（埋深大于 6000m ）碳酸盐岩中，其中又有1/2是白云岩[1]。近年来，白云岩地层逐渐成为油气勘探的重要接替领域[2]。作为中国最大的含油气盆地，塔里木盆地的深层、超深层寒武系和奥陶系广泛发育着巨厚难钻白云岩[3-4]。一方面，由于埋藏较深，白云岩地层所处井下环境苛刻，围压高达 170MPa ；另一方面，白云岩的岩石强度大、研磨性强，并且部分层段发育燧石结核。以上2个原因导致传统的平面齿PDC钻头在白云岩地层钻进过程中发生崩齿和掉齿等，难以满足高效钻进深部地层需求[5-6]。如LT1井鹰4段、蓬莱坝组、下秋里塔格组的难钻白云岩厚度达1480m ，单只钻头平均进尺仅 114m ，平均机械钻速只有 1.33m/h 。井下高围压环境与白云岩地层的难钻特性严重制约了深部油气资源的高效勘探开发。

近年来，许多学者和钻头厂家设计的一些具有非平面结构的PDC异形齿突破了传统PDC平面齿几何形状的限制，其不仅改变了PDC齿在破碎岩石时的受力状态和破岩方式，也改变了PDC齿的抵抗外界破坏的能力（耐磨性、抗冲击性等）[7-12]。为了给在高围压环境下能高效钻进白云岩地层的PDC钻头布齿设计提供新思路，降低钻井成本、缩短钻井周期，有必要探究高围压环境下异形齿在白云岩地层中的破岩特征。笔者基于自主研制的真三轴PDC单齿切削破岩装置，以塔里木盆地采集的硅质白云岩为试验对象，选取4种PDC齿，开展了围压条件下的室内切削试验，研究了单次切削后得到的岩屑的质量分数、表面粗糙度和分形维数等，并比较了不同PDC齿的破岩比功。……