Study On Cuttings Transportation Of Large Diameter Horizontal Directional Drilling

Horizontal Directional Drilling(HDD),with origins in the oil and gas industry,is a trenchless technology employed to install underground pipelines with minimal impacts on the environment or damage to existing infrastructure such as roadways and other surface structures.In order to meet the requirement of energy(such as oil,gas,electricity and water etc.),the diameter of pipelines is increasing.And the borehole diameter could be 10 times larger than the drill rod,which can result in very low mud velocity in the annular space(for example,if the borehole diameter reaches 1000 mm,the mud velocity in the annular space could be as low as 0.01m/s-0.02m/s).Based on this situation,the cuttings could hardly be moved out from the borehole.As common measures,the constructor would use the high-performance mud(which can provide high viscosity at low shear rate)and high pump rate(which can provide high mud velocity in the annular space)to improve the cuttings transportation performance.However,the efficiency of these methods is not significant,and could lead to exorbitant pressure loss along the circulation,also may cause the borehole instability,and finally results in crossing project failure.In this thesis,the theoretical analysis,experimental analysis and numerical simulation are used to investigate the cuttings transportation of large diameter HDD crossing project.Take the main influence factors(such as mud properties,mud flow field in the annular space,maximum allowable pressure in the borehole and the interaction between cuttings transportation and drill rod rotation)into consideration,this thesis provides the data base of high performance mud recipes,and reveals the characteristics of drill rod motion,and established the flow field function of annular space,and provides an new calculation method for the upper limit of maximum allowable pressure in the borehole,and finally reveals the law of cuttings transportation in large diameter horizontal directional drilling.The main study contents and achievements are summarized as follow:(1)Based on the summary of the advantages and disadvantages of mud rheological models,a series of laboratory tests have been done to investigate the impact of bentonite,soda ash and hydration time on mud and filter-cake properties.Additionally,the tests results can be the data base of high performance mud for large diameter HDD crossing projects.(2)Take the dynamic drill rod into consideration,the equations of motion and velocity field were established in cylindrical coordinates with proper boundary conditions.In addition,the numerical simulation was also employed to analysis the impact of construction parameters(such as the eccentric distance,rotation speed of drill rod,axial velocity of annular mud)on the flow field of annular space.The results also indicated the relationship between these construction parameters and cuttings transportation indirectly.(3)Established the influence relationship among the maximum annular mud pressure,flow field of annular space and cuttings transportation;then established the equation to calculation the maximum annular mud pressure and investigated the limitations of maximum annular mud pressure by laboratory tests and case study.(4)The laboratory tests and numerical simulation were employed to investigated the relationship among the cuttings sedimentation,the mud velocity in the annular space and cuttings size;the results shown that the large size cuttings could settled down in a short time;once the cuttings bed is formed,increasing the mud velocity,borehole inclination and mud properties could hardly break it.The rotating drill rod could break the cuttings bed,and partly increase the cuttings transportation performance.