Study On The Cuttings Transport Characteristics And Hydraulic Optimization In Horizontal Well Drilling

Horizontal wells are widely used in exploring offshore and unconventional reservoirs across the world.The hole cleaning,hydraulic optimization and hydraulic extension capability prediction have become some of the main challenges and concerns in drilling extended-reach horizontal wells.Cuttings tend to accumulate on the low side of the wellbore and cause stuck pipe,lost circulation,high torque and drag and many other problems,which greatly increase drill cost and risk,limit the extension of horizontal wells and decrease the drilling efficiency.Therefore,investigating the cuttings transport characteristics and then optimizing the drilling parameters to achieve maximum drill bit hydraulic power and best hole cleaning is of great importance to facilitate the field application of horizontal wells.This study focuses on the critical velocity prediction for cuttings transport,the hole cleaning evaluation method,the hydraulic optimization and hole cleaning optimization method and the hydraulic extension length calculation method for drilling horizontal wells.Some assumptions in calculating the critical cuttings transport velocity have been modified.The drag force does not necessarily pass the particle centers,but is determined by the velocity distribution around the cutting particle.The velocity distribution for laminar helical flow in eccentric annulus and the boundary velocity distribution for turbulent flow were introduced to the model to take the effect of drill pipe rotation into account.Based on the forces balance analysis,the critical velocity prediction model for cuttings transport was established.The effects of different drilling parameters on the critical cuttings transport velocity were analyzed,and the result shows that: the critical velocity reaches its largest at the 50~70°hole section;moderately increasing the drilling fluid density,increasing the drilling pipe rotation rate and the drilling fluid viscosity under laminar flow is favorable for cuttings incipient motion.The effect of drill pipe rotation was introduced into the layer model by assuming that the drill pipe rotation increases the average annular velocity because drill pipe rotation changes the annular flow state from axial flow to helical flow.A three-layer steady-state model that considers the drill pipe rotation was developed,and the model construction equations and solution strategies were presented.Integrating the critical velocity prediction model and the layer model,the hole cleaning evaluation method was established.In the case study,the hole cleaning evaluation method were used to analyze the cuttings transport behavior in horizontal,highly inclined and small inclination hole sections of a horizontal well,respectively.The result shows that: flow rate is the most influential factor that affects hole cleaning;moderately increasing the drilling fluid density,increasing the drill pipe rotation speed and maintaining a reasonable penetration rate is favorable for improving hole cleaning.Considering the interrelations between breaking rocks and hole cleaning and the constraints from the whole drilling process,based on the ant colony algorithm,two types of hydraulic optimization methods that aim to maximize the drill bit horse power and minimize cuttings bed height respectively were established.The methodology of automatic optimization with the ant colony algorithm was proposed,which greatly simplifies the hydraulic optimization procedures and extends the application potential of hydraulic optimization.The hydraulic optimization and hole cleaning optimization methods established in this study were applied to optimize the drilling parameters of a horizontal well.The result shows that: the drilling hydraulic parameters should be jointly optimized considering the process of breaking rocks and hole cleaning;improving the pressure bearing capacity of circulating system,increasing the capability of handling hole cleaning problems and cooperating the drilling parameters can achieve better hole cleaning and larger drill bit hydraulic power.Considering the constraints of formation pressure and drilling pump pressure capacity,hole cleaning and breaking rocks hydraulic requirements,the method of calculating the hydraulic extension length for drilling horizontal wells was established.Then,the effects of various drilling parameters on hydraulic extension length for drilling horizontal wells were analyzed.The result shows that: properly designing drilling fluid density and flow rate,increasing the drill pipe rotation and improving the pressure bearing capacity of circulating system are favorable to increase the hydraulic extension length of horizontal wells.