Drilling Hard Abrasive Rock Formations with Differential Hydraulic Hammers: Dynamic Modeling of Drillstring Vibrations for ROP Optimization

Abstract

Abstract In deep oil, gas, and geothermal well construction, percussion drilling is sometimes used to improve rate of penetration (ROP) and promote drilling efficiency when breaking hard abrasive rocks (with a uniaxial compressive strength (UCS) > 25 kpsi). Down-the-hole (DTH) differential pressure hydraulic hammers can be used to convert the hydraulic energy of the drilling mud into the percussion energy necessary for rock destruction. The appeal of using DTH hammers for deep hard rock drilling is that this is a highly mature, low-cost, and proven technology. Lacking, however, is a good model to describe mud hammer operation, allowing mud hammer parameters to be tuned for ROP optimization in hard rock environments. In this study, bond graph models are used to investigate the hydraulic and mechanical dynamics of typical DTH hammer stroking cycles. The Colebrook-White equation is used to derive the Darcy-Weisbach friction factor for pressure loss caused by the turbulent flow between hammer chambers. The interactions between the hammer, the bit, and the formation are represented by three mass-spring mechanical systems with various degrees-of-freedoms (DOFs). Rock breaking is modeled using an anisotropic non-linear spring, which can represent the depth of the penetration of the hammer's indenters into the rock. This spring model also allows for seamless integration with the system model. Dynamic percussion drilling behavior is simulated for various rock stiffness values while varying hammer parameters, such as input hydraulic pressure and piston-bit mass ratio. The simulation results provide estimations of both ROP and system energy transfer efficiency (ETE) expected for given hard rock drilling scenarios. The developed model can be used to optimize mud hammer parameters, which in turn will benefit ROP optimization in hard rock environments encountered while drilling deep oil and gas or geothermal wells.