Air hammers have been used to drill gas wells in West Canada and Central USA since 1980s. Field evidence has demonstrated air drilling can be significantly improved with hammer bits in terms of Rate of Penetration (ROP), hole geometry, cost per foot, etc. However, inconsistent results in different formations, risks in operation, and economic uncertainties impede hammer acceptance and development.

In an effort to improve understanding of drilling physics and predict hammer performance, a 3D numerical simulator of air hammer drilling is developed in this study. The main features include an elastoplastic material model for rock constitutive behavior, a rock damage model for strength reduction and damage accumulation because of cyclic hammer impacts, multiple rock failure criteria for initiation of rock breakage, rock dynamic characteristics for energy dissipation and non-reflective boundaries. The numerical tool is further calibrated with the results from a series of single impact tests.

The simulation describes how rock behaves during the drilling, including stresses propagation inside the rock, deformation and damage evolvement, and breakage occurrence. It produces an estimation of ROP for different hammer energy and formation properties. The records of rock failure history indicate aggressive tensile failure may be primarily responsible for rock breakage in air hammer drilling, while compressive failure (or shear failure) may only contribute as a minor player.

These developments advance the simulation technology of hammer drilling and improve fundamental understanding of the physics involved. More importantly, the simulator can serve as a new tool to achieve a more predictable hammer performance.

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