Abstract

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.

Introduction

Hammer drilling has long been recognized to have the potential of drilling faster than conventional rotary drill, especially in some hard formations such as granite, sandstone, limestone, dolomite, etc.[1,2,3,4] With the same Weight on Bit (WOB) and Rotation per Minute (RPM), it has been demonstrated that percussive-rotary method could be 7.3 times faster than the conventional rotary method.[4] Air hammers have been used to drill gas wells in West Canada and Central USA since 1980s. In 1987, air hammers were tested on 27 wells in the Waterton, Jumping Pound, and Clearwater areas of Alberta, and in the Flathead valley of British Columbia. Average time to total depth for recent air drilled wells at Jumping Pound had been 80 days (best 66 days), compared to the record of 103 days drilled with mud.[5] In the Arkoma Basin of Central USA, a cost per foot reduction of 49% was realized with hammer bits.[6] Another significant advantage of air hammer drilling is less well deviation. In Ritchie county, West Virginia, there were some wells drilled into a high dipped formation, such as 45 degree. Less than 1 degree deviation was achieved for most wells, compared to 5 degree deviation with average roller cones.

Other attractions of hammer drilling include lower requirement for WOB, less contact time between bit and rock, longer bit life, and the generation of larger cuttings.[7] Some additional applications of percussion drilling have been proposed recently, such as using hammer impacts as steady seismic signals to estimate rock properties,[8] or as a steerable drilling device to provide down-hole rotation,[9] or sources for down-hole electricity generation, etc.

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