Abstract
This study is an evaluation of the feasibility of real-time drilling performance monitoring using a near-bit AE detection tool in drilling with PDC drag bits under laboratory conditions to investigate an improved drilling performance with a new Vibration Assisted Rotational Drilling tool. This paper focuses on calibrating the micro-seismic response to rock failure mechanisms, improved Rate of Penetration (ROP) and cutting particle-size distribution. Concrete cylinders with comparable properties to natural rock were fabricated in the laboratory. Drill-Off Tests (DOT) were conducted under rigid and compliant drilling with a two-cutter PDC bit. Simultaneously, micro-crack Acoustic Emissions (AE) from the bit-rock interaction process were monitored by four symmetrically mounted Non-Destructive Testing (NDT) sensors. The fracture characteristics were investigated by analyzing acoustic events in terms of event occurrence rate, and average event energy. Analysis from the DOT indicates that some factors are correlated with improved drilling performance, including particle-size distribution, vibration compliance setting and acoustic emission. A stronger vibration compliance setting increases ROP by increasing cutting depth per revolution and bigger cuttings are generated. Higher average event energy corresponds to bigger cracking when cracking event rate increases. All of this has contributed to our understanding of the mechanisms of improved drilling performance.
1. INTRODUCTION
One of the major problems for field drilling engineers is how to drill to target formations fast and safely. In drilling subsurface formations a vertical oscillation generator was found to increase penetration rate [1] and this early stage of oscillation vibrator drilling was called Resonant Sonic Drilling. After that, both surface and offshore mechanical vibrator were introduced [2]. Vertical vibration applied to a bit leads to fluctuations in WOB which interact with rock. A new vibration tool developed in the Advanced Drilling Laboratory (ADL) at Memorial University was tested for improving ROP [3, 4]. Laboratory tests have shown that controlled amplitude and frequencies of vibration are available from this vibration tool and the effect of vibration on drilling efficiency and penetration rate was investigated.
Micro seismic events, or acoustic emission (AE), are the elastic waves produced when rock undergoes internal change, such as micro crack initialization and propagation. In non-destructive testing (NDT), piezoelectric transducers are commonly employed in detecting and monitoring micro crack propagation. As an alternative way to ‘see’ micro crack initiation and propagation, AE detection has been applied in triaxial compression tests to monitor the whole deformation process [5]. Some AE parameters such as dominant frequency, event energy and cumulative emission counts are related to the different deformation stages. Numerical simulation with PFC2d models has been used to simulate UCS tests to predict rock failure [6, 7]. In this PFC model, hit number is scaled by counting crack number under tri-axial compression tests. AE detection is commonly used in hydraulic fracturing (HF) researches. Fluids has been injected at high pressure into cylindrical rock to simulate hydraulic fracturing with AE detection of crack initiation and propagation [8].
