The design of a logging-while-drilling (LWD) sonic tool is always a challenge; the acoustic propagation along the tool cannot be ignored, nor can effects on measurements due to tool presence. It is well known that collar arrivals can interfere with compressional waves in fast formations. The interaction of the collar with other modes such as Stoneley and quadrupole must also be considered while designing an LWD acoustic tool. In this paper, we present an approach to optimize tool design for minimizing tool effects on measurements or making tool presence effects predictable for enabling a broadband use of the acquired data. Experimental results validate the design of the tool, and real log examples illustrate the quality of the acquired waveforms.
Sonic measurements provide useful information for understanding the rocks and fluids of a reservoir and surrounding formations. Therefore, sonic logging is one of the principal measurements used to evaluate the presence of hydrocarbons in the reservoir and to enable efficient and safe oil production. For example, in geomechanical analysis, sonic data can provide information for pore pressure, rock strength, formation alteration, stress direction, and magnitudes (Sinha, 2006). In petrophysical analysis, sonic data can be used to evaluate formation lithology and/or identify the fluid in the pore. LWD technology has progressed rapidly in recent years to address the needs of rig time savings and real-time decisions for drilling efficiency and risk management. Real-time LWD sonic measurements provide timely data for borehole stability analysis, drilling optimization, and assisting with pore-pressure prediction and seismic well ties. As sound waves propagate very efficiently along steel tool housing, tool arrivals are a significant issue when designing sonic logging tools, especially for LWD acoustic tools. Looking back through the history of wireline sonic logging, tool designers have made continual efforts to minimize tool arrivals by isolating the transmitters from receivers by means of a tortuous path of machined slots or grooves in the steel sonde. In the case of LWD, because the tools operate under very severe environments (torque, shock, vibration, etc.), the major structural part of the tool is a rigid drill collar (thick steel pipe), which is favorable for tool wave propagation. Minimizing tool arrivals is, therefore, one of the keys to obtaining a high-quality acoustic measurement while drilling. Field test data in different formation and borehole conditions show that LWD tools can acquire high-quality waveforms in a wide frequency band, providing reliable compressional and shear slownesses. Additionally, the tool has the capability to acquire low-frequency Stoneley data, thus enabling applications such as formation damage indicators and fracture evaluation.
To achieve high-quality measurement, modeling and experiments are essential, instead of solely relying on iterative experiences and intuition in the design process. Finite-difference method (FDM) modeling for elastic wave propagation plays an important role predicting the tool response in a borehole. By employing fine grid spacing, details of the tool design such as structure of receiver module as well as grooves on the collar are incorporated into the model.
