ROCK CHARACTERIZATION USING DRILLING PARAMETERS

Identifying natural fractures in contaminated metasandstone and metashale rock from conventional borehole investigations is an integral part of a bedrock bioremediation project in southeastern New Hampshire. A series of cored 100 mm boreholes approximately 10 m apart and 60 m deep have been used to develop protocols for bioremediation. This project used an innovative site characterization technique based on the recording of drilling parameters during the advance of each borehole. Drilling parameter recorders (DPR) are computerized systems installed on standard drilling equipment designed to continuously monitor and automatically collect data on advance rate, downthrust pressures, rod torque, rotation rate, mud/water pressure and flow, depth, and time.

For this project, carefully logged cores summarize lithologic variability, fracture attitude, spacing, and fracture surface types, as well as provide the basis for comparison with the DPR data. Drilling parameters are also compared to measurements from downhole geophysical methods such as videologging and acoustic televiewer monitoring. Data collected by the DPR significantly enhance the quality of information obtained from the boreholes and have the potential to lead to improvements in drilling equipment and techniques.

The DPR results using individual drilling parameters such as advance rate, downthrust pressure and torque, show that in cored boreholes it can also detect the presence of fractures. The drilling parameters are also combined into compound parameters to further enhance the capability of the DPR to identify rock types, fractures and other anomalies, as well as strength and deformability properties using empirical relationships. The use of drilling parameters measurements can potentially provide a significantly improved subsurface characterization at a modest investment of time and cost. For this project, the DPR has already helped improve the quality and efficiency of the drilling process and reduce the amount of fluid being introduced and lost into the aquifer. From the results of this investigation, a protocol for drilling and for the use of the DPR will be developed for future sites and for other geological conditions, providing characterization rapidly and more cost-effectively, thus limiting the need for expensive coring during rock investigations.