Hydraulic Down-The-Hole (DTH) fluid hammer systems present an innovative, field-proven technology for drilling of slim to regular size boreholes down to any depths and in any type of geological formations, including hard and abrasive rocks. Such technology uses rotary-percussive mechanism, which generates high energy axial signals in the downhole environment. Recorded signals can be directly used for the seismic while drilling (SWD) purposes. In this technology, drill bit seismic source can provide useful data during and after drilling operations for further detailed analysis of the geological and geophysical characteristics of the subsurface i.e. around the well and ahead of the drill bit. The seismic and geophysical measurements provided by SWD can help to better understand and predict the location of a geothermal reservoir. The analyzed data can provide structural information for better location of fault zones and their directions, and identify other anomalies in the geological structures.
In the conventional cross correlation-deconvolution SWD approach, the method uses seismic recordings around the well, together with reference recordings at the top of the drill string or at the bottom hole in the close proximity to the drill bit. In standard SWD applications with conventional roller-cone bits and near vertical drilling, surface pilot measurements only can be utilized to record the drill-bit signal. Advantage of this approach is that no recording tools downhole are required. This is crucial for geothermal high-temperature wells, where the use of downhole electronics and recording tools tends to be problematic in higher geothermal gradients. In the process of acquiring seismic signals from the vibrating drill bit, the seismic signature of the drill bit source varies depending on the bit type, e.g.: fluid-powered DTH hammer, roller cone or PCD bits, as well as drilling conditions, the performance of the drilling rig and experience of the drilling personnel. To monitor these highly variable conditions, quality control during automated SWD may be performed using drilling parameters, typically transmitted to the SWD system by the mudlogging technology or drill-control unit available at the rig site.
Fig. 1. Schematic configuration of DTH hammer in a SWD experiment using the drill bit as a seismic source (Poletto et al., 2015)
Fig. 2. Wassara’s water-powered DTH (Down-The-Hole) hammer (source: www.renishaw.com)
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Author: Michal Kruszewski