Characterizing, simulating, and eliminating vibration-induced counts in measurement-while-drilling gamma ray detectors

Abstract

A measurement-while-drilling (MWD) gamma ray tool will report a higher than actual count rate if its scintillation crystal package produces vibration-induced light pulses. The scintillation crystal is used as a transducer that measures the count rate of gamma rays coming from naturally occurring radioisotopes in downhole shales. Each gamma ray causes the crystal to emit a number of visible light photons that are converted into an electrical pulse by a photomultiplier tube. The pulse is shaped and digitized by tool electronics and registers as a count if it is above some threshold energy. The harsh shock and vibration conditions MWD gamma detectors experience can cause a crystal package to emit spurious light pulses which typical tool electronics cannot distinguish from genuine gamma counts, so the detector will report a higher than actual count rate. For ‘geosteering’ applications a higher count rate typically will indicate that the drill string trajectory must be changed to remain within a reservoir because such a response is indicative of a shale reservoir boundary; therefore vibration-induced counts will provide a false control input, making it difficult to steer correctly. This could cause the trajectory of the drill string to exit the reservoir. Measurements of vibration-induced counts from MWD scintillation packages were made using an electrodynamic vibration table, gamma ray spectroscopy electronics, and a digitizing oscilloscope. It was found that a scintillation package will produce counts as a function of the input g-level if the Rigid body resonant frequency of the crystal in its support structure is within the bandwidth of the operational vibration environment. These measurements provided the basis for computer simulations of MWD gamma ray logging that showed the detrimental effect of vibration-induced counts on log accuracy. It was concluded that stiffening the scintillation crystal mechanical support and, therefore, moving the rigid body resonant frequencies of the system outside the operational vibration bandwidth ensures the scintillation package will not produce vibration-induced counts in a specified bandwidth.