Apparatus and methods of evaluating rock properties while drilling using acoustic sensors installed in the drilling fluid circulation system of a drilling rig

An apparatus for determining properties of rock in a formation in real-time during drilling, the apparatus comprising:

• an acoustic sensor installed in a drilling fluid circulation system of a drilling rig, the acoustic sensor coupled to one of the following:
• (i) a bell nipple, (ii) a gooseneck, or (iii) a standpipe, wherein the acoustic sensor is operable to detect an acoustic signal generated real-time as a result of rotational contact of a drill bit with rock during drilling and transmitted through the drilling fluid;
• a data acquisition unit in communication with the acoustic sensor and a computer, wherein the data acquisition unit is operable to sample the raw acoustic sensor data and digitize the raw acoustic sensor data, the raw acoustic sensor data representing the acoustic signal generated real-time as a result of rotational contact of a drill bit with rock during drilling;
• the computer in communication with the data acquisition unit and configured to perform the following operations:
• receiving the digitized acoustic sensor data from the data acquisition unit;
• processing the digitized acoustic sensor data received from the data acquisition unit, the processing comprising:
• sending sampling commands to the data acquisition unit in communication with the one or more acoustic sensors;
• converting analog acoustic signals into digitized data through employment of the data acquisition unit;
• transforming the digitized data into Fast Fourier Transform (FFT) data using a Fast Fourier transformation;
• filtering the FFT data;
• restoring the FFT data to compensate for attenuation by the drilling fluid;
• deriving the plurality of acoustic characteristics from the filtered and restored Fast Fourier Transform data;
• deriving a plurality of acoustic characteristics from the digitized acoustic sensor data, the plurality of acoustic characteristics including mean frequency and normalized deviation of frequency;
• and
• comparing the mean frequency and the normalized deviation of frequency for the rock undergoing drilling with mean frequency and normalized deviation of frequency for a plurality of rock samples having different known lithologies;
• identifying a lithology type of the rock undergoing drilling responsive to the comparing.