Seismic while drilling system and methods

Abstract

A bottom hole assembly is configured with a drill bit section connected to a pulse generation section. The pulse generation section includes a relatively long external housing, the particular housing length being selected for the particular drilling location. The long external housing is positioned closely adjacent to the borehole sidewalls to thereby create a high speed flow course between the external walls of the housing and the borehole sidewalls. The long external housing includes a valve cartridge assembly and optionally a shock sub decoupler. While in operation, the valve cartridge assembly continuously cycles and uses downhole pressure to thereby generate seismic signal pulses that propagate to geophones or other similar sensors on the surface. The amount of bypass allowed through the valve assembly is selectable in combination with the long external housing length and width to achieve the desired pulse characteristics. The bottom hole assembly optionally includes an acoustic baffle to attenuate wave propagation going up the drill string.

1 . A seismic-while-drilling system comprising:
 a bottom hole assembly located at the end of a length of tubing inserted into a borehole for drilling, the bottom hole assembly comprising:
 a drill bit section, 
 a motor section, and 
 a pulse generation section comprising an external housing having a valve cartridge assembly configured to create repeated axial impulsive loads to impart force to the drill bit section; and 
   at least one surface receiver configured to record ground motion due to shear waves radiated from the drill bit section.   
     
     
         2 . The seismic while drilling system of  claim 1 , further comprising a surface receiver configured to record ground motions due to pressure waves radiated from the drill bit section. 
     
     
         3 . The seismic while drilling system of  claim 1  further comprising a shock sub decoupler configured between said valve cartridge assembly and said drill bit. 
     
     
         4 . The seismic-while-drilling system of  claim 1  in which the pulse generation section is configured to cycle at a rate of 10 Hz or less. 
     
     
         5 . A method of detecting natural fracture networks in subsurface formations while drilling, comprising the steps of:
 running into a borehole a drillstring comprising:
 a bottom hole assembly comprising a drill bit section and 
 a pulse generation section; 
   drilling with the drill bit section of the bottom hole assembly;   generating shear wave impulses at the drill bit section by generating pulses with the pulse generation section;   recording surface motion due to shear waves radiated from the drill bit section; and   monitoring changes in shear wave amplitude as the borehole penetrates the formation.   
     
     
         6 . The method of  claim 5 , further comprising the steps of:
 generating pressure wave pulses with the pulse generator section recording surface motion due to pressure waves radiated from the drill bit section.   
     
     
         7 . The method of  claim 6  further comprising the step of comparing the amplitude of said shear waves with said pressure waves. 
     
     
         8 . The method of  claim 6  further comprising the step of comparing the arrival time of shear waves and pressure waves. 
     
     
         9 . The method of  claim 5  further comprising the step of operating the pulse generation section at a frequency of 10 Hz or less.