Research On The Method Of Seismic Ranging While Drilling In Gas Drilling

Gas drilling has the advantage in increasing penetration rate of deep formations and alleviating formation damage of tight sandstone reservoirs.But there are strict requirements for gas drilling,especially risk formation may lead to gas drilling failure.Predicting formation information ahead of bit is important to improve safety of gas drilling.So the paper will use numerical simulation and experiment to explore feasibility and precision of measuring distance ahead of bit.Based on the above results,the controlled source pup joint of gas drilling has been designed,which was used to predict formation information ahead of bit.The following conclusions are obtained:(1)The paper deduce wave equation of viscoelastic medium and Biot two-phase medium.The principle of staggered grid finite difference is used to solve the wave equation.Then the questions of frequency dispersion,stability and boundary condition are analyzed.Finally the seismic wave numerical simulation program is compiled using matlab software.(2)Excitation frequency and ranging accuracy of the controlled source are studied by using the seismic wave numerical simulation program.First radiation shape of seismic wave are drawn by using far-field displacement expression.Then excitation firequency of the source was simulated by using the model of horizontal low velocity 2m thin layer,inclined low velocity 2m thin layer and horizontal high speed 2m thin layer.Frequency range of identitying 2m thin gas reservoir are obtained in Sichuan area.Ranging accuracy of horizontal low and high speed thin layer model are simulated under the different detection distance.Finally the identification method of the fluid layer in front of the bit is discussed.(3)In order to prove feasibility and precision of sound-ranging at source,ultrasonic transducers with different frequencies are used to detect the distance between source and interface.Experimental results show:ultrasonic transducers of 500KHz,1 MHz and 2.SMHz can detect accurately the distance of source and interface in shale;but ultrasonic transducers can’t detect accurately the distance of source and interface in sandstone.(4)Based on results of numerical simulation and experiments,the controlled source pup joint of gas drilling are designed,which are used to predict the distance between drill bit and interface.Pulse signal is excited by the source pup joint using pulse coding technique.Impact energy and frequency are controlled by flow valve and rotary valve of the controlled source.(5)Gas injection pressure and piston quality have an effect on impact frequency and energy of piston,so the effect law is simulated by using air hammer simulation program.Under the condition of satisfying the air hammer working,with the increase of the gas injection pressure,the impact frequency and energy of piston increase;with the increase of the piston quality,the impact frequency increase,but the impact energy decrease.