Study On Borehole Wavefield Simulation In Unconvetional Reservoirs

In the last decades,oil companies have been increasingly attracted to the use of horizontal wells for the exploration and development of unconventional reservoirs,which is mainly because of the high recovery expected from these reservoirs and the convenience of targeting desirable hydrocarbon-bearing zones.Due to velocity anisotropy,in-situ stress anisotropy and formation heterogeneity of unconventional reservoirs,conventional acoustic slowness logging and acoustic reflection logging methods are faced with serious challenges in horizontal and highly deviated wells.Finite difference numerical simulation technology is a powerful theoretical analysis tool for the study of acoustic logging methods and data processing algorithms.Firstly,different spatial difference operators are introduced for different partial differential quantities,and then a staggered grid finite difference scheme based on the first-order velocity-stress elastic wave equations in tilted transversely isotropic(TTI)media is deduced.With Bond transformation,highly deviated well in a VTI medium can be equivalent to horizontal well in a TTI medium,which realizes the numerical simulation of acoustic logging in horizontal well and high deviation wells penetrating VTI media;secondly,by hybriding the stretching factors of Convolutional Perfectly Matched Layer(C-PML)and Multi-axis Perfectly Matched Layer(M-PML),a new PML absorbing boundary — Hybrid-PML absorbing boundary(Hybrid-PML)is proposed.Then a three-dimensional finite-difference forward simulation program is programmed,which provides a tool to simulate acoustic logging in horizontal and highly diviated wells penetrating unconventional reservoirs.With the numerical simulation of different acoustic slowness logging models,the effects of high-velocity layers outside the borehole,thin cuttings layer inside the borehole,formation thickness,borehole dipping angle and anisotropy on acoustic slowness are analyzed.By numerical simulation of different acoustic reflection logging models,traveltime and amplitude information of the monopole reflected waves are systematically analyzed;acoustic reflection models with different azimuth of reflection interfaces are established,and cross-dipole acoustic reflection logging is simulated in isotropic/in-situ stress anisotropic formations,effects of in-situ stress anisotropy,azimuth of reflector and hetergeneousity induced by in-situ stress anisotropy on traveltime and amplitude of dipole reflected waves are summarized.The results show that the staggered grid finite-difference time-domain scheme based on TTI media,combined with the excellent Hybrid-PML absorbing boundary,can effectively realize the numerical simulation of three-dimensional TTI media,which provides a powerful forward modeling tool for the study of acoustic logging methods.Through establishing different acoustic logging models and extracting acoustic slowness,the response of acoustic slowness logging in TI medium under the conditions of horizontal and highly deviated wells is obtained and the acoustic slowness correction chart is established.Based on the numerical simulation of monopole acoustic reflection model in shale formation for horizontal well and highly deviated well,using amplitude information of q SV-q SV reflected wave in acoustic reflection imaging is proposed.Based on the numerical simulation of dipole acoustic reflection logging in isotropic and in-situ anisotropic tight sandstone formations,the response of dipole reflected waves under different azimuth of reflector and stress anisotropy is obtained.Besides,it's proposed that stress anisotropy will reduce the traveltime and amplitude of the SS reflected waves at short and medium spacings.The influence of in-situ stress anisotropy should be taken into account in acoustic reflection imaging process.