The scalar images (PP, PS, SP, and SS) of elastic reverse time migration (ERTM) can be generated by applying an imaging condition as crosscorrelation of pure wave modes. In conventional ERTM, Helmholtz decomposition is commonly applied in wavefield separation, which leads to a polarity reversal problem in converted-wave images because of the opposite polarity distributions of the S-wavefields. Polarity reversal of the converted-wave image will cause destructive interference when stacking over multiple shots. Besides, in the 3D case, the curl calculation generates a vector S-wave, which makes it impossible to produce scalar PS, SP, and SS images with the crosscorrelation imaging condition. We evaluate a vector-based ERTM (VB-ERTM) method to address these problems. In VB-ERTM, an amplitude-preserved wavefield separation method based on decoupled elastic wave equation is exploited to obtain the pure wave modes. The output separated wavefields are both vectorial. To obtain the scalar images, the scalar imaging condition in which the scalar product of two vector wavefields with source-normalized illumination is exploited to produce scalar images instead of correlating Cartesian components or magnitude of the vector P- and S-wave modes. Compared with alternative methods for correcting the polarity reversal of PS and SP images, our ERTM solution is more stable and simple. Besides these four scalar images, the VB-ERTM method generates another PP-mode image by using the auxiliary stress wavefields. Several 2D and 3D numerical examples are evaluated to demonstrate the potential of our ERTM method.

Sichuan shale gas development will move to reservoirs deeper than 3,500m TVD in the future after a production milestone breakthrough of 10 billion m3 per year from Southern Sichuan basin was achieved. 80% of Sichuan shale gas total resources will come from deep reserves compared to reservoirs at a shallower depth. Improvements in drilling efficiency are the key success factor of deep shale gas development to enhance production and cost control with the increasing activity. Due to complex engineering and geological conditions, drilling deep shale gas horizontal wells in the Southern Sichuan basin is more challenging than traditional shallower wells. The High Pressure and High Temperature (HPHT) harsh drilling environment has caused the frequent failure of the standard Rotary Steerable System (RSS), Measurement While Drilling (MWD), and Logging While Drilling (LWD) tools during recent drilling operations. The surface cooling system, combined with thermal mitigation practices, positively impacted the increasing trend of bottom hole circulating temperature (BHCT) and extended equipment life in short horizontal sections. However, thermal mitigation reduced in effectiveness with the increase in the length of the horizontal section as frictional heating increased. BHCT reached above 150degC while drilling and exceeded the operating limits of standard tools. The challenge of managing the circulating temperatures resulted in approximately 50% of the total runs in 2020 being tripped before the run objectives were met, creating non-production time (NPT) and significantly decreasing drilling efficiency. To overcome this challenge and reduce NPT, two options were evaluated. A high-temperature Motor bottom hole assembly (BHA) brought risks of poor well trajectory control, resulting in well placement issues during geosteering, and lower potential reservoir exposure. For the first time in China Shale gas, an HPHT RSS with near-bit gamma-ray imaging was selected to maximize drilling efficiency and reservoir exposure. In addition to the tool selection, an HT optimization process was created that included horizontal well BHCT modeling and prediction and deep shale gas RSS drilling best practices. The near-bit gamma imaging quality was enhanced to improve steering. These changes delivered record runs in deep shale gas long horizontal wells and significantly decreased NPT. Reducing the reliance on surface cooling systems also increased overall operating efficiency. This paper reviews the choice of equipment, implementation of HPHT RSS, and development of HT optimization process that improved the drilling efficiency, reduced well time and enhanced long horizontal well placement in this complex drilling environment.

Compared with one-way wave equation migration methods, reverse-time migration has advantages in the aspects of amplitude preservation. In this paper, we extend the angledomain true-amplitude acoustic reverse-time migration to elastic case. First, we propose to modify the source term as the boundary condition to reconstruct the vector source wavefield and use the true-amplitude wavefield separation method to obtain amplitude-preserved P-and S-waves. Then, the extended imaging condition and angle decomposition procedure are applied to obtain the relatively amplitude-preserved angle-domain common image gathers. In numerical examples, we carry out the amplitude preservation analysis by comparing the peak amplitudes of the angle-domain common image gathers with the theoretical reflection coefficients calculated by solving the Zoeppritz equations. The results demonstrate that the angle-domain common image gathers are not only kinematically correct but also relatively amplitudepreserved.

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The attractive feature of pure P-wave equations for anisotropic media is that it is completely free from shear-wave artifacts, and can alleviate the numerical instabilities caused by anisotropy. We present the first-order pure P-wave velocity-stress equation for transversely isotropic media with a vertical symmetry axis. Like most other pure acoustic anisotropic wave equations, our equation involves complicated pseudo-differential operators in space, which cannot be solved with the finite-difference method alone. For computational efficiency, we adopt an efficient hybrid pseudospectral (PS)/finite-difference (FD) scheme to solve the pure P-wave equation. The stability of the hybrid PS/FD scheme on central and staggered grids is investigated by von Neumann's method. Numerical tests on 2D synthetic examples demonstrate that the proposed pure P-wave equation provides stable and kinetically accurate simulation results for complex anisotropic media.