三维随钻反射声波成像测井的数值模拟

采用有限差分方法对基于圆弧片状声源和接收器的三维随钻反射声波成像测井进行模拟,研究了反射信号的幅度、相位等参数随方位和源距的变化规律,着重分析了利用该方法在水平井中对地层界面进行探测的可行性.研究结果表明,钻铤的存在使得圆弧片状声源能够向固定方位辐射声场,其主瓣三分贝角宽窄,旁瓣级低,向目的方位辐射的纵波场的幅度约为传统环状声源的0.6倍,适用于三维随钻反射声波成像测井;对于本文计算的井孔模型,反射波信号约为井孔导波信号的1/100;随着源距的增加,反射纵波幅度逐渐减小,转换波(P-SV、SV-P)的幅度先增加后减小,反射SV波的幅度增加,建议在实际应用中,选择合适源距的波形进行处理,并对其他非目的波动进行压制,以期获得更好的成像效果.本文模拟了在水平井中对地层上下界面进行探测的例子,结果显示,反射波纵波信号有较好的方位分辨率,能够准确获得井外波阻抗不连续界面的方位,而且对上、下界面的成像互不影响.

Numerical modeling of three-dimensional acoustic reflection logging while drilling

The technique of reflected acoustic logging while drilling (RALWD), which is the development trend of the next-generation acoustic logging tool, affords to provide important data on rock formation structures and geological bodies in real-time and in three spatial dimensions (3D), which is helpful in guiding drill direction and optimizing well trajectory into the best position in the oil (gas) reservoir to achieve the desired effect of oil (gas) production or water injection. As the RALWD needs information in 3D, existing wireline single-well reflected acoustic methods, which have limited azimuth detection capability, cannot be adopted and new methodology is required. We propose a new asymmetric arcuate source/receiver, and investigate the feasibility of adopting this sources/receiver to perform RALWD in a fluid-filled borehole.The 3D acoustic reflection LWD with the asymmetric arcuate source/receiver is numerically simulated by adopting a 2.5D time-domain finite difference method (TDFD). The amplitude and phase of reflected signal versus azimuth angles and offsets is numerically investigated and analyzed. And based on the simulation results, we numerically study the feasibility to detect formation boundaries in horizontal wells by using these arcuate sources and receivers.Simulation results show that the arcuate source is able to transmit acoustic signals in desired direction with a radiation pattern including a main lobe of small angular width and side lobes of low degrees. The amplitude of the radiation wave field for this source is about 0.6 time that of conventional ring sources. These characteristics indicate that the proposed source is suitable for 3D RALWD. For the well model investigated in this paper, the reflected signals have an amplitude of 1/100 of borehole guided waves, and the amplitude of reflected compressional waves decreases with increasing offset, while that of reflected converted waves (P-SV, SV-P) increases first and then decreases as the offset increases, and the reflected SV strengthens with increasing offset. It suggests that it is possible to obtain better images by selecting waveforms with appropriate offset and suppressing non-target wave modes in the processing of field data. We also show results for a case of a horizontal well with the purpose to detect upper and lower rock formation boundaries by the arcuate sources and receivers. Results indicate that multi-signals, including reflected P and SV signals from the top and bottom boundary, are observed. The azimuths, which reach the maxima for reflected P-waves from top boundary and bottom boundary, are 0°and 180°, respectively, agreeing well with the input borehole model. The reflected P-waves are of desired azimuth resolution with a 3 dB of 48.6° and 49.4° for top and bottom boundaries, respectively. Note that, reflect signals from the top boundary do not exist in the waveforms of 180°, while those from the bottom boundary do not exist in the waveforms of 0°. These results suggest that it is possible to image top and bottom boundaries by processing the waveforms of the azimuths 0° and 180°, respectively, without interference to each other.The existence of a drill collar makes it possible for the source to omit acoustic signal along a desired azimuth direction, and the radiation pattern of the source has a main lobe with a narrow angle width as well as side lobes of low level, indicating that the proposed source is suitable for 3D RALWD. The case analysis for a horizontal well shows that the reflected compressional waves are of desirable resolution, and thus they can be utilized to obtain the azimuth of the reflector outside of the borehole.