页岩水平井声波时差各向异性校正方法及其应用

Anisotropy correction method for acoustic time difference in horizontal shale wells and its application

声波时差是储层物性和地应力评价的关键参数,在地层可压裂性评价中具有重要作用。当前页岩储层以水平井开发为主,由于页理发育,水平井中的声波时差测量值与直井中的值存在较大差异,影响了储层评价的精度。基于时域有限差分算法,构建了大斜度井/水平井的声波时差模拟算法,系统模拟了不同相对井斜角和不同声波各向异性系数条件下的声波时差测井响应规律。当井斜角小于20°时,斜井声波时差值约等于竖直方向的声波时差值;当页岩地层声波各向异性系数大于0.04时,斜井和直井的声波时差差值较大,且差值随着声波各向异性和井斜角的增大而增大。不同竖直方向纵波时差条件下,声波时差随声波各向异性、相对井斜角的变化规律基本一致,但声波时差的变化范围随着竖直方向声波时差的增大而变大。斜井与竖直方向声波时差的差值相对值与井斜角成近似正弦函数关系、与竖直方向的时差值相关性较小。基于模拟结果,建立了不同声波各向异性系数条件下页岩水平井声波时差的校正方法。校正后声波时差计算的孔隙度与中子-密度孔隙度基本重合,且校正后声波时差计算得到的孔隙度与相邻直井的岩心孔隙度在峰值和分布区间上较为一致;校正后的声波各向异性比值为1.18,与岩石物理实验得到的声波各向异性比值(1.04~1.24)及其主峰区间(1.16~1.20)一致,表明校正方法应用效果较好,可用于页岩水平井声波时差校正。

Acoustic time difference is a key parameter for evaluating reservoir physical properties and ground stress,which is irreplaceable especially in the fracturability evaluation.At present,shale reservoirs are mainly developed by horizontal wells.Due to the development of lamellation,there is a big difference in the measured value of acoustic time difference between horizontal wells and vertical wells,in which case a large error will occur when reservoir evaluation is performed using the originally measured acoustic curve.Based on the finite-difference time-domain algorithm,this paper constructs the acoustic time difference simulation algorithm under the conditions of highly-deviated wells/horizontal wells,and systematically simulates the acoustic logging response regularity under different relative deviation angles of the well and different acoustic anisotropy coefficients.When the well deviation angle is less than 20°,the time difference of deviated wells basically equals to that of vertical wells;when the formation acoustic anisotropy coefficient is greater than 0.04,there is a big difference in the acoustic time difference between deviated and vertical wells,which also increases with the increasing of acoustic anisotropy coefficients and well deviation angles.Under the time difference of longitudinal waves in different vertical directions,the change laws of acoustic time difference with acoustic anisotropy and relative well deviation angle are basically consistent on the whole,but the change range increases with the time difference in vertical direction.The relative value of the difference between time differences of deviated and vertical wells has an approximately sinusoidal function relationship with well deviation angle,which is irrelevant to the time difference in vertical direction.Based on the above simulation results,this paper establishes the correction method for acoustic time difference in horizontal wells under different acoustic anisotropy coefficients.Finally,the above methods are applied to actual horizontal wells,and a comparison is performed between the porosity calculated based on the corrected acoustic time difference and that calculated by neutron-density,from which it can be found that the both are basically coincident,and the corrected acoustic porosity is nearly consistent with the porosity of adjacent vertical wells in terms of peak values and distribution intervals;the acoustic anisotropy ratio calculated after correction is 1.18,and falls within the range of acoustic anisotropy ratio obtained by petrophysical experiment,i.e.,1.04~1.24,consistent with its major peak interval,i.e.,1.16~1.20.This reflects that the correction effect is good,and the acoustic time difference of horizontal shale wells can be corrected.