New Techniques for Fracture Height Determination in Exploration Wells Drilled in the Xihu Sag, East China Sea
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摘要: 针对东海西湖凹陷多口探井压裂后，阵列声波交叉偶极子各向异性方法无法准确评价裂缝纵向高度的技术难题，研究了评价探井储层压裂后缝高的新方法。通过研究压后裂缝径向变化特点，利用纵波走时层析成像技术重建压裂前后井壁附近地层的二维速度剖面，如二维速度剖面压裂后较压裂前有明显差异，说明储层被压开；同时，压裂后会在主裂缝周围形成一系列微裂缝，通过偶极声波远探测成像技术对其进行成像，可以评价裂缝的纵向高度和横向深度。研究得知，与压裂前相比，压裂后形成的裂缝使井壁岩石声速降低20%，井周裂缝形成的强反射和散射区域表明井眼周围至少20 m的范围内形成了明显的“压裂体积改造”，通过对比可以准确判断裂缝的纵向高度，识别精度可达0.152 4 m。研究结果表明，纵波径向层析成像和偶极声波远探测成像2项技术可以解决无法准确评价探井裂缝高度的难题，提升压裂后的综合评价效果。Abstract: Anisotropy analyses of conventional array acoustic cross dipole data failed to determine the vertical heights of fractures formed in reservoir formations in exploration wells drilled in the Xihu Sag, East China Sea. To eliminate such technical challenges, innovative techniques have been developed to determine the fracture heights in reservoir formations after fracturing in such exploration wells. To determine the features in radial directions after fracturing operations, 2D velocity profiles for formations in vicinity of sidewalls before and after fracturing were created by using the longitudinal wave travel time tomography technique. It was found that such fracturing operations might make a huge difference such velocity profiles if the formation were fractured successfully. Meanwhile, the hydraulic fracturing operations might also create a series of micro-fractures around the major fractures. Capable of detecting those micro-fractures, the dipole acoustic far-field images could be used to determine heights and widths of the resulting fractures. Research results showed that the acoustic speed reduced by 20% after the fracture were created near the wellbore, whereas the strong acoustic reflection and scattering confirm the presence of volumetric fractures were created within 20 m around the well. The dipole acoustic far-field images could identify the fracture heights with high resolution up to 0.152 4 m. Combination of the longitudinal wave travel time tomography technique and the dipole acoustic far-field images could effectively enhance accuracy in the determination of fracture heights in exploration wells. Eventually, the comprehensive performances of fracturing operations can be assessed more accurately.
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