科罗拉多州Rulison油田多裂缝组的地震特征
Seismic characterization of multiple fracture sets at Rulison Field, Colorado
摘要：
传统的裂缝再表征方法假设在地下存在一组对齐的垂直裂缝。我们放松了这一假设，并证明了多裂缝集地震表征的可行性。我们的技术依赖于最近的数值结果，表明多个，不同取向，可能交叉的平面裂纹嵌入在一个其他各向同性的容矿岩的结果，在近正交（或正交各向异性）的有效介质。在这里，裂纹诱导的正交各向异性的控制参数估计从3D，宽方位角，多分量地震反射数据采集的致密气体在美国科罗拉多州的Rulison领域。我们翻译成间隔裂缝密度，裂缝方向，流体填充的类型，和速度的P-和S-波在未破裂的岩石的正常时差速度的强方位角变化。我们的反演程序确定了一组裂缝对齐约西北-东南方向在西部的研究区域和多个，可能在其东部的交叉断裂。我们验证了我们的基本理论模型和两个独立的测量所获得的估计：（i）估计的流体填充参数表示干裂缝，如预期的产气砂岩在Rulison;和（ii）所获得的裂纹方向的支持，在一口井中的测量。作为裂缝表征的副产品，我们建立了Rulison储层的各向异性速度模型，我们相信，这是第一个从地面地震资料构建的正交速度场。
英文摘要：
Conventional fracture-characterization methods assume the presence of a single set of aligned, vertical cracks in the subsurface. We relax this assumption and demonstrate the feasibility of seismic characterization of multiple fracture sets. Our technique relies on recent numerical findings, indicating that multiple, differently oriented, possibly intersecting planar cracks embedded in an otherwise isotropic host rock result in a nearly orthorhombic (or orthotropic) effective medium. Here, the governing parameters of crack-induced orthotropy are estimated from 3D, wide-azimuth, multicomponent seismic reflection data acquired over the tight-gas Rulison Field in Colorado, USA. We translate strong azimuthal variations of the normal-moveout velocities into interval crack densities, fracture orientations, type of fluid infill, and velocities of the P- and S-waves in an unfractured rock. Our inversion procedure identifies a set of cracks aligned in approximately WNW-ESE direction in the western part of the study area and multiple, likely intersecting fractures in its eastern part. We validate both our underlying theoretical model and the obtained estimates by two independent measurements: (i) the estimated fluid-infill parameter indicates dry cracks as expected for the gas-producing sandstones at Rulison; and (ii) the obtained crack orientations are supported by measurements in a well. As a by-product of fracture characterization, we build an anisotropic velocity model of the Rulison reservoir that, we believe, is the first orthorhombic velocity field constructed from surface seismic data.
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