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Measurements and Modeling of the Shallow Pore Pressure Regime at the Sigsbee Escarpment: Successful Prediction of Overpressure and Ground-Truthing with Borehole Measurements

机译：Sigsbee陡坡处浅孔压力状态的测量和建模：井眼测量成功预测了超压和地层修整

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摘要

Shallow overpressures, and shallow water flow (SWF), arernsignificant hazards to deepwater drilling and facilities. In thisrnpaper we demonstrate how a combination of geomorphology,rnhydrogeology, and seismic interpretation has beenrnsuccessfully used to infer shallow overpressures, to develop arntestable hypothesis of pore pressure distribution, and torndelineate where to collect in situ data. In situ pore pressurernmeasurements provide ground-truth data validating the model,rnand can be used to evaluate the hazard of internally drivenrnfailure for facilities development scenarios.rnSeveral of the slope failures on the Sigsbee Escarpment in thernMad Dog and Atlantis field areas show a geomorphologyrndistinct from “top-down” slope failures. Specifically, thesernslumps have steep, amphitheatre-shaped headscarps, shallowlyrnsloping bases, sharp inflection points with the surroundingrnseafloor at both the top and bottom of the headscarp, a linearrntrend in map view, and linear side walls. These geomorphicrncharacteristics suggest that the slumps are formed byrninternally driven failure, and that the slumps growrnretrogressively by headward migration. Within the Mad Dogrnand Atlantis field areas, Mad Dog Slump 8 and Atlantis SlumprnA are the best examples of this morphology; in this paper wernwill focus our attention on Mad Dog Slump 8 due to itsrnproximity to planned facilities.rnAt any given slope gradient, failure related to internallyrnforcing can be facilitated by a combination of increased porernpressure or decreased sediment strength. In our analysis of thernMad Dog field area, we were not able to identify a mechanismrnfor weakening surficial sediment. We therefore used thernseafloor morphology and sub-surface seismic data from MadrnDog Slump 8 to infer both the distribution and magnitude ofrnshallow overpressure through 1D and 2D models.rnBased upon the results of these models, in situ pore pressurernmeasurements were acquired from a set of boreholes locatedrnwithin, behind, and adjacent to two slumps on the SigsbeernEscarpment. In situ pore pressure measurements werernobtained from multiple horizons in each borehole.rnMeasurements indicate that the upper section in the Mad Dogrnarea is normally pressured to very slightly overpressured, butrnthat the section below a prominent regional seismic reflector isrnoverpressured. 2D numerical modeling of the measured porernpressure indicates that there must be significant anisotropyrnand/or heterogeneous permeability at or immediately belowrnthis regional horizon, and that this anisotropy/heterogeneity isrncapable of projecting overpressures to near the seafloor.rnA similar analysis of Atlantis Slump E suggested that thisrnsection has been hydrologically isolated from the rest of thernsupra-salt section by a set of salt ridges. In situ pore pressuresrnat two Atlantis boreholes indicate normal pressures throughoutrnthe section, although the presence of saline-rich pore fluidsrnfrom borehole samples (and possible brine seeps) indicatesrnthat the flow regime is density driven and related to thernpresence of salt.
机译：浅超压和浅水流量（SWF）对深水钻井和设施构成重大危害。在本文中，我们演示了如何成功地利用地貌学，水文地质学和地震解释的组合来推断浅层超压，建立可验证的孔隙压力分布假说，并确定在何处收集现场数据。现场孔隙压力测量提供了验证该模型的真实数据，可用于评估内部驱动故障对设施开发情景的危害。rn疯狗犬和亚特兰蒂斯油田Sigsbee陡坡上的几次边坡破坏都显示出与“自顶向下”的边坡破坏。具体而言，震群具有陡峭的圆形剧场形的头皮，浅倾斜的底部，在头皮的顶部和底部都具有周围海底的明显拐点，在地图视图中呈线性趋势，以及线性侧壁。这些地貌特征表明，这些塌陷是由内部驱动的破坏形成的，并且这些塌陷由于向前迁移而向后退生长。在Mad Dogrnand Atlantis野外区域，Mad Dog Slump 8和Atlantis SlumprnA是这种形态的最好例子。在本文中，我们将把注意力集中在疯狗坍落度8上，因为它对计划中的设施很近。在任何给定的坡度坡度下，增加孔隙压力或降低沉积物强度都可以促进与内部施力有关的破坏。在对疯狗场区域的分析中，我们无法确定减弱表层沉积物的机制。因此，我们使用MadrnDog Slump 8的海底形态和地下地震数据通过一维和二维模型推断浅层超压的分布和大小.rn基于这些模型的结果，从位于其内的一组钻孔中获得了原位孔隙压力测量值，在Sigsbeern悬崖上的两个塌陷处，并与之相邻。从每个钻孔的多个层进行了现场孔隙压力测量。rn测量表明，Mad Dogrnarea的上部通常被压至非常轻微的超压，但是突出的区域地震反射器下方的部分被超压。测得的孔隙压力的二维数值模型表明，在该区域地平线处或该区域地平线以下必须存在明显的各向异性和/或非均质渗透率，并且这种各向异性/非均质性可能将超压投影到海底附近。AtlantisSlump E的类似分析表明，该剖面通过一套盐脊在水文上与其余超盐段隔离开来。尽管有来自钻孔样品的富含盐水的孔隙流体（以及可能的盐水渗漏）表明，流动方式是由密度驱动的并且与盐的存在有关，但两个Atlantis钻孔的原位孔隙压力表明整个断面都处于正常压力。

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