Fracture modes in the Silurian Qusaiba Shale Play, Northern Saudi Arabia and their geomechanical implications
Fracture modes in the Silurian Qusaiba Shale Play, Northern Saudi Arabia and their geomechanical implications
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DOI：
10.1016/j.marpetgeo.2016.07.013
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发表时间：
2016-12
影响因子：
4.2
通讯作者：
M. Ameen
中科院分区：
文献类型：
--
作者：
M. Ameen
为了评估沙特阿拉伯北部志留系库塞巴页岩的非常规前景，进行了裂缝和地应力研究。研究中使用了钻孔图像测井、定向岩心、地震和钻井观测。裂缝包括天然裂缝和诱发裂缝。研究诱导断裂以评估应力状态的方向和大小。现今最大水平地应力趋势从NNW-SSE到NNE-SSW变化，呈现出以阿拉伯板块构造为主的区域格局。当前应力的相对大小是伸展到走滑状态的特征。微观（微裂缝）到宏观（宏观裂缝）尺度的天然裂缝包括伸展裂缝（节理/脉状），以及表现为剪切和混合（伸展-剪切和压缩-剪切）裂缝的断层。除非与断层带相关，否则节理聚集成区域的情况很少见。超过一半的断层（56%）显示出聚集成断层带，其宽度（厚度）变化最多5个数量级，长度和位移分别变化最多4个数量级。该研究确定了五个独特的区域性裂缝组：一个是平缓倾斜的（与层理平行或与层理呈低角度），以及多达四个中度至陡峭倾斜的裂缝组：最古老的东向裂缝组，其次是走向西北西-南SE、北-南和北北-南南西的三个较年轻的主要裂缝组。较新的裂缝几乎与当今最大水平地应力平行。裂缝封闭机制（自然水力压裂）主导着初始裂缝的生长，有些裂缝有几个阶段的部分至完全矿化或覆盖，主要是方解石、石英和白云石。水和碳氢化合物气态和流体包裹体在裂缝的矿物充填中很常见。地应力和天然裂缝的区域性质意味着它们的发生、方向、相对优势以及相对年龄和相对孔径更容易预测和操纵，以进行井规划和完井，包括水力压裂。正演模型表明，天然裂缝网络在储层条件下并未受到严重应力，但当受到大规模水力压裂刺激时，它们和层理不连续性形成了复杂水力压裂网络生长的种子，该网络可能会从假定的应力屏障中生长出来。井中断层周围缺乏应力旋转支持了建模结果。微震监测提供两种类型的延时（增量）微震事件；与最大水平地应力和主要天然裂缝趋势平行的随机和线性模式。整体微震云与高分辨率钻孔图像映射的断层趋势没有独特的联系。这一发现对大规模水力压裂的未校准微震监测以绘制断层图的可用性提出了挑战。
Fracture and in-situ stress studies were conducted for unconventional prospect evaluation in the Silurian Qusaiba Shale, northern Saudi Arabia. Borehole image logs, oriented cores, seismic, and drilling observations were used in the studies. The fractures include natural fractures and induced fractures. The induced fractures were studied to assess the stress regime in terms of directions and magnitudes. The present day maximum horizontal in-situ stress trend varies from NNW-SSE to NNE-SSW, and shows a regional pattern dominated by Arabian plate tectonics. The relative magnitudes of the current day stresses are characteristic of an extensional to strike-slip regime. Natural fractures of microscopic (microfractures) to macroscopic (macrofractures) scales include extension fractures (joints/veins), and faults manifested as shear and hybrid (extensional-shear and compressional-shear) fractures. Joints clustering into zones are rare, unless when associated with fault zones. Over half of the faults (56%) show clustering into fault zones with their widths (thickness) varying by up to 5 orders of magnitude, and lengths and displacements varying by up to 4 orders of magnitudes respectively. The study identified five distinctive, regional, fracture sets: one gently dipping (bedding-parallel or at low angle to bedding) and up to four moderately to steeply dipping fracture sets: an easterly striking set is the oldest, followed by three younger major sets striking NNW-SSE, N-S, and NNE-SSW. The younger fractures are nearly parallel to the present day maximum horizontal in-situ stress. Crack-seal mechanism (natural hydrofracturing) dominates initial fracture growth, some with several phases of partial to complete mineralization or coating, dominated by calcite, quartz, and dolomite. Aqueous and hydrocarbon gaseous and fluid inclusions are common in the fractures' mineral filling. The regional nature of in-situ stresses and natural fractures means their occurrence, orientation, relative dominance, and relative age and relative apertures are easier to predict and manipulate for well planning and completion, including hydrofracturing. Forward modeling shows that natural fracture network are not critically stressed under reservoir conditions but when subjected to massive hydrofracture stimulation they and the bedding discontinuities form the seeds for the growth of a complex hydrofracture network that potentially grows out of presumed stress-barriers. Lack of stress rotation around faults in wells supports the modeling results. Microseismic monitoring gives time-lapse (incremental) microseismic events of two types; random and linear patterns parallel to maximum horizontal in-situ stress and the predominant natural fracture trend. Bulk microseismic cloud has no unique link to fault trends mapped from high resolution borehole images. This finding challenges the usability of uncalibrated microseimic monitoring of massive hydrofracturing to map faults.