Date of Graduation
Statler College of Engineering and Mineral Resources
Petroleum and Natural Gas Engineering
Huseyin Ilkin Bilgesu
Huseyin Ilkin Bilgesu
Ali Takbiri Borujeni
Shale has been usually recognized as a transverse isotropic (TI) medium in conventional geomechanical log interpretation due to its laminated nature. However, when natural fractures (NFs) exist in the rock body, additional elastic anisotropy can be introduced, converting Shale to an orthorhombic (OB) medium. Previous study illustrates that treating the naturally fractured shale rock as a TI medium by ignoring the NF-induced anisotropy can cause the erroneous estimation of the geomechanical properties (i.e. Young’s modulus, Poisson’s ratio, brittleness index, and etc.) and in-situ stress. In this paper, the study is extended to quantify the impact of NF-induced elastic anisotropy on completion and frac designs in different actual shale reservoirs in U.S.
Published acoustic log data from five different shale formations (Bakken, Marcellus, Haynesville, Eagle Ford, and Niobrara) are collected and examined to determine their availability to generate the stiffness tensor of the representative TI background rock of each Shale reservoir. Natural fractures with different intensity values from 0 to 10 per foot, with shear wave splitting ranging from 0-5%, are introduced in the TI background rock to create the corresponding OB rock stiffness tensor. The OB stiffness tensors of different shale cases are finally converted back to the compressional and shear acoustic signals, which can be interpreted based on the TI or OB assumptions. The final output elastic moduli and in-situ stress results interpreted from different assumptions are compared, and the impact of NF-induced elastic anisotropy on completion and fracturing designs is quantified and fully understood for different shales.
The results show that the higher the natural fracture intensity within the shale rock body, the outcomes interpreted from TI and OB models are more deviated from each other. In addition to that, the impact of natural fracture induced anisotropy on geomechanical log interpretation is different in different shale reservoirs. Specifically, the magnitudes of Young’s modulus are overestimated for all five shale when ignoring natural fracture induced anisotropy in log interpretation. The overestimation is different for different layers of a single shale formation as well as different shales. Similarly, the magnitudes of the minimum horizontal stress are also overestimated by different extents for different shales. Moreover, ignoring natural fracture induced anisotropy leads to incorrect interpretation of stress contrast. The stress depth profiles of all five shales are identified for upper, middle, and lower zones. The stress difference between upper and middle zones (upper stress contrast) and between middle and lower zones (lower stress contrast) are calculated and compared for TI and OB models. It is interesting to observe a complex NF induced anisotropy impact on stress contrast interpretation. For example, both upper and lower stress
contrasts are overestimated for Bakken and Marcellus Shales, with a higher overestimation for Bakken. In contrast, both upper and lower stress contrasts are underestimated for Eagle Ford and Niobrara Shales, with a slightly higher underestimation for Niobrara. Regarding to Haynesville Shale, there is almost no impact on upper stress contrast, whereas a significant overestimation for lower stress contrast. Such complexity of impacts is believed to be closely associated with the mineralogy of the shale rocks as well as their lithology sequence across the identified upper, middle, and lower zones. Future study is needed to investigate the inherent relation between the NF induced impact and rock mineralogy. Because Young’s modulus, minimum horizontal stress, and stress contrast are all critical parameters for completion and hydraulic fracturing designs, ignoring natural fracture induced anisotropy can result in different kinds of erroneous or suboptimal designs for different shale reservoirs, which is also discussed and concluded in the thesis. The current study not only illustrates the importance of taking natural fracture induced anisotropy into account when performing geomehcanical log interpretation, but also provides guidance to the operators of the current five fields to better evaluate their current completion/fracturing design strategies and to determine if the natural fracture induced anisotropy impact should be corrected for their current designs or not, based on the monitored splitting of shear fast and slow wave slowness.
Dundar, Emre Can, "Impact of Natural Fracture Induced Elastic Anisotropy on Completion and Hydraulic Fracturing Designs" (2019). Graduate Theses, Dissertations, and Problem Reports. 3826.