Natural fractures (NFs) are commonly encountered in unconventional reservoirs, sometimes strongly impacting the hydraulic fracture (HF) propagation. The HF-NF interaction has been studied extensively as a 2D problem, and almost all studies assume NFs to be continuous with the same properties over the entire NFs. However, it is a nearly ubiquitous feature that NFs in shale reservoirs are of different sizes and fully/partially filled with mineralization, leading to spatial variations in mechanical properties. These heterogeneous features of NFs may lead to distinctive interaction behaviors that can only be understood in a 3D setting.
Inspired by field observations, this research is aimed at studying the influence of NF heterogeneity on HF propagation. It is comprised of three main parts. First, analogue laboratory experiments were carried out to demonstrate the HF-NF interaction with variations of cemented proportion and cementation strength of NFs. Experimental observations prove that the spatial heterogeneity of NFs can significantly influence the HF propagation path. Three main patterns were observed as the size of the cemented region(s) decreases: (1) complete crossing (2) crossing with mismatched crack path (3) no crossing. Furthermore, based on and benchmarked with lab observations, a new 3D analytical criterion was developed from linear elastic fracture mechanics to quantitatively assess the dependence of HF-NF interaction behaviors on NF heterogeneity. Lastly, fully-coupled DEM (distinct element method) lattice simulation was conducted on the 3D growth of HFs crossing partially/fully cemented NFs. The simulation results are consistent with experimental observations and match the analytical criterion well in an extended parametric space of NF properties including twenty sets of simulation cases.
In summary, experimental, analytical and numerical approaches demonstrate the essential consideration of NF heterogeneity when estimating the HF propagation. This work reveals the 3D interaction patterns of HFs crossing partially-cemented and non-persistent NFs, and identifies the role of spatially-varied NF properties in HF-NF interaction. Most importantly, a new analytical criterion is found to give good agreement with both experimental and fully-coupled numerical results. It therefore can be used to embed more realistic criteria in numerical simulators aimed at predicting potentially complex and multi-stranded HF propagation in unconventional reservoirs.
|School:||University of Pittsburgh|
|School Location:||United States -- Pennsylvania|
|Source:||DAI-B 80/02(E), Dissertation Abstracts International|
|Subjects:||Geophysical engineering, Civil engineering, Petroleum engineering|
|Keywords:||Analytical criterion, Fracture complexity, Hydraulic fracturing, Laboratory experiments, Numerical simulation, Unconventional reservoir|
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