Complex deformation of naturally fractured rocks

• Published in: Journal of petroleum science & engineering Vol. 183; p. 106410
• Main Authors: Mokhtari, Mehdi, Nath, Fatick, Hayatdavoudi, Asadollah, Nizamutdinov, Rustam, Jiang, Shuxian, Rizvi, Hashim
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.12.2019
• Subjects: Complex fracture; Digital image correlation; Natural fracture; Optical technique; Strain; Natural fracture; Complex fracture; Digital image correlation; Optical technique; Strain
• ISSN: 0920-4105; 1873-4715
• DOI: 10.1016/j.petrol.2019.106410

Strain is a key parameter in the evaluation of rock mechanical properties and failure behavior. Strain is usually measured using strain gauges, however, this laboratory method has inherent uncertainty in the attachment of sensitive gauges to the rock surface and it is limited to one-point strain measurement. In complex geometries such as rock samples with natural fractures or lamination, that classical approach cannot provide the whole picture of strain development in such complex geometries. To overcome these challenges, the application of optical technique with image processing is proposed. The optical technique has the advantage of being a non-contact method and it provides quantitative visualization of full-field strain measurement. To examine quantitative visualization of strain development, DIC technique was first conducted on a homogeneous sandstone sample as a benchmark. The results verify that the maximum tensile strain is developed at the central vertical line of the sample as it was expected from theoretical solutions. This DIC measurement matches the post-failure fracture pattern of the specimen. Then the technique was applied on the Buda Limestone samples with horizontal, sub-horizontal or multiple complex natural fractures (sealed or partially open) and inclusions while the samples were under diametrical compression experiment. The results of this study provide further insight into the role of natural fractures on induced fracture initiation and propagation under this specific testing condition. Horizontal natural fractures are planes of maximum compressive strain which can delay the propagation of vertical tensile fractures. The maximum amount of tensile strain usually initiates at the boundary of these natural fractures and vertical tensile fractures usually deviate toward horizontal fracture for a short distance before turning back to propagate vertically following the maximum stress path. Full-field surface deformations are mapped optically in the crack–inclusion vicinity where DIC results showed that crack front is arrested by the inclusion for the complete fracture event. In contrast to the failure pattern in homogenous sandstone specimen, complex fracture pattern was observed on several specimens with multiple fracture network. Fracture slippages along the natural fracture, fracture deviation was observed in samples with steep inclined natural fractures. The results of this study can help us to better understand the complex failure mechanism in naturally fractured media and elaborate on various mechanisms of failure under diametrical compression tests. DIC is a powerful technique in providing the dynamic strain development over the surface of the sample in real-time, thus it can provide the fracture initiation and propagation in such heterogeneous rock media. •Full-Field strain profile was measured on samples with complex natural fractures.•The role of natural fractures on strain accumulation and subsequent fracture initiation and propagation was illustrated.•The difference between the behavior of partially open and sealed natural fracture behavior was distinguished.•The role of inclusion on fracture pattern was determined.•The shortcomings of Brazilian testing in the measurement of tensile strength in naturally fractured samples were discussed.