Browsing by Subject "Cemented"

Now showing 1 - 2 of 2

Examining the effect of cemented natural fractures on hydraulic fracture propagation in hydrostone block experiments
(2012-08) Bahorich, Benjamin Lee; Olson, Jon E.; Holder, Jon
Micro seismic data and coring studies suggest that hydraulic fractures interact heavily with natural fractures creating complex fracture networks in naturally fractured reservoirs such as the Barnett shale, the Eagle Ford shale, and the Marcellus shale. However, since direct observations of subsurface hydraulic fracture geometries are incomplete or nonexistent, we look to properly scaled experimental research and computer modeling based on realistic assumptions to help us understand fracture intersection geometries. Most experimental analysis of this problem has focused on natural fractures with frictional interfaces. However, core observations from the Barnett and other shale plays suggest that natural fractures are largely cemented. To examine hydraulic fracture interactions with cemented natural fractures, we performed 9 hydraulic fracturing experiments in gypsum cement blocks that contained embedded planar glass, sandstone, and plaster discontinuities which acted as proxies for cemented natural fractures. There were three main fracture intersection geometries observed in our experimental program. 1) A hydraulic fracture is diverted into a different propagation path(s) along a natural fracture. 2) A taller hydraulic fracture bypasses a shorter natural fracture by propagating around it via height growth while also separating the weakly bonded interface between the natural fracture and the host rock. 3) A hydraulic fracture bypasses a natural fracture and also diverts down it to form separate fractures. The three main factors that seemed to have the strongest influence on fracture intersection geometry were the angle of intersection, the ratio of hydraulic fracture height to natural fracture height, and the differential stress. Our results show that bypass, separation of weakly bonded interfaces, diversion, and mixed mode propagation are likely in hydraulic fracture intersections with cemented natural fractures. The impact of this finding is that we need fully 3D computer models capable of accounting for bypass and mixed mode I-III fracture propagation in order to realistically simulate subsurface hydraulic fracture geometries.
Study of Mechanical and Flow Properties of Weakly Cemented and Uncemented Sands Using a Discrete Element Method
(2008-12) Likrama, Fatmir; Olson, Jon E.
Many natural gas and oil reserves are situated in uncemented and weakly cemented formations. These formations exhibit different mechanical and transport properties compared to cemented, consolidated ones. Phenomena such as sand production, wellbore stability, subsidence and permeability alteration during production have been reported in field experience and in numerous experimental results. Our work is intended to analyze these issues. A two dimensional discrete element method is developed using the commercial software PFC-2D, with the aim of studying the mechanical and transport properties of uncemented and poorly cemented sandstones. Unconfined compressive tests of poorly cemented sands, triaxial, hydrostatic compressive and radial extension tests of cemented and uncemented materials were simulated. Failure modes under different conditions and the effect of micromechanical properties of the rock’s constituents on the macroscopic behavior were studied. Comparison of our simulations to the experimental data was used to reaffirm the validity of the DEM method we are using. Pore collapse is observed in hydrostatic compression tests of wekly bonded sands with a critical pressure of 17 MPa. The failure mechanism of weakly cemented sands during triaxial tests changes at a critical confining pressure of about 4 MPa from brittle failure to strain thickening ductile failure. Using a pore network fluid flow model which takes into account the mechanics of the material from a microscopic point of view, permeability variation with deformation during triaxial, radial extension and hydrostatic compression tests was simulated and the results shown. The permeability of uncemented and weakly cemented sands is dependent on the stress state of the system and the stress path followed. Permeability of uncemented sands decreases by 55% at 5.4 MPa hydrostatic stress and by 35% before shear failure in triaxial tests of 1.0 MPa confining pressure. It increases up to 200% of the initial value during shear failure in 0.35 MPa confining stress triaxial tests and up to 200% during radial expansion tests. Weakly cemented sands’ permeability drops by 60% at 19 MPa hydrostatic pressure. Pore collapse which occurs at 17 MPa hydrostatic pressure speeds up the irrecoverable reduction in permeability. A 60% reduction in permeability of weakly cemented sands is also observed during 1.5 MPa confining pressure triaxial tests. Weakly bonded samples’ vertical permeability increases by 200% during radial expansion tests with an initial hydrostatic pressure of 5.0 MPa. A strong coupling between permeability evolution and stress-deformation state of the system exists and is confirmed in the results of all tests simulated.