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Abstract We have used a hydraulic fracture model to quantify fault rupturing promoted by injection and migration of fluid into a fault, which contains in-plane high-conductivity segments and out-of-plane jogs or branches. The fluid under elevated pressure can promote extension-shear fracturing. The model provides numerical results on the opening and pressure variations with position and time. High-conductivity segments aid penetration of the pressurized fluids, but limit fluid pressure increases, so as to sustain a stable rupture growth mode. In the presence of varying normal stresses, stable growth cannot be maintained by pressure variations leading to the unstable growth in shear, and rapid fault movement events can be triggered as the fault re-establishes stable growth, radiating seismic energy and accompanied by local backward slip. These coupled seismic and aseismic faulting processes are applicable to faults with jogs and branches, which interact with the main fault to produce changes in the local stress states. The opening and slip along jogs and branches, either pre-existing or induced by fluid flow, will not only contribute to fluid storage due to their suction pumping action, but also produce changes in the downstream flow rates. The slip along them can produce associated opening along the main fault, but their opening can increase the compressive stress across the main fault which restricts its opening. The deformation transfer at junctions can complicate the fracture and flow responses. 1. INTRODUCTION Injection of liquid waste has been found to result in generation of seismic events and some of these may be large enough to be felt at the surface [1, 2]. The total volume injected and the maximum size of the seismic events generated have been found to be correlated [3]. Microseismic monitoring of low-level induced seismicity generated by hydraulic fracturing is a technology applied in unconventional gas reservoirs [4- 6] for the purpose of mapping the extent of fracturing. Considerable effort has been devoted to detecting and locating seismic events associated with fluid injection. However, little attention has been paid to understanding the source mechanisms of seismic events for a pressurized fault [7, 8]. Hydraulic fracturing in lowpermeability reservoirs is strongly affected by natural fractures within the targeted rock layers. Stimulation improves the connection of these fractures to one another and to the hydraulic fracture. Slip on these natural fractures can be activated by stress changes to generate low-magnitude seismic events. Meanwhile, shearing and shear-induced dilation on the natural fractures enhances conductivity and increases the stimulated volume. The aperture distribution along a fracture, including the initial and subsequent propagation paths, controls the conductivity and pressure distributions resulting from the injection. An integrated coupled hydraulic fracture model is applied in this paper to this problem to predict the relationship among stress, pressure, deformation and rupture growth.
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (1.00)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
A 2D Experimental Method with Results for Hydraulic Fractures Crossing Discontinuities
Jeffrey, R. G. (CSIRO Energy Flagship) | Kear, J. (CSIRO Energy Flagship) | Kasperczyk, D. (CSIRO Energy Flagship) | Zhang, X. (CSIRO Energy Flagship) | Chuprakov, D. (Schlumberger Doll Research) | Prioul, R. (Schlumberger Doll Research) | Schouten, J. (BG/QGC)
Abstract Experimental methods and results are presented for two-dimensional (2D) hydraulic fracturing experiments investigating the interaction and crossing of a hydraulic fracture with a pre-existing frictional discontinuity. The 2D experimental results include direct viewing and measurement of fracture path along with fracture and interface displacement. These data are compared directly to results from a 2D numerical hydraulic fracture model. The 2D experimental method involves propagating a hydraulic fracture from the centre of a 350 by 350 by 50 mm rock sample. Transparent shims are used to transmit the normal stress to the face of the sample and allow continuous digital video records to be made of the fracture growth and fracture interaction with the orthogonal discontinuity. Displacement of a grid drawn onto the sample face is measured by optical analysis methods, allowing the fracture and interface opening to be measured to better than 10-micron resolution. The data provide information about fracture crossing/arresting, path, and fracture and interface displacement with time. The numerical model matched the experiment in the sense of correctly predicting crossing in one case and blunting in the other. The model was unable to match the pressure from the experiments in siltstone, primarily because of not including effects from the pump and injection system compliance. 1. INTRODUCTION The mechanics involved in a hydraulic fracture interacting with a natural fracture have been extensively studied over the past years because this process produces a number of first order effects on fracture growth. The crossing interaction can result in branching, offsetting, and even blunting of the hydraulic fracture tip, significantly changing the fracture growth, excess pressure, and width. A verified method to predict the outcome of a crossing interaction is therefore necessary in any fracture stimulation model intended for design and post-treatment analysis of hydraulic fractures placed in naturally fractured rock. This paper is primarily concerned with describing and illustrating the use of a new experimental method that allows two dimensional hydraulic fractures to be grown and studied as they interact with a frictional interface. Therefore, only a brief review of previous work on the topic of hydraulic fractures interacting with frictional discontinuities is provided.
- North America > United States (1.00)
- Oceania > Australia (0.68)
Abstract Finite element method (FEM) is widely used in the study of soil mechanical problems, while traditional FEM is difficult to solve the high mesh distortion for large deformation. The preload process of jack-up based on the Arbitrary Lagrangian Eulerian (ALE) algorithm is studied. An axisymmetric model with layered seabed is established according to ZUOYE-5 jack-up. And the seabed is simulated with the Mohr-Coulomb elastoplasticity constitutive model. The ALE method can accurately simulate the large deformation of spudcan penetration, the ultimate bearing capacity of foundation and the soil flow -back mechanism.
Geomechanical Study of Fractured Carbonate Reservoir – Part II, 3D Wellbore Stability Analysis and FMI Image Logs Calibration
Hozayen, M. (ADCO) | Radwan, E. S. (ADCO) | Zhang, X. (Schlumberger) | Ni, Q. (Schlumberger) | Nawrocki, P. (Petroleum Institute) | Wang, D. (Petroleum Institute) | Jebbouri, A. (Petroleum Institute)
Abstract A major geomechanical study was conducted for one of the UAE onshore oil fields with the purpose of studying reservoir rock properties and stress distribution, based on which a 3D coupled reservoir geomechanical model was built. This model was later used for wellbore stability analysis and for analyzing the effect of fractures on future reservoir production performance, including fracture deformation and permeability evolution occurring due to reservoir depletion and resulting changes in local stress field. The subject of this study was a complex fractured Upper Cretaceous carbonate reservoir characterized by high degree of heterogeneity, both vertically and laterally. Wellbore instability related drilling problems were encountered while drilling the wells in this field. The problems were mainly in drilling the horizontal wells, including breakouts and mud losses. Experimental results of laboratory tests used in this study are summarized in Part I of this paper whereas wellbore stability analysis and formulation of a3D geomechanical model that was built for that purpose is summarized herein. A 3D geomechanical reservoir model was developed, which included natural fractures. The mechanical properties of the reservoir rock, such as Young's modulus, Poisson's ratio, UCS, friction angle and tensile strength were populated based on the correlations established in Part I of this paper and the developed model was then used to simulate the 3D mechanical-property distribution and the evolution of the in situ stresses in the field due to depletion from 1983 to present-day. Based on the full field model, mud weight cubes were developed to predict the safe mud weight windows in different drilling directions. In addition, well stability prediction for two planned wells was performed using a 3D near-borehole model. Firstly, the computed stress state and mechanical properties simulated in the full field model were extracted along the well trajectory of an existing horizontal well which was drilled recently and then, a 3D near-borehole model was constructed along the well trajectory. The wellbore stability analysis along the well trajectory, including stress state, strain distribution and displacements, was performed to identify the instability events along the well trajectory. It has been seen that the presence of natural fractures is closely related to the recorded borehole instability incidents. The computed plastic strains which indicate the wellbore failure were consistent with the recorded wellbore instability events in the FMI image logs, what provide further verification of the developed model. Then, this 3D near-borehole model was applied to two planned horizontal wells and the wellbore stability analysis for the two wells was carried out. The results showed that drilling vertically in the reservoir is safer than drilling horizontally. However, the drilling mud weight window in the vertical direction is narrow, being as narrow as 18.7 pcf or 0.299 sg locally. Drilling horizontally is less stable towards the maximum far field stress direction than towards the minimum far field stress direction. The mud weight cubes were used to identify the optimal well placement and well trajectory. For planned well trajectories, this technology can also be used to determine the optimal mud weight.
- Geophysics > Borehole Geophysics (1.00)
- Geophysics > Seismic Surveying > Borehole Seismic Surveying (0.51)
- Asia > Middle East > Kuwait > Ahmadi Governorate > Arabian Basin > Widyan Basin > Umm Gudair Field > Marrat Formation > Sargelu Formation (0.99)
- Asia > Middle East > Kuwait > Ahmadi Governorate > Arabian Basin > Widyan Basin > Umm Gudair Field > Marrat Formation > Najmah Formation (0.99)
- Asia > Middle East > Kuwait > Ahmadi Governorate > Arabian Basin > Widyan Basin > Umm Gudair Field > Marrat Formation > Marrat "C" Formation (0.99)
- (3 more...)
Abstract A major geomechanical study was conducted for one of the UAE onshore oil fields with the purpose of studying reservoir rock properties and stress distribution, based on which a 3D coupled reservoir geomechanical model was built. This model was later used for wellbore stability analysis and for analyzing the effect of fractures on future reservoir production performance, including fracture deformation and permeability evolution occurring due to reservoir depletion and resulting changes in local stress field. The subject of this study was a complex fractured Upper Cretaceous carbonate reservoir characterized by high degree of heterogeneity, both vertically and laterally. Wellbore instability related drilling problems were encountered while drilling the wells in this field. The problems were mainly in drilling the horizontal wells, including breakouts and mud losses. Experimental results of laboratory tests are summarized in this paper whereas wellbore stability analysis and formulation of a 3D geomechanical model that was built for that purpose is summarized in a sister paper (Part II). A suite of tests reported includes tensile strength, uniaxial and triaxial compression and permeability under stress at simulated reservoir conditions. All tested rocks have been grouped into twelve test units and reservoir geology, rock types and challenges related to designing a viable sampling and testing program for a big reservoir are discussed first followed by sample selection criteria and details of the testing program. All test units showed brittle behavior at zero confinement and some samples showed similar behavior at low confining stresses. With confining stress increase, the behavior gradually turned to plastic but some samples showed features of ductile behavior even at low confinement. Obtained experimental data have been summarized in the form of strength criteria and correlations. The Mohr-Coulomb strength equations were obtained for each test unit and also for the averaged values. It was noticed that triaxial compressive strength (TCS) increases with decreasing porosity or increasing density but their influence on TCS is getting smaller with confining pressure increase. Good correlation of porosity and density with Young's modulus, Poisson's ratio, unconfined compressive strength (UCS), friction angle, cohesion and tensile strength has been noticed and there was an increasing trend between the confining pressure and TCS. Rock type (mud or grain supported) was not influencing the TCS as much as porosity and density and the two lithologies encountered affected TCS approximately in the same way. These results and correlations are summarized and explained in this paper. They were further used in the numerical modelling part (Part II) of this paper, Hozayen et al. (2015). It is also concluded that rock lithology is not a decisive factor and mechanical properties of tested rocks should be rather considered dependent on porosity and permeability. This helped greatly in reducing the number of tests that had to be performed.
- Asia > Middle East > Turkey (0.28)
- Asia > Middle East > UAE > Abu Dhabi Emirate > Abu Dhabi (0.15)
- Geology > Geological Subdiscipline > Geomechanics (1.00)
- Geology > Rock Type > Sedimentary Rock > Carbonate Rock (0.98)