Browsing by Subject "Fracture modeling"
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Item A new reservoir scale model for fracture propagation and stress reorientation in waterflooded reservoirs(2016-12) Bhardwaj, Prateek; Sharma, Mukul M.It is now well established that poro-thermo-elastic effects substantially change the magnitude and orientation of in-situ stresses. Fractures induced in injectors during water injection for waterflooding or produced water disposal have a profound impact on waterflood performance. These effects, coupled with injectivity decline due to plugging caused by injected particles, lead to permeability reduction, fracture initiation and propagation. Models are available for fracture propagation in single injection wells and single layered reservoirs that account for these effects. However, the impact of fluid injection and production on fracture growth in multiple wells and multi-layered reservoirs with competing fractures, has not been systematically modelled at a field scale. In this work, a three-dimensional, two-phase flow simulator with iteratively coupled geomechanics has been developed and applied to model the dynamic growth of injection-induced fractures. The model is based on a finite volume implementation of the cohesive zone model for arbitrary fracture propagation coupled with two-phase flow. A dynamic filtration model for permeability reduction is employed on the fracture faces to incorporate effects of internal damage and external filter cake build-up due to the injection of suspended solids and oil droplets. All physical phenomena are solved in a single framework designed for multi-well, field-scale simulation. The pressure distribution, saturation profile, thermal front, mechanical displacements and reservoir stresses are computed as fluids are injected and produced from the reservoir. Simulation results are discussed with single as well as multiple fractures propagating. Stress reorientation due to poroelastic, thermoelastic and mechanical effects is examined for the simulated cases. The orientation of the fractures is controlled primarily by the orientation of the stresses, which in turn depends on the pattern of wells and the rates of injection and production. The sweep efficiency of the waterflood is found to be impacted by the rate of growth of injection-induced fractures. Heterogeneities in multi-layered reservoirs strongly govern the expected vertical sweep and fluid distribution, which impacts the cumulative oil recovery. This is the first time a formulation of multiphase flow in the reservoir has been coupled with dynamic fracture propagation in multiple wells induced by solids plugging while including poro-thermo-elasticity at the reservoir scale. The model developed in this work can be used to simulate multiple water injection induced fractures, determine the reoriented stress state to optimize the location of infill wells and adjust injection well patterns to maximize reservoir sweep.Item Shale fracturing enhancement by using polymer-free foams and ultra-light weight proppants(2013-12) Gu, Ming, active 21st century; Mohanty, Kishore KumarSlickwater with sand is the most commonly used hydraulic fracturing treatment for shale reservoirs. The slickwater treatment produces long skinny fractures, but only the near wellbore region is propped due to fast settling of sand. Adding gel into water can prevent the fast settling of sand, but gel may damage the fracture surface and proppant pack. Moreover, current water-based fracturing consumes a large amount of water, has high water leakage, and imposes high water disposal costs. The goal of this project is to develop non-damaging, less water-intensive fracturing treatments for shale gas reservoirs with improved proppant placement efficiency. Earlier studies have proposed to replace sand with ultra-light weight proppants (ULWP) to enhance proppant transport, but it is not used commonly in field. This study evaluates the performance of three kinds of ULWPs covering a wide range of specific gravity and representing the three typical manufacturing methods. In addition to replacing sand with ULWPs, replacing water with foams can be an alternative treatment that reduces water usage and decreases proppant settling. Polymer-added foams have been used in conventional reservoirs to improve proppant placement efficiency. However, polymers can damage shale permeability in unconventional reservoirs. This dissertation studies polymer-free foams (PFF) and evaluates their performance. This study uses both experiments and simulations to assess the productivity and profitability of the ULWP treatment and PFF treatment. First, a reservoir simulation model is built in CMG to study the impact of fracture conductivity and propped length on fracture productivity. This model assumes a single fracture intersecting a few reactivated natural fractures. Second, a 2D fracturing model is used to simulate the fracture propagation and proppant transport. Third, strength, API conductivity and gravity settling rates are measured for three ULWPs. Fourth, foam stability tests are conducted to screen the best PFF agents and the selected foams are put into a circulating loop to study their rheology. Finally, empirical correlations from the experiments are applied in the fracturing model and reservoir model to predict productivity by using the ULWPs with slickwater or using the PFFs with sand. Experimental results suggest that, at 4000 psi with concentrations varying from partial monolayer (0.05 lb/ft²) to multilayer (1 lb/ft²), ULW-1 (polymeric) is the most deformable with conductivity of 1-10 md-ft. ULW-2 (resin coated and impregnated ground walnut hull) is the second most deformable with similar conductivity. ULW-3 (resin coated porous ceramic) is the least deformable with conductivity of 20-1000 md-ft, which is comparable to sand. Three foam formulations (A, B: regular surfactant foam, C: viscoelastic surfactant foam) are selected based on the stability results of fourteen surfactants. All PFFs exhibit power-law rheological behavior in a laminar flow regime. The power law parameters of the regular surfactant PFF depend on both quality and pressure when quality is higher than 60% but depend on quality only when quality is lower than 60%. Simulation results suggest that under the optimal concentration of 0.04-0.06 v/v (0.37-0.55 lb/gal) for both ULW-1 and ULW-2, and 0.1 v/v (1.46 lb/gal) for ULW-3, 1-year cumulative production for 0.1 µD shale reservoir is higher than sand by 127% for ULW-1, 28% for ULW-2, and 38% for ULW-3. The productivity benefits decrease as shale permeability increases for all three ULWPs. ULW-1 and ULW-2 have higher productivity benefits for longer production time, while ULW-3 has relatively constant productivity benefits over time. The economic profit of ULW-1 when priced at $5/lb is 2.2 times larger than that of sand for 1-year production in 0.1 µD shale reservoirs; the acceptable maximum price is $10/lb for ULW-1, $6/lb for ULW-2, and $2.5/lb for ULW-3. The maximum price increases as production time increases. The PFFs with a quality of 60% carrying mesh 40 sand at a partial monolayer concentration of 0.04 v/v (0.88 lb/gal) can generate 50% higher productivity, 74% higher economic profit, and over 300% higher water efficiency than the best slickwater-sand case (mesh 40 sand at 0.1 v/v) for 1-year production in 0.1µD shale reservoirs. The benefits of using the PFFs decrease with increasing shale permeability, increasing production time, or decreasing pumping time. This dissertation gives a range of field conditions where the ULWP and PFF may be more effective than slickwater-sand fracturing.Item Steel fracture modeling at elevated temperature for structural-fire engineering analysis(2015-12) Cai, Wenyu; Engelhardt, Michael D.; Helwig, Todd A.; Tassoulas, John; Ghannoum, Wassim; Ezekoye, OfodikeOne of the key elements of performance-based structural-fire safety design is the ability to accurately predict thermal and structural response to fire. For steel structures, significant advances have been made in using finite element models for predicting the response of members, connections and entire structural systems exposed to fire. However, predicting the initiation and propagation of fracture of structural steel at elevated temperatures is still very difficult and uncertain using even the most advanced finite elements software. Fracture plays a critical role in the response of steel structures to fire, and is particularly important in connection response, where fracture often controls both strength and deformation capacity. While advances have been made in computational prediction of the initiation and propagation of fracture in steel at room temperature, much less is known at elevated temperature. The objective of the research described in this dissertation was to evaluate the ability of existing ductile fracture models for metals to predict initiation and propagation of fracture in structural steel at elevated temperatures. The general finite element program Abaqus was used in this research to explore and evaluate various approaches for simulation of fracture. In the first part of this study, true stress-strain curves were developed for structural steel at ambient and elevated temperatures that extend to very large, post-necking strains. Then two different fracture criteria were studied for modeling steel fracture at ambient and elevated temperatures in Abaqus. These two fracture criteria are referred to as the ductile fracture criterion and the shear fracture criterion. Both predict the equivalent plastic strain at fracture as a function of the state of stress, most notably the stress triaxiality, but have different formulations and model parameters. Model parameters for each fracture criterion were estimated by a calibration process that involved developing finite element models of various tests reported in the literature of structural steel materials, members, and connections at ambient and elevated temperatures. To evaluate the capabilities and limitations of each model, a number of comparisons were made between tests of steel components that failed by fracture, and simulations of those tests. These evaluations were conducted for tests conducted at temperatures ranging from ambient up to 1000C. Results of this work showed that the calibrated ductile fracture model was able to reproduce the experimentally observed behavior of tension coupons at elevated temperatures, all the way up through complete fracture. However, this same calibrated ductile fracture model was significantly less accurate in predicting the experimentally observed elevated temperature behavior of bolted steel connections. The model significantly overestimated the measured deformation capacity of the connections. This implies that the model overestimated the equivalent plastic strain at fracture for the states of stress developed in the regions of the bolted connections that experienced fracture. The calibrated shear fracture model, on the other hand, was capable of predicting the observed behavior of a wide range of bolted connection tests with reasonable accuracy. At any given temperature, the same shear fracture model parameters were able to reasonably predict the fracture of a variety of steel grades as well as high strength bolts. This suggests that the fracture model parameters may not be highly sensitive to changes in steel strength. Based on information in the literature and observations from this research, neither the ductile fracture model nor the shear fracture model is applicable across a full range of stress triaxiality values. The ductile fracture model appears to be most appropriate for predicting fracture under high levels of stress triaxiality, whereas the shear fracture model appears most appropriate for states of stress characterized by lower levels of stress triaxiality. The attempts at fracture simulations in this dissertation are based on limited experimental data and should be considered preliminary in nature. Far more work is needed to further develop these capabilities. Nonetheless, the numerous comparisons between simulations and experiments provided in this dissertation offer the hope that fracture behavior of steel connections and members at elevated temperatures can ultimately be simulated with confidence.