Browsing by Author "Yu, Wei"
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Item A probabilistic workflow for uncertainty analysis using a proxy-based approach applied to tight reservoir simulation studies(2016-08) Wantawin, Marut; Sepehrnoori, Kamy, 1951-; Yu, WeiUncertainty associated with reservoir simulation studies should be thoroughly captured during history matching process and adequately explained during production forecasts. Lacking information and limited accuracy of measurements typically cause uncertain reservoir properties in the reservoir simulation models. Unconventional tight reservoirs, for instances, often deal with complex dynamic flow behavior and inexact dimensions of hydraulic fractures that directly affect production estimation. Non-unique history matching solutions on the basis of probabilistic logic are recognized in order to avoid underestimating prediction results. Assisted history matching techniques have been widely proposed in many literature to quantify the uncertainty. However, few applications were done in unconventional reservoirs where some distinct uncertain factors could significantly influence well performance. In this thesis, a probabilistic workflow was developed using proxy-modeling approach to encompass uncertain parameters of unconventional reservoirs and obtain reliable prediction. Proxy-models were constructed by Design of Experiments (DoE) and Response Surface Methodology (RSM). As preliminary screening tools, significant parameters were identified, thus removing those that were insignificant for the reduced dimensions. Furthermore, proxy-models were systematically built to approximate the actual simulation, then sampling algorithms, e.g. Markov Chain Monte Carlo (MCMC) method, successfully estimated probabilistic history matching solutions. An iterative procedure was also introduced to gradually improve the accuracy of proxy-models at the interested region with low history matching errors. The workflow was applied to case studies in Middle Bakken reservoir and Marcellus Shale formation. In addition to estimating misfit function for the errors, proxy-models are also regressed on the simulated quantity of the measurements at various points in time, which is shown to be very useful. This alternative method was utilized in a synthetic tight reservoir model, which analyzed the impact of complex fracture network relative to instantaneous well performance at different stages. The results in this thesis show that the proxy-based approach reasonably provides simplified approximation of actual simulation. Besides, they are very flexible and practical for demonstrating the non-unique history matching solutions and analyzing the probability distributions of complicated reservoir and fracture properties. Ultimately, the developed workflow delivers probabilistic production forecasts with efficient computational requirement.Item Developments in modeling and optimization of production in unconventional oil and gas reservoirs(2015-05) Yu, Wei; Sepehrnoori, Kamy, 1951-; Chin, Lee; Delshad , Mojdeh; Mohanty, Kishore K; Patzek, Tadeusz WThe development of unconventional resources such as shale gas and tight oil exploded in recent years due to two key enabling technologies of horizontal drilling and multi-stage fracturing. In reality, complex hydraulic fracture geometry is often generated. However, an efficient model to simulate shale gas or tight oil production from complex non-planar fractures with varying fracture width along fracture length is still lacking in the petroleum industry. In addition, the pore size distributions for shale gas reservoirs and conventional gas reservoirs are quite different. The diffusivity equation of conventional gas reservoirs is not adequate to describe gas flow in shale reservoirs. Hence, a new diffusivity equation including the important transport mechanisms such as gas slippage, gas diffusion, and gas desorption is required to model gas flow in shale reservoirs. Furthermore, there are high cost and large uncertainty in the development of shale gas and tight oil reservoirs because of many uncertain reservoir properties and fracture parameters. Therefore, an efficient and practical approach to perform sensitivity studies, history matching, and economic optimization for the development of shale gas and tight oil reservoirs is clearly desirable. For tight oil reservoirs, the primary oil recovery factor is very low and substantial volumes of oil still remain in place. Hence, it is important to investigate the potential of CO₂ injection for enhanced oil recovery, which is a new subject and not well understood in tight oil reservoirs. In this research, an efficient semi-analytical model was developed by dividing fractures into several segments to approximately represent the complex non-planar fractures. It combines an analytical solution for the diffusivity equation about fluid flow in shale and a numerical solution for fluid flow in fractures. For shale gas reservoirs, the diffusivity equation of conventional gas reservoirs was modified to consider the important flow mechanisms such as gas slippage, gas diffusion, and gas desorption. The key effects of non-Darcy flow and stress-dependent fracture conductivity were included in the model. We verified this model against a numerical reservoir simulator for both rectangular fractures and planar fracture with varying width. The well performance and transient flow regime analysis between single rectangular fracture, single planar fracture with varying width, and single curving non-planar fracture were compared and investigated. A well from Marcellus shale was analyzed by combining non-planar fractures, which were generated from a three-dimensional fracture propagation model developed by Wu and Olson (2014a), and the semi-analytical model. Contributions to gas recovery from each gas flow mechanism were analyzed. The key finding is that modeling gas flow from non-planar fractures as well as modeling the important flow mechanisms in shale gas reservoirs is significant. This work, for the first time, combines the complex non-planar fracture geometry with varying width and all the important gas flow mechanisms to efficiently analyze field production data from Marcellus shale. We analyzed several core measurements for methane adsorption from some area in Marcellus shale and found that the gas desorption behaviors of this case study deviate from the Langmuir isotherm, but obey the BET (Brunauer, Emmett and Teller) isotherm. To the best of our knowledge, such behavior has not been presented in the literature for shale gas reservoirs to behave like multilayer adsorption. The effect of different gas desorption models on calculation of original gas in place and gas recovery prediction was compared and analyzed. We developed an integrated reservoir simulation framework to perform sensitivity analysis, history matching, and economic optimization for shale gas and tight oil reservoirs by integrating several numerical reservoir simulators, the semi-analytical model, an economic model, two statistical methods, namely, Design of Experiment and Response Surface Methodology. Furthermore, an integrated simulation platform for unconventional reservoirs (ISPUR) was developed to generate multiple input files and choose a simulator to run the files more easily and more efficiently. The fracture cost was analyzed based on four different fracture designs in Marcellus shale. The applications of this framework to optimize fracture treatment design in Marcellus shale and optimize multiple well placement in Bakken tight oil reservoir were performed. This framework is effective and efficient for hydraulic fracture treatment design and production scheme optimization for single well and multiple wells in shale gas and tight oil reservoirs. We built a numerical reservoir model to simulate CO₂ injection using a huff-n-puff process with typical reservoir and fluid properties from the Bakken formation by considering the effect of CO₂ molecular diffusion. The simulation results show that the CO₂ molecular diffusion is an important physical mechanism for improving oil recovery in tight oil reservoirs. In addition, the tight oil reservoirs with lower permeability, longer fracture half-length, and more heterogeneity are more favorable for the CO₂ huff-n-puff process. This work can provide a better understanding of the key parameters affecting the effectiveness of CO₂ huff-n-puff in the tight oil reservoirs.Item On countermeasures of worm attacks over the Internet(2009-05-15) Yu, WeiWorm attacks have always been considered dangerous threats to the Internet since they can infect a large number of computers and consequently cause large-scale service disruptions and damage. Thus, research on modeling worm attacks, and defenses against them, have become vital to the field of computer and network security. This dissertation intends to systematically study two classes of countermeasures against worm attacks, known as traffic-based countermeasure and non-traffic based countermeasure. Traffic-based countermeasures are those whose means are limited to monitoring, collecting, and analyzing the traffic generated by worm attacks. Non-traffic based countermeasures do not have such limitations. For the traffic-based countermeasures, we first consider the worm attack that adopts feedback loop-control mechanisms which make its overall propagation traffic behavior similar to background non-worm traffic and circumvent the detection. We also develop a novel spectrumbased scheme to achieve highly effective detection performance against such attacks. We then consider worm attacks that perform probing traffic in a stealthy manner to obtain the location infrastructure of a defense system and introduce an information-theoretic based framework to obtain the limitations of such attacks and develop corresponding countermeasures. For the non-traffic based countermeasures, we first consider new unseen worm attacks and develop the countermeasure based on mining the dynamic signature of worm programs? run-time execution. We then consider a generic worm attack that dynamically changes its propagation patterns and develops integrated countermeasures based on the attacker?s contradicted objectives. Lastly, we consider the real-world system setting with multiple incoming worm attacks that collaborate by sharing the history of their interactions with the defender and develop a generic countermeasure based on establishing the defender?s reputation of toughness in its repeated interactions with multiple incoming attackers to optimize the long-term defense performance. This dissertation research has broad impacts on Internet worm research since this work is fundamental, practical and extensible. Our developed framework can be used by researchers to understand key features of other forms of new worm attacks and develop countermeasures against them.Item Performance evaluation of CO2 EOR in tight oil formation with complex fracture geometries(2016-05) Zuloaga Molero, Pável; Sepehrnoori, Kamy, 1951-; Yu, WeiThe recent development of tight oil reservoirs has led to an increase in oil production in the past several years due to the progress in horizontal drilling and hydraulic fracturing. However, the oil recovery factor expected is still very low even after the wells have been fractured and therefore, tight formations are considered good candidates for enhanced oil recovery (EOR). One of the most suitable solutions to improve the oil recovery is the carbon dioxide (CO2)-based EOR. Although the injection of CO2 is not new for conventional oil reservoirs, its practice in tight oil formations is still a relatively novel idea. Two injection-production strategies are often employed: continuous CO2 injection or flooding and CO2 Huff-n-Puff. However, it is not clear which scenario is the best strategy to achieve an optimal recovery, which highly depends on many uncertain reservoir and fracture parameters and it is not clearly understood until recently. Another challenge of the estimation of the incremental recovery of these injection approaches is to properly model the hydraulic fractures and CO2 transport mechanism. The actual hydraulic fracturing process often creates complex fracture networks, especially when the fracture propagates in a formation with a large amount of pre-existing natural fractures. In this study, the CO2-EOR effectiveness is simulated and analyzed by comparing the Huff-n-Puff and the continuous injection scenarios. The effect of matrix permeability on the comparison of well performance of these two scenarios was investigated. Subsequently, Design of Experiment and Response Surface Methodology is used to perform sensitivity studies with four uncertain parameters including matrix permeability, number of wells, well pattern, and fracture half-length to determine the best injection approach. In addition, an efficient methodology of embedded discrete fracture model (EDFM) is introduced to explicitly model complex fracture geometries. The effects of complex fracture geometries on well performance of CO2 Huff-n-Puff and CO2 continuous injection were also investigated as well as the effect of natural fractures. The analysis of the CO2-EOR effectiveness confirms that the appropriate modelling of the complex fractures geometry plays a critical role in estimation of the incremental oil recovery. This study provides new insights into a better understanding of the impacts of reservoir permeability, complex hydraulic fractures and natural fractures on well performance during CO2-EOR process in tight oil reservoirs and in the determination and design of the optimal injection-production scheme to maximize the oil recovery factor for multi-fractured horizontal wells.