Browsing by Subject "Unconventional reservoirs"
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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 Basin analog approach answers characterization challenges of unconventional gas potential in frontier basins(Texas A&M University, 2007-04-25) Singh, KalwantTo continue increasing the energy supply to meet global demand in the coming decades, the energy industry needs creative thinking that leads to the development of new energy sources. Unconventional gas resources, especially those in frontier basins, will play an important role in fulfilling future world energy needs. We must identify and quantify potential unconventional gas resources in basins around the world to plan for their development. Basin analog assessment is one technique that can be used to identify and quantify unconventional gas resources that is less expensive and less time consuming. We have developed a basin analog methodology that is useful for rapidly and consistently evaluating the unconventional hydrocarbon resource potential in exploratory basins. We developed software, Basin Analog System (BAS), to perform and accelerate the process of identifying analog basins. Also, we built a database that includes geologic and petroleum systems information of intensely studied North America basins that contain well characterized conventional and unconventional hydrocarbon resources. We have selected 25 basins in North America that have a history of producing unconventional gas resources. These are ??????reference?????? basins that are used to predict resources in frontier or exploratory basins. The software assists us in ranking reference basins that are most analogous to the target basin for the primary purpose of evaluating the potential unconventional resources in the target basin. The methodology allows us to numerically rank all the reference basins relative to the target basin. The accuracy of the results depends on the descriptions of geologic and petroleum systems. We validated the software to make sure it is functioning correctly and to test the validity of the process and the database. Finding a reference basin that is analogous to a frontier basin can provide insights into potential unconventional gas resources of the frontier basin. Our method will help industry predict the unconventional hydrocarbon resource potential of frontier basins, guide exploration strategy, infer reservoir characteristics, and make preliminary decisions concerning the best engineering practices as wells are drilled, completed, stimulated and produced.Item Development of an efficient embedded discrete fracture model for 3D compositional reservoir simulation in fractured reservoirs(2013-08) Moinfar, Ali, 1984-; Sepehrnoori, Kamy, 1951-; Johns, Russell T.Naturally fractured reservoirs (NFRs) hold a significant amount of the world's hydrocarbon reserves. Compared to conventional reservoirs, NFRs exhibit a higher degree of heterogeneity and complexity created by fractures. The importance of fractures in production of oil and gas is not limited to naturally fractured reservoirs. The economic exploitation of unconventional reservoirs, which is increasingly a major source of short- and long-term energy in the United States, hinges in part on effective stimulation of low-permeability rock through multi-stage hydraulic fracturing of horizontal wells. Accurate modeling and simulation of fractured media is still challenging owing to permeability anisotropies and contrasts. Non-physical abstractions inherent in conventional dual porosity and dual permeability models make these methods inadequate for solving different fluid-flow problems in fractured reservoirs. Also, recent approaches for discrete fracture modeling may require large computational times and hence the oil industry has not widely used such approaches, even though they give more accurate representations of fractured reservoirs than dual continuum models. We developed an embedded discrete fracture model (EDFM) for an in-house fully-implicit compositional reservoir simulator. EDFM borrows the dual-medium concept from conventional dual continuum models and also incorporates the effect of each fracture explicitly. In contrast to dual continuum models, fractures have arbitrary orientations and can be oblique or vertical, honoring the complexity and heterogeneity of a typical fractured reservoir. EDFM employs a structured grid to remediate challenges associated with unstructured gridding required for other discrete fracture models. Also, the EDFM approach can be easily incorporated in existing finite difference reservoir simulators. The accuracy of the EDFM approach was confirmed by comparing the results with analytical solutions and fine-grid, explicit-fracture simulations. Comparison of our results using the EDFM approach with fine-grid simulations showed that accurate results can be achieved using moderate grid refinements. This was further verified in a mesh sensitivity study that the EDFM approach with moderate grid refinement can obtain a converged solution. Hence, EDFM offers a computationally-efficient approach for simulating fluid flow in NFRs. Furthermore, several case studies presented in this study demonstrate the applicability, robustness, and efficiency of the EDFM approach for modeling fluid flow in fractured porous media. Another advantage of EDFM is its extensibility for various applications by incorporating different physics in the model. In order to examine the effect of pressure-dependent fracture properties on production, we incorporated the dynamic behavior of fractures into EDFM by employing empirical fracture deformation models. Our simulations showed that fracture deformation, caused by effective stress changes, substantially affects pressure depletion and hydrocarbon recovery. Based on the examples presented in this study, implementation of fracture geomechanical effects in EDFM did not degrade the computational performance of EDFM. Many unconventional reservoirs comprise well-developed natural fracture networks with multiple orientations and complex hydraulic fracture patterns suggested by microseismic data. We developed a coupled dual continuum and discrete fracture model to efficiently simulate production from these reservoirs. Large-scale hydraulic fractures were modeled explicitly using the EDFM approach and numerous small-scale natural fractures were modeled using a dual continuum approach. The transport parameters for dual continuum modeling of numerous natural fractures were derived by upscaling the EDFM equations. Comparison of the results using the coupled model with that of using the EDFM approach to represent all natural and hydraulic fractures explicitly showed that reasonably accurate results can be obtained at much lower computational cost by using the coupled approach with moderate grid refinements.Item Development of effective medium models for quantification of elastic properties and modeling of velocity dispersion of saturated rocks(2015-12) Sayar, Paul Mikhaël; Torres-Verdín, Carlos; Daigle, Hugh; Spikes, Kyle T; Olson, Jon; Sepehrnoori, KamyElastic effective medium theory (EMT) relates to quantitative rock physics modeling that calculates macroscopic properties of a mixture by incorporating the individual elastic properties, the volume fractions, and the spatial arrangement of the constituents that make up the rock. Despite the valuable merits of effective medium models, these theories exhibit limitations that require further investigation. Common instances are the non-unique configurations of the rock’s elements that give rise to identical wave velocities and the limiting assumption that rocks are purely elastic materials. Consequently, direct applications of classical EMTs can yield inaccurate and non-unique estimates of rock fabric properties that directly affect the assessment of elastic properties. The primary purpose of this dissertation is to improve the reliability of rock physics models based on the use of effective medium theories. In the first part, a rock physics model is developed for reliable estimation of velocities and elastic properties for sandstone-shale laminated rocks that are assumed to be vertical transverse isotropic (VTI). The new model is concerned with the reproduction of typical geological features and petrophysical properties of such formations that exhibit complex rock fabric. Isotropic and anisotropic versions of the self-consistent approximation and the differential effective medium theory, and Backus average are invoked to compute the effective medium’s stiffness tensor. The rock is separated into volumes of sandstone (regarded as isotropic) and shale (regarded as VTI), which are treated separately to reliably reproduce the spatial arrangement of the individual components included in the rock. Shale volumes enclose penny-shaped cracks and clay platelets aligned in the horizontal direction. Total porosity is divided into percolating porosity, isolated pores, and aligned fractures. The new simulation method is implement in three wells in the Haynesville shale and the Barnett shale. Estimates of elastic properties are verified when calculated velocities and sonic logs are in agreement. All relative differences between simulated and measured velocities are below 5.4%. To reduce non-uniqueness, electrical resistivity is calculated with modified effective medium theories and a procedure to compute Stoneley velocity is combined with the rock physics model. A method is advanced to calculate stress distribution and fracture initiation pressure around potential wellbores drilled horizontally in VTI rocks from the stiffness tensor obtained with the improved rock physics model. Effects of degree of anisotropy and elastic properties on fracture initiation pressure are investigated to determine a criterion to locate optimal depths along a vertical well to place a horizontal well. In the second part of the dissertation, an effective medium model is developed for reproduction of four of the main mechanisms of dispersion and attenuation of acoustic waves in saturated rocks. Simple and practical alternatives are introduced for effective medium modeling that account for dispersion mechanisms due to fluid flow inside the pore space. Biot’s flow and squirt flow effects are simulated by the calculation of frequency-dependent equivalent bulk and shear moduli for the solid background of the rock. When equal to the static moduli of minerals that compose the matrix of a rock at low frequencies, dynamic moduli of the solid background become complex at high frequencies and their absolute value increases. Frequency-dependent solid moduli are used as elastic properties of the matrix material in which fluid-filled porous inclusions are then added with dynamic self-consistent approximations for replication of acoustic scattering phenomena due to stiff pores and cracks. Resulting elastic features of the saturated medium calculated with the frequency-dependent effective medium model display viscoelastic behavior. Velocity predictions are conducted on synthetic examples to investigate conditions where dynamic rock physics modeling is necessary to obtain accurate elastic properties.Item Fracture abundance and strain in folded cardium formation, Alberta fold-and-thrust belt, Canada(2014-12) Ozkul, Canalp; Eichhubl, Peter; Ukar, Estibalitz, 1980-The folded and thrusted Mesozoic clastic sequence of the Canadian Rocky Mountain foothills forms important hydrocarbon reservoirs. Understanding the distribution of natural fractures, their evolution, and timing of formation relative to the evolution of the fold-and-thrust system could potentially improve exploration and development outcomes in these otherwise tight unconventional reservoirs. However, the formation of fractures and their timing relative to folding and thrusting have remained unclear. I investigated the relation between folding and fracture formation in the Upper Cretaceous Cardium Sandstone by combining field structural observations and kinematic modeling of the fold-and-thrust belt evolution. I explored the relationship between fracture intensity and fracture strain with structural position by analyzing fracture spacing or frequency and aperture data collected along outcrop and micro-scanlines in the backlimb, in the forelimb close to the crest, and in the steeper dipping forelimb away from the crest of the Red Deer River anticline. Fracture frequency and aperture data collected both at the outcrop and micro scales indicate that variation in fracture strain is small across these three structural domains of the fold, with somewhat lower fracture intensity in the forelimb close to the crest. These fracture strain measurements are qualitatively consistent with calculated horizontal strain in the tectonic transport direction obtained through kinematic numerical models that simulate fold development associated with slip along the underlying Burnt Timber thrust. The models predict roughly similar amount of horizontal extension in both the back and forelimbs, and somewhat lower extension in the upper forelimb during early development of the Red Deer River anticline. Fracture formation early during fold development is consistent with the field structural observations of shear reactivation during later stages of folding. This combined kinematic modeling and field structural study demonstrates that deforming fold and thrust belts can undergo a complex evolution of bed-parallel extension in both space and time, resulting in spatially variable fracture formation in such structurally complex subsurface reservoirs.Item Numerical modeling of complex hydraulic fracture development in unconventional reservoirs(2014-12) Wu, Kan; Olson, Jon E.; Balhoff, Matthew T.Successful creations of multiple hydraulic fractures in horizontal wells are critical for economic development of unconventional reservoirs. The recent advances in diagnostic techniques suggest that multi-fracturing stimulation in unconventional reservoirs has often caused complex fracture geometry. The most important factors that might be responsible for the fracture complexity are fracture interaction and the intersection of the hydraulic and natural fracture. The complexity of fracture geometry results in significant uncertainty in fracturing treatment designs and production optimization. Modeling complex fracture propagation can provide a vital link between fracture geometry and stimulation treatments and play a significant role in economically developing unconventional reservoirs. In this research, a novel fracture propagation model was developed to simulate complex hydraulic fracture propagation in unconventional reservoirs. The model coupled rock deformation with fluid flow in the fractures and the horizontal wellbore. A Simplified Three Dimensional Displacement Discontinuity Method (S3D DDM) was proposed to describe rock deformation, calculating fracture opening and shearing as well as fracture interaction. This simplified 3D method is much more accurate than faster pseudo-3D methods for describing multiple fracture propagation but requires significantly less computational effort than fully three-dimensional methods. The mechanical interaction can enhance opening or induce closing of certain crack elements or non-planar propagation. Fluid flow in the fracture and the associated pressure drop were based on the lubrication theory. Fluid flow in the horizontal wellbore was treated as an electrical circuit network to compute the partition of flow rate between multiple fractures and maintain pressure compatibility between the horizontal wellbore and multiple fractures. Iteratively and fully coupled procedures were employed to couple rock deformation and fluid flow by the Newton-Raphson method and the Picard iteration method. The numerical model was applied to understand physical mechanisms of complex fracture geometry and offer insights for operators to design fracturing treatments and optimize the production. Modeling results suggested that non-planar fracture geometry could be generated by an initial fracture with an angle deviating from the direction of the maximum horizontal stress, or by multiple fracture propagation in closed spacing. Stress shadow effects are induced by opening fractures and affect multiple fracture propagation. For closely spaced multiple fractures growing simultaneously, width of the interior fractures are usually significantly restricted, and length of the exterior fractures are much longer than that of the interior fractures. The exterior fractures receive most of fluid and dominate propagation, resulting in immature development of the interior fractures. Natural fractures could further complicate fracture geometry. When a hydraulic fracture encounters a natural fracture and propagates along the pre-existing path of the natural fracture, fracture width on the natural fracture segment will be restricted and injection pressure will increase, as a result of stress shadow effects from hydraulic fracture segments and additional closing stresses from in-situ stress field. When multiple fractures propagate in naturally fracture reservoirs, complex fracture networks could be induced, which are affected by perforation cluster spacing, differential stress and natural fracture patterns. Combination of our numerical model and diagnostic methods (e.g. Microseismicity, DTS and DAS) is an effective approach to accurately characterize the complex fracture geometry. Furthermore, the physics-based complex fracture geometry provided by our model can be imported into reservoir simulation models for production analysis.Item Simple mechanistic modeling of recovery from unconventional oil reservoirs(2015-05) Ogunyomi, Babafemi Anthony; Lake, Larry W.; Sepehrnoori, Kamy; Srinivasan, Sanjay; Jablonowski, Christopher J; Bickel, James EDecline curve analysis is the most widely used method of performance forecasting in the petroleum industry. However, when these techniques are applied to production data from unconventional reservoirs they yield model parameters that result in infinite (nonphysical) values of reserves. Because these methods were empirically derived the model parameters are not functions of reservoir/well properties. Therefore detailed numerical flow simulation is usually required to obtain accurate rate and expected ultimate recovery (EUR) forecast. But this approach is time consuming and the inputs in to the simulator are highly uncertain. This renders it impractical for use in integrated asset models or field development optimization studies. The main objective of this study is to develop new and “simple” models to mitigate some of these limitations. To achieve this object field production data from an unconventional oil reservoir was carefully analyzed to identify flow regimes and understand the overall decline behavior. Using the result from this analysis we use design of experiment (DoE), numerical reservoir simulation and multivariate regression analysis to develop a workflow to correlate empirical model parameters and reservoir/well properties. Another result from this analysis showed that there are at least two time scales in the production data (existing empirical and analytical model do not account for this fact). Double porosity models that account for the multiple time scales only have complete solutions in Laplace space and this make them difficult to use in optimization studies. A new approximate analytical solution to the double porosity model was developed and validated with synthetic data. It was shown that the model parameters are functions of reservoir/well properties. In addition, a new analytical model was developed based on the parallel flow conceptual model. A new method is also presented to predict the performance of fractured wells with complex fracture geometries that combines a fundamental solution to the diffusivity equation and line/surface/volume integral to develop solutions for complex fracture geometries. We also present new early and late time solutions to the double porosity model that provide explicit functions for skin and well/fracture storage, which can be used to improve the characterization of fractured horizontal wells from early-time production data.