Browsing by Subject "Natural fractures"

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A comprehensive numerical model for simulating two-phase flow in shale gas reservoirs with complex hydraulic and natural fractures
(2017-09-15) AlTwaijri, Mohammad Hamad; Sepehrnoori, Kamy, 1951-
Increase in energy demand has played a significant role in the persistent exploitation and exploration of unconventional oil and gas resources. Shale gas reservoirs are one of the major unconventional resources. Advancements in horizontal drilling and hydraulic fracturing techniques have been the key to achieve economic rates of production from these shale gas reservoirs. In addition to their ultra-low permeability, shale gas reservoirs are characterized by their complex gas transport mechanisms and complex natural and induced (hydraulic) fracture geometries. Production from shale gas reservoirs is predominantly composed of two-phase flow of gas and water. However, proper modeling of the two-phase behavior as well as incorporating the complex fracture geometries have been a challenge within the industry. Due to the limitation of the local grid refinement (LGR) approach, hydraulic fractures are assumed to be planar (orthogonal), which is an unrealistic assumption. Although more flexible approaches are available, such as the use of unstructured grids, they require significantly high computational powers. In this research, an efficient embedded discrete fracture model (EDFM) is introduced to explicitly model complex fracture geometries. The EDFM approach is capable of explicitly modeling complex fracture geometries without increasing the computational demand. Utilizing EDFM alongside a commercial simulator, a 3D reservoir model is constructed to investigate the effect of complex fracture geometries on the two-phase flow of a shale gas well. In this investigation, varying degrees of hydraulic fracture complexity with 1-set and 2-set natural fractures were tested. The simulation results confirm the importance of properly modeling fracture complexity, highlighting that it plays an integral part in the estimation of gas and water recoveries. In addition, the simulation results hint to the pronounced effect of fracture interference as fracture complexity increases. Finally, variable fracture conductivities and initial water saturation values were analyzed to further assess their effect on the two-phase production behavior of the shale gas well. This study examines the effect of non-orthogonal complex fracture geometry on the two-phase flow of shale gas wells. The work can provide a significant insight toward understanding the extent to which fracture complexity can affect the performance of shale gas wells.
Assisted history matching workflow for unconventional reservoirs
(2019-05-13) Tripoppoom, Sutthaporn; Sepehrnoori, Kamy, 1951-
The information of fractures geometry and reservoir properties can be retrieved from the production data, which is always available at no additional cost. However, in unconventional reservoirs, it is insufficient to obtain only one realization because the non-uniqueness of history matching and subsurface uncertainties cannot be captured. Therefore, the objective of this study is to obtain multiple realizations in shale reservoirs by adopting Assisted History Matching (AHM). We used multiple proxy-based Markov Chain Monte Carlo (MCMC) algorithm and Embedded Discrete Fracture Model (EDFM) to perform AHM. The reason is that MCMC has benefits of quantifying uncertainty without bias or being trapped in any local minima. Also, using MCMC with proxy model unlocks the limitation of an infeasible number of simulations required by a traditional MCMC algorithm. For fractures modeling, EDFM can mimic fractures flow behavior with a higher computational efficiency than a traditional local grid refinement (LGR) method and more accuracy than the continuum approach. We applied the AHM workflow to actual shale gas wells. We found that the algorithm can find multiple history matching solutions and quantify the fractures and reservoir properties posterior distributions. Then, we predicted the production probabilistically. Moreover, we investigated the performance of neural network (NN) and k-nearest neighbors (KNN) as a proxy model in the proxy-based MCMC algorithm. We found that NN performed better in term of accuracy than KNN but NN required twice running time of KNN. Lastly, we studied the effect of enhanced permeability area (EPA) and natural fractures existence on the history matching solutions and production forecast. We concluded that we would over-predict fracture geometries and properties and estimated ultimate recovery (EUR) if we assumed no EPA or no natural fractures even though they actually existed. The degree of over-prediction depends on fractures and reservoir properties, EPA and natural fractures properties, which can only be quantified after performing AHM. The benefits from this study are that we can characterize fractures geometry, reservoir properties, and natural fractures in a probabilistic manner. These multiple realizations can be further used for a probabilistic production forecast, future fracturing design improvement, and infill well placement decision.
Discrete fracture network modeling and simulation using EDFM
(2020-09-08) Leines Artieda, Joseph Alexander; Sepehrnoori, Kamy, 1951-
Recent advances in fracture network characterization have identified high degrees of heterogeneity and permeability anisotropy in conventional reservoirs and complex fracture network generation after well stimulation in unconventional reservoirs. Traditional methods to model such complex systems may not capture the key role of fracture network geometry, spatial distribution, and connectivity on well performance. Because of the ubiquitous presence of natural fractures in conventional and unconventional reservoirs, it is key to provide efficient tools to model them accurately. We extend the application of the embedded discrete fracture model (EDFM) to study the influence of natural fractures represented by discrete fracture network (DFN) models on well performance. Current state-of-the-art modeling technologies have been able to describe natural fracture systems as a whole, without providing flexibility to extract, vary, and group fracture network properties. Our developed implementations analyze fracture network topology and provide advanced mechanisms to model and understand fracture network properties. The first application features a numerical model in combination with EDFM to study water intrusion in a naturally fractured carbonate reservoir. We developed a workflow that overcomes conventional methods limitations by modeling the fracture network as a graph. This representation allowed to identify the shortest paths that connect the nearby water zone with the well perforations, providing the mechanisms to obtain a satisfactory history match of the reservoir. Additionally, we modeled a critically-stressed carbonate field by modeling faults interactions with natural fractures. Our workflow allowed to discretize the hydraulic backbone of the field and assess its influence on the entire field gas production. Our next application applies a connectivity analysis using an efficient and robust collision detection algorithm capable of identifying groups of connected or isolated natural fractures in an unconventional reservoir. This study uses numerical models in combination with EDFM to analyze the effect of fracture network connectivity on well production using fractal DFN models. We concluded that fracture network connectivity plays a key role on the behavior of fractured reservoirs with negligible effect of non-connected fractures. Finally, we performed assisted history matching (AHM) using fractal methods to characterize in a probabilistic manner the reservoir properties and to offer key insights regarding spatial distribution, number, and geometry of both hydraulic and natural fractures in unconventional reservoirs. In this work, we provided computational tools that constitute the foundations to conduct advanced modeling using DFN models in conjunction with EDFM in several reservoir engineering areas such as well-interference, water intrusion, water breakthrough, enhanced oil recovery (EOR) efficiency characterization, and fracture network connectivity assessments. The benefits of our work extend to conventional, unconventional, and geothermal reservoirs
Fracture growth in chemically reactive geologic systems : experimental and field studies
(2023-05) Doungkaew, Natchanan; Eichhubl, Peter; Helper, Mark; Tisato, Nicola; Espinoza, Nicolas; Gardner, James
Fractures in the subsurface play important roles in controlling mechanical and mass transport properties of the brittle crust. Following conventional linear-elastic fracture mechanics principles, natural rock fractures are expected to be elliptical in shape with small aperture-to-length or aspect ratios, in contrast to observed fracture shapes that frequently deviate from an ellipse with large aspect ratios. This research aims to test the hypothesis that deviations from linear-elastic fracture geometric characteristics reflect inelastic deformation mechanisms, especially under, although not limited to, chemically reactive subsurface conditions. The study follows two approaches, laboratory constrained sintering experiments of fracture formation and field-based structural and textural investigations of natural fractures. In constrained sintering experiments, I generated fractures under chemically reactive conditions associated with the reaction of jadeite and quartz to albite. Constrained sintering experiments conducted in a muffle furnace allowed fracture growth at atmospheric pressure and temperatures of up to 1000°C, with partial melt present at temperatures of 850°C and above. Constrained sintering experiments in a hydrothermal furnace were conducted in the presence of aqueous fluid at temperatures between 455°C and 600°C and pressure of 1 kbar. In both types of experiments, solution-precipitation creep is the main deformation mechanism associated with fracture growth. I also observed multiple stages of fracture growth concurring with the degree of sintering of the materials, resulting in the deviation of fracture shapes from an ellipse and large aspect ratios. In the absence of an externally applied tensile stress, fracture growth is driven solely by porosity reduction of the porous reacting mineral aggregate and the formation of a tensile sintering stress. In the second part of the study, I quantified shape characteristics of natural fractures, including fracture aspect ratios and fracture ellipticity or shape, and inferred their deformation mechanisms based on microscale textural and compositional analyses. The fractures are in quartz arenite (Travis Peak Fm. and Boyer Ranch Fm.), quartzite (Campito Fm.), rhyolite (Mono Craters), and sintered oxidized siliceous mudstone (Sisquoc Fm.). The fractures have non-elliptical shapes and large aspect ratios due to different rates of growth in aperture and length occurring in multiple stages due to deformation mechanisms such as solution-precipitation creep. While fractures in quartz arenite, quartzite, and oxidized siliceous mudstone formed under chemically reactive aqueous conditions, rhyolite fractures are associated with viscous flow, demonstrating that large fracture apertures are not necessarily unique to chemically reactive systems. The results demonstrate that inelastic deformation mechanisms, especially those associated with chemically reactive environments, can result in fracture characteristics that differ from those of brittle-elastic fractures. With fracture geometry affecting fracture aperture and connectivity, these results are significant for predicting the flow properties of fractured rock formation in the subsurface under chemically reactive conditions.
Hydraulic fracture modeling in naturally fractured reservoirs
(2020-03-27) Shrivastava, Kaustubh; Sharma, Mukul M.; Olson, Jon E.; Mohanty , Kishore; Prodanović , Maša; Bonnecaze, Roger
Hydraulic fracturing of horizontal wells is one of the key technological breakthroughs that has led to the shale revolution. Hydraulic fracturing models are used to engineer hydraulic fracture design and optimize production. Typically, hydraulic fracturing models treat hydraulic fractures as planar, bi-wing fractures. However, recent core-through investigations have suggested that during hydraulic fracturing in naturally fractured reservoirs, complex hydraulic fracture geometries can be created due to the interaction of the growing hydraulic fracture with natural fractures. This limits the application of planar fracture models for optimizing hydraulic fracturing design in naturally fractured reservoirs. In this research, we present a novel three-dimensional displacement discontinuity method based hydraulic fracturing simulator that allows us to model hydraulic fracture growth in the presence of natural fractures along with proppant transport in an efficient manner. The model developed in this dissertation is used to investigate the interaction of a hydraulic fracture with natural fractures and study the transport of proppant in the resulting complex fracture networks. This investigation gives us novel insight into the influence of fracture geometry and stress interference on the final distribution of proppant in fracture networks. Based on this investigation, suggestions are made to improve proppant transport in complex fracture networks. In order to correctly capture the effect of natural fractures on fracture growth, knowledge about the distribution of natural fractures in the reservoir is imperative. Typically, little is known about the in-situ natural fracture distribution, as direct observation of the reservoir is not possible. A novel technique of synthetic coring is developed to create a discrete fracture network (DFN) from core data, and it is used to create a DFN based on the Hydraulic Fracturing Test Site #1 data. Hydraulic fracture propagation is modeled in the created DFN, and the results are compared with field observations. As the reservoir may contain thousands of natural fractures, simulations in a realistic DFN can be computationally very expensive. In order to reduce the computational requirements of the simulator, we present a novel predictor step based on the local linearization method that provides a better initial guess for solving the fluid-solid interaction problem. This is shown to reduce computational time significantly. A novel technique, Extended Adaptive Integral Method, to speed up the simulator is developed. The method uses an effective medium to represent the interaction between displacement discontinuity elements and reduces the order of complexity of solving the geomechanical system of equations from O(N²) to O(NlogN). The novel formulation of this method is presented, and sensitivity studies are conducted to show the improvement in computational efficiency
Laboratory quantification and detection of pre-existing fractures and stress-induced microfracturing through combined ultrasonic and triaxial-stress testing
(2016-08) Ramos, Matthew John; Espinoza, David N.; Torres-Verdin, Carlos
Simultaneous triaxial stress testing and ultrasonic wave propagation were utilized to quantify natural fractures and microfracturing in Berea Sandstone and Silurian Dolomite. Experimental results indicate that the presence of fractures distinctly decreases wave velocities, with calculated dynamic elastic moduli decreasing by up to 7.5% in artificially fractured sandstone. Wave analysis of intact and artificially fractured Berea Sandstone reveal the nonlinear mechanical and geophysical response or fractured rocks subject to isotropic and deviatoric stress loading paths. Specifically, fractures increase hysteretic stress-strain behavior, and tend to amplify the stress dependence of wave attenuation and the filtering of high-frequency wave components. Additional deviatoric loading tests of Berea and Silurian samples provide evidence for the onset of stress-induced microfracturing, detected at a threshold of 1% shear wave anisotropy called the “shear wave crossover” (SWX). The SWX and subsequent increases in shear wave anisotropy evidence microstructural damage development well before quasi-static indicators such as the volumetric strain positive point of dilatancy (PPD) and yield/failure in all samples. Specifically, Berea and Silurian samples exhibit up to 5% and 7% shear wave anisotropy at the PPD, respectively. Additionally, stresses at the SWX and PPD were compared to peak axial stress to understand linkages between damage at several scales and ultimate rock strength. The SWX occurs at an average of 27% lower axial stresses, and 5% less shear wave anisotropy than the PPD, indicating that samples undergo irreversible microstructural changes earlier than previously thought. The SWX and PPD both provide meaningful estimates of failure stress, however samples must be subjected to higher stresses and strains to reach the PPD, making it less favorable for sample preservation. Furthermore, correlation between the SWX and peak stress under several different radial stresses, present a viable technique for using dynamic measurements to predict static rock failure properties, while also preserving sample competence for future tests. Linking the dynamically measured SWX to static rock failure properties provides an additional avenue for developing accurate transforms for several rock types. Therefore, the SWX can add value across industries for predicting rock behavior and maximizing the value of expensive samples and rock testing.
Mechanisms of fracture complexity and topology of fracture systems induced by hydraulic fracturing
(2020-12-05) Zhao, Peidong; Gray, Kenneth E., Ph. D.; Olson, Jon E; Espinoza, David Nicolas; Pyrcz, Michael J; Podnos, Evgeny G
Stimulated reservoir volume (SRV) is a prime factor controlling well performance in unconventional shale plays. In general, SRV describes the topology of induced fractures by hydraulic fracturing. Natural fractures (NFs), such as joints and faults, are ubiquitous in oil and gas reservoirs, where their tectonics, diagenesis, and hydrocarbon-generation history make the rock prone to fracturing. Being a pre-existing weak interface, NFs are preferred failure paths during hydraulic fracturing and becoming conductive under shear slip. Therefore, the interaction of hydraulic fractures (HFs) and NFs is fundamental to fracture growth in a formation. However, field observations of induced fracture systems show the necessity of modeling fracture complexity for improving completion design and interpreting drained reservoir volume (DRV). Thus, this work explains the mechanisms of HF-NF interaction and provides a physics-based method to infer SRV. First, fracture complexity results from fracture-tip processes involving stress perturbation by HF and failure of the pre-existing weak interface. Such so-called HF-NF interactions enable permeability enhancement around the HF and the development of SRV within unconventional shale reservoirs. This work proposes a two-dimensional (2D) analytical workflow to delineate the potential slip zone (PSZ) induced by an HF. An explicit description of failure modes in the near-tip region explains the complexity involved in HF-NF interaction. The results show varying influences of HF-NF relative angle, stress state, net pressure, frictional coefficient, and HF length to the NF slip. An NF at a 30±5° relative angle to an HF is analytically proved to have the highest potential for reactivation, which dominantly depends on the frictional coefficient of the interface. The spatial extension of the PSZ normal to the HF converges as the fracture propagates away and exhibits asymmetry depending on the relative angle. The proposed concept of PSZ can be used to measure and compare the intensity of HF-NF interactions at various geological settings. Second, the intensity of HF-NF interaction has been found to vary by formation and shale play. The problem of HF-NF interaction is multivariant and nonlinear, requiring conditional screening among three failure modes. By considering realistic subsurface conditions, a machine-learning (ML) model (random forest [RF] regression) is built to replicate the physics-based model and statistically investigate parametric influences on NF slip. The ML model finds the statistical significance of predicting features to be in the order of relative angle between HF and NF, fracture gradient (FG), frictional coefficient of the NF, overpressure index, stress differential, formation depth, and net pressure. The ML result is compared with sensitivity analysis and provides a new perspective on HF-NF interaction using statistical measures. The importance of formation depth on HF-NF interaction is stressed in both the physics-based and data-driven models, thus providing insight for field development of stacked resource plays. Finally, previous fracturing models either reduce model flexibility in simulating complex HF-NF interaction or require great computation cost for discrete fracture growth. This work presents a finite discrete-element model, which is a hybrid model adopting numerical setups from both continuum and discontinuous approaches, to investigate multifracture propagation in fractured reservoirs. The numerical model captures the fracture complexity, including branched, stranded, and kinked fractures, as well as the offset crossing of NFs. The results show biased fracture growth in the fractured reservoir, which is different from the numerical results of multifracture propagation in homogeneous rocks.. This work also emphasizes the control of fluid partition at the wellbore and among the intersecting fractures. Fluid partition at the wellbore is found to be a major challenge to the completion design of tight cluster spacing, which has been shown to improve production in recent years.
Solving three-dimensional problems in natural and hydraulic fracture development : insight from displacement discontinuity modeling
(2013-08) Sheibani, Farrokh; Olson, Jon E.
Although many fracture models are based on two-dimensional plane strain approximations, accurately predicting fracture propagation geometry requires accounting for the three-dimensional aspects of fractures. In this study, we implemented 3-D displacement discontinuity (DD) boundary element modeling to investigate the following intrinsically 3-D natural or hydraulic fracture propagation problems: the effect of fracture height on lateral propagation of vertical natural fractures, joint development in the vicinity of normal faults, and hydraulic fracture height growth and non-planar propagation paths. Fracture propagation is controlled by stress intensity factor (SIF) and its determination plays a central role in LEFM. The DD modeling is used to evaluate SIF in Mode I, II and III at the tip of an arbitrarily-shaped embedded crack by using crack-tip element displacement discontinuity. We examine the accuracy of SIF calculation is for rectangular, penny-shaped, and elliptical planar cracks. Using the aforementioned model for lateral propagation of overlapping fractures shows that the curving path of overlapping fractures is strongly influenced by the spacing-to-height ratio of fractures, as well as the differential stress magnitude. We show that the angle of intersection between two non-coincident but parallel en-echelon fractures depends strongly on the fracture height-to-spacing ratio, with intersection angles being asymptotic for "tall" fractures (large height-to-spacing ratios) and nearly orthogonal for "short" fractures. Stress perturbation around normal faults is three-dimensionally heterogeneous. That perturbation can result in joint development at the vicinity of normal faults. We examine the geometrical relationship between genetically related normal faults and joints in various geologic environments by considering a published case study of fault-related joints in the Arches National Park region, Utah. The results show that joint orientation is dependent on vertical position with respect to the normal fault, the spacing-to-height ratio of sub-parallel normal faults, and Poisson's ratio of the media. Our calculations represent a more physically reasonable match to measured field data than previously published, and we also identify a new mechanism to explain the driving stress for opening mode fracture propagation upon burial of quasi-elastic rocks. Hydraulic fractures may not necessarily start perpendicular to the minimum horizontal remote stress. We use the developed fracture propagation model to explain abnormality in the geometry of fracturing from misaligned horizontal wellbores. Results show that the misalignment causes non-planar lateral propagation and restriction in fracture height and fracture width in wellbore part.