bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of Hydraulic Fracturing Design with Future EOR Considerations in Shale Oil Reservoirs
Authors: Nguyen Ngoc T.B. , Dang Cuong T.Q. , Chen Zhangxin , Nghiem Long X.
Published: May 2015

Nowadays, shale oil (SO) resources gain much attention in the oil industry world-wide. SO is classified as an unconventional resource and an optimal hydraulic fracturing (HF) design with future EOR consideration is essential for achieving a high oil production rate. However, people are still faced with many issues in HF optimization and EOR implementation associated with a significant increase in capital costs in SO development. To overcome the current challenges, maximizing the oil recovery and minimizing the future investment costs, this paper presents an HF optimization and comprehensive evaluation of EOR potential in SO reservoirs in terms of maximizing oil recovery and project revenue.

We first address the key importance for achieving a successful HF design. Then, different development strategies for improving oil recovery including waterflooding, continuous gas flooding, and cycling gas flooding are systematically evaluated. Finally, HF design and EOR gas flooding are optimized through a robust procedure.

Four parameters that strongly affect the SO production have been identified: matrix permeability, fracture half-length, fracture spacing and rock compressibility. Detailed analyses of these key factors are addressed to allow the design of optimal and practical HF strategies to maximize the oil recovery.

The analysis show that an EOR application is crucial for improving the oil recovery factor in SO. Different development strategies including natural depletion, waterflooding, and EOR gas flooding are implemented. Among them, continuous gas injection after 20 years' primary production is proven as the most promising method to improve oil recovery in both technical and economical points of views. The critical effect of a hydraulically fracturing pattern is examined in this study and it is shown that oil recovery in an aligned fracturing pattern yields a superior performance (a higher oil rate and slower pressure depletion) than the one in a staggered fracturing pattern. Moreover, the distance between an injector and a producer is investigated to obtain the highest oil production and the lowest injection cost.

To maximize the oil recovery, a series of physics-based optimizations have been performed for HF design and EOR gas operation by applying the DECE algorithm in a robust optimizer. After the optimization process, the ultimate oil recovery yeilds about 13.32% for no injection and 18.58% for gas injection. The optimal values are also documented in this paper as a guideline for future HF and EOR gas flooding in SO.

One of the important contributions of this research is to present a development strategy for SO reservoirs, in which HF optimization with future EOR considerations significantly helps to address future technical challenges, investment costs, and project economy. The proposed approach can be effectively optimized for HF design and served as a guideline for investigating the EOR potential in unconventional reservoirs.
Seismic Assisted History Matching Using Binary Image Matching
Authors: Obidegwu Dennis , Chassagne Romain , MacBeth Colin
Published: May 2015

This paper presents a history matching scheme that has been applied to production data and time lapse seismic data. The production data objective function is calculated using the conventional least squares method between the historical production data and simulation predictions, while the seismic objective function uses the Hamming distance between two binary images of the gas distribution (presence of gas (1) or absence of gas (0)) sequenced over the different acquisition times. The technique is applied to a UKCS (United Kingdom Continental Shelf) field that has deep-water tertiary turbidite sands and multiple stacked reservoirs defining some degree of compartmentalisation. Thirty five parameters are perturbed in this history match, they can be classified as volumetric parameters (net-to-gross, pore volume), transmissibility parameters (permeability, transmissibility), and end points of the relative permeability curves (critical saturation points). An initial ensemble of fluid flow simulation models is created where the full range of uncertain parameters are acknowledged using experimental design methods, and an evolutionary algorithm is used for optimization in the history matching process. It is found that permeability and critical gas saturation are key parameters for achieving a good history match, and that the volumetric parameters are not significant for this match in this particular reservoir. We also observe that matching only to production data marginally improves the seismic match, whilst matching to only seismic data improves the fit to production data. Combining both sets of data delivers an improvement for the production data and seismic data, as well as an overall reduction in the uncertainties. A unique feature of this technique is the use of the Hamming distance metric for seismic data history matching analysis, as this circumvents the use of the uncertain petroelastic model. This approach is easy to implement, and also helps achieve an effective global history match.
Numerical Simulation of Gas Production From Tight, Ultratight and Shale Gas Reservoirs: Flow Regimes and Geomechanical Effects
Authors: Moghaddam Rasoul Nazari , Aghabozorgi Shokoufeh , Foroozesh Jalal
Published: May 2015

Production from unconventional gas resources (UGRs) has received great attentions because of their large reserves as well as technical advances in developing these reservoirs. The fluid flow in ultralow permeability porous media cannot be considered in the range of conventional Darcy flow as it undergoes a transition from a Darcy regime to slip flow and free molecule flow regimes. Understanding fluid flow inside the matrix and how the matrix permeability evolves over depletion are among the major challenges to unconventional gas reservoirs characterization. Considering different flow regimes in UGRs and time dependent permeability during the production of reservoir, the applicability of the availabe numerical simulatior to predict the production from unconventional reservoirs is questionable.

In this paper, a numerical approach is proposed for simulation of gas production of UGRs including geomechanical effect, slippage effect and non-Darcy flow. In this simulation, gas production is calculated using a pseudo-pressure integral for well inflow performance and material-balance for reservoir depletion. The numerical approach has been verified by comparing with the results of fine-grid compositional simulation for a typical conventional gas reservoir. The pseudo pressure-integral has been extended to include the geomechanical effect and time dependent matrix permeability. The flow regime is distinguished by Knudsen number for each regions of the reservoir during the reservoir depletion.

According to the numerical results, the matrix permeability changes depending on the flow regime determined by Knudsen dimensionless number. Slip flow and Knudsen diffusion which are dependent on net pore pressure can play important roles in the gas production. Higher-than–expected matrix permeability becomes more highlighted when the permeability of the matrix decreases and dimensionless Knudsen number is higher than 0.1. This higher permeability enhances the gas production. On the other side, the matrix permeability decrease as the net overburden stress increases during the production life of the reservoirs. This decrease in matrix permeability clearly decreases the rate of gas production.

The presented numerical simulation evaluates the significance of different flow regimes, time dependent permeability and geomechnical effect in production from UGRs. It also offers a rapid and simple tool for prediction of gas deliverability of UGRs well.
A Diffusion Controlled Model for Prediction of Long-Term Cement Sheath Carbonation
Authors: Aiex Cristina , Campos Gilson , Chiney Abhinandan , Deshpande Abhimanyu , Patil Rahul , Paiva Maria , Ravi Kris
Published: May 2015

The recent increase in production from reservoirs containing in-situ CO2 and H2S; and injection of produced CO2 for geological carbon dioxide storage and enhanced oil recovery (EOR) has prompted strict risk assessment, well construction best practices, and long-term monitoring. Isolation materials used in these wells, particularly casing and cementing slurries, are to be resistant to CO2 attack. A long-term experimental approach might be too slow to meet industry needs; therefore, model-based simulation is essential to guide a design that helps ensure zonal isolation for the life of the well.

This work presents a diffusion-controlled model for simulating the conversion of cement hydration products to calcium carbonate and subsequently to water-soluble calcium bicarbonate, predicting the depth of penetration of the two reaction fronts. Temperature, total pressure, and partial pressure of CO2 are considered, corresponding to downhole conditions. After calibration with short-term experiments, the model is able to extrapolate the degradation to long term.

A simulation tool based on this model was developed and validated using available literature results and experiments performed with this purpose. Simulation results show that progress of the carbonation reaction front is faster than the subsequent conversion to calcium bicarbonate. The extent of degradation is quantified by depth of penetration of the two reactions. Strong dependency on temperature, pressure, and partial pressure of CO2 was observed. The formation permeability also plays an important role, acting as an additional mass transfer resistance.

The results demonstrate that the need for long-term experiments can be reduced or eliminated using this model-based simulation tool. The tool does not only evaluate and compare current slurry formulations, but also guides the design of new CO2 resistant materials. The extent of cement degradation attributed to CO2 injection can be predicted in downhole conditions, providing one of the most important parameters for assessing the risk of CO2 leakage in the long term, for long-term safety of wells.
A New, Accurate and Simple Model For Calculation of Horizontal Well Productivity in Gas and Gas Condensate Reservoirs
Authors: Ghahri P. , Alatefi S. , Jamiolahmady M.
Published: May 2015

Horizontal wells are a proven and well acknowledged technology to enhance well productivity through an increase in reservoir contact compared to that of a vertical well under the same conditions. In the last three decades, a considerable effort has been directed to study flow around horizontal wells by many investigators. These studies have mainly focused on proposing practical tools (in the form of skin factor) for long-term well productivity estimation. The skin factor proposed can be applied in an equivalent (one dimensional radial) open-hole system replicating the flow around the actual complex three dimensional (3-D) flow geometry of the Horizontal well. However, all these studies concentrate on single-phase Darcy flow conditions.

In gas condensate reservoirs, in addition to the three dimensional (3-D) nature of flow geometry, the flow behavior is further complicated by the phase change and the variation of relative permeability (kr) due to the coupling (increase in kr by an increase in velocity or decrease in IFT) and inertia (a decrease in kr by an increase in velocity) effects. Therefore, simulating such a complex 3-D flow using numerical commercial simulators requires a three dimensional fine grid compositional approach, which is very impractical, cumbersome and sometimes trigger convergence problems due to numerical instability. In fact, the introduction of a quick and reliable tool for long term productivity calculation is much needed in such systems.

This work is aimed at the development of a practical, general, and easy-to-use method for defining an effective wellbore radius of an equivalent open-hole system, replicating flow around the 3-D Horizontal well in gas condensate reservoirs.

Accordingly, a 3-D compositional finite element based in-house simulator was developed to accurately model gas and gas condensate flow around horizontal wells. A large data bank was generated, covering the impact of a wide range of pertinent geometric and flow parameters on the well performance. Then a general approach is proposed for estimation of an effective wellbore radius of an equivalent open-hole radial 1-D system replicating flow around the 3-D Horizontal well system. The effective wellbore radius varies with fluid properties, velocity, IFT, reservoir and wellbore conditions. The results of the proposed formulation, which benefits from suitable dimensionless numbers, has been tested against the simulator results not used in its development confirming the integrity of the approach. Also, the proposed formulation is applicable for both single-phase non-Darcy and two-phase gas condensate flow systems.

With this approach, no numerical simulation is needed and instead a simple excel spread-sheet can predict the horizontal well performance, significantly facilitating engineering and management decisions relating to the application of horizontal well technologies.
Full System Analysis to Predict the Reliability of a Wastewater Disposal System; a Hydraulic Simulation-Based Approach
Authors: Al-Hajri Nasser M. , Al-Ajmi Mohammed D. , Al-Subaie Fehead M.
Published: May 2015

The increase in water production from mature fields calls for continuous monitoring of wastewater disposal (WWD) system capacities. The three main components of any WWD system are discharge pumps, pipeline's network and disposal wells. Situations may arise when the full system capacity is constrained by the flow performance of one or a combination of these components. Subsequently, produced water volumes are reduced by closing high water cut oil producers. The grave and critical importance of such systems necessitates proactive diagnosing and forecasting of WWD performances.

Plant-A injects 45 thousand barrels per day (MBD) of wastewater into five disposal wells. Historical system performance showed an increase in the pump discharge pressure that has nearly reached its maximum operating limit. A variation in injection performance among all the five disposal wells was observed. This variation is attributed to the different completion designs and reservoir inflow performances of the five subject wells. A 14 km flow line of varying sizes connects the disposal wells to two parallel discharge pumps. These pumps are designed to inject the abovementioned rate at a discharge pressure of 800 psi at current conditions.

A full system analysis of Plant-A disposal network, utilizing hydraulic simulation techniques, was performed to model the current injection performance. The goal of this comprehensive study of the disposal system was to explore all possible root causes of the discharge pressure increase. In addition, the hydraulic simulator forecasted several feasible and recommended scenarios to eliminate this high discharge pressure.

The five disposal well's inflow and outflow performance relationships were generated and calibrated using actual injection data. Network flow analysis was performed to account for both frictional and gravitational pressure losses in pipelines and flow restrictions using sophisticated software. This high accuracy model improved the decision making process and an optimized solution was put into effect based on the simulation results.
Reservoir Flow Simulation Coupled Rock Mechanics for Modeling Shale Liquid Rich Gas Play
Authors: Lin M. , Wang S. , Chen S. , Chen Z.
Published: May 2015

Multistage hydraulic fracturing technique now applied with horizontal wells and over large areas has enabled commercial production of oil and gas from low-permeability rock formations, changing the energy landscape in North America. In these reservoirs, tons of fracturing fluid and proppants are pumped into the reservoir matrix to create hydraulic fractures and it is important to understand the propagation mechanism of hydraulic fractures and further optimize their properties. In addition, natural fractures are often present in the shale and tight formations, which might be activated during the fracturing process and contribute to the after-stimulation well production rates.

In this paper, reservoir simulation is coupled with rock mechanics to predict the well after-stimulation production performance. Firstly, a dual-permeability geological model is built based on field data collected from a well pad in Montney formation, Canada. Fracturing fluid flow in the formation coupled rock mechanics is employed to simulate dynamic hydraulic fracturing process. More specifically, as the continuous injection of fracturing flurry, the effective stress will decrease accordingly. When the effective stress reaches the rock failure criteria, hydraulic fractures will be generated, allowing the fracturing liquid to flow along the fractures. Based on the fracturing operational schedule, dynamic hydraulic fracturing simulation is conducted and results show that hydraulic fractures tend to propagate upward first until it connects the entire formation in the vertical direction. Early production history of the stimulated well is then matched to valid the simulated fracture geometries. Finally, the effects of natural fractures and well bottom-hole pressure on well production are studied. Results show that if natural fracture can be propped or partially propped by the proppants, the production will be increased significantly for shale liquid rich gas plays. This paper provides a significant insights on the fracture propagation and can be a reference for fracturing treatments in unconventional shale reservoirs.
Minifrac Analysis Using Well Test Technique as Applied to the Vaca Muerta Shale Play
Authors: Rohmer B. , Raverta M. , Boutaud de la Combe J-L. , Jaffrezic V.
Published: May 2015

In shale gas, permeability is so low such that radial flow is rarely reached and only fracture flow is observed. Minifrac test interpretation is therefore a non unique process as the flow observed are governed by several unknown parameters, fracture geometry, formation permeability and pressure.

The industry tends to use unique fracture geometry and single pressure estimation such as linear flow extrapolation in order to reduce the interpretation to a unique case. This paper presents a new approach which enables to characterize the range of permeability and pressure. Pressure Transient Analysis tools are used in order to interpret minifrac tests before and after closure period. The common lack of radial flow results in an infinite range of possible permeability and initial pressure scenarios. These can be constrained by considering different simplified 2D fracture geometry. These constrained scenarios, resulting in a limited range of possible values for these two parameters, can then be tested with Rate Transient Analysis which otherwise has also non-unique solution.

First and foremost, this work shows that uncertainty on initial pressure can be low, as even extreme cases can yield pressure difference in the order of only a few tens of psi. However, small differences in pressure can lead to permeability estimation with a range up to several orders of magnitude.

The consequence of considering only a single pressure value is to neglect the potential uncertainty in permeability estimation. Describing the uncertainty on these parameters should be a compulsory output of a minifrac interpretation in shale formations.

This paper also demonstrates that when radial flow is not observed, a straight line analysis gives only a maximum value of permeability thickness and linear flow extrapolation is only one value amongst the possible range. This pressure can be associated to unrealistic fracture geometry.

The methodology is applied to a shale gas exploration well in the Vaca Muerta formation. The production data and the Rate Transient Analysis over the first year of production proved the validity and benefits of this approach.

In a nutshell, using this methodology enables to determine the range of possible pressure and permeability at the very early life of the well simply based on a minifrac before and after closure interpretation. This method in comparison to a commonly performed single case interpretation allows estimating the impact of permeability uncertainty on production forecast after only a few months of production.
The Convective-Diffusive Mechanism in CO2 Sequestration in Saline Aquifers: Experimental and Numerical Simulation Study
Authors: Foroozesh Jalal , Moghaddam Rasoul Nazari
Published: May 2015

Carbone Dioxide can be injected into the saline aquifers and securely stored underground if it is dissolved in water. When CO2 contacts water, it starts to diffuse through the interface and the water layer near to the interface becomes CO2 saturated and denser. The heavier saturated layer induces a gravitational instability (disturbance) which can result in fluid convection. This natural convection significantly increases the rate of CO2 dissolution. Despite a decade of research on this area, this complex mechanism is not fully understood. In addition, mathematical modelling of this process by solving the equations of momentum balance coupled with convection-diffusion is challenging.

In order to better understand and study the important parameters affecting the instability of the diffusive layer, a series of visual experiments were performed. A vertical, two-dimensional Hele-Shaw setup composed of a flat glass plate and a flat aluminum plate has been designed to investigate the instability of saturated layer. It has been partially filled with brine with salinity of 20 wt% containing pH sensitive dye of litmus powder. CO2 was introduced into the top of Hele-Shaw cell with the initial pressure of 100 psi. Experimental results clearly shows the onset of instability and the natural convection phenomenon when the start point of CO2 fingering and its propagation through the water phase were recorded.

Experimental data were also used to simulate the process using the conventional and a new proposed model. In this study, a new model has been proposed to be used instead of the conventional one. Conventionally the governing equations including momentum balance equation coupled with convection-diffusion equation are solved to simulate the disturbances. In addition, in this study, a new approach is proposed. In this model, a pseudo diffusion coefficient is introduced into the unsteady-state diffusion equation based on the second Fick's law. A so-called pseudo diffusion coefficient was defined as a function of time to capture the variable dissolution rate occurring during CO2 dissolution in brine. The proposed model was consistent with conventional model in prediction of CO2 dissolution rate.

The results of this study can help to better understand the dissolution mechanisms of CO2 in brine. Moreover the developed model here can be used as a rapid and simple method to predict the CO2 dissolution behaviour for other systems with different scales and properties. Additionally the idea of pseudo diffusion coefficient can be used in other systems when the convection can be represented by the diffusion.
Improving the Estimation of the Chance of Success of a 4D Seismic Project Based on Representative Models
Authors: Ferreira Carla Janaina , Davolio Alessandra , Schiozer Denis J.
Published: May 2015

Reservoir monitoring has constantly used 4D seismic data and has become an important tool for reservoir management. However, from the economic perspective, the information is only valuable if it influences decisions. It is necessary to decide if the acquisition of new information will be useful for field management. Thus, a methodology was developed to estimate the chance of success of a 4D seismic project. The methodology is an iterative process; it quantifies the increase of the project's value through the optimization of the production strategy for several reservoir models. As the number of reservoir scenarios representing the problem can be high, the optimization process can be time consuming. Therefore, the selection of representative models is an important step of the methodology. The present study discusses the impact of the number of representative models on the expected value of information (EVOI) and chance of success results. The chance of success methodology was applied to a synthetic model to validate the results and show its benefits. The selection of the representative models was based on production and economic results from the reservoir scenarios. Five evaluations were performed; each one considers a different number of representative models. The expected value of information and chance of success varies depending on the number of representative models. However, the EVOI and chance of success results obtained stabilize as the number of representative models increases. The methodology estimates the variation of the expected benefits due to the acquisition of 4D seismic data. The results obtained support the decision-making process, making the methodology an important tool for reservoir management. The decision-making process must consider the current uncertain scenario, the engineering and geophysical studies and also the economic aspect. Measuring and, especially, predicting the economic impact of new information is complex. Thus, the chance of success methodology is a tool to assist the decision maker in the evaluation of new 4D seismic projects.
Application of Low Salinity Water Injection in Heavy Oil Carbonate Reservoirs
Authors: Sánchez-Rodríguez José , Gachuz-Muro Heron , Sohrabi Mehran
Published: May 2015

Low Salinity Water Injection (LSWI) has received much attention recently as an Enhanced Oil Recovery (EOR) technique. Extensive research programs have been launched to investigate and identify positive effects of LSWI on oil recovery. Experiments have been performed on different formations and crude oils in order to identify the cause of wettability alteration, which is considered as the main reason for the observed additional oil recovery. The majority of the studies reported in the literature have been performed on light oils showing positive results, which represent a significant opportunity for these reservoirs. However, application of LSWI in heavy oils has received much less attention and poses a big challenge, especially in carbonate heavy oil reservoirs.

Carbonate reservoirs are generally much more complex than their sandstone counterparts. Further difficulties arise when the reservoir is naturally fractured. These features as well as the oil-wet or mixed-wet conditions that dominate carbonate formations usually result in low recovery factors. The available data suggest that the ultimate recovery factor from these reservoirs is low and in the range of 25 to 45%, depending on the applied method. Although the most widely used method for heavy oil recovery is the thermal processes, non-thermal processes such as gas injection and smart fluids are gaining increasingly more interest amongst operators.

In this paper, we present the results of an integrated experimental and simulation study performed in order to explore the potential of using low salinity water as an injection fluid for secondary and tertiary oil recovery from heavy oil carbonate reservoirs. Coreflood experiments were performed to derive measured relative permeability curves. This data were then used to numerically simulate the injection of low salinity water. A systematic sensitivity analysis was then performed to investigate the impact of reservoir parameters on the performance of LSWI in these reservoirs. The parameters considered included: reservoir thickness, well spacing, permeability, heterogeneity, and reservoir pressure. The numerical simulation results confirmed the high potential of LSWI for recovering heavy oils. Oil recoveries of up to 70% (OOIP) were predicted when LSWI was used as secondary recovery, which was consistent with the results from laboratory experiments. On the other hand, high salinity water injection just recovered less than 35%. LSWI as a tertiary recovery method also demonstrated high oil recovery which improves the economics of field applications of waterflooding as less water needs to be injected in the field for achieving the same oil recovery factor when low salinity water is used compared to high salinity water.
An Investigation of Fluid Leak-off Due to Osmotic and Capillary Effects and Its Impact on Micro-Fracture Generation during Hydraulic Fracturing Stimulation of Gas Shale
Authors: Wang Junjian , Rahman Sheik S
Published: May 2015

The fluid leak-off into the shale matrix during hydraulic fracture treatment has been a critical issue in determining fracture geometry. Furthermore, fluid intake also affects mechanical properties of the surrounding rock matrix which in turn affects fracture propagation. Conventional approaches for the prediction of leak-off have been inadequate as several important phenomena are ignored. In this paper the osmotic and capillary effects on fluid leak-off during shale gas reservoir stimulation are considered. A simplified structure is used to depict the complex pore network in shales. Different interactive forces involved in fluid displacement are taken into account in the mathematical formulation of the model. The proposed numerial model is used to study the amount of fluidfluid leak-off and the consequent pressure increase due to gas entrapment as well as potential influence of increase in pore pressure on the generation of micro-fractures are discussed.

The simulation results show excellent agreement with the previous experimental observations, and indicate that the water leak-off greatly depends on composition of shale matrix and concentration of different components of the fracture fluid. Clay minerals for example, are naturally prone to water invasion and draw water faster than other hydrophilic minerals and organic matter because of their osmotic effect. Furthermore, the invaded water significantly increases the pore pressure within the shale matrix due to gas entrapment. This leads to a strong non-linear relationship between fluid leak-off and the square root of time. An increase of pore pressure also results in a decrease in effective stress which leads to the generation of tension and shear micro-fractures in shale matrix. These Physical alterations increase the complexity of hydraulic fractures.

This study emphasizes the significance of osmotic and capillary effects as well as gas entrapment on hydraulic fracturing treatment of shale gas reservoirs. Moreover, the new leak-off model can be applied to investigate fracture propagation behaviour in shale gas reservoir.
Petrophysical Evaluation Of Water Flooding In Bohai Bay Fresh Water Reservoirs - A Case Study
Authors: Li Xingli , Li Ting , Lu Yunlong , Lv Hongzhi , Cui Yunjiang , Qi Yi
Published: May 2015

Water flooding evaluation in development wells is one of the biggest technical challenges in Bohai Bay. Due to the extremely low salinity of formation and injected water, resistivity contrast between hydrocarbon and water sands is low. Moreover, the clay mineral in the rock matrix increases overall conductivity and can lead to misinterpretation of water saturation.

In this report, we present a case study of water flooding evaluation with LWD logs using a novel approach. The distinguishing features of this approach are: 1. It removes the clay effect on porosity and resistivity measurements; 2. It computes an irreducible water saturation to assess what fraction of the pore fluid is movable. The key enabling technology in this approach is LWD nuclear capture spectroscopy. The most valuable inputs provided by spectroscopy are the matrix responses and rock lithology dry weights, particularly the clay. The following are the steps of our workflow: Derive matrix density and matrix neutron from the element logs and compute matrix-adjusted porosity.Derive clay dry weight from element logs and compute modern day water saturation Swi with a shaly sand conductivity equation.Compute irreducible water saturation Swi from rock lithology.For oil zones with original water at irreducible level, the water flooding in modern day can simply be estimated from Sw – Swi.

Included in this case study are 6 development wells in Bohai Bay logged with LWD natural GR, caliper, neutron-density, propagation resistivity and capture spectroscopy. We used the proposed approach to evaluate water flooding in these wells and our results are consistent with production data.
History Matching for Steam Drive Heavy Oil Reservoirs Using Ensemble Based Techniques - A synthetic Wafra Oil Field Case Study
Authors: Katterbauer Klemens
Published: May 2015

Heavy oil recovery has been a major focus in the oil and gas industry to counter the rapid depletion of conventional reservoirs. Various techniques for enhancing the recovery of heavy oil were developed and pilot-tested, having shown that steam drive techniques prove in most circumstances to be most economical and successful. The Wafra field in Saudi Arabia is at the forefront of utilizing steam recovery for carbonate heavy oil reservoirs in the Middle East and has led to considerable recovery of heavy oil resources. With growing injection volumes, tracking the steam evolution within the reservoir and characterizing the formation, especially in terms of porosity and permeability, have been key objectives for sound economic decisions and enhanced production forecasts. We have developed an integrated reservoir history matching framework using ensemble based techniques integrating cross-well seismic data for enhancing reservoir characterization and improving history matches. Examining the performance on a synthetic field study of the Wafra field, we could demonstrate the improved characterization of the reservoir formation, determine more accurately steam chambers and enhance forecasts of the reservoir's recovery potential. The results demonstrate the potential of the integration of full-waveform seismic data for steam drive reservoir characterization and increasing recovery efficiency.
Connection Matrices and the Definability of Graph Parameters
Authors: Kotek Tomer , Makowsky Johann A.
Published: May 2015

In this paper we extend and prove in detail the Finite Rank Theorem for
connection matrices of graph parameters definable in Monadic Second Order Logic
with counting (CMSOL) from B. Godlin, T. Kotek and J.A. Makowsky (2008) and
J.A. Makowsky (2009). We demonstrate its vast applicability in simplifying
known and new non-definability results of graph properties and finding new
non-definability results for graph parameters. We also prove a Feferman-Vaught
Theorem for the logic CFOL, First Order Logic with the modular counting
quantifiers.
Cost Preserving Bisimulations for Probabilistic Automata
Authors: Turrini Andrea , Hermanns Holger
Published: May 2015

Probabilistic automata constitute a versatile and elegant model for
concurrent probabilistic systems. They are equipped with a compositional theory
supporting abstraction, enabled by weak probabilistic bisimulation serving as
the reference notion for summarising the effect of abstraction. This paper
considers probabilistic automata augmented with costs. It extends the notions
of weak transitions in probabilistic automata in such a way that the costs
incurred along a weak transition are captured. This gives rise to
cost-preserving and cost-bounding variations of weak probabilistic
bisimilarity, for which we establish compositionality properties with respect
to parallel composition. Furthermore, polynomial-time decision algorithms are
proposed, that can be effectively used to compute reward-bounding abstractions
of Markov decision processes in a compositional manner.
State of B\"uchi Complementation
Authors: Tsai Ming-Hsien , Fogarty Seth , Vardi Moshe Y. , Tsay Yih-Kuen
Published: May 2015

Complementation of B\"uchi automata has been studied for over five decades
since the formalism was introduced in 1960. Known complementation constructions
can be classified into Ramsey-based, determinization-based, rank-based, and
slice-based approaches. Regarding the performance of these approaches, there
have been several complexity analyses but very few experimental results. What
especially lacks is a comparative experiment on all of the four approaches to
see how they perform in practice. In this paper, we review the four approaches,
propose several optimization heuristics, and perform comparative
experimentation on four representative constructions that are considered the
most efficient in each approach. The experimental results show that (1) the
determinization-based Safra-Piterman construction outperforms the other three
in producing smaller complements and finishing more tasks in the allocated time
and (2) the proposed heuristics substantially improve the Safra-Piterman and
the slice-based constructions.
Bounded variation and the strength of Helly's selection theorem
Authors: Kreuzer Alexander P.
Published: May 2015

We analyze the strength of Helly's selection theorem HST, which is the most
important compactness theorem on the space of functions of bounded variation.
For this we utilize a new representation of this space intermediate between
$L_1$ and the Sobolev space W1,1, compatible with the, so called, weak*
topology. We obtain that HST is instance-wise equivalent to the
Bolzano-Weierstra\ss\ principle over RCA0. With this HST is equivalent to ACA0
over RCA0. A similar classification is obtained in the Weihrauch lattice.
Quantified CTL: Expressiveness and Complexity
Authors: Laroussinie François , Markey Nicolas
Published: May 2015

While it was defined long ago, the extension of CTL with quantification over
atomic propositions has never been studied extensively. Considering two
different semantics (depending whether propositional quantification refers to
the Kripke structure or to its unwinding tree), we study its expressiveness
(showing in particular that QCTL coincides with Monadic Second-Order Logic for
both semantics) and characterise the complexity of its model-checking and
satisfiability problems, depending on the number of nested propositional
quantifiers (showing that the structure semantics populates the polynomial
hierarchy while the tree semantics populates the exponential hierarchy).
Global semantic typing for inductive and coinductive computing
Authors: Leivant Daniel M
Published: May 2015

Inductive and coinductive types are commonly construed as ontological
(Church-style) types, denoting canonical data-sets such as natural numbers,
lists, and streams. For various purposes, notably the study of programs in the
context of global semantics, it is preferable to think of types as semantical
properties (Curry-style). Intrinsic theories were introduced in the late 1990s
to provide a purely logical framework for reasoning about programs and their
semantic types. We extend them here to data given by any combination of
inductive and coinductive definitions. This approach is of interest because it
fits tightly with syntactic, semantic, and proof theoretic fundamentals of
formal logic, with potential applications in implicit computational complexity
as well as extraction of programs from proofs. We prove a Canonicity Theorem,
showing that the global definition of program typing, via the usual (Tarskian)
semantics of first-order logic, agrees with their operational semantics in the
intended model. Finally, we show that every intrinsic theory is interpretable
in a conservative extension of first-order arithmetic. This means that
quantification over infinite data objects does not lead, on its own, to
proof-theoretic strength beyond that of Peano Arithmetic. Intrinsic theories
are perfectly amenable to formulas-as-types Curry-Howard morphisms, and were
used to characterize major computational complexity classes Their extensions
described here have similar potential which has already been applied.
Stochastic Parity Games on Lossy Channel Systems
Authors: Abdulla Parosh Aziz , Clemente Lorenzo , Mayr Richard , Sandberg Sven
Published: May 2015

We give an algorithm for solving stochastic parity games with almost-sure
winning conditions on {\it lossy channel systems}, under the constraint that
both players are restricted to finite-memory strategies. First, we describe a
general framework, where we consider the class of 2 1/2-player games with
almost-sure parity winning conditions on possibly infinite game graphs,
assuming that the game contains a {\it finite attractor}. An attractor is a set
of states (not necessarily absorbing) that is almost surely re-visited
regardless of the players' decisions. We present a scheme that characterizes
the set of winning states for each player. Then, we instantiate this scheme to
obtain an algorithm for {\it stochastic game lossy channel systems}.
Decidability Problems for Actor Systems
Authors: De Boer Frank , Jaghoori Mahdi , Laneve Cosimo , Zavattaro Gianluigi
Published: May 2015

We introduce a nominal actor-based language and study its expressive power.
We have identified the presence/absence of fields as a crucial feature: the
dynamic creation of names in combination with fields gives rise to Turing
completeness. On the other hand, restricting to stateless actors gives rise to
systems for which properties such as termination are decidable. This
decidability result still holds for actors with states when the number of
actors is bounded and the state is read-only.
Stochastic Timed Automata
Authors: Bertrand Nathalie , Bouyer Patricia , Brihaye Thomas , Menet Quentin , Baier Christel , Groesser Marcus , Jurdzinski Marcin
Published: May 2015

A stochastic timed automaton is a purely stochastic process defined on a
timed automaton, in which both delays and discrete choices are made randomly.
We study the almost-sure model-checking problem for this model, that is, given
a stochastic timed automaton A and a property $\Phi$, we want to decide whether
A satisfies $\Phi$ with probability 1. In this paper, we identify several
classes of automata and of properties for which this can be decided. The proof
relies on the construction of a finite abstraction, called the thick graph,
that we interpret as a finite Markov chain, and for which we can decide the
almost-sure model-checking problem. Correctness of the abstraction holds when
automata are almost-surely fair, which we show, is the case for two large
classes of systems, single- clock automata and so-called weak-reactive
automata. Techniques employed in this article gather tools from real-time
verification and probabilistic verification, as well as topological games
played on timed automata.
Computing a Solution of Feigenbaum's Functional Equation in Polynomial Time
Authors: Hertling Peter , Spandl Christoph
Published: May 2015

Lanford has shown that Feigenbaum's functional equation has an analytic
solution. We show that this solution is a polynomial time computable function.
This implies in particular that the so-called first Feigenbaum constant is a
polynomial time computable real number.
An Effect System for Algebraic Effects and Handlers
Authors: Bauer Andrej , Pretnar Matija
Published: May 2015

We present an effect system for core Eff, a simplified variant of Eff, which
is an ML-style programming language with first-class algebraic effects and
handlers. We define an expressive effect system and prove safety of operational
semantics with respect to it. Then we give a domain-theoretic denotational
semantics of core Eff, using Pitts's theory of minimal invariant relations, and
prove it adequate. We use this fact to develop tools for finding useful
contextual equivalences, including an induction principle. To demonstrate their
usefulness, we use these tools to derive the usual equations for mutable state,
including a general commutativity law for computations using non-interfering
references. We have formalized the effect system, the operational semantics,
and the safety theorem in Twelf.
Effective zero-dimensionality for computable metric spaces
Authors: Kenny Robert
Published: May 2015

We begin to study classical dimension theory from the computable analysis
(TTE) point of view. For computable metric spaces, several effectivisations of
zero-dimensionality are shown to be equivalent. The part of this
characterisation that concerns covering dimension extends to higher dimensions
and to closed shrinkings of finite open covers. To deal with zero-dimensional
subspaces uniformly, four operations (relative to the space and a class of
subspaces) are defined; these correspond to definitions of inductive and
covering dimensions and a countable basis condition. Finally, an effective
retract characterisation of zero-dimensionality is proven under an effective
compactness condition. In one direction this uses a version of the construction
of bilocated sets.
Permission-Based Separation Logic for Multithreaded Java Programs
Authors: Haack Christian , Huisman Marieke , Hurlin Clément , Amighi Afshin
Published: May 2015

This paper presents a program logic for reasoning about multithreaded
Java-like programs with dynamic thread creation, thread joining and reentrant
object monitors. The logic is based on concurrent separation logic. It is the
first detailed adaptation of concurrent separation logic to a multithreaded
Java-like language. The program logic associates a unique static access
permission with each heap location, ensuring exclusive write accesses and
ruling out data races. Concurrent reads are supported through fractional
permissions. Permissions can be transferred between threads upon thread
starting, thread joining, initial monitor entrancies and final monitor exits.
In order to distinguish between initial monitor entrancies and monitor
reentrancies, auxiliary variables keep track of multisets of currently held
monitors. Data abstraction and behavioral subtyping are facilitated through
abstract predicates, which are also used to represent monitor invariants,
preconditions for thread starting and postconditions for thread joining.
Value-parametrized types allow to conveniently capture common strong global
invariants, like static object ownership relations. The program logic is
presented for a model language with Java-like classes and interfaces, the
soundness of the program logic is proven, and a number of illustrative examples
are presented.
Aspect-oriented linearizability proofs
Authors: Chakraborty Soham , Henzinger Thomas A. , Sezgin Ali , Vafeiadis Viktor
Published: May 2015

Linearizability of concurrent data structures is usually proved by monolithic
simulation arguments relying on the identification of the so-called
linearization points. Regrettably, such proofs, whether manual or automatic,
are often complicated and scale poorly to advanced non-blocking concurrency
patterns, such as helping and optimistic updates. In response, we propose a
more modular way of checking linearizability of concurrent queue algorithms
that does not involve identifying linearization points. We reduce the task of
proving linearizability with respect to the queue specification to establishing
four basic properties, each of which can be proved independently by simpler
arguments. As a demonstration of our approach, we verify the Herlihy and Wing
queue, an algorithm that is challenging to verify by a simulation proof.
Maximal ideals in some F-algebras of holomorphic functions
Authors: Mestrovic Romeo
Published: May 2015

For 1 < p < ?, the Privalov class Np consists of all holomorphic functions f
on the open unit disk D of the complex plane C such that sup 0?r<1?2?0
(log+ |f(rei?)j|p d?/2? < + ? M. Stoll [16] showed that the space Np with
the topology given by the metric dp defined as dp(f,g) = (?2?0 (log(1 +
|f*(ei?) - g*(ei?)|))p d?/2?)1/p, f,g ? Np; becomes an F-algebra. Since
the map f ? dp(f,0) (f ? Np) is not a norm, Np is not a Banach algebra.
Here we investigate the structure of maximal ideals of the algebras Np (1 <
p < ?). We also give a complete characterization of multiplicative linear
functionals on the spaces Np. As an application, we show that there exists a
maximal ideal of Np which is not the kernel of a multiplicative continuous
linear functional on Np.