bluuebunny/crossref_metadata_2025 huggingface dataset:
Accounting for uncertainty in ecological analysis: the strengths and limitations of hierarchical statistical modeling
Authors: Cressie Noel , Calder Catherine A. , Clark James S. , Hoef Jay M. Ver , Wikle Christopher K.
Published: March 2009

Analyses of ecological data should account for the uncertainty in the process(es) that generated the data. However, accounting for these uncertainties is a difficult task, since ecology is known for its complexity. Measurement and/or process errors are often the only sources of uncertainty modeled when addressing complex ecological problems, yet analyses should also account for uncertainty in sampling design, in model specification, in parameters governing the specified model, and in initial and boundary conditions. Only then can we be confident in the scientific inferences and forecasts made from an analysis. Probability and statistics provide a framework that accounts for multiple sources of uncertainty. Given the complexities of ecological studies, the hierarchical statistical model is an invaluable tool. This approach is not new in ecology, and there are many examples (both Bayesian and non‐Bayesian) in the literature illustrating the benefits of this approach. In this article, we provide a baseline for concepts, notation, and methods, from which discussion on hierarchical statistical modeling in ecology can proceed. We have also planted some seeds for discussion and tried to show where the practical difficulties lie. Our thesis is that hierarchical statistical modeling is a powerful way of approaching ecological analysis in the presence of inevitable but quantifiable uncertainties, even if practical issues sometimes require pragmatic compromises.
Invasive pathogen threatens bird–pine mutualism: implications for sustaining a high‐elevation ecosystem
Authors: McKinney Shawn T. , Fiedler Carl E. , Tomback Diana F.
Published: March 2009

Human‐caused disruptions to seed‐dispersal mutualisms increase the extinction risk for both plant and animal species. Large‐seeded plants can be particularly vulnerable due to highly specialized dispersal systems and no compensatory regeneration mechanisms. Whitebark pine (Pinus albicaulis), a keystone subalpine species, obligately depends upon the Clark's Nutcracker (Nucifraga columbiana) for dispersal of its large, wingless seeds. Clark's Nutcracker, a facultative mutualist with whitebark pine, is sensitive to rates of energy gain, and emigrates from subalpine forests during periods of cone shortages. The invasive fungal pathogen Cronartium ribicola, which causes white pine blister rust, reduces whitebark pine cone production by killing cone‐bearing branches and trees. Mortality from blister rust reaches 90% or higher in some whitebark pine forests in the Northern Rocky Mountains, USA, and the rust now occurs nearly rangewide in whitebark pine. Our objectives were to identify the minimum level of cone production necessary to elicit seed dispersal by nutcrackers and to determine how cone production is influenced by forest structure and health. We quantified forest conditions and ecological interactions between nutcrackers and whitebark pine in three Rocky Mountain ecosystems that differ in levels of rust infection and mortality. Both the frequency of nutcracker occurrence and probability of seed dispersal were strongly related to annual whitebark pine cone production, which had a positive linear association with live whitebark pine basal area, and negative linear association with whitebark pine tree mortality and rust infection. From our data, we estimated that a threshold level of ∼1000 cones/ha is needed for a high likelihood of seed dispersal by nutcrackers (probability ≥0.7), and that this level of cone production can be met by forests with live whitebark pine basal area >5.0 m2/ha. The risk of mutualism disruption is greatest in northernmost Montana (USA), where three‐year mean cone production and live basal area fell below predicted threshold levels. There, nutcracker occurrence, seed dispersal, and whitebark pine regeneration were the lowest of the three ecosystems. Managers can use these threshold values to differentiate between restoration sites requiring planting of rust‐resistant seedlings and sites where nutcracker seed dispersal can be expected.
Upper respiratory tract disease, force of infection, and effects on survival of gopher tortoises
Authors: Ozgul Arpat , Oli Madan K. , Bolker Benjamin M. , Perez-Heydrich Carolina
Published: March 2009

Upper respiratory tract disease (URTD) caused byMycoplasma agassiziihas been hypothesized to contribute to the decline of some wild populations of gopher tortoises (Gopherus polyphemus). However, the force of infection (FOI) and the effect of URTD on survival in free‐ranging tortoise populations remain unknown. Using four years (2003–2006) of mark–recapture and epidemiological data collected from 10 populations of gopher tortoises in central Florida, USA, we estimated the FOI (probability per year of a susceptible tortoise becoming infected) and the effect of URTD (i.e., seropositivity toM. agassizii) on apparent survival rates. Sites with high (≥25%) seroprevalence had substantially higher FOI (0.22 ± 0.03; mean ± SE) than low (<25%) seroprevalence sites (0.04 ± 0.01). Our results provide the first quantitative evidence that the rate of transmission ofM. agassiziiis directly related to the seroprevalence of the population. Seropositive tortoises had higher apparent survival (0.99 ± 0.0001) than seronegatives (0.88 ± 0.03), possibly because seropositive tortoises represent individuals that survived the initial infection, developed chronic disease, and experienced lower mortality during the four‐year span of our study. However, two lines of evidence suggested possible effects of mycoplasmal URTD on tortoise survival. First, one plausible model suggested that susceptible (seronegative) tortoises in high seroprevalence sites had lower apparent survival rates than did susceptible tortoises in low seroprevalence sites, indicating a possible acute effect of infection. Second, the number of dead tortoise remains detected during annual site surveys increased significantly with increasing site seroprevalence, from ∼1 to ∼5 shell remains per 100 individuals. If (as our results suggest) URTD in fact reduces adult survival, it could adversely influence the population dynamics and persistence of this late‐maturing, long‐lived species.
Drought impact on forest growth and mortality in the southeast USA: an analysis using Forest Health and Monitoring data
Authors: Klos Ryan J. , Wang G. Geoff , Bauerle William L. , Rieck James R.
Published: March 2009

Drought frequency and intensity has been predicted to increase under many climate change scenarios. It is therefore critical to understand the response of forests to potential climate change in an effort to mitigate adverse impacts. The purpose of this study was to explore the regional effects of different drought severities on tree growth and mortality. Specifically, we investigated changes in growth and mortality rates across the southeastern United States under various drought and stand conditions using 1991–2005 Forest Health and Monitoring (FHM) plot data from Alabama, Georgia, and Virginia. Drought effects were examined for three species groups (pines, oaks, and mesophytic species) using the Palmer drought severity index (PDSI) as an indicator of drought severity. Stand variables, including total basal area, total tree density, tree species richness, slope, and stand age, were used to account for drought effects under varying stand conditions. The pines and mesophytic species exhibited significant reductions in growth rate with increasing drought severity. However, no significant difference in growth rate was observed within the oak species group. Mean mortality rates within the no‐drought class were significantly lower than those within the other three drought classes, among which no significant differences were found, for both pines and mesophytic species. Mean mortality rates were not significantly different among drought classes for oaks. Total basal area, total tree density, and stand age were negatively related to growth and positively related to mortality, which suggests that older and denser stands are more susceptible to drought damage. The effect of basal area on growth increased with drought severity for the oak and mesophytic species groups. Tree species richness was negatively related to mortality for the pine and mesophytic species groups, indicating that stands with more species suffer less mortality. Slope was positively related to mortality within the mesophytic species group, and its effect increased with drought severity, indicating a higher mortality on sites of greater slope during severe‐drought conditions. Our findings indicate that pines and mesophytic species are sensitive to drought, while oaks are tolerant of drought. The observed differential growth and mortality rates among species groups may alter the species composition of southeastern U.S. forests if drought episodes become more frequent and/or intense due to climate change. The significant effects of stand conditions on drought responses observed in our study also suggest that forest management may be used as a tool to mitigate drought effects.
Trawl disturbance on benthic communities: chronic effects and experimental predictions
Authors: Hinz Hilmar , Prieto Virginia , Kaiser Michel J.
Published: March 2009

Bottom trawling has widespread impacts on benthic communities and habitats. While the direct impacts of trawl disturbances on benthic communities have been extensively studied, the consequences from long‐term chronic disturbances are less well understood. The response of benthic macrofauna to chronic otter‐trawl disturbance from a Nephrops norvegicus (Norway lobster) fishery was investigated along a gradient of fishing intensity over a muddy fishing ground in the northeastern Irish Sea. Chronic otter trawling had a significant, negative effect on benthic infauna abundance, biomass, and species richness. Benthic epifauna abundance and species richness also showed a significant, negative response, while no such effect was evident for epibenthic biomass. Furthermore, chronic trawl disturbance led to clear changes in community composition of benthic infauna and epifauna. The results presented indicate that otter‐trawl impacts are cumulative and can lead to profound changes in benthic communities, which may have far‐reaching implications for the integrity of marine food webs. Studies investigating the short‐term effects of fishing manipulations previously concluded that otter trawling on muddy substrates had only modest effects on the benthic biota. Hence, the results presented by this study highlight that data from experimental studies can not be readily extrapolated to an ecosystem level and that subtle cumulative effects may only become apparent when fishing disturbances are examined over larger spatial and temporal scales. Furthermore, this study shows that data on chronic effects of bottom trawling on the benthos will be vital in informing the recently advocated move toward an ecosystem approach in fisheries management. As bottom‐trawl fisheries are expanding into ever deeper muddy habitats, the results presented here are an important step toward understanding the global ecosystem effects of bottom trawling.
Modeling fish health to inform research and management:Renibacterium salmoninarumdynamics in Lake Michigan
Authors: Fenichel Eli P. , Tsao Jean I. , Jones Michael L.
Published: March 2009

Little is known about the interaction between fish pathogens and managed freshwater fish populations. We develop a model of chinook salmon (Oncorhynchus tschawytscha)–Renibacterium salmoninarum(Rs) dynamics based on free‐swimming Lake Michigan fish by synthesizing population and epidemiological theory. Using the model, we expose critical uncertainties about the system, identify opportunities for efficient and insightful data collection, and pose testable hypotheses. Our simulation results suggest that hatcheries potentially play an important role in Lake MichiganRsdynamics, and understanding vertical transmission will be critical for quantifying this role. Our results also show that disease‐mediated responses to chinook salmon density need to be considered when evaluating management actions. Related to this, a better understanding of the stock–recruitment relationship and natural mortality rates for wild‐spawned fish and the impact of hatchery stocking on recruitment is required. Finally, to further develop models capable of assisting fishery management, fish health surveys ought to be integrated with stock assessment. This is the first time a host–pathogen modeling framework has been applied to managed, freshwater ecosystems, and we suggest that such an approach should be used more frequently to inform other emerging and chronic fish health issues.
Fouling by External Particles and Ultrasonic Cleaning of a Porous Material
Authors: Poesio Pietro , Ooms Gijs
Published: March 2009

The aim of this paper is to investigate experimentally the fouling of a porous material by external particles and the optimal way to clean the porous material with high-frequency acoustic waves. In particular, we are interested in the fouling by mud particles of the near-wellbore region of an oil reservoir. In the experiments, therefore, we used natural sandstone as porous material and mud particles as fouling particles. To generate fouling, mud particles were flushed through a sandstone core. Next, the core was treated with very short bursts of ultrasound, and the change in permeability was measured after each burst. (Earlier papers report only the end result after applying the total amount of acoustic energy.)

Experiments were carried out under different acoustic-cleaning conditions to investigate the influence of the relevant parameters on the cleaning process. For instance, the amplitude of the acoustic waves, the duration of the bursts, and the time between the bursts were varied. During the ultrasonic-cleaning process, brine flowed through the core. The effect of this flow was studied by changing the flow rate. Also, the effect of the temperature, pressure, and initial core permeability on the cleaning process was investigated. The experimental results show that short bursts of acoustic energy are more efficient for cleaning than long bursts or continuous application of ultrasound (for the same total amount of acoustic energy). The overall conclusion is that the optimal method of ultrasonic cleaning is to apply many very short bursts of low-amplitude acoustic energy, with a short rest time between the bursts while keeping the liquid (brine) flow at a very low velocity. More acoustic energy is needed to clean a core with a high initial permeability than a core with a low initial permeability. At low pressure, cavitation occurs and prevents the generation of ultrasonic bursts. Cavitation can even have a negative effect on the cleaning process.
Robust Waterflooding Optimization of Multiple Geological Scenarios
Authors: Essen G. M. van , Zandvliet M. J. , Hof P. M. J. Van den , Bosgra O. H. , Jansen J. D.
Published: March 2009

Dynamic optimization of waterflooding using optimal control theory has significant potential to increase ultimate recovery, as has been shown in various studies. However, optimal control strategies often lack robustness to geological uncertainties. We present an approach to reduce the effect of geological uncertainties in the field-development phase known as robust optimization (RO). RO uses a set of realizations that reflect the range of possible geological structures honoring the statistics of the geological uncertainties. In our study, we used 100 realizations of a 3D reservoir in a fluvial depositional environment with known main-flow direction. We optimized the rates of the eight injection and four production wells over the life of the reservoir, with the objective to maximize the average net present value (NPV). We used a gradient-based optimization method in which the gradients are obtained with an adjoint formulation. We compared the results of the RO procedure to two alternative approaches: a nominal-optimization (NO) and a reactive-control approach. In the reactive approach, each production well is shut in when production is no longer profitable. The NO procedure is based on a single realization. In our study, the NO procedure is performed on each of the 100 realizations in the set individually, resulting in 100 different NO-production strategies. The control strategies were applied to each realization, from which the average NPVs, the standard deviation, the cumulative-distribution functions, and the probability-density functions were determined. The RO results displayed a much smaller variance than the alternatives, indicating an increased robustness to geological uncertainty. Moreover, the RO procedure significantly improved the expected NPV compared to the alternative methods (on average 9.5% higher than using reactive-control and 5.9% higher than the average of the NO strategies).
Investigating Electrical-Impedance Tomography as a Technique for Real-Time Saturation Monitoring
Authors: Stacey Robert W. , Li Kewen , Horne Roland N.
Published: March 2009

3D electrical-impedance tomography (EIT) is a technique that has the potential to provide estimates of reservoir saturation at multiple scales by determining the resistivity distribution within the subsurface. In theory, EIT is well suited for researching oil and brine systems because of the large contrast in resistivity between the two phases. Here, in our initial laboratory investigation, we have applied the EIT technique to measure the saturation distribution of water within a core.

The initial EIT experiment presented here used a Berea-sand-stone core with 48 electrodes attached in three rings of 16. The core was open to the atmosphere, with saturation occurring by natural imbibition and desaturation occurring by evaporation. The voltage-potential field was measured by applying a direct-current (DC) pulse across the core and measuring the voltage potential at all electrodes, essentially applying the four-wire resistance technique over all electrodes in turn. The result was a data set that embodies the resistivity distribution within the core, and, by inversion, the resistivity distribution was reconstructed, which allowed for the inference of the saturation.

The data processing was accomplished by using the Electrical Impedance Tomography and Diffuse Optical Tomography Reconstruction Software (EIDORS) toolkit, which was developed for application to this nonlinear and ill-posed inverse problem. The procedure uses a finite-element model for forward calculation and a regularized nonlinear inverse solver to obtain a unique and stable inverse solution.

Experiments have indicated that EIT is a viable technique for studying the displacement characteristics of fluids with contrasting resistivity and is capable of detecting displacement fronts in near to real time. The current system is also a quantitative technique able to measure saturation distributions accurately between 15% < Sw < 65% in a Berea sandstone core. These limitations were imposed because of connate-water connections to the electrodes and ion-mobility effects caused by the DC voltage source. It is anticipated that the applicability of EIT will increase with the implementation of an alternating-current (AC) voltage source.
Estimation and Removal of Tidal Effects From Pressure Data
Authors: Zhao Yong , Reynolds Albert C.
Published: March 2009

We consider field pressure data from two wells, one offshore and one onshore. The onshore well is shut-in and experiences only a very small pressure decline due to interference, whereas in the offshore case, the late time buildup data appear to be severely corrupted by ocean tidal effects. The objectives in both cases are to determine the tidal signal that can be used to estimate formation compressibility and to remove the tidal signal from the measured data.

For the onshore well, we use a Savitzky-Golay (SG) filter to smooth the pressure data. Subtracting the smooth data from the measured signal yields a modified signal that ideally represents the earth tidal signal plus measurement error. A s the main harmonic components of the earth tidal signal are known, we perform a least-squares fit of the modified pressure data with a series of sines and cosines that contain only the known frequencies of the earth tide. By subtracting the series obtained by least-squares fitting from the true signal, we remove the effect of the earth tide.

In the offshore case, the time span of data that are severely affected by tidal effects is too short to use the SG filter with confidence. As the reservoir pressure change due to the tides is well-approximated by a damped and delayed version of the seafloor response, a least-squares fit of the data with a function that follows the underlying buildup trend plus an attenuated-delayed response of the measured seafloor pressure trace allows one to obtain a good estimate of the attenuation factor and time delay so that the tidal component of the bottomhole pressure change can be determined and removed from the measured pressure.
Flow of a Polyacrylamide/Chromium Acetate System in a Long Conduit
Authors: McCool Stan , Li Xianping , Wilhite G. Paul
Published: March 2009

Typical gelled-polymer treatments to treat fractured rock consist of injecting in-line-mixed gelant into the reservoir for times usually much longer than the bulk gel time of the gelant. Flow experiments were conducted to determine the effect of shear on the flow properties of the gelant for durations greater than the bulk gel time. In-line-mixed gelant was injected through a 1,031-ftlong tube to simulate a fracture treatment. Flow resistance increased down the tubing to steady values indicating gelation of the flowing system. Similar flow experiments were conducted by injecting preformed gel through the tubing. Flow resistances decreased down the tubing to steady values, indicating breakdown of the gel structure. Steady flow resistances of the downstream sections were higher during the injection of in-line-mixed gelant compared to injection of preformed gel, and both were much lower than literature values determined where preformed gel was injected through short fractured rocks and short lengths of tubing. Gel samples underwent syneresis after they were formed during shear flow in the tubing and in a rheometer. Interpretations of flow-resistance data from the injection of in-line-mixed gelants and preformed gels in long tubing are presented.
A New Splitting Scheme and Existence of Elliptic Region for Gasflood Modeling
Authors: Dutra T. A. , Pires A. P. , Bedrikovetsky P. G.
Published: March 2009

Analytical models of gasflooding are important for enhanced-oil-recovery (EOR) screening, for interpretation of laboratory data, and for streamline modeling. Introduction of two Lagrangian coordinates linked with one of the components and with the overall two-phase flux results in splitting the compositional model into an auxiliary system and an independent scalar equation. The number of equations in the auxiliary system is less by one if compared with the compositional model, making analytical modeling possible for more practical cases. The auxiliary system contains only thermodynamic functions and is independent of transport properties. Therefore, phase transitions and minimum miscibility pressure for gas injection are also independent of transport properties. The new splitting method is applicable for both self-similar solutions of continuous gas injection and nonself-similar solvent-slug problems. Analytical solution for four-component oil displacement by a nitrogen-based solvent was obtained using the splitting technique. The compositional two-phase model contains an elliptic region if and only if an elliptic region is also present in the auxiliary system. Calculations for several four-component mixtures exhibit existence of an elliptic region in compositional modeling.
Modeling a Finite-Length Sucker Rod Using the Semi-Infinite-Wave Equation and a Proof of Gibbs' Conjecture
Authors: DaCunha J. J. , Gibbs S. G.
Published: March 2009

In this paper, we study semi-infinite-spatial-domain wave equations modeling the real-world problem of longitudinal waves propagating along a long, slender, homogeneous elastic rod of finite length. A practical conclusion from the paper is that precision of the downhole card can be increased by improving the accuracy of the friction law in the wave equation. To conclude, we provide a rigorous proof of Gibbs' theorem and illustrate its validity with an existing well.
Where is the Gap? Is it in More Reservoir Engineers or in Leveraging New Skills and Workflows that Enhances Individual Productivity?
Authors: Amudo C. , Graf T.
Published: March 2009

The primary functions of a reservoir engineer include estimation of hydrocarbons in place, the evaluation of the recovery factor and the scheduling of the recovery. These roles are central in meeting the extremely complex challenges of the life-cycle development of hydrocarbon resources.

These challenges have now assumed gargantuan proportions because of mature assets requiring more attention to squeeze the last drop of oil from them, complicated accumulation needing ingenious solutions to make them profitable, high development costs demanding greater attention to details to reduce costs and unprecedented prices that has made otherwise marginal fields more attractive to develop. Throw also in the mix the heightened sense of environmental issues, unconventional hydrocarbon resources, competing alternative sources of energy, strict compliance with regulatory bodies and uncertain political situations in most of the growth basins and you might have sympathy for those crying for more reservoir engineers in the industry.

The fact is that our traditional deterministic approach of working is people intensive and it has started to fail the industry. Therefore, recruiting many more engineers to meet the present challenges can only fuel the vicious cycle of hire and fire approach that has made our industry less attractive in the past. We need to embrace new workflows based on established statistical concepts like neural networks and experimental designs that move the focus from people to computers. These new workflows also enable greater flexibility in data handling, ensure consistency in uncertainty quantification and give results in continuous distributions that can be sampled rapidly using statistical techniques like Monte Carlo.

This paper describes how statistical techniques can help in many core reservoir engineering roles like surveillance, history match and reservoir management. In addition, it presents relevant examples that illustrate the successful application of these concepts in the industry including a possible automation of integrated reservoir studies.
Reservoir Simulation Study on Improvement of Waterflooding Effect for a Naturally Fractured Low Permeability Field in Daqing, China: A Successful Case
Authors: Pu Hui , Wang Guofeng , Li Yingxin
Published: March 2009

Xinzhao field, developed in 2000, is one of the major naturally fractured reservoirs of Daqing oilfield, China. Production is obtained from the Putaohua(P) sandstone formation at an average depth of 1,338m. After several years' waterflood development, the development effect was deteriorating, many wells were becoming to be stripper wells, and watercut increased greatly, even some wells were seriously watered out. In order to improve the development effect of Xinzhao oilfield, a reservoir study was made in sufficient depth to understand past performance and to develop a long-range development plan, and the proposed pattern modification was performed in Xinzhao oilfield.

The purpose of the study is to develop an optimization strategy for waterflood performance. The first step in the process is to better understand the reservoir. One of tools used are Sourceless Microseismic Fracture Monitoring Technology. The Sourceless Microseismic Fracture Monitoring tool is used to determine the characteristics of fractures. The results of Sourceless Microseismic Fracture Monitoring Technology show that the orientation of most of fractures is east-west.

The second step is to analyze the past waterflood production performance to know the development characteristics of existing pattern(inverted nine-spot well pattern). The production rate decline is analyzed, and the reasons causing the current poor development effect are identified. The third step is to develop optimization and surveillance recovery strategies. Reservoir simulation approaches are utilized to optimize the well pattern. The recommended plan is pattern modification with strategy of implementing conversions step by step.

A field test of waterflood pattern realignment was carried out. This paper presents the results of pattern modification, and the following tracking and modifications strategies. This field test in Xinzhao field is successful and also is an excellent opportunity to demonstrate the method to improve the development effect of naturally fractured reservoir.
Horizontal Well Sectional Frac Technique with Dual-Retrievable Packers in Daqing Oilfield
Authors: Yaning Liang , Qingfeng Yue , zhenhuan Zhang , Qingzhong Li , Jianhua Xie , Li Ban , Borui Li
Published: March 2009

Horizontal well technology is a technological revolution in oil industry, and it is one of the most important methods of thick layer sustainable production and marginal low permeable formation exploitation in Daqing Oilfield. It is commonly believed that fracturing is an effective way to recover oil from thin, low porosity and low permeability sands. However, the existing frac technique in the horizontal wells - the limited entry frac is a kind of commingled frac method that cannot guarantee the fractures' opening and extension uniformly owing to heterogeneity formation. A novel sectional frac technique with dual-retrievable packers by separately stimulating the specific intervals one by one has been developed to recover low permeable reservoirs in the horizontal wells. Sectional frac string chosen includes high pressure, high temperature tools which have been used in several wells application as well as long term tested in a laboratory environment to differential pressures of 7300 psi at 190 ° F with functions of stuck-preventing, stuck-releasing and reverse sand flushing. Five intervals treatment can be done in one trip.

Up to now, this technique has been applied on 55 horizontal wells in Daqing Oilfield and 281 intervals were fractured in the field. It usually takes one to three days to frac a horizontal well according to the numbers of target reservoirs. The initial oil production after using the sectional frac technique in horizontal wells is 5.7 times higher than that in vertical wells, and 1.04 times higher than that by using the limited entry frac technique in horizontal wells. The current oil production after using the sectional frac technique in horizontal wells is 4.3 times higher than that in vertical wells, and 1.72 times higher than that by using the limited entry frac technique in horizontal wells. Field runs and lab testing have confirmed that sectional frac technique is a viable option in recovering low permeable reservoirs in the horizontal wells.
Case History: Successful Application of Combined Rotary-Jetting and MLT to Stimulate Dual-Lateral Producer in Ghawar Field
Authors: Al-Buali Muhammad H. , Al-Arnaout Ibrahim H. , Al-Shehri Ayed M. , Haldar Surajit , Al-Driweesh Saad M.
Published: March 2009

The implementation of multilateral wells in Saudi Aramco, and in the southern area of Ghawar field in particular, is being increased for production and cost optimization. Accessing and stimulating individual laterals is a challenge that requires an optimum design in terms of cost, application and efficiency.

This paper will describe a unique utilization of a combined high energy rotary-jetting tool with a multilateral tool (MLT) to access and treat a dual-lateral oil producer in the southern part of Ghawar field. The main purpose of utilizing this combined tool was to acid wash the Arab-D horizontal section of both laterals with nitrified hydrochloric acid using coiled tubing (CT). The objectives of the treatment were to increase flowing bottom-hole pressure (FBHP), remove formation damage and return the well to production. The rotary-jetting tool is a high energy jetting tool that maximizes performance of treatment fluids while minimizing the need for large acid volumes. It can be run with the MLT to enable accessing and stimulating both laterals in a single trip.

The implementation of the combined tool resulted in accessing and stimulating both laterals successfully. The well was returned to production at a rate of 11,000 barrels of oil per day (BOPD). The volume of acid was reduced due to utilizing the rotary-jetting tool, which maximized fluid treatment performance. This paper will cover the whole cycle of candidate selection, job design, execution, post job evaluation and conclusion.
Attracting Top Students to Study Petroleum Engineering: Curtin University
Authors: Smith L. C. , Evans B. J.
Published: March 2009

Over the past 20 years, two of the main methods of training new students in petroleum engineering at Curtin University have been to provide a Bachelor of Engineering in chemical engineering with a minor/major in petroleum engineering, or to offer a Masters in Petroleum Engineering as a conversion course for graduates of an appropriate science or engineering discipline.

When the industry picks up strongly and there becomes a shortage of skilled professional engineers, the industry employs anyone who has skill and experience. A knee-jerk reaction is to turn to academics, enticing them with attractive salaries, thereby depleting the stocks of quality petroleum engineering teachers. This results in a weakening of the education system which is expected to provide the future quality graduates.

Curtin University Department of Petroleum Engineering is no exception. Our first step however was to base the department on high quality research, so that we were not dependent upon short term income from industry. The next step was to change the Masters teaching base to semester rather than block teaching. This allowed faculty to enter long term research contracts and to teach over the full semester period. Additionally we extended our Masters program to 2 years in duration to make it more attractive to international students who could subsequently apply for permanent residency in Australia following their studying. As a result applications to the Masters program has increased by 60%. Additionally, we have developed a new Master's entry point via a one year Post Graduate Diploma course, which then allows international students to enter the Masters program with the desired prerequisites- this has also removed Australian visa restrictions, making it financially prohibitive for students coming from some countries. The result has been a return to stability and optimism with faculty and staff who want to educate tomorrow's engineers.
Thermodynamic Criteria and Final Results of WAG CO2 Injection in a Pilot Project in Croatia
Authors: Novosel D.
Published: March 2009

The remaining oil reserves in Croatian oil fields amounted to 11.0 million cubic meters at the end of 2007. This, together with high oil prices on the world market, represents a good basis for the implementation of enhanced oil recovery methods. Between 1978 and 1991, detailed laboratory research of carbon dioxide usage for oil displacement was performed. That research, coupled with simulation studies, has proved that water alternate gas – WAG process achieves the best oil recovery from Croatian oilfields (e.g. Ivanić oilfield).

The CO2 pilot project on the Ivanić oilfield started in 2001 by injecting salt water into the reservoir in order to increase reservoir pressure. The injection of carbon dioxide lasted between 2003 and 2006. Final results and in situ data from the field will be presented in the paper. During the injection of carbon dioxide into the reservoir, some thermodynamic changes occurred in the tubing string as well. The most important factors during CO2 injection are the knowledge of CO2 physical characteristics and thermodynamic conditions during the injection. PT conditions during the CO2 injection on the pilot project have been measured and will be presented in the paper. Based on the results of the pilot project, a decision to set up the first Croatian full-field EOR project on Croatian oilfields was made.
Transient Pressure Behavior for a Reservoir with Continuous Permeability Distribution in the Invaded Zone
Authors: El-Khatib Noaman A. F.
Published: March 2009

A mathematical model is presented for a reservoir with a continuous permeability distribution in the invaded zone. An analytical solution of the system was obtained in Laplace space. Solutions in real time space for the pressure and pressure derivative are obtained by numerical inversion of the Laplace transform using Stehfest algorithm. Solutions were obtained for infinite reservoirs and for finite reservoirs with closed and constant pressure outer boundaries. The permeability in the invaded zone varies from a skin permeability KS at the wellbore to the original permeability Ke at the end of the invaded zone RS. The effects of the permeability ratio, the depth of invasion, the reservoir size, and the wellbore storage were investigated. Results of this study show that for this permeability distribution, the permeability ratio has a profound effect on the pressure behavior while the depth of invasion plays an insignificant role. It also showed that the use of the average permeability of the invaded zone in the steady state skin equation gives almost identical results of the additional pressure drop due to formation damage at large times as those obtained from the exact solution. An approximate solution can be used for relatively high invasion depths (Rsd ≥ 20 ) which shows that the additional pressure drop is independent of the invasion depth
A New Practical Way for Calculating the Amount of Gas Flared Based on PVs in South Pars Gas Development Projects of IRAN
Authors: Khazaei M. , Pakizehseresht M.
Published: March 2009

Flaring is used as an effective and safe method of hydrocarbon vapor disposal whenever excess pressures should be released to prevent equipment failure on major upset conditions, such as instrument malfunction, power failure, or plant fire. It is also desirable to provide safe receipt and disposal of combustible, toxic gases and vapors released from process equipment during normal operation and as well as emergencies. One of the main concerns is having a keen understanding the amount of flaring gas and precise location of relieved gas injected to flare header.

In most oil and gas refineries, PVs are the main sources of continuous flaring in normal operation; further more, this will also ensure a constant flow of sweeping gas in flaring header.In emergency cases BDV (Blow down Valve), PSV (Pressure Safety Valve) and TSV (Temperature Safety Valve) are another flaring resources which will not be considered here. In this study a new calculation procedure for released gas due to PV openings is proposed based on thorough investigations made in one of South Pars development projects in Iran. This computation uses the design information in PVs datasheet as well as actual plant data for opening percentage values have been utilized from plant DCS system and PIMS (Plant Integrated Management System). The main advantage of this method is reporting the exact location of gas conducted to flare header, flow rate, condition of gas temperature, pressure, molecular weight and composition. So this makes it possible plant personnel to find the flaring cause earlier and rectify the problem sooner.
Development of an Integrated Reservoir Surveillance Process for World's Second Largest Field in Kuwait
Authors: Desai Sameer F. , Al-Ajmi Hussain Z. , Al-Humoud Jamal H. , Gazi Naz H.
Published: March 2009

The Greater Burgan Field in Kuwait is the largest clastic reservoir and the second largest field in the world accounting for most of the oil produced in the country. Discovered in 1938 it has over 1000 wells drilled to date. New wells are being added every year to this field to meet the production strategy of Kuwait Oil Company. It is becoming extremely important to identify and perform focused surveillance work to have a better understanding of the reservoir, drainage mechanism, remaining potential and pressure decline.

A huge amount of data accumulates every year leading to questions whether acquiring so much data is really justified. Techniques and processes used to acquire surveillance data to detect anomalies and the routine monitoring needs had to be quantified to assess the value of information. Thus a detailed study was undertaken to review the historic surveillance activities and the interpretation of the acquired data. The value of increased surveillance was highlighted through a dual approach of a quantitative and a qualitative analysis through which incremental benefits were achieved both directly and indirectly such as increased oil production, reduction in produced water, finer tuning of simulation models and sub surface maps.

The study concluded that the necessity for up to date surveillance data is justified by its importance for generating value based information for: Well performance and nodal analysis for production optimization and improved recoveryReservoir management and monitoringMappingSimulation studies for reservoir characterization, depletion mechanism and reserves addition

This paper describes the approach taken in the study towards the development of an integrated reservoir surveillance plan for monitoring the massive Burgan Field and highlights the challenges and accomplishments in this process. It covers the designing, completing, and operating aspect of this successful plan which can be used in other fields around the world.
PDO FEED Office Value Proposition
Authors: Al-Balushi Abdulnabi , Jeganathan Xavier , Flewker-Barker Ian
Published: March 2009

The Petroleum Development Oman (PDO) projects portfolio is going through a significant change as its oil and gas fields mature. In addition to the large number of repetitive brownfield developments, there is now an increase of major projects as Enhanced Oil Recovery opportunities mature. In addition, a number of major gas compression projects need to be implemented.

To sustain its oil and gas production PDO require to implement several major projects over the next ten years, each in the range from $ 100 mln to $ 2.0 bln.

PDO experience demonstrated that value improving practices are linked to improved capital project outcomes. Value improving practices are out-of-the-ordinary practices used to improve project performances and they are primarily used during initial phases of project i.e. from identifying the opportunity to define Front End Engineering Design (FEED) phase.

Due to lack of in-country FEED capability, all major FEED works were carried out abroad by International Design consultants and this FEED package was issued to Engineering Procurement & Constructing Contractor for Bidding / Detail Design purposes.

These FEED packages were resulting in followings deficiencies:- Insufficient FEED definition leading to risks and uncertainties during detail designMultiple changes to design at significant cost and schedule impactLack of Adequate Analysis of Potential SolutionsUnrealistic Plan/budget (optimistic forecasting)Continued loss of potential optimisation from the leverage opportunities that existRework due to changes in parallel activities in the same area lead to discussion on commercial/contractual terms, which causes delay in absorption of changes in FEED work.Lack of standardization of designPoor operability and maintainability of plantPoor first year + availability of new plant

Ultimately these deficiencies were leading to prolonged project completion cycles and delay in production. In view of these FEED package deficiencies and increase in PDO portfolio of major projects and the scarce market resources, PDO established in-house FEED capability. This reduces project lead time, improve project definition prior to the Final Investment Decision (FID) and will provide development opportunities for Omani staff. The 60 staff FEED capability was established with in-house resources, supported by a capable contractor. Training of PDO Omani staff in the additional skills was carried out in Q2 – Q4 2007 in a highly professional FEED office, while in the same period, office, Information technology (IT) infrastructure established and a support contractor was selected.
Sweep Impairment Due to Polymers Shear Thinning
Authors: AlSofi Abdulkareem M. , LaForce Tara C. , Blunt Martin J.
Published: March 2009

Most polymers used in EOR exhibit shear thinning behavior. At least theoretically, shear thickening will improve sweep while shear thinning (pseudoplasticity) will impair it through exacerbating the velocity contrast and/or inducing instability. Despite this, the effect of pseudoplasticity on sweep has not been studied in detail.

An in-house streamline simulator has been extended to handle polymer flooding with Newtonian and non-Newtonian behavior. Three main modifications were implemented. First, the polymer mass balance was solved along the streamlines. Second, a polymer multiplier was defined to account for polymers' viscosifying and thinning effects. Finally, an iterative approach was implemented to solve the pressure field. This is needed since the pressure depends on the aqueous phase viscosity, which for non-Newtonian fluids depends on shear stress, and hence the pressure itself.

The simulator was then used to investigate pseudoplasticity effects on sweep and recovery in various reservoir models. Two cases were run. The first had a stable connate bank; hence, thinning does not induce instability and the only effect is velocity exacerbation. The second had an unstable connate bank; hence, thinning can induce instability.

The results of this work prove the importance of taking polymers' non-Newtonian behavior into account for the successful design and evaluation of polymer flooding projects. This is because pseudoplasticity will impair sweep, which can deteriorate the whole economic picture of a polymer flood. Even if instability is not induced, more pore volumes will be needed, more water will be produced, and in light of a limiting water cut, less oil will be recovered; in other words, higher operations costs, higher processing costs, but less profit. Simulations carried in a 2D heterogeneous model suggest that for unconditionally stable flooding, the injection requirement will double from 2 to 4 pore volumes. In addition, in light of an 80% limiting water cut, a thinning fluid would decrease recovery by 2% of OOIP compared to a Newtonian fluid. Furthermore, if thinning does induce instability, shear thinning will reduce ultimate recovery. Simulations carried in a 2D heterogeneous model suggest that a pseudoplastic fluid would decrease recovery by 5% of OOIP compared to a Newtonian fluid.