bluuebunny/crossref_metadata_2025 huggingface dataset:
Ovarian development in the Amazonian red discus, Symphysodon discus Heckel (Osteichthyes: Cichlidae)
Authors: Chellappa S. , Câmara M. R. , Verani J. R.
Published: April 2006

The Neotropical red discus fish, Symphysodon discus, originates from the Amazonian basin and has a discoid body. Although this species is popularly used in aquaria and is exported as an ornamental fish, it has not been intensively studied. The purpose of this work was to study the morphological and histological aspects of the ovarian development in the red discus. Forty females of S. discus of varying body sizes and different stages of gonadal development were used in this study. The ovaries were weighed and examined macroscopically in order to observe the maturation stages. Histological staining of hematoxilin-eosin was used for microscopic observations of the ovaries. Macroscopic observations showed immature, maturing, mature and partially spent stages of ovarian development. Microscopic examination of the ovaries revealed five stages and five phases of ovarian development. The ovaries of the mature females showed all the different phases of oocyte development, indicating the multiple spawning habit of this species.
Reproductive biology of Astyanax janeiroensis (Osteichthyes, Characidae) from the Ubatiba River, Maricá, RJ, Brazil
Authors: Mazzoni R. , Mendonça R. S. , Caramaschi E. P.
Published: April 2006

Aspects of the reproductive biology of Astyanax janeiroensis in the Ubatiba river in Maricá, RJ, Brazil were evaluated in order to ascertain whether the strategy adopted by the species is causally related to environmental variables. Specimens were collected on a monthly basis from October 1994 to September 1995, through electrofishing. The Sex ratio was not significantly different from the expected frequency of 1:1 (G = 1.29; p > 0.50); nonetheless, considering three standard length classes, significantly larger numbers of females were recorded for the higher SL class (G = 11.07; p < 0.01). Size at first maturation showed no significant differences between sexes. Length-weight ratio analyses showed negative allometry for males and isometry, for females. Length structure was significantly different between sexes, suggesting that females are larger than males (D = 0.027, p < 0.01). Reproductive specimens were recorded during nine months of the annual cycle. Seasonal variations of reproductive specimens belonging to two different standard length classes displayed an asynchronous behavior, with larger specimens having a longer reproductive period and smaller ones reproducing for fewer months during the annual cycle. High values of fecundity (F), varying from 3169 to 18714 oocytes, were recorded for fish of 9.1 and 10.2 cm lengths, respectively. The correlation between weight and number of oocytes/unit of weight was positive, indicating that larger specimens produced more eggs.
Fire effects on the composition of a bird community in an amazonian Savanna (Brazil)
Authors: Cintra R. , Sanaiotti T. M.
Published: April 2006

The effects of fire on the composition of a bird community were investigated in an Amazonian savanna near Alter-do-Chão, Pará (Brazil). Mist-net captures and visual counts were used to assess species richness and bird abundance pre- and post-fire in an approximately 20 ha area. Visual counts along transects were used to survey birds in an approximately 2000 ha area in a nearby area. Results using the same method of ordination analysis (multidimensional scaling) showed significant effects of fire in the 20 ha and 2000 ha areas and strongly suggest direct effects on bird community composition. However, the effects were different at different spatial scales and/or in different years, indicating that the effects of fire vary spatially and/or temporally. Bird community composition pre-fire was significantly different from that found post-fire. Using multiple regression analysis it was found that the numbers of burned and unburned trees were not significantly related to either bird species richness or bird abundance. Two months after the fire, neither bird species richness nor bird abundance was significantly related to the number of flowering trees (Lafoensia pacari) or fruiting trees (Byrsonima crassifolia). Since fire is an annual event in Alter-do-Chão and is becoming frequent in the entire Amazon, bird community composition in affected areas could be constantly changing in time and space.
Reappraisal of experimental infections with cercariae and schistosomula of a brazilian strain of Schistosoma mansoni in mice
Authors: Vilar M. M. , Pinto R. M.
Published: April 2006

The present investigation involves a reevaluation of previous results obtained after experimental infection of Swiss Webster mice with cercariae and schistosomula of the Schistosoma mansoni LE strain maintained under laboratory conditions. Three experimental groups of mice were considered: the animals of the first group were percutaneously (ring method) infected with cercariae, those of the second were subcutaneously inoculated with cercariae and the mice of the third were inoculated by the same route with schistosomula transformed in vitro. The data obtained so far indicated that the most effective method of infection is the subcutaneous injection with schistosomula, with a mean adult worm burden recovery of 54.1% when compared to the abdominal percutaneous and subcutaneous routes of infection with cercariae, in which the values were 36.7% and 32.4%, respectively. This suggests that, in experimental infections of SW mice with a LE S. mansoni strain, the skin is to be considered an effective attrition site in the percutaneous route, whereas in the case of inoculation with cercariae, a small amount of larvae fails to be transformed into viable schistosomula, possibly due to skin phase avoidance. A brief discussion about attrition sites and elimination of larval S. mansoni worms in mice is presented.
Gestão de inovação: a lei de inovação tecnológica como ferramenta de apoio às políticas industrial e tecnológica do Brasil
Authors: Matias-Pereira José , Kruglianskas Isak
Published: April 2006

O objetivo principal deste artigo é debater, a partir das experiências de alguns países bem-sucedidos na implementação de políticas industrial e tecnológica, se a construção de um moderno arcabouço institucional na área de desenvolvimento científico e tecnológico, no qual deve estar incluída a questão da inovação, é essencial para o crescimento econômico do País. Nesse esforço, buscamos aprofundar a discussão sobre as políticas de gestão da inovação no Brasil, em especial sobre a Lei de Inovação Tecnológica - LIT (Lei Federal n.º 10.973 de 02.12.2004 ) - que estabelece medidas de incentivo à inovação e à pesquisa científica e tecnológica no ambiente produtivo, com vistas à capacitação e ao alcance da autonomia tecnológica e ao desenvolvimento industrial do País, por meio da análise das principais medidas explicitadas em seu texto. Evidencia-se que a aprovação da Lei de Inovação Tecnológica, apesar de suas deficiências, representa um instrumento relevante de apoio às políticas industrial e tecnológica do Brasil.
Effect of the sodium hypochlorite and citric acid association on smear layer removal of primary molars
Authors: Götze Gabriela da Rosa , Cunha Cristiane Beatriz Costa Sales , Primo Laura Salignac de Souza Guimarães , Maia Lucianne Cople
Published: April 2006

This study aimed to assess the capacity of a sodium hypochlorite and citric acid (CA) association (the latter at different concentrations) in removing coronal smear layer (SL) of primary teeth. For this purpose, the pulp chamber roof and floor of 28 primary molars were removed to obtain enamel and dentine disks. SL was produced on the internal walls of the disks using high-speed drills. The disks were irrigated with 1% sodium hypochlorite and citric acid at different concentrations (CA-4%, CA-6%, CA-8% and CA-10%), and with 0.9% sodium chloride. The samples were split and observed under SEM. Scores were attributed to the obtained photomicrographs, according to the amount of SL present. It was noted that all the tested concentrations of citric acid used after the sodium hypochlorite were capable of removing SL. The results were analyzed by the Kruskal-Wallis test, and there was no significant statistical difference among the scores of the groups tested. However, it was observed that CA-8% and CA-10% caused peritubular dentine destruction, and that CA-4% presented a larger number of samples with dense SL. Based on these results, 6.0% citric acid in association with 1% sodium hypochlorite is suggested as auxiliary chemical substances for primary teeth irrigation.
Effects of apple orchard management strategies on the great tit (Parus major) in southeastern france
Authors: Bouvier Jean-Charles , Toubon Jean-François , Boivin Thomas , Sauphanor Beno′t
Published: April 2006

The impact of conventional, organic, and integrated pest management (IPM) strategies of apple orchards on the reproduction of the great tit Parus major was investigated during a three-year period in southeastern France. The colonization process, egg-laying dates, clutch sizes, and fledging success were similar among pairs of P. major nesting in orchards conducted under the three studied management strategies. However, the mean number of young produced per ha (orchard productivity) was significantly higher in organic orchards than in both conventional and IPM orchards. Such divergences between both fledging success and orchard productivity primarily resulted from higher densities of P. major nesting pairs, but also from lower rates of nest abandonment during incubation in organic orchards. We suggest that intensive pesticide use under both IPM and conventional managements may have resulted in a substantial reduction in insect prey availability that enhanced intraspecific competition, which then led to failure in reproduction in pairs with low competitive ability. Our results highlight the relevance of P. major in assessing the environmental impact of apple orchard management strategies.
The Effect of Heterogeneity on In-Situ Combustion: Propagation of Combustion Fronts in Layered Porous Media
Authors: Akkutlu I. Yucel , Yortsos Yannis C.
Published: April 2006

In-situ combustion is a potential method for the recovery of heavy oil. The effect of reservoir heterogeneity, a ubiquitous feature of oil reservoirs, on in-situ combustion has not been systematically addressed in prior studies, however. In this paper, we present analytical models for filtration combustion, namely the combustion of a stationary solid fuel, in the specific case where the reservoir consists of two layers of different permeability and thickness, separated by nearly impermeable shales. We investigate the conditions for the propagation of steady combustion fronts as a function of some key parameters, including the permeability-thickness contrast R between the layers, the thickness ratio η, and the external heat loss coefficient h.

We find that heterogeneity acts in two distinct ways: It reduces the temperature of the leading front in the high-permeability layer in all cases, and uncouples the propagation of the fronts in the two layers if R is smaller than a critical value Rc. The first effect may lead to low-temperature oxidation conditions, and therefore to the effective extinction of the front in the high-permeability layer. The second leads to a reduced sweep efficiency (and early breakthrough). However, if R exceeds the critical value, the fronts in the two layers travel coherently (with the same speed). This coherence is identified for the first time. The resulting thermal coupling greatly retards the front in the more permeable layer, and accelerates only slightly that in the less permeable one, until the two fronts reach a common velocity.

We study the effects of R, the heat loss rate, and the ratio of thickness η. The coupling is aided by moderate heat losses (small h), and smaller η, which affect the critical value Rc.Asinthe homogeneous case, at sufficiently high heat loss rates, steady front propagation cannot be sustained and the combustion process becomes extinct.

The work is useful for the understanding of the viability of in-situ combustion process in heterogeneous layered reservoirs and the effect of a number of injection, combustion, and reservoir parameters.
Modeling Coupled Fracture-Matrix Fluid Flow in Geomechanically Simulated Fracture Networks
Authors: Philip Zeno G. , Jennings James W. , Olson Jon E. , Laubach Stephen E. , Holder Jon
Published: April 2006

In conventional reservoir simulations, gridblock permeabilities are frequently assigned values larger than those observed in core measurements to obtain reasonable history matches. Even then, accuracy with regard to some aspects of the performance such as water or gas cuts, breakthrough times, and sweep efficiencies may be inadequate. In some cases, this could be caused by the presence of substantial flow through natural fractures unaccounted for in the simulation. In this paper, we present a numerical investigation into the effects of coupled fracture-matrix fluid flow on equivalent permeability.

A fracture-mechanics-based crack-growth simulator, rather than a purely stochastic method, was used to generate fracture networks with realistic clustering, spacing, and fracture lengths dependent on Young's modulus, the subcritical crack index, the bed thickness, and the tectonic strain. Coupled fracture-matrix fluid-flow simulations of the resulting fracture patterns were performed with a finite-difference simulator to obtain equivalent permeabilities that can be used in a coarse-scale flow simulation. The effects of diagenetic cements completely filling smaller aperture fractures and partially filling larger aperture fractures were also studied.

Fractures were represented in finite-difference simulations both explicitly as grid cells and implicitly using nonneighbor connections (NNCs) between grid cells. The results indicate that even though fracture permeability is highly sensitive to fracture aperture, the computed equivalent permeabilities are more sensitive to fracture patterns and connectivity.
An Analytical Model for Cleanup of Yield-Stress Fluids in Hydraulic Fractures
Authors: Balhoff Matthew T. , Miller Matthew J.
Published: April 2006

The retention of fracturing fluid in a proppant pack reduces the dimensionless fracture conductivity, Fcd, resulting in poor well productivity regardless of fluid type (gelled oil, crosslinked polymers, viscoelastic surfactants, or foams). Analytical expressions derived in this paper can be used to calculate the extent of fracture cleanup under a set of production conditions. Several dimensionless parameters describing the fluid, fracture, and reservoir properties are introduced that affect the equilibrium cleanup. A second, transient, fracture-cleanup model is also proposed and is used to estimate the dimensionless-parameter values.

The equilibrium model predicts that cleanup increases greatly with decreasing dimensionless yield stress of the fracturing fluid. The magnitude of cleanup is also greater for cases in which the permeability ratio of the clean portion of the fracture to the fouled portion is high. The cleanup is expected to increase with an increase in the ratio of reservoir and fracture mobility until an optimum is reached, and then decrease with increasing the mobility ratio. The critical process of parameter estimation is achieved by making experimental measurements and by history matching published fracture flowback data using the transient model.
Prediction of Capillary Fluid Interfaces During Gas or Water Coning in Vertical Wells
Authors: Johns Russell T. , Lake Larry W. , Ansari Rafay Z. , Delliste Arnaud M.
Published: April 2006

Gas and water coning significantly reduce oil production while increasing production costs. Simulation and experimental methods, coupled with simple analytical solutions or correlations, are typically used to identify the oil rate that minimizes coning and maximizes recovery. Current analytical solutions, however, are overly simplified in that they assume negligible capillary pressure, which leads to segregated flow. This paper presents new benchmark analytical solutions that relax this assumption and also allow for simultaneous two-phase flow.

The new coning solutions apply to vertical wells where in-situ fluids are in vertical equilibrium. The development identifies the important dimensionless groups that control the effect of coning on oil recovery and illustrates how simultaneous two-phase flow affects capillary fluid levels in the formation. Dimensionless two-phase production windows are constructed to identify critical rates, the largest oil rate at which water (or gas) will not be produced. From comparisons to simulation, we show that critical flow rate estimates are accurate for aspect ratios greater than approximately 10. For aspect ratios less than 10, the critical rate estimates are always conservative.
Stability of a Multilateral Junction: Experimental Results and Numerical Modeling
Authors: Papanastasiou P. , Sibai M. , Heiland J. , Shao J. F. , Cook J. , Fourmaintraux D. , Onaisi A. , Jeffryes B. , Charlez P.
Published: April 2006

We studied the stability of multilateral (ML) junctions in a combined experimental and numerical modeling program. The experiments were carried out in a true triaxial machine on large cubic blocks (40 cm) of weak triassic sandstone with two holes intersecting. Six tests have been performed with two different geometrical configurations and three different stress states. The experimental results are presented and compared with numerical modeling obtained with finite-element software developed for assessing the integrity of rock surrounding an ML junction.
High-Order Upwind Schemes for Two-Phase, Multicomponent Flow
Authors: Mallison B. T. , Gerritsen M. G. , Jessen K. , Orr F. M.
Published: April 2006

In this paper, we investigate high-order finite-difference schemes for compositional simulation. Results presented for onedimensional (1D) gas-injection problems suggest that a third-order, essentially nonoscillatory scheme achieves accurate results more efficiently than first-order methods; leads to an improved treatment of phase behavior; and is more robust than traditional total variation diminishing schemes. We use phase diagrams as well as solution profiles to analyze discretization errors. Discretization errors can strongly affect the prediction of local displacement efficiency because of the nonlinear coupling introduced by the phase behavior in the system of governing equations. The study explains the ability of the numerical schemes to predict local displacement efficiency. In all of our tests, we compare numerical results to analytical and semianalytical solutions found through the method of characteristics. Our 1D results can be extended to 3D through conventional finite-difference approaches, but we plan to apply them in a streamline-based compositional simulator.
A Mechanistic Model for Horizontal Gravel-Pack Displacement
Authors: Martins A. L. , de Magalhães J. V. M. , Calderon A. , Chagas C. M.
Published: April 2006

This article presents the detailed formulation for each of the three steps of a horizontal gravel-pack displacement operation, including sand injection and alpha/beta waves propagation. The main core of the model, aiming to define alpha wave height, is based on a well known two-layer model. Initially developed for hydrotransport applications, this kind of model has been adapted by several authors for drilled cuttings transport analysis. Additionally, a comparison between theoretical predictions and pumping charts from a field operation performed in Campos basin is presented.
Effect of Crossflow on Foam-Induced Diversion in Layered Formations
Authors: Nguyen Quoc P. , Currie Peter K. , Zitha Pacelli L. J.
Published: April 2006

Foams have been widely used to control fluid mobility in improved oil recovery as well as in near-wellbore production enhancement operations, such as acid diversion. The efficiency of fluid blockage and diversion depends strongly on the geological layering of the formation and on the occurrence of (viscous and capillary) crossflow. However, these effects have not been studied in detail in the literature. In this paper, we report an experimental study of foam-induced fluid diversion in isolated and capillarycommunicating double-layer cores. Sodium dodecyl sulfate (SDS) surfactant was used at just above its critical micelle concentration. Natural rocks were used to manufacture concentric double-layer cores, having final permeability contrasts in the range of 10 to 200. We also used capillary-communicating double-layer cores, obtained by sintering two homogeneous glass-bead layers. In addition to pressure-drop monitoring, the in-situ fluid saturation was visualized using X-ray computerized tomography. A range of foam qualities has been investigated.
A New Method for Measuring Solvent Diffusivity in Heavy Oil by Dynamic Pendant Drop Shape Analysis (DPDSA)
Authors: Yang Chaodong , Gu Yongan
Published: April 2006

This paper presents a new experimental method and its computational scheme for measuring solvent diffusivity in heavy oil under practical reservoir conditions by DPDSA. In the experiment, a see-through windowed high-pressure cell is filled with a test solvent at desired pressure and temperature. Then, a heavy-oil sample is introduced through a syringe delivery system to form a pendant oil drop inside the pressure cell. The subsequent diffusion of the solvent into the pendant oil drop causes its shape and volume to change until an equilibrium state is reached. The sequential digital images of the dynamic pendant oil drop are acquired and digitized by applying computer-aided digital image-acquisition and-processing techniques. Physically, variations of the shape and volume of the dynamic pendant oil drop are attributed to the interfacial tension reduction and the well-known oil-swelling effect as the solvent gradually dissolves into heavy oil. Theoretically, the interfacial profile of the dynamic pendant oil drop is governed by the Laplace equation of capillarity, and the molecular diffusion process of the solvent into the pendant oil drop is described by the diffusion equation. An objective function is constructed to express the discrepancy between the numerically predicted and experimentally observed interfacial profiles of the dynamic pendant oil drop. The solvent diffusivity in heavy oil and the mass-transfer Biot number are used as adjustable parameters and thus are determined once the minimum objective function is achieved. This novel experimental technique is tested to measure diffusivities of carbon dioxide in a brine sample and a heavy-oil sample, respectively. It should be noted that, with the present technique, a single diffusivity measurement can be completed within an hour and only a small amount of oil sample is required. The interface mass-transfer coefficient at the solvent/heavy-oil interface can also be determined. In particular, this new technique allows the measurement of solvent diffusivity in an oil sample at constant prespecified high pressure and temperature. Therefore, it is especially suitable for studying the mass-transfer process of injected solvent into heavy oil during solvent-based post-cold heavy-oil production (post-CHOP).
Factors Affecting Cleanup of Water Blocks: A Laboratory Investigation
Authors: Mahadevan Jagannathan , Sharma Mukul M.
Published: April 2006

Water-blocked low-permeability gas formations with drawdown pressures comparable to capillary entry pressures can take a very long time to clean up. This work aims to study the effect of permeability, wettability, temperature, and drawdown on the cleanup of cores containing brine.

Gas displacement experiments were conducted on cores fully saturated with brine. Addition of methanol, increasing temperature, and increasing core permeability by changing rock type resulted in faster cleanup after approximately 50 to 100 pore volumes (PV, the volume of vacant pores in the dry sample) of gas flow. The change of wettability of the rock from water-wet to oil-wet also resulted in faster recoveries in gas relative permeability.

The cleanup of water blocks in gas wells occurs in two regimes: displacement of the fluids from the formation, followed by vaporization by the flowing gas, which becomes undersaturated as the pressure decreases. Our observations show that the cleanup of water blocks can be improved by: (a) Influencing the displacement regime (i.e., by changing the wettability) and (b) increasing the rate of vaporization by introducing volatile solvents such as methanol.

The study quantifies the effects of factors such as rock type (permeability), wettability, surface tension/volatility, and temperature on gas relative permeability. The results of this study will help in selecting strategies for cleanup of water blocks created by various operations such as drilling, acidizing, and fracturing, as well as making recommendations for the use of surfactants or solvents for well treatments to remove water blocks.
Drift-Flux Modeling of Two-Phase Flow in Wellbores
Authors: Shi H. , Holmes J. A. , Durlofsky L. J. , Aziz K. , Diaz L. R. , Alkaya B. , Oddie G.
Published: April 2006

Drift-flux modeling techniques are commonly used to represent two- and three-phase flow in pipes and wellbores. Unlike mechanistic models, drift-flux models are continuous, differentiable, and relatively fast to compute, so they are well suited for use in wellbore flow models within reservoir simulators. Drift-flux models require a number of empirical parameters. Most of the parameters used in current simulators were determined from experiments in small-diameter (2 in. or less) pipes. These parameters may not be directly applicable to flow in wellbores or surface facilities, however, because the flow mechanisms in small pipes can differ qualitatively from those in large pipes. In order to evaluate and extend current drift-flux models, an extensive experimental program was initiated. The experiments entailed measurement of water/gas, oil/ water, and oil/water/gas flows in a 15-cm-diameter, 11-m-long plexiglass pipe at eight deviations ranging from vertical to slightly downward. In this paper, these experimental data are used to determine drift-flux parameters for steady-state two-phase flows of water/gas and oil/water in large-diameter pipes at inclinations ranging from vertical to near-horizontal. The parameters are determined using an optimization technique that minimizes the difference between experimental and model predictions for phase in-situ volume fraction. It is shown that the optimized parameters provide considerably better agreement with the experimental data than do the existing default parameters.
Analytic Wellbore-Temperature Model for Transient Gas-Well Testing
Authors: Hasan A. R. , Kabir C. S. , Lin D.
Published: April 2006

Questions arise whether bottomhole pressures (BHPs), derived from their wellhead counterpart (WHP), lend themselves to transient analysis. That is because considerable heat exchange may affect the wellbore-density profile, thereby making the WHP translation a nontrivial exercise. In other words, gas density is dependent on both spatial locations in the wellbore and time during transient testing. Fully coupled wellbore/reservoir simulators are available to tackle this situation. However, they are not readily adaptable for their numeric formulations.

This paper presents analytic expressions, derived from first principles, for computing time-dependent fluid temperature at any point in the wellbore during both drawdown and buildup testing. The simplicity of the analytic expressions for Tf (z, t) is profound in that one can compute flowing or shut-in BHPs on a spreadsheet.

Two tests were considered to verify the new analytic formulae. In one case, measurements were available at both sandface and surface, and partial wellhead information was available in the other case. We explored a parametric study to assess whether a given wellbore/reservoir system will lend itself to wellhead measurements for valid transient analysis. Reservoir flow capacity (kh) turned out to be the most influential variable.
Dimensions and Degree of Containment of Waterflood-Induced Fractures From Pressure-Transient Analysis
Authors: Hoek P. J. van den
Published: April 2006

It is well established within the industry that injection of (produced) water almost always takes place under fracturing conditions. Particularly when large volumes of very contaminated water are injected—either for voidage replacement or disposal—large fractures may be induced over time.

This paper aims to provide a methodology for injection-falloff (IFO) test analysis of fractured (produced) water-injection wells. Some essential elements of IFO for fractured water injectors include the closing fracture, (early) transient elliptical reservoir-fluid flow, finite fracture conductivity, and fracture face skin.

An exact semianalytical solution is presented to the fully transient elliptical fluid-flow equation around a closing fracture with finite conductivity, fracture face skin, and multiple mobility zones in the reservoir surrounding the fracture. This solution also captures the case that during closure, the fracture is generally shrinking from adjacent geological layers under higher in-situ stress. Based on this solution, type curves of the dimensionless bottom-hole pressure as a function of dimensionless time are provided, covering both the period during fracture closure/shrinkage and the period after fracture closure. The shape of these type curves is studied as a function of the different relevant parameters, in particular the fracture compliance, the height of in-situ stress contrasts, fracture face skin, fracture closure time, and injection period. It is shown how the fracture length and height and the degree of fracture containment (in combination with the heights of the stress contrasts) can be derived from these types of curves. It is also demonstrated that the analyses based on the storage flow and linear formation flow regimes need to be integrated into one analysis method to obtain consistent results.

Finally, the concepts developed in this paper are applied to a number of field examples, in which the dimensions and degree of containment of the induced fractures are derived from the analysis of the IFO data.
The Role of Electrofacies, Lithofacies, and Hydraulic Flow Units in Permeability Prediction From Well Logs: A Comparative Analysis Using Classification Trees
Authors: Perez Hector H. , Datta-Gupta Akhil , Mishra S.
Published: April 2006

Predicting permeability from well logs typically involves classification of the well-log response into relatively homogeneous subgroups based on electrofacies, Lithofacies, or hydraulic flow units (HFUs). The electrofacies-based classification involves identifying clusters in the well-log response that reflect "similar" minerals and lithofacies within the logged interval. This statistical procedure is straightforward and inexpensive. However, identification of lithofacies and HFUs relies on core-data analysis and can be expensive and time-consuming. To date, no systematic study has been performed to investigate the relative merits of the three methods in terms of their ability to predict permeability in uncored wells.

The purpose of this paper is three-fold. First, we examine the interrelationship between the three approaches using a powerful and yet intuitive statistical tool called "classification-tree analysis." The tree-based method is an exploratory technique that allows for a straightforward determination of the relative importance of the well logs in identifying electrofacies, lithofacies, and HFUs. Second, we use the tree-based method to propose an approach to account for missing well logs during permeability predictions. This is a common problem encountered during field applications. Our approach follows directly from the hierarchical decision tree that visually and quantitatively illustrates the relationship between the data groupings and the individual well-log response. Finally, we demonstrate the power and utility of our approach via field applications involving permeability predictions in a highly complex carbonate reservoir, the Salt Creek Field Unit (SCFU) in west Texas. The intuitive and visual nature of the tree-classifier approach also makes it a powerful tool for communication between geologists and engineers.
Field Verification: Invert-Mud Performance from Water-Based Mud in Gulf of Mexico Shelf
Authors: Aldea Catalin , Bruton James R. , Dobbs William R. , Klein Aaron L.
Published: April 2006

A uniquely designed water-based mud (WBM) was used in two shelf areas of the Gulf of Mexico to drill highly reactive gumbo sections. The drilling performance and inhibition in these shale intervals resembled that of synthetic-based muds (SBMs) previously used in offset wells. Owing to new environmental regulations, obtaining the drilling-performance characteristics of a synthetic-based fluid and the stability of a water-based fluid is a high priority for Gulf of Mexico operators.

A special characteristic of the WBM fluid is its ability to deliver SBM-like shale inhibition without use of any salts, through an ideal combination of a polyamine shale inhibitor, a polymeric encapsulator, and an antiaccretion additive.

This paper presents field data comparing the drilling performance and the inhibition in gumbo-shale sections of the new WBM and the previously used SBM. The combination of this inhibitive WBM and the rotary-steerable tool produced an 85% higher rate of penetration (ROP) than in similar wells in the area drilled with SBM and mud motors. No accretion or bit balling was noted on bottomhole assemblies (BHAs) during bit runs, and the cuttings drilled were well encapsulated and protected from hydration. Economic savings were realized by reducing the drilling time and by not requiring the essential waste-management equipment and engineering associated with SBM-cuttings discharge. Further-more, the new high-performance WBM was easily prepared at the wellsite with seawater, which also translated into cost savings by reducing the volumes handled by the workboats.

These field applications demonstrate that this uniquely designed WBM can effectively replace invert-emulsion fluids in many highly reactive shale sections without sacrificing drilling performance or increasing environmental risk.
A New Thermal Insulating Fluid and Its Application in Deepwater Riser Insulation in the Gulf of Mexico
Authors: Wang Xiaolan , Javora Paul , Qu Qi , Pearcy Ricky
Published: April 2006

Undesired heat loss from the production tubing, or uncontrolled heat transfer to outer annuli, contributes to the formation of gas hydrates and causes the deposits of paraffin and asphaltene materials that reduce the production rate. The successful application of thermal-insulating fluids in the last several years has demonstrated that such fluids can control heat loss effectively. In some cases, heat loss from the produced fluids caused by conduction and convection can be reduced by more than 90% when compared with traditional packer fluids.

Recently, a new thermal-insulating-fluid system with superior thermal properties has been developed. Compared with the existing insulation fluids, the new insulating-fluid system provides high viscosity at a low-shear-rate range to reduce the fluid-convection rate. It also provides lower viscosity at high-shear-rate range to facilitate fluid placement. The new fluid, with improved insulation properties, has proved to be an attractive alternative to the current insulation method for deepwater risers. This paper highlights the development of this new thermal-insulating fluid and its application in deepwater-riser insulation in the Gulf of Mexico (GOM). Laboratory data and field cases are also presented to demonstrate the effectiveness of this fluid system.
Fast-Fourier-Transform-Based Deconvolution for Interpretation of Pressure-Transient-Test Data Dominated by Wellbore Storage
Authors: Cheng Yueming , Lee W. John , McVay Duane A.
Published: April 2006

We present a deconvolution technique based on a fast-Fourier-transform (FFT) algorithm. With the new technique, we can deconvolve "noisy" pressure and rate data from drawdown and buildup tests dominated by wellbore storage. The wellbore-storage coefficient can be variable in the general case. In cases with no rate measurements, we use a "blind" deconvolution method to restore the pressure response free of wellbore-storage effects. Our technique detects the afterflow/unloading rate function with Fourier analysis of the observed pressure data.

The technique can unveil the early-time behavior of a reservoir system masked by wellbore-storage effects, and it thus provides a powerful tool to improve pressure-transient-test interpretation. It has the advantages of suppressing the noise in the measured data, handling the problem of variable wellbore storage, and deconvolving the pressure data without rate measurement.

We demonstrate the applicability of the method with a variety of synthetic and actual field cases for both oil and gas wells. Some of the actual cases include measured sandface rates (which we use only for reference purposes), and others do not.

Although this paper is focused on deconvolution of pressure-transient-test data during a specific drawdown/buildup period corresponding to an abrupt change of surface flow rate, the deconvolution method itself is very general and can be extended readily to interpret multirate test data.
Closing the Gap: Fracture Half Length from Design, Buildup, and Production Analysis
Authors: Barree R. D. , Cox S. A. , Gilbert J. V. , Dobson M.
Published: April 2006

It is commonly observed that hydraulically fractured wells perform as though the "effective" fracture half-length is much lower than the designed half-length. This observation has been explained by various models, including poor fracture-height containment, poor proppant transport, proppant falling out of zone (convection), ineffective proppant-pack cleanup, capillary-phase trapping, multiphase flow, gravitational-phase segregation, and non-Darcy flow, with combinations of any of these mechanisms. With recent improvements in diagnostic measurements of fracture geometry, some of these explanations have lost credibility, but the problem of low effective fracture length persists.

This paper presents detailed evaluations of hydraulically fractured well behavior with continuous production analysis, pressure-transient (buildup) analysis, and fracture-treatment evaluation by use of actual field data from a tight-gas reservoir in the Rocky Mountain Region. The various analyses explain the observed producing behavior of the well and lead to a consistent determination of the actual effective fracture half-length compared with the physically created or propped length. Problems relating to semantics and inconsistent fracture and reservoir description, especially the physical processes encompassed by various analytical techniques, will be addressed.

Methods will be outlined for predicting the useful effective length from available proppant-conductivity data. The process outlined helps to close the gap between designed-fracture and producing lengths and points out the causes for the remaining system bottlenecks that limit post-fracture well productivity. Finally, the understanding of these mechanisms provides a means to arrive at an economical optimum fracture-treatment design for a reservoir once key parameters are known.
A New Model To Obtain Representative Field Relative Permeability for Reservoirs Produced Under Solution-Gas Drive
Authors: Bauget F. , Egermann P. , Lenormand R.
Published: April 2006

Relative permeability curves (kr) control production and are of primary importance for any type of recovery process. In the case of production by displacement (waterflood or gasflood), the kr curves obtained in the laboratory can be used in numerical simulators to predict hydrocarbon recovery (after upscaling to account for heterogeneity). In the case of reservoirs produced under solution-gas drive (depressurized field, foamy oils), the experiments conducted in the laboratory depend on the depletion rate and cannot be used directly for reservoir simulations.

We have developed a novel approach for calculating representative field relative permeabilities. This new method is based on a physical model that takes into account the various mechanisms of the process: bubble nucleation (pre-existing bubbles model), phase transfer (volumetric transfer function), and gas displacement (bubble flow). In our model, we have identified a few "invariant" parameters that are not sensitive to depletion rate and are specific to the rock/fluid system (mainly the pre-existing bubble-size distribution and a proportionality coefficient relating gas and oil velocity for the dispersed-phase regime). These invariant parameters are determined by history matching one experiment at a given depletion rate.

The calibrated model is then used to generate synthetic data at any depletion rate, especially at very low depletion rates representative of the reservoir conditions. Relative permeabilities are derived from these "numerical" experiments in the same way as they are from real experiments. The calculated kr is finally used in commercial reservoir simulators.

We have tested our model by using several series of published experiments with light and heavy oils. After adjusting the invariant parameters on one or two experiments, we are able to predict other experiments performed at different depletion rates with very good accuracy. Finally, we present an example of determination of relative permeabilities at reservoir depletion rates.
A Comparison of Travel-Time and Amplitude Matching for Field-Scale Production-Data Integration: Sensitivity, Nonlinearity, and Practical Implications
Authors: Cheng Hao , Datta-Gupta Akhil , He Zhong
Published: April 2006

The traditional approach to reconciling geologic models to production data involves an "amplitude matching," that is, matching the production history directly. These include water-cut, tracer concentration, and pressure history at the wells. It is well known that such amplitude matching results in a highly nonlinear inverse problem and difficulties in convergence, often leading to an inadequate history match. The nonlinearity can also aggravate the problem of nonuniqueness and instability of the solution. Recently, production data integration by "travel-time matching" has shown great promise for practical field applications. In this approach, the observed data and model predictions are lined up at some reference time such as the breakthrough or "first arrival" time. Further extensions have included amplitude information by a "generalized travel-time" inversion. Although the benefits of travel-time inversion are well documented in the context of seismic inversion, no systematic study has been done to examine its merits for field-scale history matching.

In this paper, we quantitatively investigate the nonlinearities in the inverse problems related to travel time, generalized travel time, and amplitude matching during production data integration and their impact on the solution and its convergence. In our previous works, we speculated on the quasilinear nature of the travel-time inversion without quantifying it. Our results here show, for the first time, that the commonly used amplitude inversion can be orders of magnitude more nonlinear compared to the travel-time inversion. We also examine the resulting implications in field-scale history matching. The travel-time inversion is shown to be more robust and exhibits superior convergence characteristics. The travel-time sensitivities are more uniform between the wells compared to the amplitude sensitivities that tend to be localized near the wells. This prevents overcorrection near the wells.

We have demonstrated our results using a field application involving a multiwell, multitracer interwell tracer injection study in the McCleskey sandstone of the Ranger field, Texas. Starting with a prior geologic model, the traditional amplitude matching could not reproduce the field tracer response, which was characterized by multiple peaks. Both travel time and generalized travel time exhibited better convergence properties and could match the tracer response at the wells with realistic changes to the geologic model.
Quantitative Use of 4D Seismic Data for Reservoir Description
Authors: Dong Yannong , Oliver Dean S.
Published: April 2006

The major contribution of this paper is the description of an efficient, gradient-based method for incorporating time-lapse seismic impedance data in anautomatic history matching procedure. We use a finite-difference, black-oilreservoir simulator, with rock physics included, to predict seismic impedanceas a function of saturation and pressure. The adjoint method is used to computethe derivatives of seismic impedance change in each gridblock with respect toporosity, permeability, pressure and phase saturation. With a quasi-Newtonminimization method to efficiently minimize the difference between observedimpedance changes and the impedance changes predicted using the reservoirsimulator, our procedure can be applied to field-scale problems. Thefeasibility of this method of utilizing time-lapse seismic data is illustratedwith an application to a semi-synthetic model created from a relatively largemiddle-eastern oil field.
Development and Field Application of a New Measurement Technique for Organic Acid Additives in Stimulation Fluids
Authors: Taylor K. C. , Al-Katheeri M. I. , Nasr-El-Din H. A.
Published: April 2006

Formic and acetic acids, with and without HCl, have been used extensively over the last 40 years as stimulation fluids and as acid additives. A measurement method for these organic acids was not previously available for highly contaminated acid stimulation samples that contain high acid concentrations, dissolved oil, suspended solids, and several different acid additives. Citric and acetic acids are also used in acid treatments as iron control agents at temperatures less than 200 and 150°F, respectively. An analytical method using capillary electrophoresis was developed to simultaneously measure the concentrations of formic and acetic acids in stimulation fluids. The developed method was used to evaluate flowback samples from pickle treatments and stimulation treatments of several high-temperature wells.

The following conclusions are made:

A method for the simultaneous measurement of formic and acetic acids was developed, evaluated, and implemented for acid stimulation samples. Citric acid concentration can be determined in certain cases, which are identified in the paper. Results of formic and acetic acid concentrations in sandstone and carbonate corefloods were evaluated. Flowback samples from pickle treatments and acid fracture treatments were evaluated. Results from treatments of super chrome-13 and low-carbon steel tubing with 15 wt% HCl/ 9 wt% formic acid show that a small decrease in formic acid relative to chloride concentration occurred over time with carbon steel. No significant decrease of formic acid relative to chloride concentration was observed in the flowback of a well with a super chrome-13 completion.