bluuebunny/crossref_metadata_2025 huggingface dataset:
History Matching of the PUNQ-S3 Reservoir Model Using the Ensemble Kalman Filter
Authors: Gu Yaqing , Oliver Dean S.
Published: December 2007

The problem of reservoir characterization through automatic history matching has been extensively studied in recent years. Efficient applications have, however, required either an adjoint or a gradient simulator method to compute the gradient of the objective function or a sensitivity coefficient matrix for the minimization. Both computations are expensive when the number of model parameters or the number of observation data is large. The codes for gradient-based history matching methods are also complex and time-consuming to write.

This paper reports the use of the Ensemble Kalman Filter (EnKF) for automatic history matching. EnKF is a Monte Carlo method, in which an ensemble of reservoir models is used. The correlation between reservoir response (e.g. watercut and rate) and reservoir variables (e.g. permeability and porosity) can be estimated from the ensemble. An estimate of uncertainty in future reservoir performance can also be obtained from the ensemble.

The methodology of EnKF consists of a forecast step and an assimilation step. A finite-difference, 3-D, 3-phase black-oil reservoir simulator is used for stepping forward the reservoir states. However, unlike the traditional history matching, the source code of the reservoir simulator is not required, which allows this method to be used with any reservoir simulator. Moreover, this forward step is well suited for parallel computation since the time evolution of ensemble reservoir models are independent, hence the ensemble of reservoir models can be advanced in time simultaneously using multiple processors. Only the data assimilation step, i.e. the computation of Kalman filter, requires communication between processors.

The assimilation of the data in EnKF is done sequentially rather than simultaneously as in traditional history matching. By so doing the reservoir models are always kept up-to-date, which is important and practical when the frequency of data is fairly high as, for example, the data from permanent sensors.

The PUNQ-S3 reservoir model is used to test the method in this paper. It is a small-size (19 × 28 × 5) reservoir engineering model that was developed by a group of companies, institutes and universities in the European Union to compare methods for quantifying uncertainty assessment in history matching. One conclusion is that EnKF can sometimes provide satisfactory history matching results while requiring less computation work than traditional methods.
Integrating NMR With Other Petrophysical Information to Characterize a Turbidite Reservoir
Authors: Dastidar Rahul , Rai Chandra , Sondergeld Carl
Published: December 2007

Uncertainty lies in developing the nation's discovered oil and gas fields. Since the subsurface cannot be accessed directly, indirect methods and procedures must be developed to estimate the size and productive potential of a reservoir. The value of individual pieces of information is realized when they are integrated with other sources of information to characterize a reservoir. New and improved technologies e.g., Nuclear Magnetic Resonance1 (NMR), has been extensively used to determine the reservoir properties such as lithology independent porosity, permeability, water saturation, irreducible water saturation etc.

The Free Fluid Model2 and the Mean T2 Model3 are two NMR based methods to estimate permeability. Partitioning of pore fluid into free and bound volume depends on the precise value of transverse relaxation time (T2), T2cutoff. Infact, for a water wet rock, bound volume irreducible (BVI) is the amount of water that coats the surface of the pore space and can be referred as the irreducible water saturation (Swirr).

In this study fifteen plug samples from a turbidite reservoir (Grubb formation) from California were used. The plugs are classified as arkosic-lithic sandstones (with an average mineral composition of feldspars (52%), quartz (40%), clays (4%), carbonates (3%) and others (1%)). The porosity ranges from 6% to 19% with an average of 15% and the permeability ranges from 0.045 md to 30.42 md having an average of 9.7 md. NMR spectra were collected on saturated (25,000 ppm NaCl brine) and desaturated samples.

Using the cutoff BVI (CBVI4) technique for estimating the percentage of bound water, the measured T2cutoff for the fifteen plugs ranged from 5 msec to 15 msec. The computed free fluid index (FFI) based on the above T2cutoff range is approximately 30% to 50% higher than the FFI calculated from industry recommended T2cutoff value of 33 msec for clastic rocks. For this reservoir, the Mean T2 Model3 gives better estimate of permeability as compared to those calculated using the Free Fluid model2. The irreducible water saturation (Swirr) decreases with increase in permeability. The comparison of Swirr from capillary pressure measurement and centrifuge experiments showed a pattern that can be explained on the basis of mineralogy and microporosity. The main purpose of establishing a correlation between NMR T2 distributions with mercury injection capillary pressure (MICP) is to generate the capillary pressure curves directly from NMR relaxation time distribution during wireline logging. An average value of effective surface relaxivity (ρe) of 24.3 μm/sec has been established for the Grubb formation, which is then used to generate the pseudo capillary pressure curves. The NMR derived capillary pressure curve shows a reasonable match with the air-brine capillary pressure curves derived from MICP.
Wavelets in Petroleum Industry: Past, Present and Future
Authors: Guan L. , Du Y. , Li L.
Published: December 2007

Wavelet is first introduced by Alfred Haar in 1910, but the development of wavelet was very slow until the middle of 1980s. In 1984, Jean Morlet made the wavelet rebirth by developing a new way of analyzing the seismic signals to overcome the deficiencies in the Fourier method. Since then, the new developments of wavelets have fascinated the scientific and engineering communities.

A wavelet is a waveform of effectively limited duration that has an average value of zero and wavelets are a family of basis functions, which can separate a signal into distinct frequency packets that are localized in the time domain. Thus, wavelets are well suited to analyze nonstationary data. They can smooth the basic signals and keep the details of basic signals. Therefore, they provide a multiresolution framework for data representation.

Wavelet analysis is a rapidly developing area in many disciplines of science and engineering and it is used in a wide variety of applications in the areas of medicine, biology, data compression, etc. In recent years, wavelet analysis has found its application in the petroleum industry. This paper reviews the recent application of wavelet analysis in the industry. Various application examples are discussed, especially the examples in the areas of reservoir characterization, geological model upscaling, and well testing.
In Well Nuclear Magnetic Resonance (NMR) Multiphase Flowmeter in the Oil and Gas Industry
Authors: Ong J. T. , Oyeneyin M. B. , Coutts E. J. , MacLean I. M.
Published: December 2007

In the current climate of production monitoring, the main concern is to obtain measurement that has high accuracy and specificity. This can be achieved using a system that is independent of flow regime, having the ability to differentiate the velocity and mass flowrates of at least three phases; gas, oil and water. In addition, it should be capable of quantifying the volume fraction of each phase with respect to the overall flow volume. Currently, there is no single flowmeter that can reliably perform these tasks.

The design of an in-well flowmeter has to account for the electronic components susceptibility to high pressure (P) and temperature (T) by equipping the system with PT sensors. By monitoring the environment, the pressure and temperature are maintained below pre-defined thresholds. This ensures a higher level of accuracy in the measurements as well as prolonging instrument lifetime.

The principle of flow measurement adopted is based on nuclear magnetic resonance (NMR). NMR is suited to flow evaluation in this industry due to the fluids present, i.e. water and hydrocarbon (gas or liquid). These fluids contain 1H and 13C nuclei which are important to NMR measurements. Unlike conventional flowmeters which measure the physical characteristics of the fluid, NMR monitors the molecular environment of the fluid. NMR has the ability to distinguish the different phases through the use of intrinsic molecular properties, i.e. relaxation times and resonant frequencies.

In NMR, the volume fraction is calculated from the proton density. The flow measurement is obtained by monitoring the magnetised nuclei flow into a detection zone. The velocity distribution function is then calculated by applying an inverse algorithm to the data.

The advantages of this system are its ability to be used as a single self-sufficient instrument and its phase independence for up to a two phase flow. This flow-independent measurement technique could be extended for use in three and four (solid) phase flow.
Probabilistic Approach Takes You Back to Reality
Authors: Macary S.
Published: December 2007

One of the most important outputs of the static model is the volumetrics of the original hydrocarbon in place (OHIP). According to the type of reservoir driving mechanism, the reservoir engineer can assume the recovery factor and hence the recoverable reserves. Most simulation studies are being conducted on fields considering a period of production performance to determine the remaining reserves and the best scenario of development to produce these reserves, which is the main function of the dynamic model.

The volumetrics could be approached using a Monte Carlo technique, the end product of which is a range of probable OHIP and, consequently using the proper recovery factor, getting probability distribution of the recoverable reserves. Therefore, using the probabilistic approach is superior in green fields rather than brown fields because it captures the full range of reality and where models are not yet calibrated to dynamic data.

Sometimes, the dynamic data may take a surprise turn from the geological understanding. Different realizations, even the crazy ones, may help under the probability distribution functions in defining some certainties that have not yet even been imagined. In addition, each of the dynamic model's scenarios yields extra recovery, the certainty of which can be evaluated using the probability distribution curve of the remaining reserves. This certainty has direct influence on the economics approval of one scenario over the others.

The current work introduces actual case histories demonstrating how the probabilistic approach could be of real help in selecting the production enhancement scenario and how this tool also could be used to prioritize management interests.
Joint Estimation of Porosity and Saturation Using Stochastic Rock Physics Modeling
Authors: Bachrach R. , Dutta N.
Published: December 2007

Porosity and saturation are fundamental sedimentary properties that affect both seismic wave propagation and appraisal development and production of an oil field. Traditionally, these parameters have been estimated from seismic impedances and velocities using Biot-Gassman theory and/or or porosity-velocity relationships. However, when using theory to relate surface seismic attributes to reservoir saturation, the porosity estimation problem and its affect on the unsaturated sedimentary properties should be addressed. Similarly, the problem of porosity estimation is also affected by the hydrocarbon saturation at the target. Because bulk modulus and density are affected by both porosity and saturation of the sediment, and because the shear modulus is affected by the sediment porosities, estimating hydrocarbon saturation and reservoir porosity from seismic data is a joint estimation problem. Moreover, the uncertainties associated with saturation estimation are related to the uncertainties we have in the porosity, and vice versa.

In this paper, we present a methodology for joint porosity-saturation estimation. We show how porosities and saturations can be jointly estimated using stochastic rock physics modeling and Bayesian estimation framework. We study the interdependencies of this reservoir property estimation problem within a given lithology unit.
Prediction of Three-Phase Relative Permeabilities Using a Pore-Scale Network Model Anchored to Two-Phase Data
Authors: Svirsky D. S. , van Dijke M. I. J. , Sorbie K. S.
Published: December 2007

A first attempt has been made to predict three-phase relative permeability experimental data of a water-wet Berea sandstone obtained by Oak1 using the three-phase flow network model for arbitrary wettability developed by van Dijke and Sorbie2. First, the network model is anchored to the corresponding two-phase relative permeability and capillary pressure data using an idealised representation of the pore geometry and a simple parameter fitting procedure. Then, predictions of three-phase properties are made, which are compared with experimental data as well as previous predictions from a different network models. The present study has confirmed that the relatively simple network model, anchored to experimental data, is able to predict three-phase relative permeabilities with reasonable accuracy, comparable to the accuracy using more complex models. Based on these preliminary results a limited sensitivity study is carried out with respect to different wettability states and two combinations of interfacial tensions. This study reveals some new results with respect to the invariance of relative permeability to interfacial tension combinations and the trend of water relative permeability as a function of the fraction of oil-wet pores in systems of non-uniform wettability.
Seismic Pore-pressure Imaging in Deepwater Offshore West Africa
Authors: Banik N. C. , Schultz G. , Wool G. , Bachrach R. , DenBoer L. , Mao W. , Huggard J. D. , Nibbelink K. , Taylor N.
Published: December 2007

Pore-pressure imaging in the frontier, deepwater offshore West Africa poses enormous challenges. The presence of large and deep radial canyons in the area makes conventional seismic velocity analysis methods inappropriate. Due to relatively younger and highly water-saturated deepwater sediments in the shallow sections, no compaction trend may be discernible for empirical methods for pore-pressure prediction. Moreover, because of the absence of deepwater wells, the calibration of mo del parameters for pore-pressure prediction is not possible.

Using automated, high-resolution, and spatially consistent 3D velocity analysis and a first-principle-based rock model, we have developed a methodology for pore-pressure imaging in the frontier, deepwater basins beset with complex geology. The rock model emphasizes burial history and shale diagenesis for pore-pressure generation. The methodology has been successfully applied to provide pre-drill prediction of pore pressure over a 600-km2 area (Figure 1) in deepwater offshore West Africa. A 2D seismic line tying a remote, shallow-water well greatly helped in developing the appropriate rock model.

The results, based on primarily seismic data and the associated geological interpretations, suggest a regressive pore-pressure system in the area. The onset of overpressure takes place at the seafloor. The pressure gradient increases slowly, but almost continuously, then rapidly to a maximum value, and then gradually reverses back to a lower value. The pressure profile closely follows certain geologic horizons and structures. Radial canyons on the seafloor significantly affect pore pressure. This paper describes these results in detail. Also presented in the paper are pore-pressure uncertainty estimation and a sensitivity analysis of the rock-model parameters for pore-pressure prediction.
Integrating Diverse Measurements to Predict Pore Pressure With Uncertainties While Drilling
Authors: Malinverno Alberto , Sayers Colin M. , Woodward Marta J. , Bartman Robert C.
Published: December 2007

An accurate estimate of the formation pore pressure is a key element for safe and cost-effective drilling. We describe here a methodology that integrates information from diverse measurements to predict pore pressure in a well before and during drilling. The data we can incorporate include surface seismics, sonic and density well logs, checkshots, mud weights, and measured pore pressures. The distinctive feature of our method is that uncertainties in the predicted velocities and pore pressures are quantified and updated while drilling. Quantifying uncertainties allows us to properly integrate all data and to show how measurements acquired while drilling reduce the uncertainty in the predicted pore pressures.

In practice, we use a Monte Carlo method to sample the posterior probability distribution (given all the data) of the compressional-wave velocity profile and of coefficients in the relationships that relate velocity to pore pressures. For each sampled value of velocities and other coefficients, we compute the corresponding pore-pressure profile along the well trajectory; the range of all sampled pore pressures describes the pore-pressure uncertainty. We demonstrate the methodology in two examples where the uncertainty in pore pressure predicted before drilling (from seismic velocities obtained by reflection tomography) is reduced by additional data acquired while drilling (checkshot, sonic log, mud weights, and pore-pressure data).
Taxonomy: A Knowledge Sharing Enabler
Authors: Smith David L. , Busch Joseph A. , Daniel Ron
Published: December 2007

As companies have begun to share information using intranets and internets, more information is available than the average person can absorb. A simple internet search using any search engine can return hundreds to thousands of documents that may contain only bits of information that one is required to integrate into useful knowledge. Successful implementation of internet and intranet portals, knowledge management, and content management requires a systematic approach to classifying information. The classification framework, or taxonomy, allows content owners to categorize their content so that it becomes more useful to a broader audience. Taxonomy allows knowledge seekers, whether they are researchers or the general public, to easily find and re-use information rather than recreate it.

Taxonomies are not new; in fact most work groups organize their information around a hierarchy that developed over several years. In most corporate environments, each product line or functional group organizes its information from its internal viewpoint rather than from a broader viewpoint. Use of a broader viewpoint would however, encourage content sharing beyond the product line or group. To be successful the digital world must eventually merge these independent hierarchies toward a common taxonomy.

This paper details the development of an enterprise-wide taxonomy for a diverse, global oil and gas services company. The paper describes implementation of the taxonomy, including the application of taxonomy to a comprehensive content management system, to knowledge management initiatives, and to effective portal design. Challenges and lessons learned are also presented, the most significant of which are issues surrounding the means to get people to think differently and do things in different ways.
Dynamic Flow-Based PEBI Grids for Reservoir Simulation
Authors: Mlacnik M. J. , Durlofsky L. J. , Heinemann Z. E.
Published: December 2007

A technique for the dynamic generation of flow based PEBI (perpendicular-bisectional) grids is presented and applied. The procedure includes a fine scale flow solution, the generation of an initial streamline–isopotential grid, grid optimization, and upscaling. The grid optimization is accomplished through application of a hybrid procedure with gradient and Laplace smoothing steps, while the upscaling is based on a global– local procedure that makes use of the global solution used in the grid determination step. The overall procedure is successfully applied to a complex channelized reservoir model involving changing well conditions. The gridding and upscaling procedures presented here may also be suitable for use with other types of structured or unstructured grid systems.
History Matching of Stochastic Models of Field-Scale Fractures: Methodology and Case Study
Authors: Jenni Sandra , Hu Lin Y. , Basquet Rémy , de Marsily Ghislain , Bourbiaux Bernard
Published: December 2007

This paper focuses on the history matching of stochastic models of large-scale (field-scale) fractures, namely (sub-) seismic faults and fracture swarms. First, we propose an object-based stochastic model for describing geological features of large-scale fractures. This model accounts for static constraints derived from seismic attributes, fault-related-strain-field, structural information (curvature) etc. Second, we review an upscaling procedure for performing fluid flow simulation in the presence of networks of large-scale fractures. Third, we present an algorithm for gradually moving and deforming stochastic fractures in the reservoir field while preserving their consistency with static constraints (location of seismic fractures, fracture density and orientation maps), whereby different realizations of the stochastic fracture network can be obtained. All these elements are integrated in an inverse procedure for calibrating stochastic models of large-scale fracture networks to hydrodynamic two-phase flow data.

The above methodology is applied to an actual fractured reservoir. We build a field-scale fracture network constrained to the fracture density map and the orientations of fracture sets. Then we perform history matching to water-cut data from four different zones of the field. Different calibration procedures are tested: global optimization that allows a general improvement of the model calibration to production data, and local calibration that further improves the match of each well. The results show the validity of the proposed methodology.
A Method to Diagnose Depletion, Skin,kh, and Drive Mechanism Effects Using Reservoir Monitoring Data
Authors: Haddad Sammy , Proano Eduardo , Patel Yogesh
Published: December 2007

The use of permanent sensors provides a continuous source of downhole pressure measurement throughout the life of the well. Moreover, multiphase flowmeters provide simultaneous flow rate measurements. The significant amount of data produced by these sensors has added a new dimension to traditional well test data interpretation techniques. This new dimension is life-of-well time. During the life of the well, permanent-gauge data and flow rate measurements are affected by several dynamic factors including changes in reservoir pressure, skin, permeability-thickness (kh), and reservoir drive mechanism. A major challenge facing the industry today is how to diagnose and respond to the effect of each of these factors. In other words, it is important to know whether a change in measured downhole pressure or flow rate is caused by depletion, changes in skin, kh, or drive mechanism.

This study investigates the effects of the above-mentioned factors in two fields in the Gulf of Mexico area. In addition, experience and lessons learned from numerous wells and fields equipped with permanent sensors have been used to develop a diagnostic method. This method uses key reservoir and well performance indicators to derive techniques to distinguish between the effects of depletion, skin, kh and drive mechanism. Best practices in the collection of flow rate and pressure data with suggestions for near-future improvements are included.
Effect of Eccentricity, Voids, Cement Channels, and Pore Pressure Decline on Collapse Resistance of Casing
Authors: Berger A. , Fleckenstein W. W. , Eustes A. W. , Thonhauser G.
Published: December 2007

Conventional collapse design is based upon uniform external pressure loading. Conventional design considers pore pressure and does not account for the effects on casing stresses from the external cement sheath and surrounding formation. In addition, the conventional design does not predict the collapse load of non-uniform loaded casing that arises from imperfect cement jobs and formation voids.

This paper presents the results of a finite element study of casing subjected to external, non-uniform loading. These loads include the effects of cement channels and voids, surrounding formation voids, and pore pressure decline in those voids. A better understanding of these common downhole conditions will help to better understand the resultant stress fields and enable a more accurate determination of the necessary collapse strength of casing.
Accelerating Drilling Technology with eKnowledge Factory: A Changing Paradigm
Authors: Samuel G. Robello , McColpin Glenn
Published: December 2007

This paper summarizes the underlying process involved in increasing the efficiency of a drilling organization through use of knowledge management and technology at a time when the knowledge base of the drilling industry is eroding and the wells per engineer are increasing. The eKnowledge factory depicts a virtual space in which drilling knowledge created is captured and stored in different buckets for distribution and reuse. It is stored by way of knowledge insertions at various stages of field development in a corporate environment. Stored knowledge can be shared across an organization. In this paper, we present various statistical data about the graying workforce in the drilling industry and discuss highlights of the results. Based on the findings we discuss three major stages of eKnowledge factory development: operational, engineering and strategic. The relationships between discrete drilling engineering analyses and the ability of modern digital technology to dynamically interact with these engineering models will be presented with examples. Also the bane of the graying work force and boon of the knowledge insertions, mining and tracking will be discussed. The ultimate premise of this paper is that through proper preparation and use of technology, something like eKnowledge factory can allow organizations to overcome the demo graphic's battle that the petroleum industry is facing in coming years.
Evaluation of Pipeline Configurations with Active Heating for Export of Waxy Crude Oil
Authors: Fleyfel Fouad , Hernandez Oris , Sturgis Rusty , Meng Weihong
Published: December 2007

Insulation and active heating options evaluated for transportation of waxy crude oil in an 8-inch flowline from a platform 10 miles offshore. Two bundled pipeline configurations are studied, and the results are compared with conventional pipe-in-pipe and a non-jacketed insulated pipe. The crude has a high pour point (PP, 91 °F) and a high wax appearance temperature (WAT, 105°F).

A solution is provided that allows the crude to stay above WAT. The non-jacketed pipe option is not thermally efficient; the pipe-in-pipe option can keep the crude oil warm during normal flow; but high storage capacity is required at the platform, which makes the solution impractical. The bundled pipeline configurations maintain the crude oil temperature above WAT thus avoiding waxing and gelling during steady state flow and during restart operations.

Despite being more expensive, the bundled pipeline configuration proved to be the right solution for this particular field. The results of this study are applicable for evaluations of similar field developments.
Streamline-Based History Matching Using Geostatistical Constraints: Application to a Giant, Mature Carbonate Reservoir
Authors: Gross H. , Thiele M. R. , Alexa M. J. , Caers J. K. , Kovscek A. R.
Published: December 2007

This paper describes the combined use of streamline and geostatistical sequential simulation to a Middle Eastern reservoir with a long production history and a significant number of wells.

In the algorithm presented, streamlines are used to delineate drainage regions for producers that are to be history matched. The time-of-flight of the streamlines is used to establish the relationship between historical rate mismatch and permeability of the drainage region. Rather than correcting permeability directly, we use the corrections supplied by the streamlines to constrain the geostatistical algorithms and thereby ensure a consistent geological scenario at every iteration in the process.

Before applying the methodology to a real field case, we use a simple synthetic example to explain the main points. Finally, we show the application of the history-matched model for evaluating an infill well prospect and how streamline simulation and finite-difference simulation are used as complementary tools to increase the predictive power of the numerical model of the reservoir.
Impact of Drilling Activities in Warm Sea: Seabed Re-Colonization
Authors: Dalmazzone C. , Blanchet D. , Lamoureux S. , Durrieu J. , Camps R. , Galgani F.
Published: December 2007

Since the onset of offshore drilling, the drill cuttings and their disposal have received much attention in the oil and gas industry. Numerous studies performed in the laboratory on the foreseeable fate of cuttings have led to the search for new formulations with the objective of achieving lower toxicity and better intrinsic biodegradability.

The study of the marine environmental impact associated with the discharge of OBM (oil base muds) drill cuttings has already given rise to numerous works and surveys related to the North Sea, but it is clear that knowledge is still limited. Further information is required on the monitoring of sediments polluted with OBM cuttings, especially in warm sea conditions.

On this topic, a research project was conducted in 2002 and 2003 in Congo (Africa) in order to study the modes of regeneration of the benthic fauna and the recolonization of the seabed after the discharge of OBM cuttings. Previous sampling and analyses were performed in 1995 (after the first step of drilling) and 2000 (one year and a half after the stop of cuttings discharge). In 2002, a systematic study of the abundance and diversity of the benthic fauna was conducted. In addition, a methodology based on classical physical chemical analyses on sediment, tests of toxicity, biodegradability and lixiviation was applied, in order to explain the evolution of the recolonization by the benthic fauna (SPE 77471). In the more general context of the environmental constraints of oil industry, our work demonstrated the containment of the impacted zones, the absence of significant risks of modification of the adjacent ecosystems and the possible application to other discharge sites.

We present here the results related to the last phase of sampling carried out in 2003. It is noteworthy that the whole results confirm the main evolutions observed in the previous survey.
Measurement and Theory of Gas Trapping in Porous Media During Steady-State Foam Flow
Authors: Tang Guo-Qing , Kovscek Anthony R.
Published: December 2007

Aqueous foams are useful to reduce gas mobility during steam or CO2 flooding and for aquifer remediation. Trapped or stationary gas contributes significantly to the extent of gas mobility reduction. There is no consensus regarding the functional dependence of trapped gas fraction, or conversely the flowing gas fraction, but it is believed to vary with pressure gradient, pore geometry, foam-bubble size (i.e., texture), surfactant type, and concentration. This work uses a gas-tracer technique in combination with X-ray CT scanning to measure the fraction of flowing gas and the in-situ aqueous phase saturation during steady-state foam flow in Berea sandstone. Statistical network concepts are used to interpret the mechanisms that govern the mobile gas fraction.

The experiments show that as gas velocity increases, the flowing gas fraction of the foam increases. Similarly, as injected gas-liquid ratio increases, the fraction of gas flowing increases. Hence, the absolute velocities of gas and aqueous surfactant solution are fundamental to foamed-gas mobility reduction. Effluent foam-bubble sizes were measured in a visualization cell installed at the core outlet. Bubbles range in size from about 60–120 μm in diameter. The smaller the effluent bubble size, the less the fraction of mobile gas. Lastly, experiments show that as surfactant concentration increased, the flowing gas fraction decreased.

While it is likely that an empirical correlation could be deduced from the experimental results, such an approach does not engender a mechanistic understanding of the role of the various parameters. Instead, statistical network theory is employed incorporating the parameters identified as important in the experimental program: pressure gradient and bubble size. A closed form expression is developed for the fraction of mobile gas. It is predicted that the fraction of gas flowing is a weak function of pressure gradient, foam-bubble size, and the permeability of the porous medium. Moreover, the theory shows excellent ability to reproduce the newly obtained experimental data
Water Saturation Estimation Using Equivalent Rock Element Model
Authors: Shang Bruce Z. , Hamman Jeffry G. , Caldwell Donald H.
Published: December 2007

The second Archie equation relates water saturation to formation resistivity index by a power function. This important relationship has been widely used to evaluate hydrocarbon saturation. However, many rocks don't obey this empirical rule. The best Archie fit to these data may not comply with physical bounds and create significant bias in computed hydrocarbon saturation.

This paper develops a new method for water saturation estimation based on the equivalent rock element model (EREM) that has been demonstrated to work well for rock transport properties to the first order effect. EREM contains two orthogonal pore components, one parallel and the other perpendicular to the direction of transport. Pore structure efficiency is defined as the volume ratio between the two components. When pores contain conductive and non-conductive fluid phases, the pore structure efficiency for the conductive phase changes with saturation due to fluid and mineral properties and pore structure variations. Approximating conductive phase pore structure efficiency as a function of saturation, we formulated a theoretical relationship between resistivity index and water saturation. The conductive phase may become discontinuous below a certain critical saturation due to pore structure, wetting characteristics, interfacial tension or a combination of different factors. The proposed algorithm readily incorporates this critical saturation phenomenon and explains the empirical relationships between resistivity index and water saturation for both Archie and non-Archie rocks. It fits core measurements better than Archie's second equation as demonstrated in our real examples and provides more accurate estimation for water saturation. This new approach expands the applications of the equivalent rock element model.
PCP Axial Load: Theory and Lab Results
Authors: Alhanati F. J. S. , Skoczylas P.
Published: December 2007

Accurate estimation of pump axial load in Progressing Cavity Pump (PCP) applications is an important aspect of the installation design. It affects the calculation of rod stresses, and therefore the determination of the rod string torque capacity. In deviated wells, it also directly affects the calculated rod-tubing contact loads, and therefore the evaluation of wear related issues.

This paper discusses the current theory related to the calculation of axial load in PCP applications, in light of laboratory test results obtained with 25 different pump models.

The axial load is normally estimated by multiplying the differential pressure across the pump by an effective pump cross sectional area. However, given the complexity of the pump geometry, it is not straightforward how to properly calculate this effective area. In the paper, it is shown that intuitive choices, such as the circle defined by the rotor minor diameter, the circle defined by the rotor major diameter, or the pump cavity area, do not yield adequate results.

A new method is suggested that not only matches quite well the extensive laboratory data for single-lobe pumps but also matches reasonably well the limited data available for multi-lobe pumps.
Estimation of Saturation Pressure through Wellbore Measurements
Authors: Millán Arcia Einstein , Rodriguez F. , Madail W.
Published: December 2007

Reliable fluid property data is always required in reservoir description. Normally, the most accurate fluid data is available from PVT studies, based on surface or bottomhole collected fluid samples. Specifically for Gas Condensate systems, fluid sample reliability poses a strong challenge mainly due to thermodynamic-instability-related problems. Very often, due to the extremely low liquid to gas ratio, even in high yield fluids, inaccuracies related to surface sampling links to erroneous results in PVT data. Despite recent advances in pressure compensate bottomhole fluid samples, temperature dependency yielding to phase envelope migration poses a strong challenge.

Tedious and time consuming laboratory procedures adversely impact field operations and hence final operating costs. Without implying to substitute laboratory results, a simplistic approach has been developed to determine saturation pressure based on easily acquired downhole wellbore data. The method involves basic flowing pressure versus depth sensor-gauge-data to estimate the dynamic gradient derivative, and offers a fast, reliable, and inexpensive approach to identify saturation pressure, fluid segregation and convection events in under saturated reservoirs. Actual PVT and dynamic gradient data belonging to gas condensate reservoirs were used. Measured saturation pressure was available for two of the three wells evaluated. The fluid data was validated and reproduced by means of the PVTi Eclipse module. PVT consistency verification was performed via, well completion, and production data, mass balance and thermodynamic stability checks. Results suggest good match between the PVT and dynamic gradient derived saturation pressure values.
Fracturing Fluid Viscosity Profiles Determine Well Productivity in a Remedially Stimulated Tight Gas Reservoir
Authors: Miller Jason D. , Ponto David A. , Reasoner Scott J.
Published: December 2007

Since 1998, over 4000 wells completed in the low-permeability Codell formation of the 300,000 acre Wattenberg Field in northeast Colorado have been remedially stimulated with massive hydraulic fracturing treatments. This remedial program has been extremely successful. A majority of the refracturing treatments in this study have yielded higher oil and gas rates than experienced following the original treatment, performed many years previously. Fluid and proppant volumes of the remedial treatments have been very similar, but certain characteristics of the fracturing fluid correlate significantly with variability in well productivity. The main correlating parameter has been the fracturing-fluid viscosity profile (FVP), which is defined by the early-time rate of viscosity buildup, the peak viscosity developed, and the rate of viscosity degradation following the peak. For each treatment, the FVP has been evaluated with a HTHP (high temperature/ high-pressure) rheometer, using a shear and thermal history representative of the Codell treatments. Within the Wattenberg Field, there is evidence that FVP has a multi-faceted impact on well productivity that is more than just being able to place the proppant into the formation.
Interfacial Interactions of Crude Oil-Brine-CO2 Systems under Reservoir Conditions
Authors: Yang Daoyong , Gu Yongan
Published: December 2007

In this paper, an experimental technique is developed to study the interfacial interactions of the crude oil-brine-CO2 systems at different pressures and temperatures. The major component of this experimental set-up is a see-through windowed high-pressure cell. By using the axisymmetric drop shape analysis (ADSA) for the pendant drop case, this new technique makes it possible to determine the interfacial tension (IFT) and to visualize the interfacial interactions among the crude oil, brine and CO2 under practical reservoir conditions. More specifically, the equilibrium IFT of the crude oil-CO2 or the brine-CO2 system is measured as a function of pressure and temperature, respectively. For the crude oil-CO2 system, there always exists a constant low IFT (i.e., the partial miscibility) as long as the pressure is higher than a threshold value. No ultra low or zero IFT between the crude oil and CO2 is found at the operating pressures and temperatures tested in this study. For the brine-CO2 system, the complete miscibility between the brine and CO2 can be achieved. In addition, a number of important physical phenomena are observed after the crude oil or brine is made in contact with CO2. In particular, the swelling effect, light-ends extraction, initial turbulent mixing and wettability alteration are the major characteristics of the crude oil-CO2 system, whereas the swelling effect and the complete miscibility are the primary features of the brine-CO2 system. For the crude oil-brine-CO2 systems, wettability between the crude oil and the stainless steel needle in CO2-saturated brine phase is rather different from that of the crude oil-CO2 system. This wettability difference may considerably improve the oil recovery in a previously waterflooded water-wet reservoir during CO2 flooding processes. The immiscibility between the crude oil and CO2-saturated brine phase is still observed up to P=28.196 MPa and T=58°C.
Interpretation of Borehole Measurements Acquired in Laminated Clastic Sequences Subject To Mud-Filtrate Invasion
Authors: Chowdhury Subhadeep , Torres-Verdín Carlos , Mezzatesta Alberto
Published: December 2007

The effect of mud-filtrate invasion on resistivity, nuclear, and formation tester wireline measurements is poorly understood when the formation of interest is laminated on a scale below the vertical resolution of borehole logging instruments. This paper describes a quantitative study to assess the effect of mud-filtrate invasion on well-log measurements acquired in thinly laminated clastic sequences.

Synthetic models are constructed with varying proportions of shale and two types of sand. These models are used to simulate the process of mud-filtrate invasion. Each type of sand is assumed isotropic, homogeneous, water wet, and saturated with oil to the level of irreducible water saturation. Simulations of the process of mud-filtrate invasion yield 2D cross-sections of water saturation, salt concentration, and electrical resistivity in the invaded rock formations. These cross-sections are then used to simulate wireline resistivity, density, neutron, and formation tester measurements. Using standard interpretation techniques, saturation of original oil in place is calculated with all the simulated log measurements.

Results indicate that for laminated sandy and shaly sand rock formations, induction (electrical resistivity), nuclear (porosity), and formation pressure tester (pressure) measurements are significantly affected by both the relative proportion of lithology and the invasion of mud filtrate. Water saturation in a laminated sandy rock formation can vary significantly when calculated with saturation and porosity exponents perpendicular and parallel to the bedding plane, respectively. Permeability calculated from dual-packer formation tester measurements can significantly depart from the effective average permeability in a composite and laminated sandy formation.

Conventional induction resistivity instruments cannot distinguish between resistivities in the flushed and invaded zones when the volume of shale is higher than 50%. Presence of shale also causes a significant error in the estimated permeability.
Application of a New Mechanistic Decline Curve Method to Kern County Oil Fields
Authors: Reyes Jericho L. P. , Li Kewen , Horne Roland N.
Published: December 2007

Field production data from Kern County acquired from the California state database was analyzed for production decline. A decline curve methodology based on fluid flow mechanisms was used. The model reveals a linear relationship between the oil production rate and the reciprocal of the oil recovery. Existing decline curve methods often either overestimate or underestimate reserves, and are mostly empirical in origin. The current approach not only provides a method to estimate reserves, but because of its basis on fluid flow mechanisms, can also be used to infer regional reservoir properties that may be useful in production decisions.

Oil wells from six producing oil fields in Kern County were analyzed using this decline curve method and two parameters, a0, the production rate index, which measures the recovery rate, and b0, the gravity index, were mapped regionally. Since the two parameters are associated with capillary pressure and relative permeability, conclusions can be made about the geological properties of the reservoirs from the derivation of the a0 and b0 parameters. This will aid in determining which regions are more prolific, and provide a correlation based on physics to infer reserves, and reservoir rock and fluid properties. It was found that fields with higher a0 and b0 values have higher oil recovery.

This study demonstrates the applicability of this decline curve analysis and provides an alternative to empirical methods.
Real Time Optimization: Classification and Assessment
Authors: Mochizuki S. , Saputelli L. A. , Kabir C. S. , Cramer R. , Lochmann M. J. , Reese R. D. , Harms L. K. , Sisk C. D. , Hite J. R. , Escorcia A.
Published: December 2007

The Real-Time Optimization Technical Interest Group (RTO TIG) has endeavored to clarify the value of real-time optimization projects. RTO projects involve three critical components: People, Process, and Technology. Understanding these components will help to establish a framework for determining the value of RTO efforts. In this paper, the Technology component is closely examined and categorized. Levels within each Technology category are illustrated using spider diagrams, which help decision-makers understand the current status of operations and the impact of future RTO projects. Uncertain value perception in our industry has been one of the critical issues in adopting RTO systems. Therefore, case histories are reviewed to demonstrate the impact of RTO projects.

To assist RTO project promotion, we list lessons learned through case histories, suggest a justification process, and present a simple economic example.