bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of Multiple Echo-Time NMR Measurements for Complex Pore Geometries and Multiphase Saturations
Authors: Toumelin E. , Torres-Verdín C. , Chen S.
Published: December 2007

We develop a numerical algorithm to simulate nuclear magnetic resonance (NMR) measurements in the presence of constant magnetic field gradients. The algorithm is based on Monte Carlo conditional random walks in restricted and unrestricted space. Simulations can be performed of 3D porous media that include both arbitrary bimodal pore distributions and multiphase fluid saturations. The ability to account for the presence of a constant external magnetic field gradient allows us to replicate actual well-logging conditions that include the effect of Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences at a microscopic level. This is accomplished by simulating ideal pulse-acquisition techniques that include multiple interecho times (TE) similar to those currently used by the well-logging industry. Benchmark examples are presented to validate the accuracy and internal consistency of our algorithm against previously published results for the case of a null magnetic field gradient. Validation examples also are presented against actual NMR measurements performed on core samples of carbonate rock formations.

Interpretation work is focused on the petrophysical assessment of both partial oil/water saturations and pore structures exhibiting diffusive coupling. Simulation examples are designed to quantify whether the inclusion of diffusion under a magnetic field gradient can improve the interpretation of multiphase fluid saturations when diffusion coupling is significant. The simulation algorithm sheds light on new NMR data-acquisition strategies that could be used to improve the detection and quantification of fluid types, complex fluid saturations, and complex pore geometries.
Integrated Core Analysis for Fractured Reservoirs: Quantification of the Storage and Flow Capacity of Matrix, Vugs, and Fractures
Authors: Bona N. , Radaelli F. , Ortenzi A. , De Poli A. , Peduzzi C. , Giorgioni M.
Published: December 2007

Answering general questions such as "Where is the oil?," "How much oil is there?," and "Can we extract it?" is a challenging task for a large fractured field in southern Italy. Various studies were conducted to gain more insight into the way oil is distributed in the rock and the producibility of the different structures observable on the cores (matrix, vugs, and fissures).

These included the cryogenic scanning electron microscope (CryoSEM) and gas chromatography (GC)-pyrolysis, pore-size-distribution measurements, SEM analysis on thin sections, and a number of nonconventional techniques that were designed specifically for that type of rock. Nuclear magnetic resonance (NMR) imaging was conducted on several whole core samples and the different porosity contributions (microporosity, vugs, and fissures) defined on a 3D basis. An analytical approach based on the percolation theory was used to separate the permeability contributions and define the conditions under which vugs and fissures may form a conducting system. The inputs were distributions of pore throats, throat length, coordination number, fissure orientation, and porosities. Wettability is a key parameter for production estimates, and we used a technique for measuring it in both microporosity and fissures, which makes use of dielectric constant measurements. All the data contributed to our current understanding of the reservoir.
Discussion of Reservoir Characteristics, Cedar Creek Anticline Fields, Montana
Authors: Hols Arnold , Bethel Frank T.
Published: December 2007

At the present time, over one-half the total oil production in Montana originates from Williston Basin fields in Eastern Montana. Six of the seven foremost producing fields in this area are located along the Cedar Creek anticline. The characteristics of the reservoir rocks which constitute these fields, the characteristics of the reservoir fluids and the behavior of the fluids in the rocks are discussed.

The conclusions drawn from the data available are that the carbonate reservoirs of the Cedar Creek anticline are under-saturated, of poor-to-fair permeability and of fair-to-good porosity with some localized fracturing. By comparison of various fields on the anticline, a general improvement in reservoir rock and fluid properties is shown to exist in a south-to-north direction. The valuable aid to reservoir interpretation and operation which can be obtained through a systematic gathering of data and modern mechanical methods of statistical analysis is emphasized.

Introduction

The nation's attention was focused on the Williston Basin potential in early 1951 by significant discoveries in Manitoba, Canada, and on the Nesson anticline in North Dakota. Later during the same year discoveries were made on the Cedar Creek anticline of Eastern Montana; namely, the Glendive and Pine fields. Rapid developments along the Cedar Creek anticline led to the discovery of 12 oil fields by 1955, one of which is located on the southern end in South Dakota (Fig. 1). Production from the Montana portion of the Cedar Creek anticline currently amounts to 34,000 BOPD which represents 44 per cent of the total Montana oil production. The first and third largest oil fields in Montana from the standpoint of production are found here, respectively, Pine and Cabin Creek.

The purpose of this paper is to discuss the different reservoir characteristics in those fields where Shell is operating, e.g., those from Glendive to Buffalo. The discussion will not deal with stratigraphic detail but is limited to those aspects which are of practical value to the reservoir engineer, and are presented in the following order:reservoir rocks,reservoir contents, andreservoir pressures.
Second-Order Directional Seas and Associated Wave Forces
Authors: Sharma J. N. , Dean R. G.
Published: December 2007

Most methods for wave force computation incorporate either the nonlinearities of the ocean surface for a single fundamental component or the random and/or directional characteristics using superposition of linear wave components. One exception is the intuitive "hybrid" method1 which combines elements of linear and nonlinear waves. This paper describes and applies a method correct to the second order in wave height for calculating waves and wave forces caused by a directional wave spectrum on an offshore structure.

Starting with a prescribed linear spectrum of directional waves, a set of random phases is generated and the second-order spectrum computed with phases defined by all contributing pairs of first-order components. Thus, with one realization of the spectrum complete up to the second order, the wave profile and water particle kinematics can be simulated in the time domain. The wave forces also are computed in the time domain, taking full account of their nonlinear and directional properties. The resulting wave forces at any level vary in direction and magnitude. The total wave forces summed over all piling of a structure are less than those for a unidirectional train of waves with the same one-dimensional spectrum.

Several examples are presented to illustrate reductions in maximum wave forces caused by the directional distribution of waves. We found that for a single piling the maximum force decreases by a factor ranging from 1.0 to 0.61 as the directional spread increases from unidirectional to uniformity over a half plane. For a four-pile group on a square array of 300-ft (91.4-m) spacing, the corresponding decrease in the factor is 1.0 to 0.51 for a Bret-schneider spectrum with a peak period of approximately 12 seconds. The results of this complete model are compared with the more intuitive and approximate hybrid method and are found to agree quite well. Force spectra are presented and discussed for the inline and transverse directions.
Phase Envelope Calculations for Hydrocarbon-Water Mixtures
Authors: Lindeloff Niels , Michelsen Michael L.
Published: December 2007

The present work describes how the pressure-temperature phase diagrams for hydrocarbon-water mixtures can be calculated. This requires a thermodynamic model that can account for the polar interactions caused by the aqueous compounds and an algorithm that can trace the two-and three-phase boundaries encountered in such systems. Models and algorithms that can handle this task are described. The paper further describes a number of examples in which the methods are applied.
Fatigue Analysis of the Cognac Platform
Authors: Kinra R.K. , Marshall P.W.
Published: December 2007

This paper describes preliminary and detailed fatigue analyses of the 1,000-ft-deep Cognac platform. Dynamic structural analysis is used to generate local member stress transfer function data, and probabilistic spectral techniques are employed to evaluate long-term stress statistics and fatigue lives using Miner's cumulative damage rule. Directional spreading of wave energy is considered in the analysis.

Introduction

The Cognac platform is located in 1,025 ft (310 m) of water in the Gulf of Mexico. Fatigue damage considerations played an important role in the structural design of the platform. Many of the heavy-wall joints in the structure were controlled by fatigue rather than by punching shear under the design wave. The reason is that the natural period of the Cognac platform is in the 4- to 5-second range, whereas the natural periods of other structures in the gulf are generally less than 2 seconds. Thus, the Cognac platform responds dynamically to everyday waves in the gulf, which typically have zero-crossing periods between 3 to 5 seconds. In comparison, other Gulf of Mexico structures tend to respond statically. To make matters more difficult, the hydrodynamic damping due to low waves is very small. Recent studies by Ruhl indicate that the total damping - including structural, foundation, and hydrodynamic contributions - may be only about 20% of critical for platform response in low sea states. Low damping values result in large dynamic amplifications when the period of the forcing function (the wave period) is close to the natural period of the structure. The combination of low damping and a natural period close to that of frequently occurring Gulf of Mexico sea states results in a fatigue-sensitive structure .The steps taken to ensure the integrity of the structure against fatigue damage consisted of (1) a preliminary fatigue analysis that served as a basis for the design of the structure and (2) a detailed member-by-member check of the final design using probabilistic techniques described here.

Preliminary Fatigue Considerations

The overall strategy of fatigue design consists of a two-stage approach:initial design of members and joints using allowables derived from generic analysis, andfinal hot-spot by hot-spot fatigue analysis,

which requires that all details of the structure be established a priori.Of necessity, the detailed analysis is done after all other design work has been completed and often after fabrication has begun. If any areas with undesirably short calculated fatigue lives are found, these can be upgraded - either by grinding welds to improve fatigue performance, locally building up the section thickness to reduce cyclic stress, or making other minor modifications.This general approach has been used on several platforms in the North Sea, the Gulf of Mexico, and offshore southern California.

JPT

P. 374＾
High- and Low-Cycle Fatigue Behavior of Prestressed Concrete in Offshore Structures
Authors: Gerwick B. C. , Venuti W. J.
Published: December 2007

The expanding use of prestressed concrete in offshore structures, both fixed and floating, in increasingly hostile environments has generated intense interest in its fatigue endurance capabilities, even though no known fatigue problems have arisen in actual structures. Although concrete does suffer progressive loss of strength with increasing number of cycles, a comparison of the Wohler curves developed on the basis of laboratory tests with the probable distribution of compressive stresses during a service life in an environment such as the North Sea shows extremely low cumulative usage at the high-cycle end of the spectrum. However, significant damage can occur at the low-cycle, high-amplitude end of the spectrum under a relatively small number of cycles of very high magnitude. This damage is displayed by a reduction in stiffness and by rapidly increasing axial and lateral strains that lead to cracking and spalling.

Repeated cycling into high compressive ranges causes a substantial increase in creep, reducing the effective prestress. Confining reinforcement resists lateral deformation and delays compressive fatigue failure. Cycling into the tensile range a large number of times can produce cracking due to tensile fatigue at about half the static tensile strength. Cracking also can occur due to overload, accident, construction procedures, and thermal strains. Repeated excursions of submerged concrete into the crack-opening range leads to pumping of water in and out of the crack and hydraulic wedging, leading to concrete splitting.

Cracking subjects the reinforced and prestressed steel to cyclic tension. Loss of bond ensues and may lead to eventual fatigue failure. Adequate endurance can be ensured by prestressing to avoid a large number of cycles extending into the crack-opening range and by providing adequate percentages of steel across the section plus transverse and confining steel.

Cyclic membrane shear may produce diagonal tension cracking at about half the static strength. Conventional reinforcing in an orthogonal grid pattern is very inefficient in resisting such cracking. Crack widths grow rapidly. Repeated loading may lead to abrasion of concrete surfaces and failure of the steel due to combined axial force and bending. Vertical prestress is an efficient and practical method of resisting high-amplitude cylic shear.

For the typical concrete sea structure, high-cycle, low-amplitude, cumulative fatigue is not a significant problem. However, low-cycle, high-amplitude fatigue requires consideration, especially when there are numerous cycles into the tensile cracking range. In this latter case, fatigue of the steel and/or concrete may occur unless adequate amounts of steel are provided to ensure that crack widths and steel stresses are kept within allowable values.
Combining Pulsed Neutron Spectroscopy and LWD Resistivity for Analysis Behind Casing in Small Holes
Authors: Beaumont-Smith N. , Webber G. D. , Shepherd P. J. , Kalyanaraman R. S.
Published: December 2007

Formation analysis from behind casing is gaining more relevance in assessing reservoirs, not only in aging fields but also in new wells completed either without the full complement of openhole petrophysical measurements or with openhole measurements of inadequate quality. Before this technology was available, formation evaluation in freshly drilled holes was dependent on the availability of openhole log measurements. This paper discusses field examples in which reservoirs were evaluated without a full suite of openhole log measurements. Examples from two sidetracks in a mature North Sea field supported by water injection are presented. The small hole size limited openhole data acquisition to resistivity and gamma ray (GR) measurements, which were acquired using logging-while-drilling (LWD) technology.

In both cases, pulsed neutron (PN) measurements were recorded through casing to augment the existing data. Porosity and formation sigma (macroscopic thermal neutron capture cross section) measurements were obtained. Fluid saturations were computed using this porosity and the LWD resistivity. Saturations also were computed from the carbon/oxygen measurement, and these agreed with the resistivity-based answers. Lithology analysis was performed using elemental spectral capture yields and was reliable and consistent fieldwide.

The results show that, in a mature oil field, PN measurements can provide a complete lithology and saturation analysis in circumstances in which insufficient openhole petrophysical measurements are available.
A Linear Programming Model for Scheduling Crude Oil Production
Authors: Lee A.S. , Aronofsky J.S.
Published: December 2007

Introduction

In recent years considerable progress has been made in the development and application of mathematical techniques for the solution of certain problems involving economic "strategies". Such a problem might involve, for example, the scheduling of shipments of a commodity from a number of sources to a number of destinations. The object would be to schedule the shipments in a manner so as to satisfy the destination requirements and at the same time minimize the transportation costs. The solution to such a problem is not necessarily intuitively obvious. The "obvious" solution is frequently far from optimum. If the shipments are to be made from, for example, only two sources to four destinations, the optimum schedule is readily found. However, if shipments are to be made from, say, 10 sources to several hundred destinations, even a competent and experienced scheduler may spend considerable time in finding a reasonable answer. Even then he is not sure that he has the optimum solution. Furthermore, he has no way of knowing how far from optimum the answer is. Consequently, he does not know whether he should accept this solution or seek a better one.

Prior to the advent of large high-speed digital computers, little more could be done with such problems because of their great size and multiplicity of possible solutions. A problem involving 20 sources and 50 destinations would require choosing, from a very large number of possible combinations, the optimum combination of 1,000 variables. The best one could do was to utilize intuition, extrapolation from past experience, and other non-exact approaches. With a high-speed computer, however, such problems can be solved providing a reasonable computational procedure (or algorithm) can be utilized. The purpose of this paper is to describe such a procedure (linear programming) and to apply this procedure to a production scheduling problem.
A Systematic Approach for Characterizing Hydrocarbon Solids
Authors: Jamaluddin* A. K. M. , Nighswander J. , Joshi N.
Published: December 2007

The potential for hydrocarbon solid formation and deposition adversely affecting flow assurance in deepwater production systems is a key risk factor in assessing deepwater developments. To reduce this risk, a systematic approach to defining and understanding the thermodynamic factors impacting flow assurance is required. The development of experimental techniques to provide this information is a current focus of attention in the reservoir fluid-phase behavior community. This paper provides a systematic experimental approach to characterization of the hydrocarbon solids in crude, and the impact of this knowledge on potential production and flow-assurance strategies.
Producer/Injector Ratio: The Key to Understanding Pattern Flow Performance and Optimizing Waterflood Design
Authors: Hansen C. E. , Fanchi J. R.
Published: December 2007

When designing a waterflood, the important aspect of processing rate with respect to pattern selection has not been addressed adequately in the literature. It has not been possible, using the existing analytical background, to determine and compare the flowrate performance of all the patterns for any given rock/fluid (mobility) system. While general two-phase equations exist for the five-spot and line-drive patterns, flow equations for other patterns either have not been presented (e.g., the seven-spot) or are not sufficiently general (e.g., the nine-spot) to apply to all mobility conditions. A general treatment of skin effect also has not been given.

This paper presents a new, comprehensive analytical treatment of pattern flow for isotropic, homogeneous reservoirs that applies to all patterns and mobility conditions. This is made possible by a new equation for the volumetric average reservoir pressure of regularly spaced, repeated patterns. The new formulas demonstrate how the producer/injector ratio (P/I) controls reservoir pressures and flow rates and (for the first time) allow the differences in the flow capacity of patterns to be quantified. The mobility characteristics that control flow rates are described fully by a newly defined total mobility ratio, MT. The form of the new equations allows for modifications for skin effect and reservoir heterogeneity. We show how skin effect can act disproportionately with injectors and producers because of its interaction with P/I and MT.
A Report on the Displacement Log
Authors: Winn R.H.
Published: December 2007

Introduction and History

Campbell and Martin described a new exploratory logging tool which was developed as the result of a new concept of the phenomenon that takes place when a porous formation containing mobile hydrocarbons and irreducible interstitial water is invaded by mud filtrate. They postulate that invasion will create a bank of interstitial water between the flushed zone and the undisturbed formation. This postulate was confirmed by Wyllie.

Thus, if Rmf is greater than Rw and mobile hydrocarbons are present, the resistivity profile with increasing radii from the borehole is high:low:high, as shown in Fig. 1. If mobile hydrocarbons are absent, the bank of interstitial water is indistinguishable from the undisturbed formation and the resistivity profile, when Rmf is greater than Rw, is high:low, as shown in Fig. 2. The zone containing the bank of interstitial water and residual oil was named the "low zone".

Campbell and Martin conducted extensive electrolytic wedge and resistivity analog model studies to select a logging system that would reflect the presence or absence of the low zone. The system they selected was that of a family of lateral curves. The model studies indicated that the presence of a low zone could be detected in approximately 60 to 70 per cent of the producing sand reservoirs with a family of lateral curves provided that the sonde effectively occupies 90 per cent of the diameter of the hole. These studies were conducted with Di = 5d. The presence or absence of the low zone is best determined by a plot of the apparent resistivity vs spacing as shown in Figs. 3 and 4. A dip or inflection in the profile that repeats at successive depth intervals is considered diagnostic. To reduce the shunting effect of the hole and mud, a sonde of insulating material that is surrounded by a series of flexible rubber bumpers was designed. The bumper efficiency is considered to be the ratio of the bumper diameter to the hole diameter. Field tests conducted by Campbell and Martin on 10 wells with a tool that provided for station readings indicated that dips or inflections could be detected in 19 out of 25 known producing zones.
Determination of Thermal Conductivity of Soils: A Need for Computing Heat Loss Through Buried Submarine Pipelines
Authors: Rawat P. C. , Agarwal S. L. , Malhotra A. K. , Gulhati S. K. , Rao G. Venkatappa
Published: December 2007

An important parameter required for computing heat loss through buried submarine pipelines transporting crude oil is the thermal conductivity of soils. This paper describes an apparatus designed for determination of the thermal conductivity of soils at the desired moisture/density condition in the laboratory under steady-state conditions. Experimental results on the three soils studied show that thermal conductivity increases as dry density increases at a constant moisture content and that it increases as water content increases at constant dry density. These results confirm the trends isolated earlier by Kersten,1 The experimental results are compared with the available empirical relationships. Kersten's relation1 is observed to predict the thermal conductivity of these soils reasonably. The predictions from Makowski and Mochlinski's relation (quoted by Szilas2) are not good but improve if the sum of silt and clay fractions is treated as a clay fraction in the computation.
Artificial Neural Network Models for Identifying Flow Regimes and Predicting Liquid Holdup in Horizontal Multiphase Flow
Authors: Osman El-Sayed A.
Published: December 2007

This paper presents two artificial neural network (ANN) models to identify the flow regime and calculate the liquid holdup in horizontal multiphase flow. These models are developed with 199 experimental data sets and with three-layer back-propagation neural networks (BPNs). Superficial gas and liquid velocities, pressure, temperature, and fluid properties are used as inputs to the model. Data were divided into three portions: training, cross validation, and testing. The results show that the developed models provide better predictions and higher accuracy than the empirical correlations developed specifically for these data groups. The developed flow-regime model predicts correctly for more than 97% of the data points. The liquid-holdup model outperforms the published models; it provides holdup predictions with an average absolute percent error of 9.407, a standard deviation of 8.544, and a correlation coefficient of 0.9896.
Chemical Osmosis, Shale, and Drilling Fluids
Authors: Schlemmer R. , Friedheim J. E. , Growcock F. B. , Bloys J. B. , Headley J. A. , Polnaszek S. C.
Published: December 2007

This paper describes continuing efforts to develop a water-based drilling fluid that will provide the osmotic membrane behavior and wellbore stability of an oil-based drilling fluid. A pore-pressure-transmission technique in use for several years as a tool to measure osmotic behavior has been refined for improved measurement of changes in shale permeability and pore pressure in response to interaction with drilling fluids. Conventional invert-emulsion and water-based drilling fluids containing selected additives were tested with outcrop and preserved shale specimens using an innovative screening method.

Observed pressure differences across each shale specimen were compared with the values predicted by osmotic theory. From this comparison, an empirical concept of "membrane efficiency" was developed. Three distinct types of "membranes" are postulated to describe the interaction of various drilling fluids with shales. Type 1 membranes are generally characterized by coupled flows of water and solutes between fluid and shale. Type 2 membranes greatly reduce the near-wellbore permeability of shale and restrict the flow of both water and solutes. Type 3 membranes transport water more selectively, but shale permeability and fluid chemistry may alter performance measurements. Invert-emulsion fluids tend to form efficient, Type 3 membranes; however, under certain conditions, these fluids can yield lower capillary pressures than described previously and invade the interstitial fabric of high-permeability shales.

Several water-based mud formulations were prepared that achieve approximately one-quarter to one-half the measured osmotic pressure of a typical oil-based mud (OBM). Fluid additives that supplement or reinforce a Type 1 membrane, such as saccharide polymers (especially in combination with calcium, magnesium, or aluminum salts), can induce relatively high efficiencies. As expected, fluids that form a Type 2 membrane, such as silicate and aluminate muds, provide the highest membrane efficiencies.
A Mixed-Wet Hysteretic Relative Permeability and Capillary Pressure Model for Reservoir Simulations
Authors: Delshad M. , Lenhard R. J. , Oostrom Mart , Pope G. A.
Published: December 2007

A crucial component of all multiphase flow models is the relationship among relative permeabilities, fluid saturations, and capillary pressures. Relative permeability and capillary pressure parametric models can be very useful for predicting fluid behavior in porous media. However, relative permeabilities and capillary pressures used in oil reservoir simulators are commonly determined through interpolation between laboratory measurements. A problem with this approach is that the relations are valid only for the specific saturation path measured. Therefore, simulations of oil production using different saturation paths from those measured are likely to be in error and can limit the investigation of alternative production scenarios. In this paper, saturation-history-dependent relative permeability and capillary pressure functions for two-phase flow in mixed-wet rocks are discussed. Relative permeabilities are predicted by integrating a pore-distribution model between limits that reflect how oil and water are distributed in mixed-wet porous media. The proposed model was tested against mixed-wet capillary pressure data. The model then was incorporated in the U. of Texas Chemical Compositional Simulator (called UTCHEM) to compare waterflood simulations in water- and mixed-wet reservoirs. The simulation results agree qualitatively with laboratory core and field observations. The model and its implementation also were validated against a sandpack experiment.
Determining Fracture Closure Pressure in Soft Formations Using Post-Closure Pulse Testing
Authors: Upchurch E. R.
Published: December 2007

This paper discusses the theory and application of a pressure-pulse testing technique to determine the fracture closure pressure (Pc) in soft formations. This technique is intended for use only after a data fracture has been pumped and is closed. Implementing the pulse technique post-closure rather than while the fracture is still open allows it to be used as an independent determination of Pc without diminishing the accuracy of the fluid leakoff coefficient (CL) determined from the data-fracture. Previous pulse-testing techniques1 require pumping into a fracture while it is still open. This practice reduces the accuracy of CL by altering the time necessary for the fracture to close (tc). Furthermore, previous pulse-testing techniques yield only a possible range for Pc, whereas the post-closure pulse technique yields a better-defined, singular value of Pc.

The accuracy of post-closure pulse testing is validated with several field examples from a soft formation on Alaska's North Slope by comparing a definitive Pc determined using pressure decline analysis with Pc determined from post-closure pulse testing.
Productivity Decline in Oil Wells Related to Asphaltene Precipitation and Emulsion Blocks
Authors: Kokal Sunil , Al-Dawood Naseem , Fontanilla Jerry , Al-Ghamdi Abdullah , Nasr-El-Din Hisham , Al-Rufaie Yousef
Published: December 2007

Several oil wells in a Saudi Arabian field have shown a significant decline in oil productivity in recent years. A few have "died" prematurely, while others have become intermittent producers. The oil productivity decline is aggravated by water encroachment and has occurred with relatively low water rates and without any significant drop in reservoir pressure. These wells have low productivity indices (PIs), resulting in relatively low flowing bottom-hole pressures (FBHPs).

This paper presents the results of an investigative case study to determine the causes of productivity decline in these wells. A multidisciplinary team was set up with engineers and scientists from reservoir management, production engineering, and the R&D Center for the investigative study. The team focused on multiple aspects, including reservoir and production engineering as well as a comprehensive laboratory and field investigation.

The results of this study indicate that one of the main causes of productivity decline in these wells is related to asphaltene precipitation and the subsequent formation of tight emulsions downhole. The emulsions block the pore throats and cause formation damage, which leads to productivity decline. Another factor that further aggravates the productivity of these wells is poor rock quality in the area. Possible causes of formation damage by inorganic scaling and leakage and mixing of gas from a deeper reservoir have been eliminated.

Well-test analyses on some of the affected wells show that the formation-damage mechanism in the affected area is further aggravated by poor reservoir rock quality. The time-lapse pressure-transient analysis also indicates a deterioration of skin and productivity with time.

On the basis of these findings, a special solvent treatment was recommended and designed as a pilot trial for one of the dead wells. The treatment included squeezing xylene and demulsifier to dissolve the asphaltenes and break the tight emulsions around the wellbore area. The treatment resulted in only a slight improvement in the PI, and the well died after a few days. Currently, a stimulation treatment with acid and demulsifier is being implemented in selected wells. The results of the field trials are described here.
The Dynamics of Electrical-Submersible-Pump Populations and the Implication for Dual-ESP Systems
Authors: Sawaryn S. J.
Published: December 2007

This paper contains analytical expressions illustrating the growth, renewal, and decline of single electrical-submersible-pump (ESP) populations and their transition to dual-ESP systems. Critical assumptions are justified with published data, and the commercial implications of the conclusions are presented. Discussion starts with possible misinterpretations of instantaneous run time (IRT), which is a commonly adopted measure of pump reliability. Statistical renewal theory is then used to quantify the sensitivity of workover-delay time to rig availability and to determine the expected change in failure rates when replacing an existing pump population with more reliable units. The transition is shown to be relatively insensitive, even to large improvements in reliability.

Dual ESPs are considered as a means of improving the run life. When analyzed as a standby redundant system, they are predicted to exhibit a gamma distribution. Under certain conditions, the replacement of an existing pump population with dual ESPs is predicted to result in a faster reduction in the workover rate and confer additional commercial benefits compared with a single ESP with the same mean time to failure (MTTF). The first dual-ESP system for the Milne Point field, Alaska, was successfully run in a Kuparuk well in April 2000.

It is concluded that dual-ESP systems deserve greater consideration. The material is of interest to vendors designing ESP systems and those managing fields that use ESPs, particularly those involved in incentive-based contracts.
Predicting Present and Future Well Productivities for Solution-Gas-Drive Reservoirs (includes associated paper 9019 )
Authors: Al-Saadoon Faleh T.
Published: December 2007

One of the first and most important steps in analyzing the performance of any conventionallycompleted flowing well is to determine the well'sproduction rate for any given flowing bottomhole pressure. The flow of oil (and/or water) from theformation into the bottom of the well is governedmainly by the productivity index (PI) of the well. In other words, the PI is a measure of the well'spotential or ability to produce fluids as related to areduction in the flowing bottomhole pressure. The PIis defined at any given time (i.e., at fixed reservoirpressure pe) as the rate of change of the productionrate with drawdown, or

dq dqPI = J =..........(1)d(Pe-Pwf) dpwf

Muskat, Gilbert, Vogel, and others indicatedthat in a two-phase reservoir liquid and gas now thePI is not constant. They concluded that a plot of pwfvs. qo will not yield a straight line but a curved line concave to the origin. Gilbert termed this completegraph the inflow performance relationship (IPR) ofthe well.It appears, therefore, that the PI is subject tochange at any specific time under differingdrawdown conditions. It also appears that the PIchanges throughout the life of a well for a particulardrawdown condition.Vogel developed a general dimensionless relationship for solution-gas-drive reservoirs toprovide accurate calculations for oilwellproductivity. Standing developed a procedure forcalculating the future IPR.This paper is a response to some common errorsthat appear in the literature. Standing, for instance, defines the PI of a well as J= qo/ (Pe - pwf) (Eq.3). This definition is true, however, only if J is assumed to be constant. This unwarranted assumptionhas led to erroneous results as proved later.It is very important to emphasize that, in general,

If J is assumed to be constant (as is the case for oilwells when the oil is undersaturated throughout theproducing formation), then

q = J(Pe - P wf).............................(2)

Eq. 2 holds only when J is (or is assumed to be)constant.If J cannot be assumed to be constant (as forsolution-gas-drive reservoirs operating at or belowthe saturation pressure), then J can be obtained fromVogel's equation.Vogel's equation may be written as

..........(3)

As J= - (dqo)/(dP wf), then by differentiatingboth sides of Eq. 3 with respect to pwf, yields

..........(4)

It is evident from Eq. 4 that as the drawdownincreases (i.e., as Pwf decreases at a given Pe) Jdecreases. When Pwf approaches Pe (i.e., zerodrawdown), then J becomes equal to J *, where

9 qo maxJ* =........................(5)5 P e

Thus,

J 1 P wf (1+8 .....................(6)J* 9 P e

Eqs. 4 and 6 are different from those obtained byStanding. He assumed that J = qo/(Pe - Pwf), which is not true for solution-gas-drive reservoirs.Vogel's equation may be rearranged in thefollowing form.

P. 868＾
Phase Behavior of a Gas-Condensate/Water System
Authors: Kokal Sunil , Al-Dokhi Mohammad , Sayegh Selim
Published: December 2007

Gas-condensate reservoirs are an essential part of Saudi Arabia's hydrocarbon resources. A good understanding of the effect of water on the phase-behavior properties of these hydrocarbons is essential for accurately forecasting the performance of the reservoirs with numerical simulators. In addition, the scaling and corrosion tendencies of the produced brine are strongly influenced by mass transfer with the hydrocarbon phase.

This paper presents unique experimental phase-behavior data for a typical Saudi Arabian gas-condensate three-phase (aqueous/ condensate/gas) system. The objective of this work is to quantify the effect of the aqueous phase on gas-condensate fluid properties.

The results show that appreciable amounts of carbon dioxide and methane are partitioned from the gas-condensate phase into the aqueous phase. Another important observation is the mass transfer of water into the condensate phase. The mass transfer between the condensate and aqueous phases results in a slight decrease in the gas/condensate ratio (GCR). The carbon dioxide in solution makes the brine acidic and can dissolve carbonate minerals from the formation (e.g., calcium carbonate). In addition, the acidic or sour brine will be quite corrosive. The experimental results are compared with equation-of-state (EOS) and other correlations published in the literature.
Integrated Optimization for Rate Allocation in Reservoir Simulation
Authors: Davidson Jeffrey E. , Beckner Bret L.
Published: December 2007

When using a reservoir simulator to predict future field production, one must determine objectives and apply constraints to manage wells and surface facilities. Sequential Quadratic Programming (SQP) methods are used to set well rates in a facility network of a reservoir simulator so that production objectives are maximized (or minimized) subject to constraints on pressures, flow rates, and stream compositions. Actions taken for infeasible conditions are discussed. This method is called Optimized Rate Allocation (ORA). Black-oil and compositional examples are given.
The Effect of Cement Heat of Hydration on the Maximum Annular Temperature of Oil and Gas Wells
Authors: Dillenbeck R. L. , Heinold T. , Rogers M. J. , Mombourquette I. G.
Published: December 2007

Recent advances in electronics technology have made it possible to monitor and record real-time annular temperatures in operational wells, both during and after primary cementing. The developments have allowed operators to record the entire annular-temperature history of their wells, including the critical period when cement hydration occurs. The ability to record these actual temperatures can significantly impact the oilfield cementing industry in several ways. Most significantly, currently accepted practice within the industry is to test certain critical aspects of set cement, such as compressive and tensile strength, at the bottomhole static temperature (BHST). If the short-term maximum annular temperature is significantly different from the later BHST of the well, laboratory tests run on cement at a steady BHST may prove to be inaccurate when based on the actual temperature encountered by a cement slurry downhole. Also of concern is the fact that the magnitude of any temperature change after the initial set may have profound effects on the induced stress in the cement sheath as well as on the casing and formation because the maximum temperature spike from hydrating cement may not occur until after the cement has achieved an initial set.

On the basis of actual field measurements of annular temperatures, this paper details how the variable factors of individual heat of hydration (HOH), relative annular geometry, and final BHST interact to produce short-term maximum temperatures in the cement sheath. In some instances, these maximum temperatures can vary significantly from the stabilized BHST in a well. The actual annular-temperature data were recovered from wells in both North and South America and include shallow and deep well applications.
Successfully Replacing Oil-Based Drilling Fluids With Water-Based Drilling Fluids: Case Study Demonstrates Value of Extensive Planning and Execution in an Extended-Reach Well
Authors: Stawaisz R. , Taylor S. , Hemphill T. , Tare U. , Morton K. , Valentine T.
Published: December 2007

An extended-reach well was planned and successfully drilled with a water-based drilling fluid in an area where wells historically have been drilled with mineral oil- and ester-based drilling fluids—the North Sea central Graben. The high cost of cuttings drilled with inverts that must be shipped to shore for processing and disposal coupled with the potential shutdown of drilling operations in times of bad weather (when cuttings could not be transferred from the rig to supply vessels) led the operator to revisit the use of water-based drilling fluids, especially in the large-diameter hole section in which large amounts of cuttings are generated. However, many water-based systems have been used in the central Graben with little success. The need to develop a stable, shale-inhibiting, waterbased mud (WBM) system capable of performing well in extended-reach drilling (ERD) applications was identified. Extensive prewell modeling of wellbore stability in problematic shale zones and of hole cleaning in the large-diameter interval contributed to the success of this challenging project. This article discusses the planning, preparation, and successful drilling of the intermediate interval with a water-based drilling fluid.
Numerical Studies of Gas Production From Methane Hydrates
Authors: Moridis G. J.
Published: December 2007

EOSHYDR2 is a new module for the TOUGH2 general-purpose simulator for multicomponent, multiphase fluid and heat flow in the subsurface. By solving the coupled equations of mass and heat balance, EOSHYDR2 can model the nonisothermal gas release, phase behavior, and flow of fluids and heat under conditions typical of common natural hydrate deposits (i.e., in permafrost and in deep ocean sediments) in complex formations, and it can describe binary hydrocarbon systems involving methane.

EOSHYDR2 includes both an equilibrium and a kinetic model of hydrate formation and dissociation. The model accounts for up to four phases (gas phase, liquid phase, ice phase, and hydrate phase) and up to nine components (hydrate, water, native CH4 and CH4 from hydrate dissociation, a second native and dissociated hydrocarbon, salt, water-soluble inhibitors, and a heat pseudocomponent). The mass components are partitioned among the phases. The thermophysical properties of the various mass components can be described at temperatures as low as −110°C. Dissociation, phase changes, and the corresponding thermal effects are fully described, as are the effects of salt and inhibitors. The model can describe all possible hydrate dissociation mechanisms (i.e., depressurization, thermal stimulation, salting-out effects, and inhibitor-induced effects).

Results are presented for four test problems of increasing complexity that explore different mechanisms and strategies for production from typical CH4-hydrate accumulations. The results of the tests indicate that CH4 production from CH4-hydrates could be technically feasible and has significant potential. In particular, thermal stimulation is capable of producing substantial amounts of hydrocarbons, and its effectiveness can be enhanced when coupled with depressurization and the use of inhibitors.
Analytical Solutions for 1D Countercurrent Imbibition in Water-Wet Media
Authors: Kashchiev Dimo , Firoozabadi Abbas
Published: December 2007

Analytical solutions for the initial stage of 1D countercurrent flow of water and oil in porous media are presented. Expressions are obtained for the time dependence of the water saturation profile and the oil recovered during spontaneous countercurrent imbibition in rod-like, cylindrical, and spherical cores, for which water is the wetting liquid. Some of the analytical solutions are found to be in good agreement with existing numerical solutions and available experimental data for oil recovery from cores with strong water wettability.
A Model for Foam Generation in Homogeneous Media
Authors: Kam S. I. , Rossen W. R.
Published: December 2007

Recent laboratory research in a wide range of porous media shows that creating foam in steady flow in homogeneous media requires exceeding a minimum pressure gradient ∇pmin. Data fit trends predicted by a model in which foam generation depends on attaining sufficient ∇p to mobilize liquid lenses present before foam generation. Data show three regimes: a coarse-foam regime at low ∇p, strong foam at high ∇p, and, in between, an unstable transient regime alternating between weaker and stronger foam.

Here, for the first time, a population-balance foam model incorporates a bubble-creation function that depends on pressure gradient. The new model reproduces the three foam regimes seen in the laboratory, the abrupt occurrence of foam generation at a threshold velocity or pressure gradient, hysteresis in experimental results, the interplay between foam stability and foam generation, the effect of injected-liquid fractional flow on foam generation, and foam behavior in the high-quality and low-quality steady-state strong-foam regimes. Once strong foam is created, the details of the lamella-creation function have little further effect on its rheology, which is controlled by other mechanisms. The fractional-flow curves predicted for foam are complex. This model is a necessary step toward quantitative prediction of foam performance in the field.
Fast and Robust Algorithm for Compositional Modeling: Part II - Two-Phase Flash Computations
Authors: Pan Huanquan , Firoozabadi Abbas
Published: December 2007

In Part I of our study, stability analysis testing in the reduced space was formulated, and its robustness and efficiency in comparison to the conventional approach was explored. In this paper, we present formulations including, first, direct solution of the nonlinear equations, and second, minimization of Gibbs free energy for two-phase flash computations in the reduced space. We use various algorithms including the successive substitution (SS), Newton's method, globally convergent modifications of Newton's method (line searches and trust region), and the dominant eigenvalue method (DEM) for direct solution of the nonlinear equations defining two-phase flash and the minimization of Gibbs free energy. We also suggest a criterion based on the tangent-plane-distance (TPD) for the initialization from the equilibrium ratios. The proposed criterion has a significant effect on reducing the number of iterations.

The results from various algorithms reveal that the direct solution of the nonlinear equations in the reduced space, combined with the use of the TPD criterion for initialization in the combined SS and Newton's method, can make flash computations extremely efficient. The efficiency and robustness of flash computations in the critical region are especially remarkable.