bluuebunny/crossref_metadata_2025 huggingface dataset:
Phenomenological Modeling of Critical Condensate Saturation and Relative Permeabilities in Gas/ Condensate Systems
Authors: Li Kewen , Firoozabadi Abbas
Published: December 2007

The effects of gravity, viscous forces, interfacial tension, and wettability on the critical condensate saturation and relative permeability of gas condensate systems are studied using a phenomenological simple network model. The results from the simple model show that wettability significantly affects both critical condensate saturation and relative permeability. Relative permeability at some saturations may increase significantly as the contact angle is altered from 0° (strongly liquid-wet) to 85° (intermediately gas-wet). The results suggest that gas well deliverability in condensate reservoirs can be enhanced by wettability alteration near the wellbore.
Estimating Mechanical Properties of Shale From Empirical Correlations
Authors: Horsrud Per
Published: December 2007

Borehole-stability prediction requires knowledge of the mechanical properties of the formations which is rarely available in shale sections. This paper presents empirical correlations to assist in predicting shale mechanical properties. The correlations are based on extensive laboratory testing of shale cores primarily from the North Sea. The acoustic P-wave velocity is a primary input parameter in several of the correlations; thus, various sources of the P-wave velocity such as sonic wireline, sonic measurement while drilling (MWD), and acoustic measurements on cuttings may be used to obtain somewhat continuous estimates of shale mechanical properties. Borehole-stability evaluations can be made at different stages in the drilling process (planning, while drilling, and post-analysis). Other applications of the correlations where shale mechanical properties are required are evaluation of overburden compaction during depletion and optimization of the drilling process (selection of bit type, bit parameters, etc.).
On-site Fracturing Disposal of Oilfield-Waste Solids in Wilmington Field, California
Authors: Hainey B. W. , Keck R. G. , Smith M. B. , Lynch K. W. , Barth J. W.
Published: December 2007

With an active drilling program that was generating over 90,000 barrels of drilling waste each year, the THUMS Long Beach Unit in Wilmington Field, California, was spending over 3.5 million dollars per year to dewater and ship these solid wastes to on-shore landfills for disposal. In 1994, THUMS implemented an environmentally safe and economic program of hydraulic fracturing for the long-term, onsite disposal of drilling mud, drill cuttings, and tank bottoms. By reinjecting the drill cuttings downhole, transportation, the major portion of drilling waste disposal costs, was eliminated. This paper reviews the regulatory permitting process and addresses injection interval assessment, well candidate selection, fracture containment, and offset well seismic monitoring. The equipment, injection history, and economics of disposing over one million barrels of slurry over a 3-yr period are detailed.
Fluid-Leakoff Delineation in High-Permeability Fracturing
Authors: Valko´ P. P. , Economides M. J.
Published: December 2007

Starting from the original concept proposed by Carter, Howard and Fast, this paper reviews the description of fracturing fluid leakoff in view of modeling flow in porous media. It is shown how various linear leakoff models have been developed and why a new, radial leakoff concept is necessary for high-permeability fracturing, where the injection time is commensurable to the response time of the reservoir. Using Laplace space methods, the new radial leakoff law is calculated and compared to linear leakoff. For comparison purposes a calibration test executed in high-permeability formation is interpreted using several approaches, namely: linear leakoff+bulk leakoff coefficient; filter-cake resistance+linear flow in the formation and finally, filter-cake resistance+radial flow in the formation.
Novel Oxidizing Breaker for High-Temperature Fracturing
Authors: Terracina J. M. , McCabe M. A. , Shuchart C. E. , Walker M. L.
Published: December 2007

A novel oxidizing breaker system has been developed for fracturing fluids at high temperatures. Below 200°F, the system is not active, but above 200° F, the oxidizing system aggressively attacks the polysaccharide backbone of the fracturing fluids, resulting in a complete break of the crosslinked fluids. In the presence of a gel stabilizer, an intermediate, reactive oxidizing species is formed. The result of this formation is a delayed, soluble, high- temperature oxidizing system.

Controlled viscosity reduction at 200 to 300°F in crosslinked gelled fluids with and without a gel stabilizer will be demonstrated. Testing included model 50 viscosity profiles, high- temperature static break tests, and conductivity testing. Results from all testing showed the effect of oxidant concentration in producing a predictable, controlled break of the thermally stabilized crosslinked systems. Data were obtained in low-pH and high-pH Zr-crosslinked fluids as well as in borate-crosslinked fluids. The delayed mechanism of the new breaker system provides fluids with excellent crosslinked viscosity properties at early times with predictable, long-term viscosity reductions. Case histories show that the breaker system can be used throughout the treatment in the pad fluid, proppant-laden fluid, and flush.

This article provides data that allow significant improvements in job design. The operations engineer can obtain predictable, controlled gel degradation by using the data provided for temperature, gel type, gel stabilizers, and breaker concentration. The results are optimized treatment designs with rapid fluid recovery, improved proppant-bed conductivity, and increased well productivity.
Modeling Foams for Acid Diversion
Authors: Rossen W. R. , Wang M. W.
Published: December 2007

Successful foam diversion in matrix acid treatments depends on foam mobility during foam injection and liquid (acid) mobility after foam injection. We present new coreflood data for both flow regimes. During foam injection in one core, for qualities below about 80%, when gas and liquid flow rates were varied separately, pressure gradient was nearly independent of water flow rate, as reported by Osterloh and Jante for foam below a threshold foam quality. Upon liquid injection after foam, pressure gradient falls to a steady-state value that varies little with liquid flow rate. The transition to this steady-state pressure gradient is strongly affected by gas expansion as pressure falls, which makes extrapolation from laboratory coreflood to field application complex. However, although some gas escapes during this transition period, the foam front does not advance; that is, this escaping gas moves as free gas, not foam.

A simple new model accounts for many of the trends observed. This model assumes that foam mobility is controlled, not by foam coalescence and capillary pressure as for high-quality foams, but by trapping and mobilization of foam bubbles of fixed size. This model explains how foam mobility can be independent of liquid flow rate; why steady-state pressure gradient falls moderately during liquid injection following foam; and why this pressure gradient is relatively insensitive to liquid flow rate. Implications of the model for design of foam processes are discussed.
Effect of Drillpipe Rotation on Hole Cleaning During Directional-Well Drilling
Authors: Sanchez R. Alfredo , Azar J. J. , Bassal A. A. , Martins A. L.
Published: December 2007

The effect of drillpipe rotation on hole cleaning during directional-well drilling is investigated. An 8 in. diameter well-bore simulator, 100 ft long, with a 412 in. drillpipe was used for the study. The variables considered in this experimental work are: rotary speed, hole inclination, mud rheology, cuttings size, and mud flow rate. Over 600 tests were conducted.

The rotary speed was varied from 0 to 175 rpm. High viscosity and low viscosity bentonite muds and polymer muds were used with 14 in. crushed limestone and 110 in. river gravel cuttings. Four hole inclinations were considered: 40°, 65°, 80°, and 90° from vertical.

The results show that drillpipe rotation has a significant effect on hole cleaning during directional-well drilling, contrary to what has been published by previous researchers who forced the drill-pipe to rotate about its own axis. The level of enhancement due to pipe rotation is a function of the simultaneous combination of mud rheology, cuttings size, and mud flow rate. Also it was observed that the dynamic behavior of the drillpipe (steady state vibration, unsteady sate vibration, whirling rotation, true axial rotation parallel to hole axis, etc.) plays a major role on the significance in the improvement of hole cleaning.

Generally, smaller cuttings are more difficult to transport. However, at high rotary speed and with high viscosity muds, the smaller cuttings seem to become easier to transport. Generally, in inclined wells, low viscosity muds clean better than high viscosity muds, depending on cuttings size, viscosity, and rotary speed level.
Unconventional Miscible Enhanced Oil Recovery Experience at Prudhoe Bay
Authors: McGuire P. L. , Redman R. S. , Mathews W. L. , Carhart S. R.
Published: December 2007

This paper summarizes the conceptual development, reservoir simulation, and field testing of an unconventional application of the miscible enhanced oil recovery (EOR) process at Prudhoe Bay. The miscible injectant stimulation treatment (MIST) process involves completing a well at the bottom of a thick, continuous, watered-out interval. A horizontal lateral is drilled along the base of the reservoir, and a large slug of miscible injectant (MI) is injected, followed by a small slug of chase water. The MI sweeps rock not contacted by previous MI injection and drives EOR oil to offset producers. MI is injected sequentially into several intervals along the lateral to mobilize EOR oil from previously unswept areas. The lateral well is then returned to normal production or injection service.

Simulation data indicate that each MIST well could recover up to 1,000 MSTB of incremental oil at initial rates of up to 1,000 STB/D. Field results have been encouraging. Incremental EOR from the three MIST lateral wells is over 2.1 MMSTB, with peak production rates of about 6,000 STB/D.
Effects of Foam Quality and Flow Rate on CO2-Foam Behavior at Reservoir Temperature and Pressure
Authors: Chang Shih-Hsien , Grigg R. B.
Published: December 2007

This paper reports a series of steady-state CO2-foam flow experiments performed at reservoir conditions of 101° F and 2100 psig. Three flow rates (total injection rates), 4.2, 8.4, and 16.8cm3/h, and five foam qualities, 20%, 33.3%, 50%, 66.7%, and 80%, were used to study how flow rate and foam quality affect foam mobility.

Results from these experiments show that foam mobility (total mobility of CO2/surfactant solution) increases with increasing flow rate and foam resistance factor decreases with increasing flow rate. Results also show that foam mobility decreases with increasing foam quality. Foam resistance factor increases with increasing foam quality ranging from 33.3% to 80%; there appears to be a minimum foam resistance factor between foam qualities of 20% and 33.3%.
Mechanisms of Shallow Waterflows and Drilling Practices for Intervention
Authors: Alberty M. W. , Hafle M. E. , Mingle J. C. , Byrd T. M.
Published: December 2007

Geopressured water sands near the mudline in deep water, greater than 1,000 ft, have been shown to be hazards when these sands are permitted to flow outside structural and 20-in. conductor pipe. Special drilling practices are required to contain the pressure during drilling and casing operations.

Four mechanisms have been identified as causes of shallow waterflows: (1) induced fractures, (2) induced storage, (3) geopressured sands and (4) transmission of geopressure through cement channels. Geoscience techniques have been developed to aid in the detection of the shallow waterflow mechanisms, prior to drilling. These techniques include seismic stratigraphic interpretation of shallow hazard airgun data and specially processed three-dimensional surveys, and special pore pressure and fracture gradient prediction methods.

Drilling and cementing practices have been developed to minimize the risk of inducing flow behind structural and conductor casings. Each of these flow mechanisms require adapted drilling and cementing practices to prevent potentially damaging flow.

This paper presents best practices developed by this operator along with our contractors to detect, drill, case, and cement shallow waterflows in the deepwater Gulf of Mexico. These practices should be transferable to other similar sites around the world.
Rig-Site Monitoring of the Drilling Fluid Solids Content and Solids-Control Equipment Discharge
Authors: Davison J. M. , Daccord Gerard , Prouvost L. P. , Gilmour Alan
Published: December 2007

Many of the key properties of a drilling fluid are adversely affected by the solids entrained during the drilling process. To control the impact of the drilled solids on the fluid properties the engineer at the rig site requires an effective monitoring tool. This paper describes the development and implementation of a technique based on X-ray fluorescence (XRF) spectrometry. The results are shown to be more accurate than traditional methods of analysis.

The speed and ease of use of the XRF technique facilitates multiple sampling from various points in the circulating system and effluent discharges. Two case histories are cited which demonstrate the use of the technique for monitoring the deployment of solids control equipment at the rig site and optimizing its configuration. The overall result is to improve well quality, control costs, and limit the environmental impact of the drilling process.
The Role of a Worldwide Drilling Organization and the Road to the Future
Authors: Boykin G. F.
Published: December 2007

After struggling for the past few years with the question "How can we get the highest return for our drilling expenditures?," major operators have arrived at very different answers. Some have responded by moving the drilling function to business units, some by retaining a mixture of centralized and decentralized drilling staffs, and some by outsourcing the function almost entirely.

The purpose of this paper is to describe why one major operator has chosen to keep drilling management as one of its core competencies, and how it continues to use a centralized drilling group to meet its exploration and production customers' needs. Also discussed is the deliberate strategy employed by this operator to meet the challenges posed by the changing exploration and development environment. This strategy included elements to manage people, business and technical processes, and to ensure that the drilling organization captured and shared its experience. This paper describes this strategy, the organization's successes and remaining challenges, and how this organization plans to address these challenges.
Influence of Reservoir Stress Path on Deformation and Permeability of Weakly Cemented Sandstone Reservoirs
Authors: Ruistuen H. , Teufel L. W. , Rhett D.
Published: December 2007

The influence of production-induced changes in reservoir pore pressure on compressibility and permeability of weakly cemented sandstones has been analyzed. Laboratory experiments simulating reservoir depletion have been conducted over a range of stress paths that a reservoir may follow. The results suggest that compressibility of weakly cemented sandstones is stress path dependent. Compressibility measured under uniaxial strain conditions, or a stress path defined by a lower ratio of the rate at which the effective horizontal to effective vertical stress were increased than the one associated with uniaxial strain, is more than twice the corresponding value found from the hydrostatic loading experiment. In contrast, matrix permeability measured in the maximum stress direction show no significant stress path dependence.

Experimental results suggest that a better understanding of the stress-sensitive behavior of weakly cemented sandstones can only be gained by dealing more directly with the microstructure of the rock. The stress-path-dependent nonlinear behavior of weakly cemented sandstones is related to effects of shear-enhanced compaction. Increasing cementation has been experimentally shown to reduce stress sensitivity. The observed nonlinearity is attributed to dilatancy rather than shear-enhanced compaction, also reflected by permeability measurements made in the maximum stress direction.
Determination of Residual Oil Saturation by Use of Enhanced Diffusion
Authors: Akkurt Ridvan , Marschall Dave , Eyvazzadeh R. Y. , Gardner J. S. , Mardon Duncan , Dunn K. J.
Published: December 2007

The enhanced diffusion method (EDM) exploits the diffusion contrast between oil and water separating their respective nuclear magnetic resonance (NMR) signals. Unlike standard NMR logs acquired with short interecho time (TE), measurements, EDM data are acquired using long TE accentuating diffusion. Fundamentally the EDM establishes an absolute upper bound for the T2 of water, thus any T2's greater than this limit unambiguously indicates that oil is present.

The EDM's best application is with intermediate viscosity oils (approximately 1 to 50 cp) complementing other NMR hydrocarbon-typing applications designed for lighter hydrocarbons (i.e., the differential spectrum method). While expanding the viscosity range of NMR hydrocarbon-typing applications, the EDM also provides a method by which to determine residual oil saturation (ROS), which is the main focus of this article.

The potential use of NMR as a direct indicator of hydrocarbon saturation via techniques such as the differential spectrum method (DSM) has generated significant interest in the petrophysical community in recent years. Although originally developed for applications involving natural gas, the DSM has also been used successfully in light hydrocarbon environments. However, success has been limited to the low end of the viscosity spectrum because of the T1 separation requirements between the brine and hydrocarbon phases.

The T1 separation requirement imposed on diffusion applications in higher viscosity oils can be eliminated by using the EDM, where diffusion is turned into the dominant relaxation mode for the wetting brine phase. Given that brine is more diffusive than the hydrocarbons, the longest apparent T2 from the brine phase can be made short enough to cause separation between the two phases in T2 space, thereby eliminating the need for T1 separation. Wait time manipulation can then be used to quantify hydrocarbon volumes when the two phases are separated in the T2 domain.

In this article we focus on determination of the residual oil saturation using the EDM, while also providing guidelines for job screening and acquisition parameter selection. Several case histories that are provided are used to illustrate the basic concepts and different methodologies available.
Air Injection LTO Process: An IOR Technique for Light-Oil Reservoirs
Authors: Ren S. R. , Greaves M. , Rathbone R. R.
Published: December 2007

The paper describes an air injection improved oil recovery (IOR) process for the recovery of residual oil from light-oil reservoirs. Unlike the air injection technique applied in heavy-oil reservoirs, the main concern for a light-oil reservoir is to remove the oxygen from the injected air by some kind of spontaneous reaction between the oil and oxygen. In-situ combustion in heavy oil reservoirs is a very effective reaction pathway to achieve complete oxygen consumption, as well as to generate heat for enhanced oil recovery. For deep light-oil reservoirs, in-situ combustion is not necessary and may not be readily sustained. More likely, so-called low temperature oxidation (LTO) will prevail. In this study, the potential for LTO reactions to consume oxygen, the reaction rate, and the reaction pathways are investigated. A simplified LTO reaction model has been established based on experimental data obtained from a batch reactor experiment. The model was validated against high-pressure flow displacement experiments in an oxidation tube. A scoping simulation study on a reservoir scale has enabled a sensitivity assessment of the process to be made. The effect of air injection rate, reservoir dip, oil viscosity, formation permeability, numerical grid size, and reservoir temperature on oil recovery and the thermal effect were investigated. Compared with what is normally understood to be conventional air injection (insitu combustion), the air injection LTO process is flexible in terms of injection rate, stable because of spontaneous reaction (if the reservoir temperature is high enough), and also an economic alternative to hydrocarbon, nitrogen, or carbon dioxide gas. It can also be used as a secondary recovery method in reservoirs that are not suitable for water injection.
Utilization of Mud Weights in Excess of the Least Principal Stress to Stabilize Wellbores: Theory and Practical Examples
Authors: Ito Takatoshi , Zoback Mark D. , Peska Pavel
Published: December 2007

In this paper, we address the theoretical possibility of drilling with mud weights in excess of the least principal stress for cases of particularly high pore pressure or severe wellbore instability. Because lost circulation caused by hydraulic fracturing is to be avoided, we consider three critical wellbore pressures, pfrac, plink, and pgrow. Tensile fractures initiate at the wellbore wall at pfrac, link up to form large axial fractures that are subparallel to the wellbore axis at plink, and propagate away from the wellbore at pgrow. It is obvious that lost circulation cannot occur if the wellbore pressure during drilling is below pfrac. However, even if pfrac is exceeded and tensile fractures are initiated at the wellbore wall, fracture propagation (and, hence, lost circulation) will be limited as long as the wellbore pressure is below plink. Finally, if the wellbore pressure is greater than plink, the fractures will not grow away from the wellbore (and significant lost circulation will not occur) if the wellbore pressure is below pgrow, which must exceed (if only slightly) the least principal stress. In general, our modeling shows that pfrac and plink can be maximized by drilling the wellbore in an optimally stable orientation, and pgrow can be maximized with noninvading drilling muds that prevent fluid pressure from reaching the fracture tip. We apply the model that uses in-situ stress data collected in real fields, such as the South Eugene Island field in the Gulf of Mexico and the Visund field in the northern North Sea.
New Model of Acid-Fracture Conductivity Based on Deformation of Surface Asperities
Authors: Gong Ming , Lacote Stephan , Hill A. D.
Published: December 2007

Acid-fracture conductivity is affected by the aperture and contact area of the fracture under closure stress. This, in turn, depends on the surface asperities created by the acidizing process and the mechanical strength of these asperities. Acid contact time, acid leakoff, rock mechanical properties, and formation heterogeneity all affect the creation of hydraulic conductivity of an acid fracture.

We have conducted a systematic experimental study to investigate the mechanisms of the creation of hydraulic conductivity in acid fractures, including characterization of the surface roughness created by acid etching, investigation of the damage to the rock compressive strength by acid, and measurement of hydraulic conductivity under closure stress. Experimental data show that longer acid contact results in rougher fracture surfaces and, in turn, higher hydraulic conductivity. However, acidizing also reduces the rock compressive strength causing the surface asperities to easily deform under stress.

Based on our experimental results, a new fracture deformation model was derived which considers both the surface roughness and the rock mechanical properties. The fracture closure under stress is modeled as plastic deformation of asperities. Finally, a cubic law is used to calculate the fracture conductivity. The prediction of acid-fracture conductivity using this model with appropriate parameters shows good agreement with the experimental data.
Ram/Powell Deepwater Tension-Leg Platform: Horizontal-Well Design and Operational Experience
Authors: Lester G. Scott , Lanier Gary H. , Javanmardi Kaz , Bernardi Tony , Halal Afif S.
Published: December 2007

A key focus of the Ram/Powell project is to utilize high-rate (>20,000 BOPD), high-volume wells (i.e., reserves several times greater than that of shelf wells) to enhance project profitability. Completions with large 5½-in. production tubing and horizontal openhole gravel packs (OHGP's) exceeding 2,000 ft in length have been implemented successfully. The horizontal OHGP design, the first implemented in the deepwater Gulf of Mexico (GOM), was selected as a means to improve well reliability over prior horizontal completion designs and sustain high production rates without plugging. A high rate of sand-control failure and plugging have plagued prior GOM horizontal non-gravel-packed completions. Achieving high-quality completions is a major project emphasis. Drilling, completion design, and operational experience of the horizontal wells are discussed.

Production from the Ram/Powell tension-leg-platform (TLP) development, Shell's third TLP, began in September 1997. The TLP is located at Viosca Knoll Block 956 in 3,214 ft of water. Four wells were predrilled prior to TLP installation. Ten wells are completed. Five are horizontal OHGP's, one is a single-zone cased gravel pack, and the remainder are two- and three-zone-selective cased gravel-packed completions. Rig operations concluded in January 2000.
Effects of Pressure Drop in Horizontal Wells and Optimum Well Length
Authors: Penmatcha V. R. , Arbabi Sepehr , Aziz Khalid
Published: December 2007

As the length of a horizontal well is increased, its contact with the reservoir increases. But at the same time, the resistance to flow in the well also increases, which has a direct negative effect on the productivity of the well. The overall performance of a horizontal well depends on the balance of these two opposing factors. A semi-analytical well model is developed for homogeneous reservoirs which can quantify the effects of both single-phase oil and two-phase oil/gas flow pressure loss in the well on the overall well performance. The model is quite flexible and can incorporate any friction factor correlation. A methodology is developed to show the effects of various reservoir, fluid, and well parameters on well productivity. We demonstrate that ignoring frictional effects could lead to unrealistically higher production estimates and longer breakthrough times for water or gas. As a result of pressure drops in the well, breakthroughs occur first at the heel of the well. A methodology is also developed to calculate the optimum horizontal well length.
Comprehensive Reservoir/Wellbore Model for Horizontal Wells
Authors: Penmatcha V. R. , Aziz Khalid
Published: December 2007

Comprehensive, three-dimensional, anisotropic and transient, reservoir/wellbore coupling models are developed for infinite-conductivity and finite-conductivity wellbores to calculate the productivity of a horizontal well placed in a box-shaped drainage volume under single phase flow conditions. The finite-conductivity well model considers frictional and accelerational pressure drops in the wellbore. It also takes into account the effects of fluid inflow into the well. These models are semianalytical in nature and can be used under transient or pseudosteady state flow conditions. The infinite-conductivity well model is compared with the uniform flux model to show the differences in well productivity calculations. The effects of wellbore pressure drop on well productivity are shown using the finite-conductivity well model. These models can provide reference solutions to calculate well indices or to check the accuracy of a numerical simulator. Because of their analytical nature they are easier and faster to use than numerical simulators. Using superposition in space and time, these models can be run under a variety of constraints. We believe that our finite-conductivity well model is more general than any published previously.
Production Logging in Horizontal Wells by Use of Ultrasonics
Authors: Nyhavn Fridtjof , Bang Jon , Øyangen Terje
Published: December 2007

This paper discusses technical and practical aspects on the future needs for monitoring of flow conditions in deviated and horizontal production and injection wells. A monitoring technique based on ultrasound has been developed. Examples of measurements in various North Sea wells are presented. The results are promising, and show the potentials of nonintrusive techniques for flow monitoring. It is expected that a combination of several sensor principles would facilitate the interpretation of downhole flow conditions.
Sulfide Removal in Reservoir Brine by Indigenous Bacteria
Authors: Jenneman G. E. , Moffitt P. D. , Bala G. A. , Webb R. H.
Published: December 2007

A field test was performed at the Coleville field to evaluate the ability of indigenous bacteria to remove sulfides from reservoir brine. Ammonium nitrate and sodium phosphate were injected continuously at two injectors for nearly 50 days. Sulfide levels at the two injectors declined by 42 to 100% and by as much as 50 to 60% at two adjacent producers. Concentrations of indigenous sul- fide oxidizing, nitrate-reducing bacteria increased at injectors and producers while concentrations of sulfate-reducing bacteria remained unchanged or decreased slightly. Stimulation of indigenous beneficial bacteria has potential application as a costeffective, low toxicity means to remove and control sulfides in reservoir brines.
Natural Downhole Separation and Its Impact on Production Performance
Authors: Asheim H. A.
Published: December 2007

This article deals with phenomena associated with gravity-induced separation along the completed interval of inclined wells. The model developed computes liquid accumulation and production performance. From the results it is shown that both the overall inflow performance and the gas-liquid ratio observed at the surface are affected by the downhole separation.

It is shown that downhole separation may be constrained by either backseepage to the reservoir or countercurrent flooding along the wellbore. The liquid backflow rate limit due to flooding has been derived explicitly, providing a design parameter for downhole separation devices.
Incorporating Seismic Data of Intermediate Vertical Resolution Into Three-Dimensional Reservoir Models: A New Method
Authors: Behrens R. A. , Tran T. T.
Published: December 2007

Three-dimensional (3D) earth models are best created with a combination of well logs and seismic data. Seismic data have good lateral resolution but poor vertical resolution compared to wells. The seismic resolution depends on seismic acquisition and reservoir parameters, and is incorporated into the 3D earth model with different techniques depending on this resolution relative to that of the 3D model. Good vertical resolution of the seismic data may warrant integrating it as a continuous vertical variable informing local reservoir properties, whereas poor resolution warrants using only a single map representing vertically averaged reservoir properties. The first case best applies to thick reservoirs and/or high-frequency seismic data in soft rock and is usually handled using a cokriging-type approach. The second case represents the low end of the seismic resolution spectrum, where the seismic map can now be treated by methods such as block kriging, simulated annealing, or Bayesian techniques. We introduce a new multiple map Bayesian technique with variable weights for the important middle ground where a single seismic map cannot effectively represent the entire reservoir.

This new technique extends a previous Bayesian technique by incorporating multiple seismic property maps and also allowing vertically varying weighting functions for each map. This vertical weighting flexibility is physically important because the seismic maps represent reflected wave averages from rock property contrasts such as at the top and base of the reservoir. Depending on the seismic acquisition and reservoir properties, the seismic maps are physically represented by simple but nonconstant weights in the new 3D earth modeling technique.

Two field examples are shown where two seismic maps are incorporated in each 3D earth model. The benefit of using multiple maps is illustrated with the geostatistical concept of probability of exceedance. Finally, a postmortem is presented showing well path trajectories of a successful and unsuccessful horizontal well that are explained by model results based on data existing before the wells were drilled.
Uncertainty Analysis in Predictive Reservoir Simulation Using Gradient Information
Authors: Lépine O. J. , Bissell R. C. , Aanonsen S. I. , Pallister I. C. , Barker J. W.
Published: December 2007

We demonstrate how gradient-based techniques can be used to estimate uncertainty in predictive reservoir simulations made following history matching. We discuss how the gradient calculations can help in finding the parameters that make the largest contributions to the uncertainty, estimating the uncertainty on these parameters, and estimating the uncertainty in predicted quantities by use of a linear analysis. The methodology is illustrated on two field examples, and compared with other methods of uncertainty quantification.
Analysis of Pressure/Flow-Rate Data With the Pressure-History-Recovery Method
Authors: Gilly Patrick , Horne Roland N.
Published: December 2007

This study presents a new way to integrate flow rate history and pressure history data in well-test analysis. The method used is an extension of the deconvolution principle to a chosen period of time before the well test. It is based on the recovery of missing parts of the pressure history by taking into account the flow rate history and the available sections of measured pressure history. The method has the advantage of working without making any assumption about the reservoir by using the data itself to define the "model." A real example with 71 pressure points and 70 corresponding flow rate points shows how it is possible to recover the first 20 pressure points correctly considering that only the flow rate history and the last 51 pressure points are known.

The main purpose of this procedure is to have an alternative to treat the problem of flow rate variations both before and during the well test. In such a case, neither the usual deconvolution (only applicable for flow rate variations during the well test, when flow rate and pressure are both known), nor the multirate superposition plots (which assume specific reservoir behavior) can be applied properly. In addition, the principle of the method enables us to extend its use to the analysis of well tests with pressure recording errors or missing pressure recording. Examples show that it is possible to make a good analysis of a well test with missing pressure regions when current methods give only parts of the information.
Numerical Solution of the Material Balance Equations of Compartmented Gas Reservoirs
Authors: Hagoort Jacques , Hoogstra Rob
Published: December 2007

This paper presents a robust and rigorous method for the numerical solution of the material balance equations of compartmented gas reservoirs. The method is based on the integral form of the material balance equations and employs an implicit, iterative solution procedure. The proposed method enables extension of traditional p/z analysis of single gas reservoirs to complex, compartmented gas reservoirs.

Example calculations of the depletion of a compartmented reservoir show how the p/z is affected by crossflow, reservoir size, and depletion rate. The depletion behavior can be rationalized by the observation that depletion of a compartmented reservoir at a constant rate tends to develop a semisteady state. A field example is presented that illustrates the capabilities of the extended material balance for the analysis of the past performance of compartmented reservoirs.
Transient Dynamic Characteristics of the Gas-Lift Unloading Process
Authors: Tang Yula , Schmidt Zelimir , Biais R. N. , Doty D. R.
Published: December 2007

This paper presents a new comprehensive transient model and dynamic simulator that describe the complicated characteristics of the gas-lift unloading process. The model uses conservation equations of mass, momentum and energy, for both the cocurrent and countercurrent multiphase flow, along with the necessary closure relationships. The effect of liquid flowing back into the formation is also considered. The transient temperature changes in the gaslift unloading process are studied and the transient tubing pressure and two-phase flow parameters are incorporated into the temperature calculation. Two heat transfer mechanisms within the wellbore annulus are considered in the temperature model.

An explicit numerical scheme is used for the mass and momentum partial differential equations. A double iteration procedure on both the temperature and pressure numerically solves the three conservation equations simultaneously.

Several example calculations are given to illustrate the characteristics of the unloading process and to underscore the unreliability of conventional design methods. Gas-lift instability is also analyzed with the dynamic simulator. In addition, field data from 25 wells support the accuracy of the transient model. Finally, the simulator can be used to perform gas-lift optimal design, stability analysis, and gas-lift operation troubleshooting.
Simplified Mechanistic Simulation of Foam Processes in Porous Media
Authors: Rossen W. R. , Zeilinger S. C. , Shi J.-X. , Lim M. T.
Published: December 2007

The mechanisms controlling transport and mobility of foam in porous media are complex, but in some cases the mechanisms that dominate foam properties can be represented in simple models. For instance, fractional-flow methods can reproduce the predictions of more-complex foam simulators while highlighting the mechanisms controlling foam behavior. Fractional-flow methods indicate that the effectiveness of foam processes that alternate injection of liquid and gas ("SAG" processes) depends on foam strength at extremely high foam quality, conditions difficult to control in the laboratory.

A numerical simulator that incorporates the relation between capillary pressure and foam stability extends simplified foam modeling to cases where fractional-flow methods do not apply. Applications of this simulator to one-dimensional (1D) foam displacements match predictions of analytical models based on laboratory data and illustrate the numerical artifacts that challenge foam simulation. Applications to 2D flow through layers in capillary contact show that the interplay between capillary crossflow and foam collapse depends on both the dimensions of the layers and the relative magnitudes of the capillary and viscous pressure differences.
Influence of Pressure Drop Along the Wellbore on Horizontal-Well Productivity
Authors: Ozkan Erdal , Sarica Cem , Haci Marc
Published: December 2007

This study investigates the effect of wellbore pressure drop on horizontal-well productivity and presents guidelines to design horizontal-well completions. The central new result of this study is a general semianalytical model that couples wellbore and reservoir hydraulics. The model enables the rigorous evaluation of horizontal-well productivity and can be used to determine optimum well length, wellbore diameter, and production rate. Field examples are presented to emphasize the consequences of the infinite-conductivity well assumption. The extension of the model to the flow of real gases is also discussed. It is shown that the pressure losses due to acceleration may play an important role in the wellbore hydraulics of horizontal gas wells and should not be ignored.
Characterizing Curing-Cement Slurries by Permeability, Tensile Strength, and Shrinkage
Authors: Backe K. R. , Lile O. B. , Lyomov S. K. , Elvebakk Harald , Skalle Pål
Published: December 2007

This work was carried out to obtain more knowledge about the transition period of curing oilwell cements. The results show that the curing characteristics are a function of temperature and that there is a correlation between shrinkage and cement content. The paper also introduces a new mechanism for gas migration and discusses how the studied parameters can be used to predict gas migration.
The Use of Slimhole Drilling and Monobore Completions To Reduce Development Costs at the Kuparuk River Field
Authors: Pearson C. M. , Petrash D. K. , Zernell J. T. , Zanghi Michael
Published: December 2007

In 1994, a low-cost development and operating plan for future wells at the Kuparuk River Field was proposed with the goal of reducing well costs 30% by the end of 1996. This paper presents this plan of a slimhole drilling and monobore completion program, a discussion of the critical issues addressed to successfully make the change, and field results from the first 16 wells drilled in 1994 and 1995.

A review of historical cost information is included which covers drilling and completing wells using the conventional well designs that had become standard in the field for the previous 10 years. A variety of new slimhole well plans are presented for both production and injection wells together with a discussion of their potential application and cost savings. The paper concludes with a look at future work plans and opportunities for additional cost savings through the use of slimhole wells.
Enhancement of Liquid Flow Through a Porous Medium by Ultrasonic Radiation
Authors: Aarts A. C. T. , Ooms Gijs , Bil K. J. , Bot E. T. G.
Published: December 2007

A new method to enhance the oil production rate is irradiation of the near-wellbore region by an ultrasonic source. The limited success for the application of ultrasound in well stimulation to date, however, is due to the lack of understanding of the physical mechanisms between an acoustic wave field and a porous medium. In this paper the mechanism in which ultrasonic radiation deforms the walls of the pores in the shape of traveling transversal waves is studied. A quantitative description of the mechanism indicates that these waves induce a net flow of the liquid inside the pores, identical to peristaltic transport. Numerical calculations show a damping effect of liquid compressibility and a strong influence of pore wall hardness and power output on the net flow rate induced.

The occurrence of a flow induced by ultrasound has been confirmed by laboratory experiments. In these experiments a water velocity of almost 1 cm/s through a rubber capillary of 0.15 mm radius could be induced by an ultrasonic source of frequency of 20 kHz. By a comparison between the trends observed in the experiments and the theoretically predicted trends, the role of the peristaltic transport mechanism in acoustic well stimulation is discussed.
Screening Criteria to Evaluate the Development Potential of Remaining Oil in Mature Fields
Authors: Weber K. J. , Dronkert Hans
Published: December 2007

Continuing reservoir management at mature stages often concentrates on delineating pockets of remaining mobile oil. This is becoming a major task for reservoir geologists and petrophysicists. Many old fields are coming up for reactivation as investment opportunities and there is an overall expectation that modern techniques can lead to additional recovery of between 10 and 20%. In this article we will discuss the screening criteria related to reservoir architecture, accumulation condition and production history.

The mobile oil remaining can be found in a number of predictable locations in reservoirs depending on their structural style and facies.

Attic oil along faults is perhaps the most simple configuration but sizeable volumes of remaining oil can also occur as a function of reservoir stratification and lateral discontinuity. A systematic overview of the different play types has been compiled based on structural or stratigraphic lateral continuity and vertical reservoir connectivity. Screening criteria have been derived on the basis of field examples and models for four play types.

The screening criteria specify minimum conditions which may lead to economic re-development with horizontal sidetracks from existing wells. In addition recommendations are given with respect to data gathering to confirm the presence of economically viable targets.
Wettability Effects in Thermal Recovery Operations
Authors: Rao Dandina N.
Published: December 2007

Do the three-phase oil-water-rock systems remain unaltered in wettability or do they become more strongly water- or oil-wet when the reservoir temperature is increased during thermal recovery operations? The published literature provides an affirmative support to each of the three possible answers to this question. Should we let this controversy continue or is it time to put together different pieces of the puzzle to reach at least a semblance of a solution? The latter is the purpose of this paper.

This paper aims to shed more light on the controversy surrounding the effect of temperature on wettability by bringing together the various schools of thought on this subject under a simple mechanistic explanation involving an interrelationship between the spreading behavior in solid-liquid-liquid (SLL) systems and the liquid-liquid interfacial tension, and thin wetting film stability considerations. This paper extends the Zisman-type correlation between contact angles and surface tension in solid-liquid-vapor (SLV) systems to the SLL systems. The success achieved in establishing such a correlation in this study is of practical significance in that it shows a way of identifying crucial wettability changes that may occur in secondary and tertiary recovery processes through simple laboratory experiments involving interfacial tension and contact-angle measurements. This would help in recovery process selection and optimization.

The paper also describes a technique, which was discovered during the course of this experimental study, to prevent adverse wettability shifts in thermal operations. The technique involves in-situ deposition of small quantities of calcium carbonate (scale) particles near the wellbore region to maintain a strongly water-wet region of high relative permeability to crude oil. Encouraging results from an Alberta field test of this technology are also presented.