bluuebunny/crossref_metadata_2025 huggingface dataset:
3D Stress Characterization from Hydraulic Fracture and Borehole Breakout Data in a Faulted Anticline, Wyoming
Authors: Miskimins Jennifer L. , Hurley Neil F. , Graves Ramona M.
Published: December 2007

The North LaBarge Shallow Unit (NLBSU) is a geologically complex field located in the Green River basin of southwest Wyoming. It produces from shallow Mesaverde (Cretaceous) sandstones at a depth around 2,000 ft. 3D seismic data was acquired in the field that confirmed the presence of several faults including major thrust faults, a tear fault, four major strike-slip faults, and a "horsetail" splay termination.

Fourteen wells were chosen for the study based on their locations relative to these faults and quality of certain data. Borehole breakout data provided maximum horizontal stress orientation. G-function analysis was used to determine minimum horizontal stress magnitudes. Integration of density logs provided overburden stresses. Well testing provided pore pressure data. Maximum horizontal stress magnitude windows were determined from combinations of minimum horizontal stress magnitudes, pore pressures, and fracture breakdown data.

Combined stress and orientation results led to complete or partial three-dimensional (3D) stress profiles for six wells. The results of the study indicate that complex faulting in the field influences horizontal stress magnitudes and orientations. Increased minimum horizontal stress magnitudes are observed near faults. Stress-controlled production is evident and suggests a horizontal well would likely provide a significant reserve increase. Results also indicate great care should be given to producer and injector locations to improve sweep efficiency and limit water breakthrough.
Optimized Performance by the Use of Rotating Separators as an Alternative to Shale Shakers
Authors: Botnmark K. , Mikalsen K. , Jensen O. D. , Saasen A. , Hoset H. , Kalvenes Ø. , Sørgård E. , Mathiassen Svein
Published: December 2007

Occupational hygiene requirements for operations on the Norwegian shelf when drilling with oil based drilling fluids favours the use of enclosed system for solids control. Personnel working with shale shakers are often exposed to a base oil mist and vapour at an unacceptable level. A principle with the use of rotating separators as an alternative to shale shakers was therefore developed.

The paper describes the principle with rotating separators as an alternative to shale shakers in detail. The paper focuses on tests where the separator efficiency from the rotating separator is compared with the efficiency from conventional shakers. The separator principle is to feed the drilling fluid with cuttings from the mud line into a slowly rotating separator that forces the cuttings to move along a very long screen section with selected screen sizes. The drilling fluid is effectively removed from the cuttings since the area outside the screen is slightly vacuumed. This principle result in an apparatus with an effective solids separation being among the better separation results obtained by conventional shale shakers.

The paper also describes the improvement on the occupational hygiene and the environmental impacts resulting from the use of the rotating separator compared to the current status from using conventional shakers. Norwegian drilling equipment standards require that the noise level from a device in the drilling area should be less than 83 dB. The paper describes how this requirement is satisfied and that the noise level is far below the requirement level when using the rotating separator.
Aggressive Drilling Parameter Management Based on Downhole Vibration Diagnostics Boosts Drilling Performance in Difficult Formation
Authors: Hood J. A. , Leidland B. T. , Haldorsen H. , Heisig G.
Published: December 2007

Exploiting the reservoir under the Ula platform in the Norwegian North Sea requires directional drilling through very challenging inhomogeneous formations, which may contain chert. In offset wells, drilled with steerable motor systems, an average of 6 to 7 runs and 3 to 4 bits were required to drill the 12 ¼" sections.

In two recent wells, the 12 ¼" sections were drilled with rotary steerable systems with aggressive PDC bits and a new downhole dynamics tool that provided continuous diagnostics of the downhole drilling conditions to the surface. This tool combination was applied for the first time worldwide. The versatility of the rotary steerable system allowed for the instantaneous adjustment of drilling parameters in response to the downhole feedback on the drilling conditions. As a result, integrity of the drilling system could be maintained and bit life extended. Both 12 ¼" sections were drilled with just two bits, in three runs, and in half the time of the best offset well.

This case study describes the planning and execution phase of the wells, and presents several examples of rigorous drilling parameter changes to overcome drilling dynamics problems such as stick-slip and BHA whirl.
Wellbore Position Errors Caused by Drilling Fluid Contamination
Authors: Wilson Harry , Brooks Andrew G.
Published: December 2007

Recent improvements in magnetic survey quality control have revealed the presence of systematic azimuth anomalies, affecting the calculated trajectories of a significant number of wells. Although these anomalies display some features commonly associated with drillstring magnetic interference, they cannot be removed by traditional magnetic correction techniques. New magnetic survey quality control procedures have identified the problem as an attenuation of the transverse component of the earth's field, similar to that caused by magnetically permeable material surrounding the magnetometers. The drill collars housing the sensors were inspected but showed no magnetic susceptibility, which led to suspicion that the problems were caused by magnetic particles suspended in the drilling fluids. Laboratory testing has confirmed the magnetic properties of these fluids, and chemical analysis has verified the presence of iron in significant quantities. Field examples demonstrate the use of a multiple-survey quality control and correction method for identification and removal of these survey anomalies.
Practical Issues in Using Risk-Based Decision Analysis
Authors: Davidson Lynn B.
Published: December 2007

Mark Twain once said, "The art of prophecy is difficult, especially with respect to the future."

Mark Twain's remark summarizes management's challenge in deciding which E&P projects to pursue. Project outcomes are always prophecies fogged in uncertainty.

Risk-based methods have been developed to help pierce the fog and make better decisions.

This paper examines practical barriers to effectively using risk-based decision tools and provides guidelines to overcome the barriers.
Microbiology of Oil Degradation in Reservoirs
Authors: Blanchet D. , Grabowski A. , Vandecasteele J-P
Published: December 2007

The microbial alteration of crude oils in reservoirs is a widespread phenomenon. Its knowledge is of prime importance from an economical point of view, specially in offshore prospecting. The understanding of conditions that favored hydrocarbon biodegradation constitutes key information to conduct exploration. This understanding requires a better knowledge of the microbial communities present in these specific ecosystems. Our views of the microbial life and of its involvment in biodegradation of hydrocarbons in such deep anoxic environments have entirely been transformed these last two decades. First, the use of the recent molecular techniques have permitted to discover the large diversity of this microbial world. Second, undeniable proofs of attack of alkanes in strict anaerobic conditions by various groups such as sulfate-reducers, denitrifying bacteria and consortia including acetogenic bacteria and Archaea have been presented. Further progress will involve the study of key limiting factors of in-reservoir overall microbial activities, through better elucidation of available sources of nutrient and electron acceptors.
Permeability Prediction Using Hydraulic Flow Units and Hybrid Soft Computing Systems
Authors: Soto B. R. , Garcia J. C. , Torres F. , Perez G. S.
Published: December 2007

In the last years, the concept of hydraulic flow units (HFU) has been used in the petroleum industry to improve prediction of permeability in uncored interval/wells. This concept is strongly related to the flow zone indicator (FZI) which is a function of the reservoir quality index (RQI). Both measures are based on porosity and permeability of cores. It is assumed that samples with similar FZI values belong to the same HFU. Thus the FZI, along with other significant variables such as porosity, can be used directly to estimate the permeability in a zone; however, the FZI has to be estimated first. In this paper, a novel method based on hybrid soft computing techniques is used for permeability predictions. The technique is known as adaptive network-based fuzzy inference systems or ANFIS, and is based on adaptive neural networks and fuzzy inference systems (FIS). The final inference on FZI is made by a FIS but the parameters of this FIS will be estimated through a learning procedure based on input data; such procedure is typically used in neural network training. The technique is applied in 3 steps: (1) Identification of the dominant variables in rock type behavior, (2) Development of an ANFIS which best suits the real model, using the dominant variables as input and the FZI as output. (3) Estimation of permeability from FZI and porosity values.

These steps are applied on a sample case and the results show that hybrid soft computing techniques offer powerful tools for further improving permeability predictions.
Water In Crude Detecting Devices and Their Application to Royalty and Custody Transfer Measurements
Authors: Basrawi Y. F.
Published: December 2007

The paper talks about the technology and operations of the various water-in-crude detecting devices and their possible application to royalty and custody measurements of crude and hydrocarbon products. It highlights the operations, current research, field test studies and viability of such devices for better precision and higher accuracy in the measurement of volumetric through puts for exports and domestic consumption. Through finer precision and higher accuracy, proper accountability can be achieved and tremendous cost savings realized. The contents of the paper will contain a brief introduction and will talk about the principles of operations of water in crude detection Devices, their types, applications, evaluations and a final conclusion.
Improved Determination of Oil/Water Drainage and Imbibition Saturation Functions of Chalk Core Plugs from Flooding Experiments
Authors: Bech N. , Nielsen C. M. , Olsen D.
Published: December 2007

This paper presents improvements to a recently developed experimental method, which is particularly suitable for the determination of oil/water capillary pressure and relative permeability for water wet samples of chalk for both drainage and imbibition. Inherently chalk has high capillary pressure, and conventional laboratory determinations of saturation functions on such material are complicated by strong scale effects, particularly by the capillary end effect. The end effect does, however contain detailed information about the saturation functions of the sample and this information is utilized in the method.

The saturation functions are represented by analytical expressions that are determined by matching simulated results and measured data. A five-step core-flooding scheme provides the fluid distributions and production data from which the relative permeabilities and capillary pressures are computed for both drainage and imbibition by a least squares technique. A chemical shift NMR technique is used for fluid distribution determination while the simulation of the experiments is carried out by a commercial reservoir simulator.

In our previous work with this method simple power functions were used to represent relative permeability functions. The present paper demonstrates the use of more flexible analytical expressions for the oil relative permeability that enables a better representation of certain function shapes. Also an improved representation of capillary pressure is incorporated in the method. The new technique is applied to a chalk sample from the North Sea.
Determining Relative Permeability Exponents Near the Residual Saturation
Authors: Laroche Catherine , Chen Min , Yortsos Yanis C. , Kamath Jairam
Published: December 2007

In a recent study1 it was shown that the ratio of the flow rates of the produced fluids in an immiscible displacement can be used to identify geometrical and petrophysical characteristics of a reservoir. In this paper we develop an extended version of this approach and apply it to displacements in 1-D laboratory cores. Because of the possible importance of capillary effects, we pay attention to the capillary end effect at the outlet and to late times.

It is shown that the exponent in the power-law dependence of the relative permeability to saturation, near its residual value, can be determined from a segment of the curve of the outlet fractional flow sometime after breakthrough. The asymptotic predictions are verified by obtaining an analytical solution to the full problem, including end-effects, for a model case corresponding to Burgers’ equation. Experimental results for the high-rate displacement of some gas-liquids pairs are reported. In agreement with the theoretical predictions, a power-law segment in the outlet fractional flow curve can be identified, before capillary effects set in. The exponents obtained are discussed in terms of pore-scale models for gas-liquid displacement.
Multicomponent Fractionator Calculations Using The Relative Operability Technique
Authors: Maddox R.N. , Takaoka Shigeyoshi
Published: December 2007

Publication Rights Reserved

This paper is to be presented at the 38th Annual Fall Meeting of the Society of Petroleum Engineers of AIME in New Orleans, La., on Oct. 6–9, 1963, and is considered the property of the Society of Petroleum Engineers. Permission to publish is hereby restricted to an abstract of not more than 300 words, with no illustrations, unless the paper is specifically released to the press by the Editor of the Journal of Petroleum Technology or the Executive Secretary. Such abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in Journal of Petroleum Technology or Society of Petroleum Engineers Journal is granted on request, providing proper credit is given that publication and the original presentation of the paper.

Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and considered for publication in one of the two SPE magazines with the paper.

Introduction

Distillation is one of the most generally used methods for performing the separation of a liquid mixture into various products. Calculational methods for simple fractionators (one feed and two products) are well known and widely applied. For the case of complex fractionators (more than one feed and/or more than two products) there are rigorous calculation techniques available in the literature. The work reported here fills a particular need in this area - a calculational procedure that can be applied to multicomponent complex fractionators and yet is short and quick enough to be used on a small size digital computer or even a desk calculator.

LITERATURE SURVEY

Underwood and Gilliland almost simultaneously, but independently, proposed a calculational method for multicomponent fractionation in a simple distillation column. They proposed to develop a series of equations very similar to the Fenske equation. The difference being that the Fenske equation applies to a column operating at total reflux, and the proposed equations were to describe the fractionation obtainable in a column operating at some finite reflux rate. This was accomplished through the "relative operability" which for the rectifying section of the column is defined as:

(1)

The term "relative operability" comes from the combination of material balance terms into a ratio for two components.

The relative operability can be used to relate the composition of passing streams in a distillation as follows:

(2)

If we include now the equilibrium calculation across a theoretical plate, we have

(3)
The Effect of Fracture Spacing on Gas/Oil Gravity Drainage in Naturally Fractured Reservoirs
Authors: Clemens Torsten , Wit Krijn
Published: December 2007

Fractured carbonate reservoirs contain a large amount of the world oil reserves. A large number of these reservoirs are produced by gas/oil gravity drainage (GOGD).

Historically, it was assumed that the spacing between vertical fractures is so small that gas/oil gravity drainage can be approximated by one-dimensional vertical flow in the matrix. Recently, the emphasis of research shifted towards reservoirs with larger fracture spacing.

This article describes the effects of the fracture spacing on reservoirs under GOGD for a matrix column with a sealed bottom. Up to a fracture spacing/formation column height ratio of 0.3, the flow in the matrix can indeed be approximated by one-dimensional flow.

For larger values of the ratio, simulations using the onedimensional flow approach are significantly overestimating the gravity drainage rate (for a fracture spacing/formation column height of 2.5, the gravity drainage rate reduces effectively to 30 %!).

In this article, a correlation has been derived to determine a reduction factor in the drainage rate as a function of the fracture spacing/column height ratio. This reduction factor can be incorporated in the gravity drainage shape factor of dualpermeability simulations.
Characterizing Disproportionate Permeability Reduction Using Synchrotron X-Ray Computed Microtomography
Authors: Seright R. S. , Liang J. , Lindquist W. Brent , Dunsmuir John H.
Published: December 2007

X-ray computed microtomography was used to investigate why gels reduce permeability to water more than that to oil in strongly water-wet Berea sandstone and in an oil-wet porous polyethylene core. Although the two porous media had very different porosities (22% versus 40%), the distributions of pore sizes and aspect ratios were similar. A Cr(III)-acetate-HPAM gel caused comparable oil and water permeability reductions in both porous media. In both cores, the gel reduced permeability to water by a factor 80 to 90 times more than that to oil. However, the distributions of water and oil saturations (versus pore size) were substantially different before, during, and after gel placement.

The disproportionate permeability reduction appeared to occur by different mechanisms in the two porous media. In Berea, gel caused disproportionate permeability reduction by trapping substantial volumes of oil that remained immobile during water flooding. With this high trapped oil saturation, water was forced to flow through narrow films, through the smallest pores, and through the gel itself. In contrast, during oil flooding, oil pathways remained relatively free from constriction by the gel.

In the polyethylene core, oil trapping did not contribute significantly to the disproportionate permeability reduction. Instead, oil films and a relatively small number of pore pathways provided conduits for the oil. For reasons yet to be understood, the small pore pathways appeared largely unavailable for water flow.
Networking Intelligent Subsea Completions Using Industrial Standards
Authors: Hiron Stéphane
Published: December 2007

Intelligent completions are expected to play a growing role in all future major subsea fields, giving the capability to maximize hydrocarbon recovery while reducing production costs and number of interventions. In the present mature "pressure monitoring" market, single-point pressure sensors are traditionally acquired by third-party subsea control pods. There is no dedicated software associated with the tool at topside for any computation more complicated than applying polynomial equations. The communication protocols over umbilicals are generally proprietary to each pod vendor, which requires a new interface card for each application, increasing risks and lead time by months, and affecting system reliability by lack of repetitiveness. Intelligent completions require typically 10 times more communication bandwidth than currently available, 10 to 100 times more power than pressure gauges, dedicated software processes at topside, and increased reliability of the interface links. Traditional practices need to be revisited for accommodating such new requirements.

We propose a modern networking approach for intelligent subsea completions using common practices of the process control and information technology (IT) industries. Downhole sensing and control would operate on fieldbus-like protocols, widely used in automated plants and petroleum refineries. For interfacing intelligent completions from the seabed to topside supervision, our approach uses transmission control protocol running on top of internet protocol (TCP/IP). These communication layers (Levels 3 and 4) in the open systems interconnection (OSI) model bring full flexibility for dynamic device configuration and reconfiguration, data file handling capability, compact and modular software interfaces at topside, and flexible interface with various vendors. This initiative is now endorsed by the Intelligent Wells Interface Standardization (IWIS) panel1, which is now drafting an International Standard Organization (ISO) standard for the industry.

For subsea operators, the networking approach using industry standards offers a seamless integration between topside systems and various downhole vendors, and opens future options of object-oriented protocols among all topside processes. Seamless integration reduces interface costs and decreases software interface developments. The overall system reliability will increase, thanks to standards and mature equipment widely supported by all the telecom and computer industries.
An Experimental Study on Mechanics of Wax Removal in Pipeline
Authors: Wang Qian , Sarica Cem , Chen Tom X.
Published: December 2007

Pigging has been recognized as the most cost-effective method for preventing flow restriction by wax deposits in subsea flowlines. However, the piping mechanics for wax removal in pipelines is still very poorly understood.

A unique test facility was designed and constructed for experimental studies on the mechanics of wax removal in pipelines. The test facility consisted of a test section, a support structure, an apparatus to pull the pig through the test pipe, and a computer-based data acquisition system. The test section was 21-ft long and was made from 3-in. inner diameter schedule-40 steel pipe. The mixure of commercial wax and mineral oil was casted inside the test section at different wax thicknesses and oil contents. A series of experiments were performed to investigate the wax removal mechanics with three different types of conventional pigs, i.e., disc, cup and polly pigs.

The experiments showed that a typical wax removal process using a pig followed four distinct phases, namely, wax breaking, plug formation, accumulation and production phases. Wax accumulating can be very significant, and is expected to be the dominating factor for the force required for moving a pig in long pipelines. As wax thickness and hardness increases, the required force to move the pig increases. The shape and material of the pig have a profound effect on the wax removal performance. While the disc pig provides the most efficient wax removal, the force requirement is excessive, especially for thicker and harder wax deposits. The wax removal performance of a cup pig is very similar to that of a disc pig. However, the cup pig can withstand higher load without mechanical damages than the disc pig. The polly pig offers the poorest wax removal performance.
New Analytical Solutions for Predicting Pressure Distribution and Transient Behavior in Wedges and Truncated Wedges
Authors: Rahman N. M. Anisur , Bentsen Ramon G.
Published: December 2007

Reservoirs of various shapes are encountered in practice. Evidence of wedges and truncated wedges has been found in Texas, Louisiana and elsewhere. In most situations, predicting the pressure distribution and transient behavior becomes very difficult due to a lack of necessary mathematical solutions. However, analytical solutions are often used for such purposes, even though it may have been necessary to compromise the geometry of the actual reservoir to obtain a solution. In the literature, analytical solutions for wedges are available. Application of these solutions very often forces the use of an approximate value of the wedge angle instead of the actual value. As a result, a considerable amount of error may be introduced into the predicted pressure responses. Moreover, no solutions for truncated wedges have been reported in the literature.

In this study, an integral-transform technique (ITT) has been used to develop new analytical solutions that overcome the limitations of the existing solutions in the literature.
Forward-Inverse Adaptive Techniques for Reservoir Characterization and Simulation Theory and Applications
Authors: Doss S. K. , Ezzedine S. , Gelinas R. J. , Ziagos J. P. , Chawathé A.
Published: December 2007

A novel approach called Forward-Inverse Adaptive Techniques (FIAT) for reservoir characterization is developed and applied to three representative exploration cases. Inverse modeling refers to the determination of the entire reservoir permeability under steady state single-phase flow regime, given only field permeability, pressure and production well measurements. FIAT solves the forward and inverse partial differential equations (PDEs) simultaneously by adding a regularization term and filtering pressure gradients. An implicit adaptive-grid, Galerkin, numerical scheme is used to numerically solve the set of PDEs subject to pressure and permeability boundary conditions. Three examples are presented. Results from all three cases demonstrate attainable and reasonably accurate solutions and, more importantly, provide insights into the consequences of data undersampling.
Fast and Accurate Correlations to Compute the Perforation Pseudoskin and Productivity of Horizontal Wells
Authors: Tang Yula , Ozkan Erdal , Kelkar Mohan , Yildiz Turhan
Published: December 2007

This paper provides fast and accurate correlations to compute the perforation pseudoskin and productivity of horizontal wells. Perforation pseudoskin is defined as the difference between the dimensionless pressures of the perforated and openhole completed horizontal wells. Simple formulas are presented to relate the perforation pseudoskin to the productivity ratio (PR) of the perforated horizontal wells. The productivity ratio, PR, is the relation between the productivities of the perforated and openhole completed horizontal wells. A semi-analytical model has been used to develop the PR correlations. It has been found that the relative error of the correlations is less than 6%. The PR correlations can be used as a quick tool to optimize the perforated completion of horizontal wells. Also using the relation between the pseudoskin for inclined perforations (non-zero phasing angle) and PR, the PR correlations provide a fast means of obtaining the pseudoskin for inclined perforations. These pseudoskin terms can be used to model perforated horizontal wells in semi-analytical and numerical reservoir simulators.
Laboratory Experiments Provide New Insights into Underbalanced Perforating
Authors: Walton Ian C. , Johnson Ashley B. , Behrmann Larry A. , Atwood David C.
Published: December 2007

Perforating underbalanced has become the primary means of removing perforation damage and maximizing productivity through a cased and perforated completion. The level of static underbalance (wellbore to pore pressure difference prior to perforating), considered the critical parameter in achieving acceptable productivity, has been investigated theoretically and experimentally through testing on outcrop and reservoir cores under laboratory conditions.

We now report on a new series of laboratory experiments conducted on outcrop cores under more representative downhole conditions. Investigation of previously neglected parameters produced profound differences in perforation productivity. In some circumstances static overbalanced perforating proved to be as effective as static underbalanced perforating. The surprising results on underbalanced and overbalanced tests suggest a new approach to minimize perforating damage. Not only do these results impact our understanding of the mechanism of perforation damage removal, but they also have important implications for future design of perforating jobs.
Hydraulic Fracturing for Reservoir Management: Production Enhancement, Scale Control and Asphaltine Prevention
Authors: Norris M. R. , Gulrajani S. N. , Mathur A. K. , Price J. , May D.
Published: December 2007

Productivity in the Scott reservoir, located in the Central North Sea, is subject to impairment by scale and asphaltine disposition in the near wellbore region. Several intervention options have been attempted to restore the production potential (ie. re-perforation, fractured re-perforation, scale dissolution). These techniques have proved largely unsuccessful since due to the severity of the damage mechanism. This left hydraulic fracturing as the final option to bypass the damage and reconnect the wellbore to the productive sands.

A technical and economic success was particularly desirable given the high operating costs associated with offshore operations and was achieved through a combination of well analysis and novel enabling technologies which are described in detail within this paper. The target well identified (J9) was selected based on criteria of production potential, drainage, pressure support and wellbore access. The fracture orientation plane was identified using existing open hole imaging logs allowing perforations to be oriented along the preferred fracture azimuth to minimize near wellbore effects during proppant placement and production. Specialized scale inhibitor impregnated proppant was employed to ensure deep and long term productivity protection for the fracture and tubulars. Placing a fracture in the reservoir also assisting in mitigating the effect of asphaltine disposition within the formation matrix. The large production string necessitated a new approach to lifting out the excess proppant within the wellbore. This was accomplished by reverse flow up a coiled tubing string in a process which was engineered to be a safe and effective.
Kinetic Rheology of Hydraulic Fracturing Fluids
Authors: Walters Harold G. , Morgan Ronnie G. , Harris Phillip C.
Published: December 2007

The rapid advancement in computing power is helping the development of more advanced fracturing models. These advancements include expanded dimensional capabilities as well as improvements in handling more complex rock and fluid mechanics models. Rheology of the fracturing fluid is fundamental for predicting fracture growth and geometry. Most hydraulic fracture models assume a steady-state power law rheology model with only an implied temperature dependence and no shear history dependence. Newer models have attempted to model shear history effects, but still do not accommodate the thermal kinetics of crosslinking and breaking chemistry. A definite need exists for more comprehensive rheological models for crosslinked fracture fluids.

This work entails development of theory and experimental confirmation of a comprehensive rheological model of a complex fracturing gel prepared with a guar derivative and dual crosslinkers. The Kinetic Rheological Modeling approach uniquely integrates the nonlinear kinetics of crosslinker chemistry with the in-situ changes in fluid rheology. The model accurately predicts the effects of shear rate, shear history, temperature, temperature-time history, pH, gel loading, crosslinker concentration, and breaker concentration on the apparent viscosity function. The model predicts the early noncrosslinked pseudoplastic behavior of the linear gel fluid, and then accurately predicts the rapid increase in apparent viscosity caused by the thermal and shear induced kinetic crosslink reaction. The model also accurately predicts thermal thinning at higher temperatures and the exponential decline in apparent viscosity that occurs when the breaker chemistry begins to dominate.

The model enables researchers to develop more comprehensive fracturing models, and the kinetic theory serves as an investigative tool for determining the effect of fluid chemistry and pumping history on the rheology of new and existing fracturing fluids.
A Rheological Criterion for Fracturing Fluids to Transport Proppant during a Stimulation Treatment
Authors: Goel Naval , Shah Subhash
Published: December 2007

One of the primary functions of a fracturing fluid is to transport proppant through a fracture and to keep the particles suspended till reservoir closes on the fracture. The proppant when suspended and uniformly distributed in the fracture improves the productivity of the reservoir. Therefore, a question is frequently asked regarding the minimum fluid viscosity required for satisfactory proppant transport through a fracture. The knowledge of this viscosity value is useful for evaluating fluids prior to their use in the stimulation treatment, and for demonstrating the effectiveness of new fluid formulations for fracturing applications. The present study addresses this question.

The study was performed by characterizing crosslinked guar gels for their viscoelastic properties. Then, the gels were evaluated for their proppant transport capabilities in two slot models using a field-scale test facility. The result shows that the proppant transport capability of crosslinked gels correlates with the fluid's elasticity, not with the fluid's viscosity. Based on these results, this investigation proposes a minimum elastic modulus criterion required for a satisfactory performance of the crosslinked fluids in a fracturing treatment. The criterion was developed from characterizations of borate-crosslinked gels, and then validated with zirconate-crosslinked gel.

The criterion proposed in this work was developed from the proppant transport behavior evaluated on a large-scale test facility. Thus, this criterion provides a useful and simple method to evaluate fracturing fluids in a laboratory, without the need for extensive fluid testing.
Mudcake/Screen Cleanup Investigation in Openhole Gravel-Packed Completions
Authors: Asadi Mahmoud , Conway Michael W.
Published: December 2007

A large-scale wellbore model is used to evaluate the effect of mudcake on screen plugging and to study various mudcake and screen cleanup methods such as acid spotting, enzyme breaker, and oxidizer. A 6 ft long wellbore with 4 in. diameter was formed at the middle of an unconsolidated formation housed in the model. Mudcake was built onto the face of the unconsolidated formation by having a sized linear log 2% KCl based carbonate mud in the wellbore for 24 hrs at 160°F and at 150 psi. Carbonate mud was then drained from the wellbore and a screen was placed into the wellbore. The effect of mudcake on screen plugging was then evaluated with and without the presence of gravelpack around the screen by measuring the pressure drop across the screen during a production process. The model was fully instrumented and included pressure sensors, a densiometer and flow meter to assist in electronically recording data. Based on the pressure drop measurements, the degree of screen plugging was measured as a result of mudcake invasion into the gravelpack and screen. Finally, various cleanup methods were applied to the system containing unconsolidated formation, mudcake, gravelpack, and screen to evaluate mudcake and screen cleanup.
Laboratory Evaluation of In-Situ Gelled Acids for Carbonate Reservoirs
Authors: Taylor K. C. , Nasr-El-Din H. A.
Published: December 2007

Three different commercial formulations (A,B,C) of in-situ gelled acids are compared in detail at temperatures up to 150 °F and at acid concentrations from 5 to 20 wt% HCl. In-situ gelled acids are claimed to work by a gelation mechanism that occurs at the rock surface as the acid is neutralized. These acids contain a polymer, a crosslinker, and a breaker in addition to other additives. Detailed viscosity measurements of each in-situ gelled acid were made as a function of pH. A new experimental procedure was developed to partially neutralize the in-situ gelled acid, then the viscosity was measured as a function of shear rate in the range 1 to 3000 s-1. Relative reaction rates with reservoir rock of the three in-situ gelled acids were compared at 100 °F. Coreflood experiments were conducted with small acid volumes so that injectivity could be measured before acid breakthrough occurred.

In-situ gelled acids significantly retard the rate of acid reaction with reservoir rock. Results showed that acids B and C had similar behavior, while acid A was more reactive with reservoir rock. Viscosity measurements of partially neutralized in-situ gelled acids showed that only with acid C was a significant viscosity maximum observed as the pH of the acid increased. This pH maximum occurred at a value of approximately 2.2 with 5 wt% HCl at 100 °F. In the absence of breaker and crosslinker, acids A and B were very similar, while acid C showed as little as a tenth of the viscosity at low shear rates. Coreflood experiments showed that in some cases, the in-situ gelled acids irreversibly reduced the permeability of reservoir rock.
Geomechanical Analysis and Decision Analysis for Mitigating Compaction Related Casing Damage
Authors: Bruno Michael S.
Published: December 2007

Reservoir compaction and associated bedding plane slip and overburden shear has induced damage to hundreds of wells in oil and gas fields throughout the world. Critical casing damage mechanisms observed in a variety of structural settings include: 1) overburden shear damage on localized horizontal planes; 2) shearing at the top of production and injection intervals; and 3) compression and buckling damage within the production interval primarily around perforations.

Analytical solutions are readily available to estimate compaction, subsidence, and casing damage risks. These should be applied as initial screening tools at an early stage in reservoir development planning. They can also be applied to estimate relative risks for various well locations and trajectories.

Geomechanical models of increasing complexity, including two-dimensional and three dimensional finite element type techniques have been used with good success to assess formation deformation and casing damage risks in several reservoirs, and are described herein. Three dimensional geomechanical models at the wellbore scale are required to evaluate shearing deformation on specific well designs, and are used to assess damage mitigation effectiveness for varying completion strategies.

An economic decision tree model is applied to compare the costs and benefits of alternative well designs, while taking into account inherent uncertainties in model input data, well damage location, and the effectiveness of various mitigation strategies. In some instances the appropriate action is not to change completion design and simply accept damage risk.
Direct Measurement of Formation Resistivity through Steel Casing Solves a North Sea Production Question
Authors: Ferguson Roy , White Jim , Kadi Antoine El
Published: December 2007

Almost every oilfield well is logged with a resistivity tool prior to setting casing. This measurement enables interpreters to locate hydrocarbon and estimate water saturation. Once steel casing is run, conventional resistivity tools no longer function, so any estimate of formation saturation has then needed a neutron based measurement, for example formation sigma or carbon/oxygen ratio.

This situation has now changed. A new generation of resistivity logging tools is now available that can make formation resistivity measurements in cased holes. Such a measurement was recently successfully run in a key well in a major North Sea field.

In this well there was concern about near-wellbore damage affecting production. Sigma measurements were already available, but these shallow readings could not confirm if the oil saturations were truly representative of saturations far from the wellbore. This question could only be resolved by a deep reading saturation tool.

The Cased Hole Formation Resistivity Tool was therefore run in this key well, along with sigma tools to allow comparison with earlier sigma logs. The wellsite cased hole formation resistivity curve highlighted the areas that were water processed or unswept, however comparison of the shallow and deep saturations showed that the true situation was more complex than originally thought. This paper will review the results of this study and the impact of this new technology on the future life of the field.
Determination of Fit-For-Purpose Saturation Exponents
Authors: Worthington Paul F.
Published: December 2007

Algorithms that are commonly used for the evaluation of water saturation can require different input values of saturation exponent for meaningful application to the same reservoir rock. Further, the differences in the required saturation exponent can vary according to the reservoir properties themselves. By taking account of these different input requirements, it is shown that the discrepancies between predicted water saturations obtained using dissimilar petrophysical algorithms can be confined to a restricted range. The procedure is synthesized for easy operational application of an ordered, interactive method that allows a fit-for-purpose saturation exponent to be identified. Through this process, the sensitivity associated with the choice of an interpretative model for the evaluation of water saturation is demonstrably contained. The application of the method therefore increases confidence in petrophysical interpretation by limiting uncertainty.
Flow Diagnosis and Production Evaluation in High Flowrate Oil-Water Producers Using Optical-Fibre Holdup Sensors
Authors: Jackson R. R. , Ayan C. , Wakefield J.
Published: December 2007

In the North Sea (UKCS), Amerada Hess Ltd. is the operator of several oil fields which are developed using subsea technology (Fife, Hudson, Telford etc) and with a fixed platform in the case of the Scott field. In almost all wells, production is above the bubble-point pressure, at a wide range of water-cuts in multilayered reservoir systems. Many of the wells have monobore completions with 5″ production tubing and surface flowrates in excess of 15000 stb/d and up to 35000 stb/d. The downhole environment of these wells which comprises of high-velocity multiphase flow, has presented significant problems for conventional production logging sensors and analysis methods to evaluate. Friction and jetting effects on gradiomanometer type tools and small bubble sizes, which can not be detected by electrical probes are the well known complications.

To obtain reliable stabilised flow and transient data for diagnosis and production evaluation, a hold-up sensor based on optical principles has been applied. This sensor was recently introduced as a gas hold-up measuring tool. The application described here is the first use of this principle for measuring oil-water hold-up in high flow velocity wells.

Flow profile data and interpretation results from several wells are presented. Sensor performance was evaluated by comparing conventional and new generation sensors, including electrical and optical probes. In all cases, the optical probes gave reliable and consistent water holdup values, which were used for the interpretation of the data. Decisions involving water shutoff operations were based on the analysis of these results. We also review pressure and flow transient data recorded during the operations, which has the potential for providing skin factors and reservoir parameters for individual zones in a layered reservoir system.
Improvements in a Through-Casing Pulsed-Neutron Density Log
Authors: Odom Richard C. , Tiller Don E. , Wilson Robert D.
Published: December 2007

The practice of measuring the formation density with a standard pulsed-neutron system has been described in the literature1,2,3,4,5, and has been successfully applied in many reservoirs. This success leads to the obvious question: Can a system be developed that is optimized for this measurement?

To understand which parameters should be measured, characterized, or require improved response, a more exact analytical model is required. The pulsed-neutron density measurement is based on the transport of gamma rays created by inelastic scattering of fast neutrons. The current density algorithm uses a diffusion model for the gamma ray transport process. This simple model does not include the effect of variations in fast neutron parameters on the process of gamma ray production. As a result, an ambiguity is introduced in the density log that is associated with the dynamic nature of the gamma source. The initial distribution of gamma rays is a function of atom density and hydrogen content and must be properly described as a first step toward a more exact transport model.

The pulsed-neutron density measurement is analyzed and optimization is achieved using theoretical models, responses from laboratory formations and test wells, and through computer modeling. Along with data supporting an improved analytical model of the measurement, data is shown for incorporating fast neutron detectors and for optimizing spacings of the gamma ray detectors. As a result of this study, a prototypical detector array is described for improved density response.
Using Pseudocases to Interpret P10 for Reserves, NPV, and Production Forecasts
Authors: Murtha James A
Published: December 2007

This article is intended to promote a discussion about how one reduces a lot of data to a few representative values while recognizing what gets preserved and what might be lost. Specifically, the phrase, "P10 production forecast" is shown to be ambiguous. By acknowledging this ambiguity and asking the intended use of the values, we examine some possible definitions and illustrate them with examples. The same scrutiny is then applied to P10 values for reserves and NPV, which are presumed to be unambiguous. Although raising more issues than it answers our goal is to open or expand the dialog and to suggest some alternate procedures. Managers as well as analysts need to understand the ambiguity presented below so they can ask better questions and make more informed decisions.