bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of a Unique Fluorosilene–KF Complex Bearing Bulky Substituents
Authors: Mieda Eiko , Sasamori Takahiro , Sase Shohei , Goto Kei , Tokitoh Norihiro
Published: January 2011

We report here the attempted synthesis of a stable 1-fluoro-2-hydrosilene bearing a Bpq group, which is a bulky aryl group bearing a cavity-shaped framework. We have successfully synthesized an overcrowded benzyldifluorosilane as a precursor. Treatment of the benzyldifluorosilane with 2 equiv of t-BuOK in THF afforded the corresponding benzyl anion, the structure of which showed a unique feature as a fluorosilene–KF complex.
Mechanical Reinforcement of Supramolecular Hydrogel through Incorporation of Multiple Noncovalent Interactions
Authors: Komatsu Harunobu , Ikeda Masato , Hamachi Itaru
Published: January 2011

Supramolecular hydrogel, comprising a three-dimensional network of self-assembled nanofibers, has recently emerged as a unique material for a variety of potential applications such as tissue engineering. One serious drawback of supramolecular hydrogels that sometimes limits their practical usage is their mechanical weakness compared with conventional polymer gels. Here we describe a stiff supramolecular hydrogel made of a zwitterionic amino acid-tethered amphiphilic molecule that self-assembles into nanofibers through multiple and orthogonal noncovalent interactions.
Sperm preservation in children: a prescription for consistency with both child protection and the criminal law
Authors: Wheeler Robert , Kohler Jan , Paget Julia , Major Jessica
Published: January 2011

People under the age of 18 with cancer sometimes require treatments that render them infertile. It may therefore be necessary, during the brief interval between diagnosis and the commencement of cancer treatment, to offer them the opportunity to produce sperm, through masturbation, for cryopreservation.

The article provides a framework for justifying this as a clinical activity, and avoiding it being construed as a sexual activity under the terms of the criminal law. Of equal importance, the article puts into place measures to ensure that this clinical activity is entirely consistent with the safeguarding principles of child protection.
The management of stress urinary incontinence in women
Authors: Shaxted Edward
Published: January 2011

Stress urinary incontinence is a common, but frequently unreported, problem for women. This article explores the management and treatment options that are available.
Consent to antipsychotic drugs and tardive dyskinesia after Chester v Afshar
Authors: Joyce Joseph P
Published: January 2011

In general, doctors must obtain a patient's consent to treatment before delivering the treatment. Most of the legal cases claiming negligence in treatment relate to surgical procedures but the same principles hold true for non-invasive or medical treatments. Some medical treatments, such as antipsychotic drugs, may have long-term side-effects. A doctor may have to pay compensation to a patient who suffers adverse effects from treatment which was delivered negligently or if the doctor has not given sufficient information to the patient who then suffers the adverse event of which they were not warned was a possibility.
Expert witness immunity: the end of the road?
Authors: Passmore Colin
Published: January 2011

Anyone reading this article who practises as an expert witness, whether in the criminal or the civil courts, and no matter in what area of expertise, needs to keep an eye on the outcome of a case that is being heard on 11 January – before the Supreme Court (the successor court to the House of Lords) called Jones v Kaney. It may be an exaggeration to say that its likely importance is demonstrated by the fact it was featured on the Today programme on 25 November 2010, but nonetheless the Supreme Court's decision, whichever way it goes, will end once and for all any debate around the question whether expert witnesses should be immune from negligence actions in respect of their court work. As to this, the ‘smart money’ is, I fear, on the Supreme Court bringing to an end this somewhat anomalous immunity. This article looks at the nature of the expert's immunity, the facts of Jones v Kaney and assesses what it might be mean for experts in future if their immunity is indeed removed in 2011.
Down's syndrome: screening and antenatal diagnosis regionally in England and Wales 1989–2008
Authors: De Souza E , Alberman E , Morris J K
Published: January 2011

Objective

To illustrate regional changes that occurred in screening for Down's syndrome (trisomy 21) in England and Wales from 1989 to 2008.

Methods

The National Down Syndrome Cytogenetic Register has collected data on all ante- and postnatal diagnoses of Down's syndrome in England and Wales since 1989 ( n = 27,954). The percentages of (i) diagnoses made antenatally, (ii) antenatal diagnoses that had nuchal translucency (NT) measured, and (iii) antenatal diagnoses in mothers aged 37 and over with advanced maternal age as the sole recorded indication for diagnosis are presented according to where the mother lived (Government Office Region), year of diagnosis (1989–1994, 1995–2000, 2001–2006, 2007–2008), and maternal age (<37 years, ≥37 years).

Results

The percentage of cases diagnosed antenatally has increased in younger women but varies between regions. It remained relatively constant at approximately 70% in older women. The use of NT measurement in antenatal screening has expanded rapidly but varies regionally, being most common in London and the South East where, in 2007–2008, over 75% of antenatal diagnoses in older women had NT measured. The sole indication of advanced maternal age has substantially reduced, and was less than 10% in older mothers in all regions in 2007–2008.

Conclusions

There are regional and maternal age variations in Down's syndrome screening and diagnosis. Some regions used NT measurements, and eliminated advanced maternal age as sole reason for antenatal diagnostic testing more quickly than others. The reasons for variations need to be identified and addressed to ensure that when new screening techniques become available, regional differences are minimized.
Disease-specific mortality may underestimate the total effect of prostate cancer screening
Authors: van Leeuwen Pim J , Kranse Ries , Hakulinen Timo , Roobol Monique J , de Koning Harry J , Bangma Chris H , Schröder Fritz H
Published: January 2011

Objectives

To study the difference between the disease-specific and excess mortality rate in the European Randomized Study of Screening for Prostate Cancer section Rotterdam.

Methods

A total of 42,376 men were randomized to systematic screening or usual care. The excess number of deaths was defined as the difference between the observed number of deaths in the prostate cancer (PC) patients and the expected number of deaths up to 31 December 2006. The expected number was derived from mortality of all study participants before a possible diagnosis with PC. The disease-specific mortality rate was based on the number of men who died from PC. The excess mortality rate based on the arm-specific excess number of deaths and the disease-specific mortality rate were compared between the two study arms.

Results

The overall mortality rate was not significantly different between the intervention and the control arms of the study: RR 1.02 (95% CI 0.98–1.07). The disease-specific mortality rate was 0.42 men per 1000 person-years in the intervention and 0.48 men per 1000 person-years in the control arm: RR 0.86 (95% CI 0.64–1.17). The excess mortality rate was 0.40 per 1000 person-years in the intervention arm and 0.61 men per 1000 person-years in the control arm, and the RR for excess mortality was 0.66 (95% CI 0.39–1.13).

Conclusions

In contrast to the disease-specific mortality rates an increased difference in the excess mortality rates was observed between the two arms. This observation may be due to a systematic underestimation of the disease-specific deaths, and/or an additional disease-related mortality that is measured by an excess mortality analysis but not by a disease-specific mortality.
Age-specific frequency of screen-detected hepatitis C virus seropositivity in men from the Punjab province of Pakistan
Authors: Hyder Omar , Ijaz Mohsin , Arshad Muhammad Ateeq , Zahira Tahseen
Published: January 2011

Pakistan is a South Asian country with a high population seroprevalence of anti-hepatitis C virus (anti-HCV) antibody. In administrative districts of its most populous province, Punjab, the frequency of seropositivity for HCV in different age groups of adult men, the usual bread winners for their families, is not known. This study describes the frequency and age-distribution of seropositivity for anti-HCV antibodies in 58,680 clinically healthy, adult men, from nine districts of the Punjab province in Pakistan, undergoing pre-employment health screening. Laboratory records belonging to these men, screened by enzyme-linked immunosorbent assay 3 for antiHCV antibodies in serum, were analysed and frequencies were calculated. A total of 4034 (6.9%) were seropositive for HCV. The highest frequency of HCV seropositivity was noted in men from Multan (9.6%), followed by Lahore (9.4%) district. The lowest (5.0%) was noted among men residing in Bahawalpur district. There was a progressive increase in seroprevalence with increasing age in men from all districts. Further research needs to be initiated to describe differential HCV risk factor distribution in various districts and sustainability of relatively low seroprevalence noted in younger men.
Moisture Measurements and Condensation Potential in Wood Frame Walls in a Hot-Humid Climate
Authors: Weston Theresa A. , Minnich Liza C.
Published: January 2011

It has long been noted that interior vapor barriers in wood frame walls in hot-humid climates can lead to interstitial condensation within walls. The bases for this recognition are predictive simulations, anecdotal observations, and a limited number of experimental studies. This paper describes an experimental study conducted in a hot-humid climate that investigated the influence of an interior vapor retarder and compares observed performance with simulation predictions. The wall performance data reviewed here was gathered as part of a larger test program evaluating the performance of a range of typical wood frame, residential wall constructions in a hot-humid climate. The approach chosen was to use real-time field exposure using a “test hut” located in Tampa, Florida. The test hut had two long sides, which provided the ability to test 16 wall specimens each. Wall specimens were instrumented with a variety of temperature, humidity, and moisture sensors. In addition to natural weather exposure, the wall specimens could be manually wetted by a water injection system to simulate rain leakage. More specifically, this paper focuses on using the data collected before and after the installation of an interior vapor barrier (vinyl wallpaper) to show the change in moisture loading and the potential condensation within the walls resulting from the installation. The field data is compared with predictions of the wall behavior using a commonly available hygrothermal model. There is increasing reliance on the use of predictive models to assess the moisture performance of building assembly designs. These predictive models need to be validated against real data to test their variance from real systems.
Gradivus földművesei (Valerius Flaccus 5, 142: ruricolae, Gradive, tui)
Authors: Nagyillés János
Published: January 2011

Valerius Flaccus Argonauticája 5, 142 ruricolae, Gradive, tui kifejezésének értelmezése vitatott: az apollóniosi eredeti mögött valószínűleg egy a görög költő által még ismert, a vasérc mosására vonatkozó forrás állt. Apollónios Rhodios Chalybsökről adott, általa még szó szerint értett információit Valerius Flaccus fordítása a vaskorhoz kapcsolódó római képzetek jegyében romanizálva, metaforikus értelemben adja vissza.
Revisiting Pressure Transient Testing of Hydraulically Fractured Wells: A Single, Simple and Exact Analytical Solution Covering Bilinear, Linear, and Transition in between Flow Regimes
Authors: Lietard Olivier , Spivey John P.
Published: January 2011

Quite a lot has been published about pressure transient testing of hydraulically fractured wells since the pioneering work of Heber Cinco-Ley in 1976. A common approach of the problem since 1981 has been to solve the ordinary differential equations (ODEs) of rate and pressure versus time in the Laplace domain and to invert the Laplace solution back into the time domain. This method has led to exact asymptotic solutions for the so-called early time bilinear and formation linear flow regimes. Unfortunately, the inversion of the general Laplace solution was shown to be too complex. Therefore, the pressure and pressure derivative type curves of the transition period between the bilinear and linear regimes have always been numerically generated, most often by making use of Stehfest's inversion algorithm, sometimes by discretization methods in the time domain.

This paper shows that a single, simple and exact analytical solution of the ODEs can be obtained directly in the time domain, providing that the flow within the fracture is assumed to be incompressible (no fracture storage effects). This general solution has two asymptotes for early and late times that agree perfectly with the previously published solutions for bilinear and formation linear flow regimes.

The contribution of the present paper is that it provides exact pressure and pressure derivative type curves throughout the long (six log cycles) transition period between the bilinear and linear asymptotes. It therefore allows for rigorous definitions of transition times, which had been so far numerically approximated. In addition, the proposed solution handles without any difficulty the presence of skin at fracture walls (fracture face skin).
A General Correlation for Proppant Settling in VES Fluids
Authors: Malhotra Sahil , Sharma Mukul M.
Published: January 2011

Polymer-free viscoelastic surfactant-based (VES) fluid systems are used to minimize damage to the proppant pack and to efficiently transport proppants into fractures. Proper selection of proppants and fracturing fluids for maximum propped fracture length requires reliable data and correlations for the impact of fracture fluid viscoelasticity and the effect of fracture walls on proppant settling. Current models and correlations neglect the important influence of fluid elasticity on proppant transport. This paper presents an experimental study that investigates the impact of fluid elasticity and fracture width and presents a general correlation for proppant settling in VES fluids.

Proppant settling experiments are performed in shear thinning VES fluids. Experimental data is presented to show that fluid elasticity plays an important role in controlling the settling rate of proppants. Increasing fluid elasticity can either increase or decrease the settling velocity depending on the rheological properties of the fluid and the properties of the proppants. A new experimental correlation is presented to quantify the settling velocity of proppants in VES fluids as a function of the fluid rheology and proppant size. It is shown that the VES fluids should be designed such that the relaxation time is greater than the critical relaxation time (Tcrit).

The productivity of fractured wells depends strongly on proper placement of proppants in the fracture. Experimental data/correlations are presented for the first time to show that the settling velocity of proppants is significantly impacted by the fracture width and in VES fluids this dependence is different than for the non-elastic fluids. Data is presented to show that settling velocity is reduced as proppant size becomes comparable to the fracture width. Results show that elasticity reduces the retardation effect caused by fracture walls. An experimental correlation to quantify the retardation effect due to fracture walls is presented. Proposed correlations highlight the advantages and limitations of using VES fluids for efficient proppant transport. These correlations can be directly used in fracture simulators for proppant selection and for the design of fracturing fluids.
Multi-Phase Fluid-Loss Properties and Return Permeability of Energized Fracturing Fluids
Authors: Ribeiro Lionel H. , Sharma Mukul M.
Published: January 2011

With the growing interest in low-permeability gas plays, foam fracturing fluids are now well established as a viable alternative to traditional fracturing fluids. Present practices in energized fracturing treatments remain nonetheless rudimentary in comparison to other fracturing fluid technologies because of our limited understanding of multi-phase fluid- loss and phase behavior that occurs in these complex fluids. This paper assesses the fluid-loss benefits introduced by energizing the fracturing fluid to estimate when and why energized fluids may be suitable.

A new laboratory setup has been specifically designed and built for measuring the leak-off rates for both gas and liquid phases under dynamic fluid-loss conditions. This paper provides experimental leak-off results for linear guar gels and for N2- guar foam-based fracturing fluids under a wide range of fracturing conditions. In particular, the effects of the pressure drop, the rock permeability, the fluid shear rate, the polymer concentration, and the foam quality are investigated. Analysis of dynamic leak-off results provide an understanding of the complex mechanisms of viscous invasion and filter-cake formation occurring at the pore-scale.

This study presents data supporting the superior fluid-loss behavior of foams, which exhibit minor liquid invasion and limited damage. It also shows direct measurements of the ability of the gas component to leak-off into the invaded zone, thereby increasing the gas saturation around the fracture and enhancing the gas productivity during flowback. The conclusions not only confirm but add to the findings of McGowen & Vitthal (1996) for linear gels and the findings of Harris (1985) for nitrogen foams.
Better Injectivity Through Science: Case Histories of Stimulating Water Disposal Well Efficiency
Authors: Durey Daniel , Bell Charles , Weiss Stephanie
Published: January 2011

One downside of prolific oil and gas production is the often associated increase in produced water volumes. As hydrocarbon prices and production increase, so do water production and the need for efficient water disposal. This need plus increasing regulatory scrutiny and operators’ environmental stewardship goals have compelled the search for economic methods of safely maximizing - and maintaining - disposal well efficiency.

This paper will relate case histories of several salt-water disposal wells in northwestern Louisiana, in which the conventional remediation solution, matrix acid treatments, did not effectively remove near-wellbore damage. Instead, precision bypass fracs were designed to maximize injectivity without fracturing rock beyond the narrow zonal limits set by state regulators. The strict regulations required a precise, scientific approach to stimulation design and operations, including the use of fracture modeling, on-site diagnostics and radioactive tracers.

The paper will present regulatory issues, stimulation design process, operational highlights and post-stimulation results for some representative disposal wells. It will also describe improvements made at producing well locations to minimize future stimulation requirements. Finally, it will conclude with some recommended best practices for economical, safe, effective bypass frac design and operations in water disposal wells.
Hydraulic Fracture Stimulation Design Considerations and Production Analysis
Authors: Stegent Neil , Ingram Stephen , Callard Jeff
Published: January 2011

When the word "shale" is included in the name of a formation, a high-rate water frac is often instantly assumed to be the correct choice for hydraulic fracture treatment. This is because high-rate water fracs are the common treatment in the Barnett shale in northeast Texas, and that is the shale reservoir with which industry professionals are most familiar. When a new shale development, such as the Eagle Ford shale in south Texas, is discovered, engineering tends to take a back seat, and a high-rate water frac is often the initial treatment design chosen simply because that is what has been done in the Barnett. Reservoir understanding acquired from core analysis, geomechanical tests, formation evaluation, proppant-embedment testing, stress analysis, and other data are sometimes ignored in the design process.

This paper discusses the thought process that engineers are encouraged to follow when deciding what type of completion should be used to fracture stimulate a shale reservoir (water frac, hybrid, or conventional). Considerations, such as the type of hydrocarbon that is expected to be produced, fracture complexity of the reservoir, lithology and mineralogy of the rock, and other reservoir parameters, should be included as part of the completion design. To close the loop, the production also should be evaluated to make good engineering-design changes as the asset is developed. Quantifiable analysis of a stimulation treatment in a horizontal well can be a daunting task because of the complexity and lack of data. Production analyses for several stimulated wells are presented. This application can be directed toward any horizontal completion in low/ultra-low permeability reservoirs and can be performed in a timely manner. The immediate goal of this process is to determine a qualitative measurement of stimulation effectiveness; the eventual goal is a quantitative tool.
Hydraulic Fracture Crossing Natural Fracture at Non-orthogonal Angles, A Criterion, Its Validation and Applications
Authors: Gu H. , Weng X. , Lund J. , Mack M. , Ganguly U. , Suarez-Rivera R.
Published: January 2011

Hydraulic fracturing treatments have become an indispensable part of well completion in shale gas field development. Shale formations often contain natural fractures and complex hydraulic fracture networks may form during a treatment. The complex fracture network is strongly influenced by the interaction between the hydraulic fracture and the pre-existing natural fractures. A criterion has been developed to determine whether a fracture crosses a frictional interface (pre-existing fracture) at non-orthogonal angles. This criterion is an extension of the one for orthogonal crossing originally developed by Renshaw and Pollard (1995). The dependence of crossing on the intersection angle is shown quantitatively using the extended criterion. The fracture is more likely to turn and propagate along the interface than to cross it when the angle is less than 90°. The validation of the criterion using laboratory experiments for various angles is described and discussed. When applied to laboratory experiments, good agreement between the criterion and experiments is observed for a wide range of angles. The criterion can be used to determine whether hydraulic fractures cross natural fractures under particular field conditions, and it has been incorporated in a hydraulic fracture model that simulates hydraulic fracture propagation in a natural fractured formation.
Utilizing Fluid and Proppant Tracer Results to Analyze Multi-Fractured Well Flow Back in Shales: A Framework for Optimizing Fracture Design and Application
Authors: King George E. , Leonard Dick
Published: January 2011

Fluid and proppant tracers and other simple measurements in returning load water flow back can be very useful in helping to describe fracture development in shales; including such parameters as fracture complexity, frac conductivity, height growth, frac barrier effectiveness, well-to-well and frac-to-frac interference, water entry points and general fracturing execution. For most engineers, flow back ion charts have often had little relevance beyond estimating frac stage flow back activity; however, combining fluid tracer information with low-level gamma emitting proppant tracers, microseismic, simple salinity measurements and water return rate can help describe fracture and formation behaviors that lead to faster optimization of fracturing design and application. This paper will use fluid and proppant tracer results from over a hundred shale frac stages in horizontal wells along with other measurements of frac flow back and blend them with microseismic, frac pumping records, production logging and production results to build a framework for better analysis of frac flow back.
Redesigning Fracturing Fluids for Improving Reliability and Well Performance in Horizontal Tight Gas Shale Applications
Authors: Bell Charles E , Brannon Harold D.
Published: January 2011

Fracture stimulation is necessary in tight gas and shale formations to ensure commercial production volumes. Variations of slickwater are the predominant fluid used in shale stimulation because their low viscosity and relatively low costs. These stimulation treatments commonly use ceramic proppants or sand to create, develop and maintain the fracture network connectivity to the wellbore. Low viscosity fluids generate fractures of more narrow width and thereby, increased complexity of the created fracture network, which is thought to provide better stimulation of tight gas shales. However, proppant settling in low-viscosity fluid can reduce the chance of achieving optimal fracture conductivities. Friction reducers allow surface equipment to achieve high pump rates and fluid velocities that can overcome some proppant settling. High proppant concentrations during slickwater treatments tend to create settling and banking problems in surface equipment and in the horizontal wellbore. Conventional crosslink systems have been used to improve placement of the proppant by minimizing proppant settling. However, in addition to potential conductivity damage imparted by the conventional crosslinked fluids, the inherent high viscosity of such systems may significantly reduce the amount of desired fracture complexity created in the tight gas formations.

Recent advances in polymer refining offer the capability of pumping an instant-crosslink system with polymer loadings as low as 8 pptg. This ultra-low polymer crosslink system provides a surface viscosity that maintains effective proppant transport through the surface equipment, perforations, and into the near-wellbore area. Additionally, programmed viscosity degradation converts the fluid to a low viscosity within minutes of introduction to the reservoir to provide a desired complexity to the created fracture network. The low polymer loadings minimize polymer volumes introduced to the formation, and therefore formation damage.

This paper will detail the effects of new system's viscosity on proppant transport through the surface equipment and horizontal wellbore, and the development of a viscosity break profile that can deliver a low viscosity, near- Newtonian fluid that is ideal for developing the intricate fracture system to optimize production from tight gas formations.
Long Term Stability Of Proppants Exposed To Harsh Shale Reservoir Conditions
Authors: LaFollette Randy F. , Carman Paul S.
Published: January 2011

Laboratory experiments under reasonable Haynesville Shale reservoir temperature conditions have shown that an uncoated ceramic proppant and shale samples changed significantly in key physical properties. Proppant tensile strength was reduced significantly and Brinell Hardness of the shale was also reduced significantly from long-term exposure to broken fracturing fluid. The water chemistry of the fluid changed substantially during the entire 6 month time at temperature test period. It was concluded that the time dependency of these changes may have important implications for well drawdown practices for similar reservoirs, fracturing materials, and conditions. SPE 131782 presented an early progress report on these experiments, using single proppant crush, proppant type and a single fracturing fluid chemistry.

The purpose of this paper is to present the next progress report. New experiments have used both low and high-pH make-up water. Tests completed since the first report and including results from both coated and uncoated sand and different ceramic proppants will be reported. Changes in water chemistry from the laboratory experiments have been compared to long-term post-fracturing water chemistry changes in Haynesville Shale wells. Results show good agreement between the chemistries of the produced water and the water sampled from the laboratory tests.
Hydraulic Fracturing of Horizontal Wells: Issues and Insights
Authors: Daneshy Ali
Published: January 2011

The unparalleled spread and variety of completions in use by the industry have clouded the selection of suitable completion and best fracturing strategies for optimizing productivity of hydraulically fractured horizontal wells. There also exists a gap in our understanding of the mechanics of fracture initiation and extension in horizontal wells. This has resulted in uncertainty in our response to large variations observed within and between different fracturing stages in the same horizontal well, and in fact within the reservoir.

This paper is based on review of many horizontal well hydraulic fractures. It includes a discussion of multiple completion options for both open and cased holes. It shows that in open hole completions there is strong tendency for fractures to initially grow axially (longitudinally) before re-orienting to become perpendicular to the least in-situ principal stress. This can cause fracture extension across open-hole packers and possibility of randomly located transverse fractures (well orientation permitting). Infrequent screen-outs usually occur within the open hole section itself. There is evidence indicating that the sealing balls used for separating different fracturing stages may sometimes break during the treatment. This may prohibit fracturing all well segments, and may also interfere with production. In cases of open hole fractures created by hydrajetting techniques one of the issues is propensity to proppant flowback.

In cased hole completions the tendency for axial initiation and early growth still exists. One of the popular techniques is creation of multiple closely-spaced limited entry fractures within each isolated wellbore interval. Treatment data shows that many of these fractures soon join and coalesce towards a single fracture. Furthermore, proppant and fluid have different distributions between various perforations. The issues with techniques based on hydra-jetting include inadequate perforations, hardware problems, and tendency for proppant flowback.
Evaluation of After-Closure Analysis Techniques for Tight and Shale Gas Formations
Authors: Mohamed I. M. , Nasralla R. A. , Sayed M. A. , Marongiu-Porcu M. , Ehlig-Economides C. A.
Published: January 2011

Optimized hydraulic fracture design requires formation permeability as an input, but it is difficult to quantify in tight gas and shale gas reservoirs. After closure analysis (ACA) following a minifrac or fracture calibration test may offer a means to determine the formation permeability in cases for which both a formation test and a conventional pressure buildup test are impractical and/or unable to provide the permeability. However, ACA techniques use a variety of specialized plots, and there is a risk that apparent straight lines may lead to erroneous results.

This paper proposes a technique that provides a simple way to calculate formation permeability, initial reservoir pressure, fracture length, and closure pressure from a single specialized plot. The proposed technique is compared with the G-function method for the estimation of the closure pressure. In addition, it is compared with 3 ACA techniques (Benelkadi, Gu, and G-Function) used in the literature to calculate formation permeability for tight gas and shale gas wells.

Three field examples of pressure fall-off tests (in tight sand gas wells and shale gas well) are analyzed. The results show that the proposed technique provides a clear and rigorous analysis procedure for determination of permeability and other parameters required for the hydraulic fracture design. This proposed technique uses only a single plot comparing to the multiple plots required by the other techniques.
Evaluation of a High-Density Brine Viscosifier for Ultra-Deep Fracture Stimulation
Authors: Gupta D. V. , Carman Paul S.
Published: January 2011

With the oil and gas industry looking ever deeper for hydrocarbons, well control and completion have become important challenges to overcome. Ultra-deep wells usually have pressured or over-pressured formations and require heavyweight brines to control hydrostatic pressures. Completions that once used fresh water or sea water are no longer feasible due to the much higher pressure requirements, which surface treating equipment cannot sustain. Consequently, the industry has started using heavyweight brines as a base for fracturing fluids. However, the practice is limited by the polymer and the base brine. The most commonly used systems employ NaBr, usually in the range of 11.0 to 12.5 ppg, with a modified guar, buffer, crosslinker and other additives. The systems are finicky and hard to break, and the NaBr brines along with calcium and zinc brines, have increased in cost, prompting a search for alternative, inexpensive solutions.

A newly developed system will viscosity any brine from 11.0 to 19.2 ppg with only gellant and breaker added to the base brine. No buffer, crosslinker or other additives are necessary, which greatly enhances operational efficiency, especially offshore. The unique polymer system creates an association with the salt solution to generate viscosity. The system can be used with lower-cost brine blends such as NaCl/NaBr, CaCl2 and CaBr2. The resulting fluid is very elastic and has excellent proppant transport characteristics. The system can be broken efficiently to achieve good regain conductivity. This paper will present the chemistry and results of complete laboratory testing to evaluate the fluid for fracturing applications.
Fracturing Fluid for Extreme Temperature Conditions is Just as Easy as the Rest
Authors: Gupta D. V. , Carman Paul
Published: January 2011

As exploration goes deeper, most conventional methods have to be modified or completely re-engineered to hold up to these extreme conditions. Fracturing fluids are no exception to these changes. Fluids have evolved rapidly to adjust to an ever changing market. The trick is to make them as easy as possible to deliver on location. There have been several attempts at developing new high temperature fluids that will give good stability and deliver the fracture properties desired. Most of these to date require special handling and have operational difficulties.

A new high temperature synthetic low-pH crosslinked fluid system has been developed to avoid these pitfalls. The system employs a synthetic co-polymer in an environmentally compliant oil-based emulsion that hydrates very rapidly with excellent fluid rheology properties. Proppant pack regain conductivity using encapsulated oxidizers has also shown very good cleanup at these temperatures. The system uses exceedingly low polymer concentrations (18 to 40 pptg) compared to others (60 to 100 pptg) at temperatures from 350 to over 450°F. The fluid can also be energized with nitrogen or carbon dioxide or foamed if needed. Operationally, fracturing with this fluid can be performed with conventional equipment without any modifications. The paper will present the chemistry and technology of this new fluid including rheology and cleanup data.

This new technology has the potential to change the manner in which ultrahigh-temperature wells are completed.
The Unexpected Rheological Behavior of Borate-Crosslinked Gels
Authors: England K. W. , Parris M. D.
Published: January 2011

Fracturing fluid testing methodology and equipment has evolved, as have industry-accepted standards for testing. These standards have undergone a number of revisions over the years and were published first as a recommended practice under the American Petroleum Institute (API RP-39) and subsequently under the International Standards Organization as ISO-13503-1. These standards define the rotor/bob geometry (for a concentric-cylinder viscometer), shear-rate, temperature control and other parameters selected to determine the fluid performance under in situ conditions. Little focus was given to the pressure applied to the fluid, other than to ensure that it would not boil under test conditions. This lack of emphasis on pressure is completely understandable as condensed matter (solids and liquids) typically have a limited response to the pressures experienced during hydraulic fracturing operations.

It has been discovered that pumping pressures encountered during hydraulic fracturing treatments can materially reduce the viscosity of a borate-crosslinked fluid. The loss in viscosity, depending on fluid formulation, temperature, and pressure can range from negligible to complete resulting in a fluid with the viscosity of an uncrosslinked polymer solution (linear gel). This phenomenon has been studied using high-pressure rheometry and high-pressure nuclear magnetic resonance (NMR).

This paper presents the results of recent testing of a variety of aqueous-based fracturing fluid systems under a range of pressures typical of hydraulic fracturing treatments. Detailed results for various borate-crosslinked fluids commonly commercially available will be presented as well as non-typical systems.
A New Correlation of Acid Fracture Conductivity Subject to Closure Stress
Authors: Deng J. , Mou Jianye , Hill A. D. , Zhu D.
Published: January 2011

The conductivity of an acid-etched fracture depends strongly on void spaces and channels along the fracture resulting from uneven acid etching of the fracture walls. In this study, we modeled the deformation of the rough fracture surfaces acidized in heterogeneous formations based on the synthetic permeability distributions and developed a new correlation to calculate the acid-etched fracture conductivity.

In our previous work, we modeled the dissolution of the fracture surfaces in formations having small-scale heterogeneities in permeability. The characterization of the correlated permeability fields of rock includes the average permeability, normalized correlation lengths in both horizontal and vertical directions, and normalized standard deviation. These statistical parameters have a significant influence on the fracture etching profiles obtained from the model. Beginning with this fracture width distribution, we have modeled the deformation of the fracture surfaces as closure stress is applied to the fracture. The elastic properties of the rock, such as the Young’s modulus and the Poisson’s ratio, have effects on the size of the spaces remaining open after fracture closure. After the model yields the width profile under closure stress, the overall conductivity of the fracture is then obtained by numerically modeling the flow through this heterogeneous system.

In this paper, we introduce our models and investigate the effects of both permeability and mineralogy distributions, and rock elastic properties on the overall conductivity of an acid etched fracture. A new acid-fracture conductivity correlation is developed based on many numerical experiments.
Impacts of Diverse Fluvial Depositional Environments on Hydraulic Fracture Growth in Tight Gas Reservoirs
Authors: Cuba Patricia H. , Miskimins Jennifer L. , Anderson Donna S. , Carr Mary M.
Published: January 2011

In an attempt to enhance the understanding of fracture growth in fluvial systems, this paper provides an analysis of the impact of depositional environments and associated heterogeneities on hydraulic fracturing growth in fluvial tight gas reservoirs. A 3D geostatistical reservoir model, representing a 160-acre field area, was created based on a 3D meandering fluvial tight gas geologic model developed from outcrop. This detailed geologic model differentiates between sandstone-dominated channel- belt environments including point bars, crevasse channels, and crevasse splays as well as the intervening overbank environments consisting of mudstone and coal deposits. Petrophysical properties and reservoir conditions, used in the reservoir model, were based on subsurface data from nearby producing fields with comparable fluvial systems.

Two different well locations were then chosen within the 3D model in an effort to capture various sandstone body distributions. A range of hydraulic fracture orientation planes, associated with the two well locations, were chosen and loaded into a 3D hydraulic fracture modeling package. Various stimulation treatment sensitivities, representing eight cases, were performed.

Results show that consideration of both vertical and lateral reservoir changes is critical to understanding fracture growth in fluvial systems. When comparing a layered system, with no lateral variation, to a system with lateral variation, one-year cumulative production can be different by ~9%. Subtle lithofacies variations, present in significant quantities in these complex depositional systems, can have a large effect on fracture growth. Additionally, how depletion is treated in the fracturing model, i.e. whether the entire interval is considered depleted or just a singular sand body, can also have a significant effect on fracture propagation.
Parameters Effecting the Interaction among Closely Spaced Hydraulic Fractures
Authors: Bunger A. P. , Zhang X. , Jeffrey R. G.
Published: January 2011

Placing multiple hydraulic fractures at intervals along horizontal wells has proven to be a highly effective method for stimulation. However, the mechanical interaction between a growing hydraulic fracture and one or more previous hydraulic fractures can affect the fracture geometry such that the final fracture array is suboptimal for stimulation. If the fracture array geometry is idealized as a set of regular and planar fractures, history matching and production forecasting may be inaccurate. During the treatments, the fractures can curve towards or away from one another, potentially intersecting one another. A detailed parametric study of this phenomenon using a coupled 2D numerical fracturing simulator shows that the curving is associated with a combination of opening and sliding along the previously placed hydraulic fracture, as well as the previous fracture’s disturbance of the local stress field because of its propped width. Dimensional analysis and scaling techniques are used to identify the key parameters that are associated with suppression of each mechanism that can lead to hydraulic fracture curving. The analysis, which is in agreement with available data, results in a clarification of the conditions under which attractive and repulsive curving are expected, as well as the conditions under which curving is expected to be negligible or even completely suppressed. This last case of planar hydraulic fracture growth is of practical importance and will usually be considered desirable. We present a straightforward method for determining whether planar fracture growth is expected that additionally gives insight into how design parameters can be modified to promote planar hydraulic fracture growth.
Hydraulic Fracture Propagation in Pre-Fractured Natural Rocks
Authors: Meng Chunfang , de Pater J.
Published: January 2011

We conducted a series of hydraulic fracture initiation tests with fractured natural rock samples. The objective is to characterize the interaction between hydraulic fracture initiation and natural fracture infiltration and opening. The natural fracture was simulated by cleaving a rock cube into two or three layers and putting the parts back together. The hydraulic fracture is initiated by pressurizing the borehole drilled perpendicular to the layers. The test parameters include rock type (sandstones and a tight limestone), confining stress and fluid type (silicon oil and cross-linked gel). A detailed model was built to simulate the hydraulic fracture interaction with a natural fracture. Qualitative agreement was found with the test results.
Interpretation of Wellbore Temperatures Measured using Distributed Temperature Sensors during Hydraulic Fracturing
Authors: Hoang Hai , Mahadevan Jagannathan , Lopez Henry
Published: January 2011

Tight gas plays often have multiple lenses of producing formations. Limited entry completions became the most cost effective method to complete and produce tight gas wells in these layered reservoirs. The rate and volume of fracturing fluid injected into the different layers has an important role in determining the fracture characteristics. However, due to the restriction of down-hole equipments, it is very challenging to obtain specific injection rate for each perforated zone. Temperature variations in the well bore, outside the casing, are available using new technology such as distributed temperature sensor (DTS) fiber optic cables. The main objective of this study is to relate the well bore temperature changes as measured by DTS data to the wellbore and fractured interval injection rates during a multizone fracturing process.

We develop a forward simulation model, based on mass and energy conservation, for calculating the temperature profile and temperature history in the wellbore and in the rock surrounding the wellbore. The model allows for liquid flow into the fractured interval. Subsequently, the model is integrated with an inverse estimation algorithm which is used to estimate flow rates both in the wellbore and into the fractured interval. The estimation algorithm is based on a gradient search method.

Our estimation results show a good comparison of the temperature profiles with that observed in the field by using DTS. Also, the model is able to estimate a flow rate history consistent with total field injection volume. This work enables an accurate and quick interpretation of the wellbore DTS data to determine the interval injection rates during a hydraulic fracturing process. Knowledge of accurate interval injection rates and the corresponding fracture characteristics can be useful in designing better limited entry completion that can optimize the fracture length using rate control.
Characterizing Dynamic Fracture Growth and Azimuth using Seismic Moment Tensor Inversion Analysis of Microseismicity
Authors: Mace Kaitlyn , Urbancic Ted , Baig Adam
Published: January 2011

Based on traditional fracture models it is generally considered that fractures develop along a single fracture azimuth or along a plane of fracturing controlled by regional stresses (along the maximum principle stress direction), even within the context of a three-dimensional fracture network. Recently, the utilization of distributed geophone arrays around treatments has provided for an opportunity to investigate the way these fractures develop by examining the microseismic events recorded during a stimulation, utilizing a Seismic Moment Tensor Inversion (SMTI) approach.

SMTI provides additional information on the events beyond location and magnitude, such as the relative contribution of isotropic versus shearing components of failure, the potential fracturing plane orientations and the localized strain axes responsible for the failures. Additionally, SMTI data can be further used in Deformation State Analysis (DSA) to define the stress-strain field responsible for the growth and development of the hydraulic fracture.

In this study, we examined microseismicty associated with an open-hole completion program in a tight sand formation in North America. We observed that the DSA-derived localized stress-strain field was perturbed by the stimulation program, being highly variable both spatially and temporally. The results further suggest that fractures progress in a complex system of both macro- and micro-fractures containing several different azimuths, with progressive growth occurring as a result of stress shedding to adjacent fractures or zones of weakness. This localized fracturing behavior led to the development of an interconnected fracture network, which resulted in hydraulic fracture growth in the direction of the regional maximum horizontal shear stress.