Hydraulic Fracturing Chemicals and Fluids Technology PDF
Preview Hydraulic Fracturing Chemicals and Fluids Technology
Hydraulic Fracturing Chemicals and Fluids Technology Hydraulic Fracturing Chemicals and Fluids Technology Johannes Karl Fink AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Gulf Professional Publishing is an Imprint of Elsevier Gulf Professional Publishing is an imprint of Elsevier 225 Wyman Street, Waltham, MA 02451, USA The Boulevard, Langford Lane, Kidlington, Oxford, OX5 1GB, UK First edition 2013 © 2013 Elsevier Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means electronic, mechanical, photocopying, recording or otherwise without the prior written permission of the publisher. Permissions may be sought directly from Elsevier’s Science & Technology Rights Department in Oxford, UK: phone: ( 44) (0) + 1865 843830; fax: ( 44) (0) 1865 853333; email: permissionselsevier.com. Alternatively visit + the Science and Technology website at www.elsevierdirect.com/rights for further information. Notice No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library For information on all Gulf Professional Publishing publications visit our website at www.elsevierdirect.com ISBN: 978-0-12-411491-3 Printed in the United States Preface This manuscript focuses on the recent developments in fracturing fluids with respect to chemical aspects. After a short introduction to the basic issues of fracturing, the text focuses mainly on the organic chemistry of fracturing fluids. The nature of the individual additives and the justification why the indi- vidual additives are acting in the desired way are explained. The material pre- sented here is a compilation from the literature, including patents. In addition, as environmental aspects are gaining increasing importance, this issue is also dealt carefully. HOW TO USE THIS BOOK Index There are four indices, an index of tradenames, an index of acronyms, an index of chemicals, and a general index. In a chapter, if an acronym is occurring the first time, it is expanded to long form and to short form, e.g., acrylic acid (AA) and placed in the index. If it occurs afterwards it is given in the short form only, i.e., AA. If the term occurs only once in a specific chapter, it is given exclusively in the long form. In the chemical index, bold face page numbers refer to the sketches of struc- tural formulas or to reaction equations. Bibliography The bibliography is given per chapter and is sorted in the order of occurrence. After the bibliography, a list of tradenames that are found in the references and which chemicals are behind these names, as far as laid open, is added. xi Acknowledgments The continuous interest and the promotion by Professor Wolfgang Kern, the head of the department is highly appreciated. I am indebted to our university librarians, Dr. Christian Hasenhttl, Dr. Johann Delanoy, Franz Jurek, Margit Keshmiri, Dolores Knabl, Friedrich Scheer, Christian Slamenik, and Renate Tschabuschnig, for support in literature acquisition. This book could not have been otherwise compiled. Thanks are given to Professor I. Lakatos, University of Miskolc who directed my interest to this topic. Last but not least, I want to thank the publisher for kind support, in particular Katie Hammon. J.K.F. xiii Chapter 1 General Aspects Hydraulicfracturingisatechniqueusedtostimulatetheproductivityofawell.A hydraulicfractureisasuperimposedstructurethatremainsundisturbedoutside thefracture.Thustheeffectivepermeabilityofareservoirremainsunchanged bythisprocess. The increased wellbore radius increases its productivity, because a large contactsurfacebetweenthewellandthereservoiriscreated. STRESSESAND FRACTURES Hydraulic fracturing is one of the newer techniques in petroleum sciences, notbeingusedformorethanapproximately60years.Theclassictreatmentof hydraulicfracturingstatesthatthefracturesareapproximatelyperpendicularto theaxisoftheleaststress(Yew,1997).Formostdeepreservoirs,theminimal stressesarehorizontal,henceverticalstresseswilloccurinfracturing. Theactualstresscanbecalculatedbybalancingtheverticalgeostaticstress and the horizontal stress by the common tools of the theory of elasticity. For example,thegeostaticstressmustbecorrectedinaporousmediumfilledwith aliquidhavingaporoelasticconstantandhydrostaticpressure.Thehorizontal stress can be calculated from the corrected vertical stress with the Poisson ratio.Undersomecircumstances,inparticularinshallowreservoirs,horizontal stresses aswell asvertical stresses canbecreated. Thepossible stress modes aresummarizedinTable1.1. Fracture Initialization Pressure Knowledgeofthestressesinareservoirisessentialtofindthepressureatwhich initiationofafracturecantakeplace.Theupperboundofthispressurecanbe estimatedusingaformulagivenbyvonTerzaghi,1923,whichstatesthat pb =3sH,min−sH,max+T − p. (1.1) The variables are explained in Table 1.1. The closure pressure indicates the pressureatwhichthewidthofthefracturebecomeszero.Thisisnormallythe minimalhorizontalstress. HydraulicFracturingChemicalsandFluidsTechnology.http://dx.doi.org/10.1016/B978-0-12-411491-3.00001-7 ©2013ElsevierInc.Allrightsreserved. 1 2 HydraulicFracturingChemicalsandFluidsTechnology (cid:2) (cid:4) TABLE1.1 ModesofStressesinFractures p Fractureinitializationpressure b 3sH,min Minimalhorizontalstress sH,max Maximalhorizontalstress (=minimalhorizontalstress+tectonicstress) T Tensilestrengthofrockmaterial p Porepressure (cid:3) (cid:5) Pressure DeclineAnalysis Thepressureresponseduringfracturingprovidesimportantinformationabout the success of the operation. The fluid efficiency can be estimated from the closuretime. COMPARISON OF STIMULATIONTECHNIQUES Inadditiontohydraulicfracturing,thereareotherstimulationtechniques,such as acid fracturing or matrix stimulation, and hydraulic fracturing is also used in coal seams to stimulate the flow of methane. Fracturing fluids are often divided into water-based, oil-based, alcohol-based, emulsion, or foam-based fluids. Several reviews are available in the literature dealing with the basic principles of hydraulic fracturing, and the guidelines that are used to select a formulationforaspecificjob(EbingerandHunt,1989;Ely,1989;Lemanczyk, 1991). Polymer hydration, crosslinking, and degradation are the key processes thatthesematerialsundergo.Technologicalimprovementsovertheyearshave focusedprimarilyonimprovedrheologicalperformance,thermalstability,and cleanupofcrosslinkedgels. Action of a Fracturing Fluid Fracturing fluids must meet a number of requirements simultaneously. They mustbestableathigh ● temperatures, ● pumpingrates,and ● shearrates. These severe conditions can cause the fluids to degrade and prematurely settle out the proppant before the fracturing operation is complete. Most commerciallyusedfracturingfluidsareaqueousliquidsthathavebeeneither gelledorfoamed. CHAPTER | 1 GeneralAspects 3 Typically,thefluidsaregelledbyapolymericgellingagent.Thethickened orgelledfluidhelpskeeptheproppantswithinthefluidduringthefracturing operation.Fracturingfluidsareinjectedintoasubterraneanformationforthe followingpurposes(Kellyetal.,2007): ● Tocreateaconductivepathfromthewellboreextendingintotheformation. ● Tocarryproppantmaterialintothefracturetocreateaconductivepathfor producedfluids. Stages in a Fracturing Job A fracturing job has several stages, including injecting a prepad, a pad, a proppantcontainingfracturingfluid,andfinally,atreatmentwithflushfluids. Aprepadisalow-viscosityfluidusedtoconditiontheformation,whichmay containfluidlossadditives,surfactants,andhaveadefinedsalinitytoprevent formationdamage.Thegenerationofthefracturestakesplacebyinjectingthe pad,aviscousfluid,butwithoutproppants. After the fractures develop, a proppant must be injected to keep them permeable. When the fracture closes, the proppant left there creates a large flow area and a highly conductive pathway for hydrocarbons to flow into the wellbore.Thus,theproppantisutilizedtomaintainanopenfracture.Viscous fluidsareutilizedtotransport,suspend,andeventuallyallowtheproppanttobe trappedinsidethefracture.Thesefluidstypicallyexhibitapowerlawbehavior fortherangeofshearratesencounteredinhydraulicfracturingtreatments. A uniform proppant distribution is needed in order to get a uniformly conductivefracturealongthewellboreheightandfracturehalf-length,butthe complicated nature of proppant settling in non-Newtonian fluids often leads to a higher concentration of proppant in the lower part of the fracture. This often leads to a lack of adequate proppant coverage of the upper portion of thefractureandthewellbore.Clusteringofproppant,encapsulation,bridging, andembedmentareallphenomenathatlowerthepotentialconductivityofthe proppantpack(Wattersetal.,2010). Thejobendseventuallywithacleanupstage,inwhichflushfluidsandother cleanupagentsareapplied.Theactualdetailedtimescheduledependsonthe particularsystemused. After the completion the fluid viscosity should decrease to allow the placement of the proppant and a rapid fluid return through the fracture. It is importanttocontrolthetimeatwhichtheviscositybreakoccurs.Inaddition, thedegradedpolymershouldproducelittleresiduetorestricttheflowoffluids throughthefracture. SIMULATION METHODS The estimation of the fracture geometry is one of the most difficult technical challengesinhydraulicfracturingtechnology(Zhangetal.,2010). 4 HydraulicFracturingChemicalsandFluidsTechnology A discrete element simulation for the hydraulic fracture process in a petroleum well that takes into account the elastic behavior of the rock and the Mohr-Coulomb fracture criterion has been presented (Torres and Munoz-Castano, 2007). The modeling of the rock was done as an array of Voronoi polygons joined by elastic beams that were submitted to tectonical stressesandthehydrostaticpressureofthefracturingfluid.Thefluidpressureis treatedsimilartothatofahydrauliccolumn.Theresultsshowthatthesimulation processfollowsarealsituation. A three-dimensional nonlinear fluid solid coupling finite element model was established based on the finite element software ABAQUS® (Zhang et al., 2010). The staged fracturing process of a horizontal well in the Daqing Oilfield, China was simulated with the model, taking account to perforation, wellbore, cement casing, one pay zone, two barriers, micro-annulus fracture, andtransversefracture. These experimental data were used in numerical computation. Micro- annulusfractureandtransversefracturegeneratesimultaneouslyandatypical T-shapedfractureoccursattheearlystageoftreatmenthistory,thenthemicro- annulusdisappearsandonlythetransversefractureremainsandpropagates. Theporepressuredistributionintheformationandthefractureconfiguration during the treatment history can be obtained. The evolution of bottomhole pressure as the direct output of simulation coincides with the experimental data(Zhangetal.,2010). Productivity Amodelhasbeendevelopedthatcalculatestheproductivityofahydraulically fractured well, including the effect of fracture face damage caused by fluid leakoff(Friehaufetal.,2010). Theresultsofthismodelhavebeencomparedwiththoseofthreeprevious models. These models assume either elliptical or radial flow around the well, with permeability varying azimuthally. Significant differences in the calculated well productivity indicate that earlier assumptions made regarding theflowgeometrycanleadtosignificantoverestimatesofthewellproductivity index. AnagreementwiththeanalyticalsolutiongivenbyLevineandPrats,1961 hasbeenshownforfinite-conductivityfracturesandnofracturedamage. The simple and discrete nature of the model makes it ideal for implementationinspreadsheetsandtoconnecttofractureperformancemodels. Cleanupofthedamageintheinvadedzonedependsonthecapillaryproperties of the formation and the pressure applied across the damaged zone during production(Friehaufetal.,2010).Ithasalsobeenfoundthattheinvadedzone willcauseasignificantdamagewhenthepermeabilityofthedamagedzoneis reducedbymorethan9%. CHAPTER | 1 GeneralAspects 5 Fracture Propagation Thepropagationofatwo-dimensionalpre-existingfractureinpermeablerock bytheinjectionofaviscous,incompressibleNewtonianfluidhasbeenmodeled (FareoandMason,2010).Inparticular,themethodofFittetal.,2007forthe solutionofhydraulicfracturinginanimpermeablerockhasbeenextendedtoa permeablerock. Thefluidflowinthefractureisassumedtobelaminar.Bytheapplicationof thelubricationtheory,apartialdifferentialequationrelatingthehalf-widthofthe fracturetothefluidpressureandleakoffvelocityhasbeenderived.Thesolution ofthisequationyieldstheleakoffvelocityasafunctionofthedistancealong thefractureandtime.Thegroupinvariantsolutionisderivedbyconsideringa linearcombinationoftheLiepointsymmetries.Theboundaryvalueproblem hasbeenreformulatedasapairofinitialvalueproblems.Themodelinwhichthe leakoffvelocityisproportionaltothefracturehalf-widthisconsidered(Fareo andMason,2010). Amodelofahydraulicfracturewithatipcohesivezoneaccordingtothe Barenblattapproach(Barenblattetal.,1990)hasbeenpresented(Mokryakov, 2011).Theparticularcaseofaninviscidfracturingfluidandimpermeablerock hasbeenstudied.Aninviscidfluidisassumedtohavezeroviscosity.Ithasbeen provedthatassumingfinitecohesivestresses,thecohesivezonelengthcannot alsoexceedacertainlimit.Thelimitformofthecohesivezoneisobtained,and thelimitfracturetoughnessisthusevaluated.Theeffectivefracturetoughness tendstothelimitvaluewithpowerof−0.5. Proppants The proppant is the most critical issue of a fracture treatment for a well, as it largely defines the ultimate deliverability. The most efficient and effective fracture stimulation designs create an optimal effective fracture area. This is theareaofacreatedfracturewithenoughconductivitycontrasttopromotean accelerateddrainageofthereservoir(BrannonandStarks,2010). A strong correlation was observed among all the designs between the effective fracture area and predicted cumulative production of 360d. The cumulative production was significantly less sensitive to the conductivity of the fracture than to the effective area of the fracture. Stimulation designs employingproppantpartialmonolayerscanbecompetitiveorlesscostlythan fracturejobsthatuseconventionalproppants. Fluid Loss The modeling of the fluid loss is important for planning a fracturing job and analysisofthefinishedprocess.Duetoitscomplexity,theoreticalmodelsare
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