FINAL REPORTSTRENGTH AND FRACTUREGRADIENTSFORSHALLOW MARINE SEDIMENTSAndrew K. Wojtanowicz, Adam T. Bourgoyne, Jr., Desheng Zhou, Kathy BenderLouisiana State UniversitySubmitted to:US Department of InteriorMinerals Management Service381 Elden StreetHerndon, Virginia 20170-4817Baton Rouge, LouisianaDecember 15, 2000

TABLE OF CONTENTSEXECUTIVE SUMMARY 11. BACKGROUND INFORMATION .31.1Shallow Blowout Statistics 31.2Significance of Sediment Strength Determination .42. CHARACTERIZATION OF SHALLOW MARINE SEDIMENTS (SMS) .52.1Shear Strength of Upper SMS .52.2Horizontal Stress - Stress Ratio - Elastic Behavior 62.3Overburden Stress in SMS 92.3.1 Overburden stress as a function of porosity 102.3.2 New Method for Overburden Stress Estimation in SMS 112.4Density and Shear Strength of SMS from Soil Borings 112.4.1 Geotechnical Tests 122.4.2 Bulk Density and Shear Strength Data .152.5Rock Mechanics Properties of SMS from Soil Borings 172.6In situ Stress in SMS . . 222.6.1 In situ Elastic Stresses 222.6.2 In situ Plastic Stress Model 232.6.3 In situ Stresses of SMS in Green Canyon, GOM .233. SMS FAILURE MECHANISM AROUND OVERPRESSURED BOREHOLES .243.1Stress Distribution Induced by Drilling - Plastic Zone . 243.1.1 Maximum Tangential Stress . 263.1.2 Maximum Vertical Stress . .273.2Vertical Fracturing of SMS at Pressurized Wellbore 293.2.1 Vertical Fracture at Elastic Wellbore .313.2.2 Vertical Fracture at Plastic Wellbore .323.2.3 Verification with Finite Element Method .333.3Horizontal Fracturing of SMS at Pressurized Wellbore 373.4Cement Parting at Casing Shoe in SMS 404. PRESSURE TESTING OF SMS STRENGTH AT CASING SHOE .424.1Conventional Leak-off Tests (LOT) 424.1.1 LOT Analysis Techniques Used by Operators .454.1.2 LOT Data Recording Procedures 534.2Leak-off Tests in Shallow Sediments .544.2.1 Field Data from Shallow LOTs .564.2.2 Shallow LOT Database 624.2.3 Analysis of Shallow LOT Data 635. LEAK-OFF TEST MODEL AND SOFTWARE 695.1Mathematical Modeling of LOT in SMS .69ii

45.3.5Wellbore Expansion Volume .70Well Fluid Loss to Rock .71Volume of Plastic Fracture .72Volume of Cement Parting Channel .74Leak-off Test Model .76LOT Simulation Software .78Use of LOTUMS Software for Simulation Studies .82Data Input 83Stress Analysis .87Fracture Analysis 88Leak-off test Analysis .89Example Simulation Study .936. LEAK-OFF TEST ANALYSIS PROCEDURE IN SMS .986.1Analysis of Maximum Stabilized LOT Pressure .1006.2Analysis of LOT Pressure Fall-off .1046.2.1 Normalized Time Scale 1056.2.2 Equivalent Plot of LOT Pressure Fall-off .1066.2.3 Graphical Analysis of LOT Pressure Fall-off .108CONCLUSIONS .111REFERENCES 113APPENDIX A:FINITE ELEMENT ANALYSIS .A1APPENDIX B:DETERMINATION OF OVERBURDEN PRESSURE IN SMS .B1APPENDIX C:LOTUMS SOFTWARE INSTALLATION AND USE .C1ATTACHMENTS: Shallow LOT Database from SMS: OffshoreLOT.xls Shallow LOT Database from land drilling: OnshoreLOT.xls Software: LOTUMSiii

EXECUTIVE SUMMARYStrength of conductor/surface casing shoe cemented in shallow marine sediments (SMS) isanalyzed in this study. In this work, SMS are defined as sedimentary deposits below the sea floorto a depth of about 3000 ft. Qualitatively, SMS are soft and ductile compared with the sedimentsat depth.The upper part of SMS is known better from soil borings. The soil is rather soft withPoisson’s ratio around 0.4, Young’s modulus about 3 104 psi at 300 ft, and low density withpressure gradient around 0.73 psi/ft. A plastic zone will appear around a well drilled in shallowmarine sediments. Compared with leak-off tests (LOTs) in deep wells, LOTs form SMS displaynon-linearity of early pressure buildup plots, fewer tested points, and maximum pressurestabilization with continuing mud pumping. Also, the maximum stabilized pressure oftencorresponds to pressure gradients close to overburden pressure.Properties of SMS are evaluated in this work using soil borings data from within theupper 1000-ft depth range below sea floor. Typical geotechnical data (bulk density, shearstrength) have been collected, first. Then, rock mechanics properties (Young modulus, PoisonRatio, Cohesion, and friction angle) were calculated. The properties were used to evaluate in-situstress in SMS.Conventional LOT from deep wells and modified procedures used by operators in SMShave been compiled and analyzed in this report. The analysis of shallow LOTs, included the landand offshore operations. Although this report pertains mostly to marine sediments, somestatistical observations regarding strength of shallow sediments onshore (in Canada) provideuseful perspectives to this study. Analysis of shallow LOTs from land drilling used the attacheddatabase file OnshoreLOT.xls containing data from tests performed at the surface casing shoe inover 10,000 wells in Canada. Most of the results indicate large values of pressure gradient –close to the value of overburden pressure in the area.The analysis of shallow LOTs from SMS offshore has been performed using data fromthe attached database, OffshoreLOT.xls. The objective was to – similarly to deep wells - identifytrends in formation strength with depth, so a correlation could be developed and used for SMSstrength prediction. All data showed that for SMS dispersion data is so large that no correlationor trend could be identified based on the data. Thus, it was concluded that SMS strength shouldbe estimated from direct testing using a theoretically sound methods and simple-to-useprocedures.Traditional method of in-situ stress analysis is based on elastic theory and is notapplicable to SMS. An analytical mathematical model of SMS stress in-situ, based on elasticplastic theory, was derived in the study. Also, a finite element program was set up and used tosimulate the transition process from elasticity to plasticity at depth. The analytical model issupported by the finite element analysis results. In addition to the theoretical modeling, empiricalformulas for overburden stress in SMS were developed using bulk density data from soil borings.The formulas consider any combination of clayey or sandy sediments subsea.Also reported is theoretical work addressing change of stress in SMS around drilledwells. Again the process was studied analytically. Based on elasto-plastic theory, formulas wereset up to determine the critical condition for transition from an elastic to a plastic wellbore, thesize of the possible plastic zone, and stress distribution around the wellbore. Stress variationduring leak-off test was also analyzed.Three types of possible failures caused by pressurization of the casing shoe (LOT ormigration) were studied: vertical fracture, horizontal fracture, and cement-rock parting. (For1

clarity, no fluid loss to the rock matrix was assumed in this study.) It has been provedtheoretically that vertical fracture is the most unlikely failure of the three. Although horizontalfractures are initiated at low pressure in the plastic zone around the wellbore, they cannotpropagate beyond the plastic zone until wellbore pressures exceed overburden pressures. On theother hand, an annular channel resulting form the cement-rock parting may propagate upwards atpressures lower than overburden pressure. (Unlike a conventional cement channel that occursduring cementing, cement-rock parting represents conditions when separation may be initiatedbetween cement and rock by high wellbore pressure with no pre-existed channels assumed.) Thestudy shows that cement parting is initiated at pressures equal to the contact stress betweencement and rock and their propagation pressure is, on average, 3.5 - fold greater than contactstress. These findings were again supported by the results from the finite element method.The study identified two factors, related directly to drilling technology, that controlcritical pressure of cement parting in SMS: contact stress at casing shoe – resulting fromcementing operations, and rock penetration by liquid – an invasion of drilling fluid into the rockaround the casing shoe. The results show that contact stress is developed during the process ofcement setting as a result of volumetric changes in cement annulus. A mathematical model ofcontact stress around casing shoe was set up based on cement volume reduction andcompensation from casing string, and cement and wellbore compressibility. It was shown in thestudy that changes in cementing and drilling practices may increases casing shoe integrity andreduce the need for cement squeeze treatments.A general pressure-volume model of LOT and computer software are presented includingvolumetric effects of wellbore expansion, mud loss into the rock, and propagation of both cementparting and plastic fracture. The proposed model describes all possible mechanisms and thereforecould explain linear, non-linear or combination patterns of the LOT plots.Software LOTUMS - attached to this report - was developed to simulate LOT in SMS. Adetailed instruction on using LOTUMS is provided in Chapter 5 and Appendix C. The softwareenables simulation studies of LOT or a casing shoe pressurization caused by fluid migrationoutside the well. Also presented is a LOTUMS simulation study of various mechanisms affectingthe shape of the pressure-volume plots. The study shows that all mechanisms give very similarresponse patterns so qualitative analysis is difficult. Moreover, quantitative analysis with thesoftware would require a detailed input data. However, since most of the system properties areunknown there is no reliable data input for the application software. Thus, LOTUMS softwarecould only be used either for simulation studies or demonstration and training.A simple method for analyzing LOTs in SMS is presented in this report. The methodconsiders separately the pressure stabilization and the pressure buildup and fall-off sections ofthe LOT plot. Also used in the analysis is drilling data that includes overburden pressure(calculated from the SMS correlations developed in this project) and the maximum cementingpressure at the time of slurry placement.The analysis of stabilized LOT pressure identifies the maximum strength of the casingshoe controlled by the rock stress (overburden pressure) and the failure by rock fracturing. Alsopresented is a graphical procedure for comparing the plots of actual and equivalent pressure falloff. The graphical procedure provides a pattern-recognition method to distinguish the mechanismof cement-rock parting from the fluid loss mechanism.2

1. BACKGROUND INFORMATIONA flow from unexpected shallow gas sand is one of the most difficult well control problemsfaced by oil and gas well operators during drilling operations. Current well control practice forbottom-supported marine rigs usually calls for shutting in the well when a kick is detected ifsufficient casing has been set to keep any flow underground. Even if high shut-in pressuresoccur, an underground blowout is preferred over a surface blowout. However, when shallow gasis encountered, casing may not be set deep enough to keep the underground flow outside thecasing from breaking through to disturbed sediments near the platform foundations. Once theflow reaches the surface, craters are sometimes formed that can lead to loss of the rig andassociated marine structures.1.1Shallow Blowout StatisticsNumerous disastrous blowouts have occurred after gas unexpectedly flowed into the well from ashallow formation. By the time the rig crew can recognize the problem and react to it, gas mayhave already traveled a considerable distance up the open borehole. Closing the blowoutpreventers may allow the wellbore pressure to build up to a value exceeding the formationbreakdown pressure. When this happens, one or more flow paths can travel to the surface or todisturbed soil near a platform leg. In some cases, a crater forms in the seafloor. If the rigstructure is bottom supported, the entire rig may be lost.Hughes (1986) compiled information on 425 Gulf Coast blowout events that covered theperiod between July 13, 1960 and January 1, 1985. The data, broken down by area, included 242blowouts in Texas, 56 in Louisiana, 121 in the Outer Continental Shelf (OCS), 3 in Mississippi,and 3 in Alabama. Gas was present in 82% of the Texas blowouts. The two major operations thatwere underway when the blowout occurred were (1) coming out of hole (27%) and (2) drilling(25%). Seventeen Texan blowout reports (7.02%) noted that the well blew out around the casing.A total of twenty events (8.26%) reported that the underground flow reached the surface either toform a crater around the well, at a nearby surface site, or caused blowouts from nearby waterwells. All the blowouts that reached the surface outside of casing had drilling depth to casingdepth ratios greater than four.Of the 56 Louisiana blowouts included in the Hughes study, gas was present in 73% ofwells that reported the type of blowout fluid. The rig operations reported to be underway at thetime of the blowout included: workover operations, 37%; coming out of hole, 21%; circulating,13.2%; and drilling, 13.2%. Hughes does not give details about flows around casing or crateringfor the Louisiana blowouts.Of the 121 OCS blowouts reported by Hughes, gas was present in 77% of the cases.Descriptions of the operations were available for 46 events. The rig operations reported to beunderway included: workover operations, 28%; coming out of hole, 24%; and drilling, 20%.Descriptions of the procedure used to control the blowout were given for 66 of the wells. Themajority (55%) of the blowouts bridged naturally. Both the date the blowout occurred and thedate the well was killed were given for 70 wells; about 49% of these were controlled within oneday.Danenberger (1993) performed a study of blowouts that occurred during drillingoperations on the Outer Continental Shelf of the United States during the period from 1971 to1991. During this period, 21,436 oil and gas wells were drilled; eighty-three blowouts occurred.Four additional blowouts occurred during sulfur drilling operations. Eleven of the blowoutsresulted in casualties with 65 injuries and 25 fatalities. Fifty-eight of the blowouts that occurred3

while drilling for oil and gas came from shallow gas zones. Exploratory wells accounted for37.4% of the wells drilled and 56.9% of the shallow-gas blowouts. Conversely, developmentwells accounted for 62.6% of the wells drilled and 43.1% of the shallow-gas blowouts.According to Danenberger (1993), a shallow gas blowout in 1980 was the most seriousblowout in the OCS, accounting for 6 of the 25 fatalities and 29 of the 65 injuries. However, nocasualties due to blowouts were reported during the last seven years of the study.Oil was not associated with the shallow gas blowouts and environmental damage hasbeen minimal. Two blowouts prior to 1971 are known to have caused oil pollution in the portionof the Outer Continental Shelf under U.S. jurisdiction. An estimated 80,000 bbl of crude oil wasreleased into the Santa Barbara Channel, and about 1,700 bbl of condensate was released into theGulf of Mexico.Although no statistics are given for the OCS on crater developments affecting rigfoundations, Danenburger (1993) reported that 71.3% of the blowouts stopped flowing when thewell bridged naturally. This bridging is thought to be due to the collapse of the uncased portionof the borehole. Flow from 57.5% of the blowouts ceased in less than a day, and flow from83.9% ceased in less than a week. A list of shallow gas blowouts compiled by Adams (1991)indicates that 18 bottom-supported structures were damaged on the OCS by shallow gasblowouts during the 1971-91 period of the Danenburger study. Seven of the U.S. structuresshown in the Adams study were reported to be a total loss, and extensive damage was reportedfor another three cases. These ten cases account for 17.2% of the 58 shallow gas blowoutsreported by Danenburger (1993). Thus 10 lost structures out of 21,436 wells drilled is a roughestimate of the risk from significant cratering.We were not successful in compiling an estimate of economic loss associated withshallow gas blowouts. However, an operator reported that the cost due to one event outside theU.S. was approximately 200 million dollars.1.2Significance of Sediment Strength DeterminationModern contingency plans for handling a shallow gas flow call for diverting the flow away froma bottom-supported rig using a diverter system. The diverter system is used to reduce thewellbore pressure so that it does not exceed the formation breakdown pressure. However, resultsof this study indicate that using diverter systems does not always prevent cratering. Craterformation during diversion can occur when the diverter is too restricted, thus allowing formationbreakdown pressure to be exceeded even though the well is not shut-in.Cratering and underground blowouts can be prevented only through appropriate casingdesign, cementing practices, and shut-in procedures. A successful well design must be based on arealistic estimate of the fracture resistance of the sediments. Thus, improvements in the analysisand procedure of leak-off testing for wells in shallow marine sediments (SMS) are needed.The sediment failure mechanisms around wells traditionally have been poorlyunderstood. In addition, the best choices of well design parameters and well control contingencyplans that will minimize the risks associated with a shallow gas flow are not always clear. Theobjectives of this study were: To identify properties of SMS that control the sediment strength and failure aroundboreholes; To study and model mathematically several possible sediment failure mechanismsaround over-pressured wellbores;4

To survey and analyze currently used procedures for testing strength of SMS at thewellbores;To collect industry data from leak-off testing (LOT) in SMS;To develop mathematical models and software for simulating stress-strain (pressurevolume) relationship at the pressurized wellbore (LOT simulator);To formulate guidelines for LOT procedure and result analysis in SMS.2. CHARACTERIZATION OF SHALLOW MARINE SEDIMENTS (SMS)For the purpose of this work, we define SMS broadly as the depositional environment below thesea floor to a depth of about 3000 ft. According to widely accepted thinking, SMS are soft andductile compared with the sediments at depth.2.1Shear Strength of the Upper SMSProperties of the upper layers of SMS, just below the seafloor, can be visualized using wellknown concepts of effective stress:σz s p(1)in which s is the total overburden stress and p is the formation pore pressure, and modifiedMohr-Coulomb failure criteria, as shown in Figure 1.σrσzσθσrσθσrσrσzMUDCAKEσ maxis maximum ofσ minis minimum ofσθ , σ r , σzσθ , σ r , σzMUDCAKEφfricσfailτfail c - σntanφfri