borehole stability analysis considering spatial variability and poroelastoplasticity abstract typically stability analysis of oil producing wells is carried out considering that both the hydraulic and mechanical parameters of the rock mass are deterministic when analyzing borehole stability two failure mechanisms are generally considered namely failure due to either tensile or compressive shear stresses these mechanisms are produced respectively by either too high or too low drilling fluid pressure mechanical and hydraulic properties of rock masses and in particular sedimentary rock masses may show a considerable degree of spatial variability this paper focuses on the evaluation of the limits for the internal pressure associated with a target probability of failure taking into account both the spatial variability of hydraulic and mechanical properties and the simple variability of the initial pore pressure and in situ stresses the analysis is performed with a finite element program that incorporates coupled fluid mechanical effects and elastoplastic behavior of the rock in this way the proposed borehole stability analysis can be set into a reliability based framework described in the paper examples are shown using stochastic data from the literature and conclusions are drawn regarding the effect of spatial variability on the borehole stability

stress directions in eastern north america determined to 4 5 km from borehole elongation measurements stress directions have been determined to depths of 4 5 km in eastern north america from borehole elongation measured by dipmeter calipers in 47 wells the average maximum horizontal stress directions are eastern canada n54 e 7 appalachian basin n58 e 8 and the illinois basin n89 w 5 stress directions determined for the illinois and appalachian basins are in agreement with published interpretations from hydraulic fractures and earthquake fault plane solutions new results from eastern canada and the appalachian basin show that stress directions are everywhere consistent with the midcontinent stress province these data suggest that the maximum horizontal stress does not rotate from the ene midcontinent trend to nw near the atlantic coast a directional analysis of hydraulic fractures petal centerline fractures and natural fractures from the appalachian basin shows that borehole elongation is systematically aligned perpendicular to the directions of hydraulic fractures centerline fractures and northeast striking natural fractures but is not systematically aligned with northwest striking natural fractures these observations indicate that borehole elongations provide reliable stress orientations

multiporosity multipermeability approach to the simulation of naturally fractured reservoirs this paper presents an array of deformation dependent flow models of various porosities and permeabilities relevant to the characterization of naturally fractured reservoirs a unified multiporosity multipermeability formulation is proposed as a generalization of the porosity or permeability oriented models of specific degree some new relationships are identified in the parametric investigation for both single porosity and dual porosity models a formula is derived to express skempton s constant b by biot s coefficient h and relative compressibility it is found that the recovery of the original expression for skempton s constant b is largely dependent on the choice of representing relative compressibility the dual porosity dual permeability model is evaluated through an alternative finite element approximation the deformation dependent fracture flow mechanism is introduced where the rock matrix possesses low permeability and fracture flow is dominant a preliminary study of the reservoir simulation identifies the strong coupling between the fluid flow and solid deformation

electrical conductivity models for the continental crust based on laboratory measurements on high grade metamorphic rocks summary deep electrical conductivity soundings are increasingly being used as an additional source of information regarding the nature of the lower continental crust however a lack of relevant laboratory based conductivity measurements makes interpretation of such soundings difficult laboratory measurements have been made on saturated and unsaturated samples of possible lower crustal rock types subjected to confining pressures up to 0 4 gpa temperatures up to 300 c and with variable pore fluid pressure up to the confining pressure extrapolation of these results suggests that the surprisingly high conductivities deduced for depths of approximately 20 km in certain stable continental areas may result from a combination of basic rock type and high pore fluid pressures for whereas the conductivities measured in acid rock types can be explained in terms of conduction through the pore fluid alone the conductivities measured in basic rock types imply enhanced conduction presumably through the matrix or along grain boundaries the lower conductivities deduced from field experiments for the upper crust may be due to more acid rock types and or lower pore fluid pressures perhaps due to hydration reactions in areas where the high conductivity layer is coincident with a low velocity layer an explanation in terms of changing pore fluid pressure i e low to high with increasing depth seems more likely

influence of secular variation of the geomagnetic field on production rates of in situ produced cosmogenic nuclides abstract scaling factors are presented that allow a full description of the cosmic ray flux modulation during secular variation of the geomagnetic field these scaling factors are analogous to those determined in a similar work dunai t j earth planet sci lett 2000 176 157 169 that were accurate for field strengths equivalent to present day field strength but cannot accommodate field strength variations strategies to derive the relevant geomagnetic field parameters are discussed and are tested with existing production rate calibrations for 3 he a production rate of 103 4 2 atoms g olivine yr sea level and for zero cutoff rigidity was derived from calibration sites from hawaii western usa and canary islands covering the time range of 2 281 kyr

anisotropic damage mechanics as a novel approach to improve pre and post failure borehole stability analysis anisotropic damage distribution and evolution have a profound effect on borehole stress concentrations damage evolution is an irreversible process that is not adequately described within classical equilibrium thermodynamics therefore we propose a constitutive model based on non equilibrium thermodynamics that accounts for anisotropic damage distribution anisotropic damage threshold and anisotropic damage evolution we implemented this constitutive model numerically using the finite element method to calculate stress strain curves and borehole stresses the resulting stress strain curves are distinctively different from linear elastic brittle and linear elastic ideal plastic constitutive models and realistically model experimental responses of brittle rocks we show that the onset of damage evolution leads to an inhomogeneous redistribution of material properties and stresses along the borehole wall the classical linear elastic brittle approach to borehole stability analysis systematically overestimates the stress concentrations on the borehole wall because dissipative strain softening is underestimated the proposed damage mechanics approach explicitly models dissipative behaviour and leads to non conservative mud window estimations furthermore anisotropic rocks with preferential planes of failure like shales can be addressed with our model

upper mantle structure along a profile from oslo noress to helsinki to leningrad based on explosion seismology abstract waveforms from the noress array were analyzed for 147 industrial explosions during the 1985 to 1988 period along a profile running east from oslo noress to helsinki to leningrad ohl profile the events were 250 to 1300 km from noress and had local magnitude in the range 2 0 to 3 5 event locations and origin times constrained by the university of helsinki9s regional seismic network provide a reliable basis for travel time estimation at noress we also used data recorded by norsar in 1979 for three shots on the fennolora north south long range seismic profile which were near the ohl profile analysis of mantle p wave signals from the explosions showed that first arrivals could be traced continuously to a distance of 750 to 800 km where there is a cutoff and shift of approximately 2 0 to 2 5 sec in the travel time curve and an increase in average apparent velocity interpretation of the observed travel times and waveforms for this profile suggests a low velocity zone from approximately 105 to 135 km depth combined analysis of the seismic data with a bouguer gravity map indicates the presence in the upper mantle of a high velocity high density body of linear extent approximately from 200 to 300 to 500 to 600 km east of the noress array it is postulated that this body may represent the root of an ancient volcanic system in which lighter silicic constituents were depleted from the upper mantle during the eruptive phase

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

stability analyses for deviated wellbores in unconsolidated cross anisotropic formations production of oil from shallow reservoirs typically involves drilling highly deviated wells through unconsolidated or poorly lithified rocks or clays this paper describes numerical analyses of t

multiscale insights into borehole instabilities in high porosity sandstones this paper presents a multiscale analysis of the classical borehole instability problem in high porosity sandstones using a hierarchical multiscale approach a rigorous two way message passing coupling of finite element method and discrete element method is employed where the finite element method is employed to solve a boundary value problem and the constitutive material responses required at each of its integration points are derived by the discrete element method solution to an embedded representative volume element instead of by using an assumed phenomenological model we employ this multiscale approach to examine the successive failure of a borehole subjected to gradually decreased support stresses at the inner borehole wall or increased far field stresses to reproduce the material behavior of high porosity sandstone representative volume elements with a high porosity structure and interparticle bonds are generated it is found that stress concentration triggers the initial failure at the borehole wall and the subsequent failure in form of deformation bands is driven by further stress concentration ahead of the band tips the failure pattern around the borehole varies with initial stress state and global loading path the comparison of the local stress paths of the initial failure points in various cases reveals that the change of failure pattern from shear failure to mixed mode failure and to compaction failure is dominated by the increased mean stress cross scale parametric studies show that the failure mode changes from clear compaction bands to multiple arrays of shear bands by changing the high porosity specimen to a low porosity one the increase in cohesion strength expands the yield locus and enlarges the critical mean stress between different failure patterns hence the failure mode may change from compaction failure to mixed mode failure or from mixed mode failure to shear failure due purely to the increased cohesion strength the qualitative comparisons indicate greater length larger area and more severe damage of the diametrically opposite failure with the increase in minor principal stress 0 and principal stress ratio the diametrically opposite failure mode is found a good indicator for 0 direction but it may also occur under hydrostatic far field stress due to material anisotropy

initiation of a secondary hydraulic fracture and its interaction with the primary fracture laboratory and field experiments have shown that secondary fracture treatment along a different direction to the primary hydraulic fracture can increase the recovery of gas from tight sands success of a restimulation treatment secondary fracture treatment is however highly dependent on transient stress distribution around the well in this paper a numerical poroelastic reservoir model is used to provide a more realistic picture of stress field around a stimulated well during post fracture production initiation of a re oriented hydraulic fracture and its interaction with the primary hydraulic fracture are also studied using a wellbore re pressurization model that includes a poroelastic formation a wellbore and a pair of finite conductive hydraulic fractures numerical results have shown that during post fracture production the directions of maximum and minimum horizontal stresses are reversed within a large area around the wellbore and the hydraulic fractures we have also shown that relatively small areas exist at the wellbore wall where the original directions of horizontal principal stresses remain unchanged it is shown that the presence of these small areas could act against initiation of a secondary hydraulic fracture along the most desired directions in terms of recovery enhancement this study also shows that oriented perforations prior to refracture treatment help initiate secondary fractures in a preferred direction before the primary hydraulic fractures start propagation during re pressurisation of the wellbore

the influence of second scale convection on the thickness of continental lithosphere and crust abstract the effect of sublithospheric convection on the thickness and structure of the continental lithosphere is studied in numerical models assuming different rheologies newtonian non newtonian and temperature and pressure dependent heat fluxes and heating modes bottom versus internal heating the stagnant regions near the top of the models are identified with the lithosphere we distinguish a seismic and thermal lithosphere controlled by temperature an elastic lithosphere controlled by viscosity and a mechanical lithosphere controlled by strain rate strong lateral thickness undulations with a thick lithosphere above the downwelling regions and a thin lithosphere above the upwelling regions develop the thickness of the elastic lithosphere is about two thirds of the mechanical lithosphere while the ratio of the elastic to the thermal lithosphere varies between 0 4 and 0 8 the time dependence of some models with internal heating and a surface heat flow of 20 mw m 2 is characterized by long periods of the order of 1 ga close to steady state and short periods 100 ma of changing cell patterns and lithospheric thicknesses models showing thick lithospheric roots suggest that the mantle beneath some old shields may be associated with cold slowly downwelling convective flow rather than being firmly attached to the lithosphere the missing gravity and topography signals from such regions can be explained by elastic bending of the lithosphere and by dynamic adjustment of the moho observed lithospheric thickness variations in europe and their minimum to maximum spacing agree well with the models suggesting sublithospheric convection to be the cause compared to other observed features thickness undulations of the lithosphere seem to provide the strongest indications for sublithospherical convection relatively low viscosities in the lower crust may lead to a long term decoupling of surface topography from moho topography and mantle dynamics the moho depth is capable of adjusting to mantle stresses within relaxation times of the order of 100 ma while surface topography may flatten out such a mechanism may be important in regions with a deep moho such as fennoscandia or with a shallow moho such as the pannonian basin neither region showing a pronounced surface topography

analysis of geomagnetic field along seismic profile p4 of the international project polonaise 97 abstract this paper presents relative secular variations of the total intensity of the geomagnetic field against a background of results of magnetic anomaly interpretation along seismic profile p4 profile p4 crosses a variscan folding zone in the paleozoic platform plz the trans european suture zone tesz and the polish part of the east european craton eec secular geomagnetic field variations t measured in 1966 2000 along a line adjacent to seismic profile p4 were analysed the study of secular variations reduced to the base recordings at the belsk magnetic observatory showed that the growth of geomagnetic field at the east european craton was slower than in the trans european suture zone and the paleozoic platform a 2d crustal magnetic model was interpreted as a result of magnetic modelling in which seismic geological and geothermal data were also used the modelling showed that there were significant differences in the magnetic model for geotectonic units which had been earlier determined based on deep seismic survey data it should be noted that a fundamental change of trend of the relative secular variations was observed at the slope of the precambrian platform after analysing the geomagnetic field observed along profile p4 the hypothesis that the contact between phanerozoic and precambrian europe lies in poland s territory can be proven

transient poroelastic response of equivalent porous media over a mining panel a transient poroelastic formulation based on the biot consolidation theory is presented to study the impact of underground longwall mining of coal on strata deformation and the subsequent modification of hydrogeological properties a finite element model is used where the geometric conditions may be complex the important parameters in this model are identified as pore pressure ratio 1 and relative compressibility 2 with the latter exerting far greater influence on the resulting fluid pressure field than the former this study indicates that the fluid pressure distribution is strongly controlled by the stress distribution or the strata deformation induced by mining has a great impact on the change in hydrogeological environment in the mining region

effective stress coefficient for seismic velocities in carbonate rocks effects of pore characteristics and fluid types the concept of effective stress is key for understanding the dependence of rock elastic and compaction behaviors on stress and pore fluid pressure previous studies on the concept largely used data acquired on siliciclastic rocks carbonate rocks however display elastic and compaction behaviors that can be very different than those of siliciclastic rocks for example applying most velocity to pore pressure transforms in the context of carbonate reservoirs can be quite challenging our study used an experimental approach a disequilibrium compaction scenario to assess effective stress coefficient n for velocities in three carbonate samples displaying comparable porosities but different dominant pore types in terms of shape and compliance different saturating fluids nitrogen and distilled water were used one at a time which allowed us to compare both pore and fluid type effects on the coefficient n between these rocks we found that n is generally bounded by unity the exception is with nvs n derived from shear wave velocities obtained under nitrogen saturated conditions nvs is higher than 1 on the three studied samples under nitrogen saturated conditions the less compliant the main pore types in a given rock are the higher the value of nvs is higher than unity values of nvs indicate a deviation from the behavior predicted by existing theories this could stem from i the fact that theoretical analyses assume a pore fluid whose properties are not comparable to those of nitrogen and or ii the way the bulk volumetric strain a main factor in elastic wave propagation is incorporated into those theories

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

relationships between the petrophysical properties density seismic velocity heat generation and mineralogical constitution the petrophysical properties density p compressional wave velocity v p and heat generation a exhibit considerable variation with rock type to characterize a given rock quantitatively a single parameter the cation fraction expressed by the k value has been found to be suited best 100 crystalline rocks have been sampled to cover the widest possible range in k value from silicic types like granophyres and granites through granodiorites tonalites diorites gabbros amphibolites hornblendites pyroxenites to ultrabasic rocks p k a and v p were determined experimentally the latter at 0 5 kbar 50 mpa the interrelations between these properties were established by regression analysis the relation k and v p k show a linear dependence whereas a k a and a v p follow exponential laws in a w m 3 12 20 230 5 k mole cm 3 in a 22 5 8 15 g cm 3 and in a 16 5 2 74 v p km s all correlation coefficients are better than 0 9

deep temperature distribution along three profiles crossing the teisseyre tornquist tectonic zone in poland abstract the teisseyre tornquist tectonic zone is a large deep fracture zone separating the precambrian east european craton from the variscan system of central europe with the use of ample deep seismic sounding data the detailed crustal temperature distribution was calculated for three profiles crossing this zone in its northern central and southern segments the conversion technique of transforming the reported seismic velocities p into heat production values a was used together with the temperature dependent thermal conductivity structure and the observed surface heat flow pattern in addition to the obtained 2 d temperature distribution this work allowed a more reliable estimate of the crustal contribution to the surface heat flow and thus the characteristic moho heat flow q m 26 mw m 2 to be assessed in this region the results can also serve as an independent confirmation of the necessity of applying the p a conversion separately for phanerozoic and for precambrian rocks

combining 3 he cosmogenic dating with u th he eruption ages using olivine in basalt this paper presents new u th he crystallization and 3 he exposure ages on olivine for a suite of basalt samples from the snake river plain idaho the results provide the first demonstration that u th he geochronology can be successfully applied to late quaternary basalts using measurements of olivine phenocrysts as opposed to u rich minerals the approach employs abrasion to remove the outer rinds of olivine crystals to avoid problems with 4 he implantation from the groundmass and a specially designed extraction system that allows u th and he concentrations and isotopic compositions to be measured on the same olivine grains we demonstrate the viability of the u th he technique on basalts with ages of 373 ka and 3400 ka previously dated by ar ar and show that concurrent measurements of u th 4 he ages and 3 he cosmogenic exposure ages improve the accuracy of both methods the combinedu th 4 he 3 hetechniqueisappliedtoaseriesofbouldersandflowsfromboxcanyonnearhagerman idaho toevaluate the ages of the basalts and to use exposure ages to make inferences about the age and mode offormation of the canyon the u th he ages of the box canyon basalts are between 86 12 ka and 130 12 ka 1 sigma errors and suggest at least two periods of eruption the ages are similar to a ca 95 ka ar ar age previously assigned and distinguish the box canyon basalts from nearby ca 395 ka basalt exposure ages of boulders within the canyon at the mouth head and middle of box canyon are 21 1 19 3 and 48 3 ka scoured bedrock at the canyon head has an exposure age of 45 5 ka these data indicate that the canyon was carved before the

lithology dependent minimum horizontal stress and in situ stress estimate abstract based on the generalized hooke s law with coupling stresses and pore pressure the minimum horizontal stress is solved with assumption that the vertical minimum and maximum horizontal stresses are in equilibrium in the subsurface formations from this derivation we find that the uniaxial strain method is the minimum value or lower bound of the minimum stress using anderson s faulting theory and this lower bound of the minimum horizontal stress the coefficient of friction of the fault is derived it shows that the coefficient of friction may have a much smaller value than what it is commonly assumed e g f 0 6 0 7 for in situ stress estimate using the derived coefficient of friction an improved stress polygon is drawn which can reduce the uncertainty of in situ stress calculation by narrowing the area of the conventional stress polygon it also shows that the coefficient of friction of the fault is dependent on lithology for example if the formation in the fault is composed of weak shales then the coefficient of friction of the fault may be small as low as f 0 2 this implies that this fault is weaker and more likely to have shear failures than the fault composed of sandstones to avoid the weak fault from shear sliding it needs to have a higher minimum stress and a lower shear stress that is the critically stressed weak fault maintains a higher minimum stress which explains why a low shear stress appears in the frictionally weak fault

borehole stability analysis accounting for anisotropies in drilling to weak bedding planes abstract borehole instabilities pose significant challenges to drilling and completion operations particularly in regions with weak bedding planes and pre existing fractures where formations have strong anisotropies the bedding planes rock anisotropy and their impacts on horizontal stresses are considered in the proposed model to improve borehole stability modeling this improved model enables to calculate borehole failures and minimum mud weight along borehole trajectories with various drilling orientations versus bedding directions laboratory test data of rock compressive strengths are analyzed and a new correlation is developed to allow for predicting uniaxial compressive strengths in weak rocks from sonic velocities time dependent rock compressive strength is also examined to analyze the wellbore failure evolution with time the slip failure gradient in the weak planes is derived which can be used to model wellbore sliding shear failure in the planes of weakness the mud weight applied to prevent borehole shear failures in both intact rocks and ones with weak bedding planes can be obtained from the proposed model

dating geomorphologic surfaces using cosmogenic 3he cosmogenically produced 3he in rocks can be used to date geomorphologic surfaces using the late pleistocene bonneville flood event the subsequent development of the provo shoreline and associated volcanic rocks a production rate of 432 atoms g yr in olivine is determined for 38 56 n latitude present geomagnetic latitude 46 5 at 1445 m altitude integrated over the last 14 400 years measurements of cosmogenic helium in flood deposits on river scoured surfaces and lava flows show that it is possible to date directly large scale events such as volcanic eruptions and catastrophic floods olivine and pyroxene are both satisfactory minerals for cosmogenic dating by the 3he method however in most cases plagioclase and quartz are unsuitable because they do not quantitatively retain helium isotopes in their lattices

deep geothermal structure and mantle heat flow along the barents sea east alps geotraverse the deep geothermal structure of the continental crust along the barents sea eastern alps profile is investigated numerical geothermal models are constructed the mantle heat flows and the moho temperatures are computed for different tectonic blocks of the crust within ancient platform areas the mantle heat flow varies from 15 28 mw m 2 in the baltic shield to 6 14 mw m 2 in the stable russian plate in the baltic syncline the mantle heat flow is about 25 33 mw m 2 the maximum mantle heat flow value of 40 m w m 2 characterizes the central european platform mantle heat flow values vary from 15 to 35 mw m 2 in the hercynian part of central europe for the thermal model the seismic velocity data are used the moho temperature values vary from 400 c in the ancient platform area to 900 1000 c in the alpine zone in the northern baltic shield tm 600 c

geothermal modelling of the lithosphere in the central baltic shield and its southern slope lithospheric temperature and heat flow density hfd were studied in the central baltic fennoscandian shield and its subsurface continuation to the south along a transect trending from eastern finland to southern estonia the transect represents an example of a low hfd 30 mw m 2 archaean craton on a thick 150 190 km lithosphere surrounded by early and middle proterozoic mobile belts on a thinner 110 150 km lithosphere with slightly elevated hfd 35 55 mw m 2 numerical 2 d conductive models were constructed in which peridotite solidus temperatures were assigned to those depths which correspond to the seismically determined lithosphere asthenosphere boundary this technique was found to reduce the effect of uncertainties in heat production and thermal conductivity values on the simulation results upper crustal heat production values for the finnish terrain were taken from published geochemical analyses of outcropping rocks for the estonian terrain new heat production values were measured from core samples representing nineteen deep boreholes middle and lower crustal lithologies were estimated with the aid of the deep seismic vpvs data and corresponding heat production values were adapted from global xenolith averages and from data for granulites cropping out in other precambrian areas the results of the modelling suggest that the lithosphere and moho depth variations are only weakly reflected in the measured surface heat flow data which are mainly controlled by heat sources in the upper crust the simulated heat flow densities at 50 km depth approximately at the moho are relatively low and range from 12 mw m 2 at the archaean northeastern end to 19 mw m 2 on the proterozoic southwestern end of the transect simulated temperatures at 50 km depth increase from northeast to southwest ranging from 450 550 c in eastern finland to about 650 c in estonia sensitivity of the simulations to parameter changes was studied by varying the heat production and thermal conductivity values the extreme values for the moho temperature estimates thus obtained may be about 50 k lower or 100 k higher than the values above the corresponding sensitivity of the moho hfd is about 6 mw m 2 and of the surface hfd 5 20 mw m 2 respectively

time dependent brittle creep as a mechanism for time delayed wellbore failure abstract by drilling a wellbore cavity high stresses arise at the wellbore wall leading to the formation of breakouts which enlarge the hole to an elongated shape oriented along the direction of the minimum principal stress if formation of breakouts is delayed rock debris falls on the drill bit which may lead to stuck pipe problems or even abandonment of the drill string reasons for such time delayed failure of the wellbore may be due to chemical fluid rock interaction especially in swelling clays however such delayed instabilities have also been observed e g in gneiss formations at the ktb borehole germany that are not known to exhibit a swelling behavior we propose to explain observations of delayed wellbore failure by time dependent brittle creep which has been observed for many types of rocks following this approach rock fails under loads less than their short time strength but after a long enough time span this time is in exponential relation to the load applied to the rock we implement a model developed for the creation of shear bands on the basis of time dependent brittle creep by amitrano and helmstetter 2006 here progressive damage of the formation is captured by a damage parameter d and the time to failure ttf young s modulus e is decreased by a factor every time ttf is expired i e when failure is reached subsequently stresses are redistributed according to the new distribution of e in the formation using this approach we obtain closure of the well with primary and secondary creep phases wellbore breakouts are formed progressively with deepening and widening of the initially damaged zone after a certain time the formation of breakouts comes to an end with an omori like decay of failure approaching a steady state

dual porosity poroelastic analyses of wellbore stability this paper presents a numerical solution of dual porosity poroelastic formulations that couple solid deformations with fluid flow in both matrix and fracture systems of naturally fractured reservoirs the finite element numerical method was applied to solve the inclined wellbore problem in which mud weight was considered for both permeable and impermeable boundary conditions failure criteria relevant to tensile collapse and shear failure modes were introduced into the numerical model and resulting failure areas around boreholes were subsequently defined several related applications including the stability of inclined and horizontal wellbores under various in situ stress regimes were evaluated upper and lower mud weight bounds and the most stable borehole orientations were determined the best trajectory selections for horizontal boreholes were also investigated

mantle flow and the evolution of the lithosphere abstract the evolution of the lithosphere is mainly controlled by time dependent forces due to 1 plate tectonic processes and 2 sublithospheric mantle flow plate tectonic processes like continental collision may provide strong thermal disturbances and after completion may trigger secondary convection beneath the lithosphere without such mantle flows lateral variations of temperature and associated variations of lithospheric thickness hl or seismic velocities will equilibrate after time scales which are considerably shorter than the geologic ages of several provinces in europe we will discuss the role of sublithospheric mantle flows on the evolution of the lithosphere and the asthenosphere and compare the results with observations from the european lithosphere steady state convection models with a rheology based on laboratory data on lherzolite show that there exists a simple relationship between mantle heat flow and hl however steady state may be reached only after transition times of the order of 1 ga during such times hl shows strong lateral variations if such variations were inherited from plate tectonic events the role of sublithospheric convection would be to prolong their lifetime considerably at higher heat flows sublithospheric mantle convection becomes strongly time dependent with a time scale of 25 50 ma on this time scale variations of hl are small altogether we observe no strong direct correlation between upwelling mantle flows and thin lithosphere in some cases the correlation is even reversed on the other hand the presence or absence of a partially molten asthenosphere may be strongly affected by sublithospheric convection and may show strong lateral variations as well these variations in space and time can be correlated with the laterally heterogeneous lvz with seismic tomography data and with intraplate volcanism in europe mantle convection mantle diapirs and variations in thickness of the lithosphere may exert forces on the lithosphere and crust they will lead to bending of the lithosphere changes in surface topography and thinning or thickening of the crust induced by lateral flows within the lower crust depending on the time scale of loading and unloading of the lithosphere and the relaxation time associated with lower crustal flows characteristic relations between moho depth and surface topography are predicted comparison with observed correlations between moho depth surface topography and lithospheric thickness of european crustal age provinces show some trends in agreement with the proposed crustal deformation model

fully coupled analysis of interaction between the borehole and pre existing fractures abstract the coupling of rock and thermal stresses along with fluid pressure are particularly important in fractured rock masses since stress induced changes in permeability can be large and irreversible under perturbations resulting from various natural and induced activities a new method is presented to model fracture permeability changes during drilling in fractured rocks the approach includes finite element method fem for fully coupled thermo poroelastic analysis of stress distribution around borehole and displacement discontinuity method ddm to model fracture deformation three cases of overbalanced underbalanced and balanced drilling fluid pressure conditions are employed the application of the approach illustrates that the maximum variation of aperture occurs near to the borehole and become negligible at large distances away from the borehole it was shown that mechanical stresses caused by excavation of the rock contribute to short time while fluid pressure and thermal stresses are responsible for long term permeability variation of fractures the results show that the fracture permeability decreases with depletion of the fracture and the rock matrix while increases with pressurization and cooling of fracture during overbalanced condition the fracture permeability reduces during balanced fluid pressure condition within a short time then enhances by cooling of the rock surrounding the borehole

a new geodynamic model for the seismicity and in situ stresses of the mediterranean region abstract the application of plate tectonic concepts to mediterranean seismicity involves contradictions and anomalies which explains why there is such a great number of conflicting theories each of them based on different geophysical data a new model designed to take into account and explain all the data known to date is presented this is a magmatic vertical chimney where the magma is moved upwards while rotating thus exerting vertical pressure on the periphery of the lithospheric lid moreover it has some relevance to geological structures if this model is correct earthquake prediction can be viewed from an entirely different angle

discussion on integrating borehole breakout dimensions strength criteria and leak off test results to constrain the state of stress across the chelungpu fault taiwan abstract the in situ stress computations presented by haimson et al 2010 overestimated the maximum horizontal stress in the vicinity of the chelungpu fault taiwan our analyses from extended leak off tests drilling induced tensile fractures and anderson s faulting theory show considerable lower values for the maximum horizontal stress our results suggest that closer to the ruptured fault more s h is released implying a strong earth crest in the shallower section which is far away from the ruptured fault however it would require more detailed and intensive work in order to achieve an accurate interpretation of the in situ stress in this region

linear pressurization method for determining hydraulic permeability and specific storage of a rock sample summary we studied the methodology and applicability of linear pressurization for determining simultaneously hydraulic permeability and specific storage of a rock sample we analytically solved the governing equation for 1 d pressure diffusion through a homogenous isotropic cylindrical rock specimen located between a downstream and an upstream reservoir considering linear pressurization of the upstream fluid as the boundary condition the solution consists of a transient and a steady state for which both differential pressure between the two reservoirs and injection rate at the upstream reservoir assume constant values since the steady state differential pressure is linearly proportional to the compressive storage of the downstream reservoir tests conducted with a systematic variation of the size of the downstream reservoir permit determining permeability and specific storage from the intercept and slope of the linear relationship alternatively simultaneous measurement of differential pressure and fluid injection rate at steady state conditions provides a basis for calculation of the hydraulic properties as previously presented for linear injection experiments were performed on fontainebleau sandstone samples to document the feasibility of the method the determined storage capacity value is used to calculate various poroelastic parameters such as bulk and pore space compressibility skempton and the effective pressure coefficient

cosmogenic 3he and 21ne age of the big lost river flood snake river plain idaho the big lost river flood in southeastern idaho occurred 20 500 calibrated yr b p on the basis of dates derived from cosmogenic 3 he and 21 ne measurements of samples from flood deposited boulders and from scour features this date corresponds to a date of 16 900 14 c yr b p and is close in age to several other cataclysmic flood events in western north america it may mark evidence for widespread warming at the end of the pleistocene in western north america the big lost river flood was smaller than some other late pleistocene floods such as the bonnevi1le flood and the missoula floods thus some samples exposed after the flood had significant amounts of cosmogenic 3 he and 21 ne that was acquired before the flood occurred

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

poroelastic response of a borehole in a non hydrostatic stress field abstract this paper deals with the solution of the stress displacement and pore pressure field induced by the drilling and or the pressurization of a vertical borehole the rock which is permeated by fluid is assumed to behave as a poroelastic material with compressible constituents following the biot theory the analytical solution is derived in the laplace transform space and is transformed to the time domain using an approximate numerical inversion technique the solution reveals a full range of coupled poroelastic effects which provide potential mechanisms for delayed borehole instability and shear fracture initiation inside the rock

two dimensional inverse modeling of crustal thermal regime with application to east european geotraverses the modeling of crustal thermal regimes is basically the inference of subsurface distributions of heat flow density and temperature by continuing downward the near surface measurements of heat flow density temperature thermal conductivity and heat production to account for the uncertainties in the near surface data and in the subsurface distributions of thermal conductivity and heat production and to provide explicit estimates for the uncertainties in the resulting models we have employed a bayesian inverse formulation that incorporates the a priori information on model parameters in the form of a joint gaussian probability density function and that gives the inverse solution in the form of the most probable estimates and the a posteriori covariance matrix for the model parameters to illustrate the method we apply it to the modeling of thermal regimes along east european geotraverses eegt 5 and eegt 1 using both a linear 5 point finite difference and a quadrilateral isoparametric finite element discretization two features are emphasized first we introduce an a priori autocovariance function for the heat flow density at model base to help stabilize the solution second we demonstrate the improvement in solution accuracy and resolution when a priori temperature information is available and incorporated in the inversion for the geotraverses studied the finite difference and finite element formulations yield essentially identical results although there is a substantial difference in the extent of their parameterization in general the surface and basal heat flow densities are similar in shape implying a relatively uniform total crustal heat production across the geotraverses both the heat flow densities and the crustal temperatures exhibit strong lateral variations that appear to correlate with the tectonic age the heat flow densities and temperatures are low in the precambrian east european platform relatively higher in the variscan units high in the alps and very high in the pannonian basin the results support the presence of lateral variations of up to 40 m w m 2 in the moho heat flow density provided that one accepts as reasonable an a priori standard deviation of 25 for the surface heat flow density and an uncertainty of a factor of two for the heat production finally the a posteriori and the observed surface heat flow densities match well but exceptions occur in localized regions suggesting the presence of perturbing effects that cannot be accounted for in a steady state purely conductive model

helium isotopic variability within single diamonds from the orapa kimberlite pipe abstract the distribution and isotopic composition of helium has been measured in a suite of well characterized one carat diamonds from the orapa kimberlite botswana crushing of the diamonds in vacuo indicates that most of the helium is contained by the matrix generally greater than 90 rather than by the inclusions step heating experiments performed on inclusion free fragments remaining after crushing indicate that the3he 4he ratio is variablewithin individual diamonds the fragments as small as 10 mg were heated in two timed steps both at 2000 c in every case lower3he 4he ratios are observed in the first graphitization step 0 05 3 atmospheric while the last heating step releases helium with systematically higher3he 4he ratio 30 80 atmospheric we suggest that this internal isotopic variability is the result of stepwise graphitization the first heating step initiates graphitization which nucleates around defects and the second heating step graphitizes the relatively defect free regions of the diamond the3he 4he ratio measured using the partial graphitization technique differs by up to a factor of 100 within a single specimen the inclusion free fragments release small quantities of helium below 2000 c which suggests that helium release is obtained only by graphitization the3he contents of the monocrystalline diamonds are relatively constant at 3 10 13 cm3 stp gram and indicate that most of the isotopic variability is due to radiogenic4he the variations in4he content are either related to zoning of th and u in the diamonds i e in situ decay to zoning of inherited4he or to implantation of particles from a th and u rich environment i e kimberlite because the orapa diamonds were mined from roughly 40 m depth in the kimberlite spallation reactions from cosmic ray interactions are not a significant source of3he however calculations based on the age of the kimberlite 90 m y and reasonable th and u abundances suggest that most of the3he in the orapa diamonds could be produced by6li n t in the diamond although this may not be true of all diamonds nuclear reactions in the crust and mantle including spallation reactions at the surface can explain many of the high3he 4he ratios previously reported for diamonds

effect of anisotropy and hydro mechanical couplings on pore pressure evolution during tunnel excavation in low permeability ground abstract continuous monitoring around drifts excavated in the underground research laboratory url of the french national radioactive waste management agency andra in callovo oxfordian claystone revealed the development of a fractured zone extensional and shear fractures induced by the excavation moreover the field monitoring during the excavation showed an anisotropic pore pressure field around the drifts with zones of marked overpressure for drifts parallel to the horizontal major principal stress even though the initial stress state in the drift section is quasi isotropic the pore pressure evolution and the mechanical response are anisotropic these observations suggest that the intrinsic anisotropy of the material plays a key role in the response of rock formation a poroelastic analysis of the pore pressure change induced by drift excavation is performed the principal objective is to simulate the main trends of the pore pressure evolution with a simple model taking into account the elastic anisotropy of the material it is shown that depending upon the permeability and the degree of anisotropy of the rock strong overpressures can be induced by the excavation furthermore it is observed that the distribution of overpressures around drifts depends also on the drifts orientation finally an analysis of the onset of failure shows the key role of the hydro mechanical coupling on the extension of the failed zone around the drifts

paleomagnetic results from the snake river plain contribution to the time averaged field global database 1 this study presents paleomagnetic results from the snake river plain srp in southern idaho as a contribution to the time averaged field global database paleomagnetic samples were measured from 26 sites 23 of which 13 normal 10 reverse yielded site mean directions meeting our criteria for acceptable paleomagnetic data flow ages on 21 sites range from 5 ka to 5 6 ma on the basis of 40ar 39ar dating methods the age and polarity for the 21 dated sites are consistent with the geomagnetic reversal time scale except for a single reversely magnetized site dated at 0 39 ma this is apparently the first documented excursion associated with a period of low paleointensity detected in both sedimentary and igneous records combining the new data from the srp with data published from the northwest united states between the latitudes of 40 and 50 n there are 183 sites in all that meet minimum acceptability criteria for legacy and new data the overall mean direction of 173 normally magnetized sites has a declination of 2 3 inclination of 61 4 a fisher concentration parameter of 58 and a radius of 95 confidence 95 of 1 4 reverse sites have a mean direction of 182 4 declination 58 6 inclination of 50 and 95 of 6 9 normal and reversed mean directions are antipodal and indistinguishable from a geocentric axial dipole field at the 95 confidence level virtual geomagnetic pole dispersion was found to be circularly symmetric while the directional data were elongate north south an updated and corrected database for the northwestern u s region has been contributed to the magnetics information consortium magic database at http earthref org

stress orientations from borehole wall fractures with examples from colorado east texas and northern canada azimuthally aligned breakouts in oil wells are explained as shear fractures in the zone of amplified stress difference near the borehole in a stress field having unequal horizontal principal stres

porothermoelastic effect on wellbore stability in transversely isotropic medium subjected to local thermal non equilibrium abstract provided that the hydraulic boundary conditions at the wellbore wall are identified as permeable pbc and impermeable impbc and that the assumption of plane strain holds the present paper formulates a newly porothermoelastic analytical solution which is based on the thermodynamic principles in laplace space domain for inclined borehole the adopted linear porothermoelastic model embodies the transient heat transfer process in the presence of local thermal non equilibrium ltne during the derivation the solution is available for the cases where the wellbore axis of inclined wellbore is perpendicular to the isotropic plane of the low permeable transversely isotropic porous medium under non hydrostatic stress field condition the wellbore problem is decomposed into axisymmetric and deviatoric loading cases the numerical results of the time dependent distributions of field variables are obtained by virtue of performing the inversion technique for laplace transforms an integrated geomechanical wellbore stability model is proposed which is not only comprised by the abovementioned new porothermoelastic solution but also associated with the theoretical derivations of strength criteria to evaluate the rock to shear failure which include mogi coulomb criterion and single weak plane criterion when a horizontal borehole is drilled along the direction of in the minimum horizontal in situ stress the numerical comparative analyses in the present paper examine the combined influence of material anisotropy and thermal effect which embody in the ltne and local thermal equilibrium lte models on the thermally induced pore pressure stresses and the shear and tensile failure regions around the wellbore

transient thermo poroelastic finite element analysis of borehole breakouts abstract a thermo poroelastic model which is fully coupled to conductive and convective transport processes is employed to analyze wellbore stability and borehole breakout using the two dimensional finite element method stress distribution around an open borehole subjected to non isothermal non hydrostatic loading is investigated four strength criteria mohr coulomb drucker prager modified lade and mogi coulomb are utilized to the stability of boreholes borehole breakout propagation is assumed to occur as successive spalling of thin layers of rock caused by stress concentration and redistribution around boreholes and breakouts stabilization of breakouts is examined by deploying mogi coulomb failure criterion for different wellbore fluid pressures and temperatures to evaluate the influence of various prominent factors on borehole breakouts a parametric study is conducted results indicate that mohr coulomb criterion offers the highest breakout depth so it underestimates rock strength and requires the highest value of the least mud weight necessary for wellbore stability and drucker prager provides the lowest estimation of the breakout depth borehole breakout stabilization is concluded to occur as consequences of reduction in effective tangential stress and increase in confinement effect of radial stress around breakout periphery moreover confinement effect of borehole fluid appears to be more influential in borehole stability than pore pressure effect parametric analysis reveals that drilling a borehole with small radii applying overbalanced fluid pressure condition and maintaining drilling fluid temperature lower than rock temperature boost the creation of a favorable borehole stability condition