variations in mode of formation and temperature of oceanic deep waters over the past 125 000 years marked changes in the deep water temperature of the different oceanic basins during the past 125 kyr are revealed by a detailed comparison of the oxygen isotopic composition ofbenthicforaminiferafrom sediment cores taken from the norwegian sea and the pacific atlantic and southern oceans the last interglacial deep water temperatures were similar to the modern ones at the 5e 5d transition around 115 kyr bp the deep water temperatures dropped below approximately 0 c in the pacific and southern indian oceans in the atlantic ocean the deep water temperature dropped in 2 successive steps at 115 and 75ky bp the global ocean deep water temperatures were in the 0 to 1 c range for the whole of the last ice age isotope stages 4 2

late pleistocene holocene evolution of the doce river delta southeastern brazil implications for the understanding of wave influenced deltas abstract brazil s coast displays many wave influenced deltas that are still incompletely understood despite their significance for reconstructing the history of late pleistocene holocene sea level changes in this region and the potential to advance in the knowledge of wave influenced deltas among them the doce river delta is the most expressive given its larger geographic extension the present work shows that this is a wave influenced delta that evolved within the context of late pleistocene holocene sea level fluctuations rather than a mid holocene lagoonal delta as proposed elsewhere the delta had an initial progradation at an osl age of 132 7 9 1 ka after the marine isotope stage mis 5e which was followed by major progradation between 45 775 49 391 and 29 678 29 226 cal yr bp beach ridges spits formed during these phases were in part destroyed during an ensuing transgression between 8161 7933 and 4974 4850 cal yr bp the central delta remained dominated by fluvial processes which kept pace with the rising sea level due to relatively high rainfall in the doce river drainage basin the doce river delta started to grow again after the late holocene transgression a process still on going at the present time during this evolution the doce river was deflected northward and then southward a process that did not result in any significant facies change when the updrift and downdrift sides of the river s mouth are compared such a pattern is in disagreement with previously proposed asymmetry index models for wave influenced deltas which predict an uneven distribution of sandy or muddy rich deposits in the updrift and downdrift sides of the river s mouth respectively several other deltas along the brazilian coast also display facies configuration similar to the doce river delta or have patterns opposed to those proposed by the asymmetry index models based on these analogs it can be stated that the geometries and facies patterns of wave dominated deltas are not determined simply by the interplay of fluvial and marine processes instead they can be strongly influenced by the fluvial influx sea level changes and or tectonic reactivations as it appears to be the case of brazilian wave influenced deltas including the doce river delta such a conclusion calls for a word of caution when applying processes based facies models to reconstruct wave influenced deltas in the sedimentary record

surface deformation of westerly granite during creep holographic interferometry has been used to observe the surface deformation of westerly granite during creep experiments at 50 mpa 500 bars confining pressure interference fringes on the holograms produced by this technique constitute contour maps of the sample surface shape change between exposures several distinctive characteristics of the surface shape change have been observed in general surface deformation during primary and secondary creep while highly inhomogeneous is distributed fairly uniformly no large scale pattern is seen in the surface shape changes at approximately the onset of tertiary creep surface displacements become more locallized eventually approx 85 90 of time to failure a distinct ridge is formed which exactly coincides with the location of the final fracture

stress concentrations and failure modes in horizontal wells accounting for elastic anisotropy of shale formations abstract this study presents a systematic analysis of stress changes near the horizontal well sections drilled in shale and clay formations using a new approach most of the prior analyses of stress concentrations and wellbore stability in anisotropic shale formations use the traditional lekhnitskii amadei solutions we present a faster set of analytical stress functions based on largely overlooked solutions by green and taylor first the full theoretical range of possible compliances for transverse isotropic materials is evaluated using data for 5 shale clay samples and 12 additional test materials contour plots show the principal stress trajectory patterns and variation in the spatial stress concentration due to deviation from isotropic elasticity next we show how uni axial far field stress solutions can be mutually superposed and combined with stress function solutions for local stress concentration due to internal pressure on the wellbore the reappraised set of green and taylor equations is then applied to horizontal wells in three andersonian stress states for compressional strike slip and extensional basins the stress concentrations in the horizontal wellbore are evaluated for well depths between 0 and 10 000 ft using the elastic stiffnesses from five shale and clay samples to show the stress deviations from the isotropic reference case finally a full wellbore stability evaluation model combines the stress concentrations around the wellbore with failure criteria to determine the safe drilling window at each well depth for each andersonian stress state practical graphs and supporting equations are provided to assess wellbore stability in transverse isotropic shale

mass wasting triggered by the 5 march 1987 ecuador earthquakes on 5 march 1987 two earthquakes ms 6 1 and ms 6 9 occurred about 25 km north of reventador volcano along the eastern slopes of the andes mountains in northeastern ecuador although the shaking damaged structures in towns and villages near the epicentral area the economic and social losses directly due to earthquake shaking were small compared to the effects of catastrophic earthquake triggered mass wasting and flooding about 600 mm of rain fell in the region in the month preceding the earthquakes thus the surficial soils had high moisture contents slope failures commonly started as thin slides which rapidly turned into fluid debris avalanches and debris flows the surficial soils and thick vegetation covering them flowed down the slopes into minor tributaries and then were carried into major rivers rock and earth slides debris avalanches debris and mud flows and resulting floods destroyed about 40 km of the trans ecuadorian oil pipeline and the only highway from quito to ecuador s northeastern rain forests and oil fields estimates of total volume of earthquake induced mass wastage ranged from 75 110 million m3 economic losses were about us 1 billion nearly all of the approximately 1000 deaths from the earthquakes were a consequence of mass wasting and or flooding

an analytical model of coal wellbore stability based on block limit equilibrium considering irregular distribution of cleats abstract an analytical model of coal wellbore stability is established based on the block limit equilibrium method taking the irregular distribution of cleats into account because coal seam is cut into discontinuous blocks with different shapes and sizes block slip can be treated as the symbol of coal wellbore instability according to the geometric characteristic of blocks blocks surrounding the wellbore can be divided into 3 categorizes type i type ii and type iii to evaluate the block slip the stress distribution near the wellbore is determined based on the continuous medium mechanics and the fracture mechanics using mohr coulomb criterion and limit equilibrium method the critical conditions of block slip are derived by defining the failure coefficient including type i type ii and type iii the proposed model is verified by comparison of the obtained results from the proposed analytical method with the discrete element method as well as the actual drilling it can be proved that the proposed model can predict accurately block slip additionally the effect of drilling fluid pressures on the wellbore stability is specially investigated the results show that with the increasing of drilling fluid pressure the degree of wellbore collapse may be more severe meanwhile the analysis method of reasonable drilling fluid pressure for coal drilling is proposed based on the interval theory combining the slip area of blocks finally the parametric study is focused on investigating the effects of the internal friction angle of face cleats the cohesion of face cleats and the tensile strength of butt cleats on the failure coefficients of blocks

borehole breakouts and compaction bands in two high porosity sandstones we compare the shape and mechanism of failure around vertical boreholes drilled in blocks of two high porosity sandstones subjected to unequal far field principal stresses tablerock sandstone has a porosity of 28 and is composed of 55 quartz and 37 weaker feldspar grains grain cementation is substantial through microcrystalline quartz critical far field stresses induce failure around boreholes in the form of v shaped dog eared breakouts the result of dilatant intra and trans granular microcracking subparallel to both the maximum horizontal far field stress and to the borehole wall on the other hand boreholes in mansfield sandstone which has similar porosity 26 but contains mainly quartz grains 90 held together primarily by spot sutured contacts fail by developing fracture like breakouts we show evidence suggesting that mansfield sandstone breakouts are preceded by the development of a narrow several grain diameters localized compaction zone along the direction of the least horizontal stress which is where the maximum compressive tangential stress concentration occurs failure mechanism here is simply the removal by the circulating drilling fluid of mainly intact grains loosened during the formation of the compaction band the type of cementation mineral homogeneity grain strength and sphericity appear to be major factors in the formation of compaction bands some breakout dimensions in both rocks are correlatable to the far field principal stresses and can be potentially used in conjunction with other information as indicators of their magnitudes

borehole stability analysis in a thermoporoelastic dual porosity medium the problem of diffusion and mass transfer in dual porous media is considered in a threephase framework the solid phase is assumed to contain two distinct cavities filled with fluid the porous mixture is composed of two overlapping media the porous blocks and the fissure network the fluid can transfer between the cavities due to fluid pressure difference in addition hydraulic and thermal diffusions take place through the mixture a global understanding of mass transfer diffusion and deformation is provided the governing equations associated with these phenomena are presented for a mixture in thermal equilibrium the finite element approximation of the governing equations is formulated and applied to the stability analysis of a vertical borehole a parametric analysis is carried out to evaluate the influence of mass transfer on the pressure profiles of the fluids around the borehole permeable and a semipermeable boundary conditions are compared to predict the potential for failure of the wellbore under drained and partially undrained conditions

isrm suggested methods for rock stress estimation part 3 hydraulic fracturing hf and or hydraulic testing of pre existing fractures htpf contents1 introduction 10112 summaryoftestmethods 10123 assumptionsandlimitations 10123 1 hydraulicfracturing hf 10123 2 hydraulictestsonpre existingfractures htpf 10124 apparatus 10134 1 fractureorientationdetectiondevices 10145 personnelquali cation 10146 equipmentperformance 10147 hydraulicfracturingtestprocedure 10148 hydraulic fracturedelineation 10168 1 impressionpackertechnique 10168 2 geophysicalimagingtechnique 10179 obtainingcriticalpressureparametersfromthepressure timerecords 101710 stresscalculations 101810 1 hydraulicfracturing hf 101810 2 hydraulictestsonpre existingfractures 101911 reportingofresults 1019references 1020

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

quaternary fault control on the coastal sedimentation and morphology of the sao francisco coastal plain brazil abstract the role of neotectonic faulting in shaping topography and controlling sediment deposition has been increasingly documented along the stable continental margin of south america but the level of the present day tectonic stability of this margin is still a matter of debate this paper presents an investigation of the effects of quaternary fault reactivation on the sao francisco coastal plain sfcp this feature is a 56 km long progradational coastal plain located in the sergipe alagoas basin along the continental margin of brazil the sfcp is bounded on the west by a system of normal faults striking n s to ne sw the horst that bounds this plain is capped by the miocene barreiras formation and quaternary debris flow and fluvial deposits whereas the graben is filled with braided stream deposits and sandy beach ridge sediments the faulting contributed to the generation of accommodation space for the deposition of coastal sediments on the plain these processes started with the reactivation of cretaceous faults during the miocene during the quaternary fault reactivation and associated soft sediment deformation occurred between 82 and 8 3 ka with minor pulses between 1700 and 60 cal yr bp several faults along this margin are tectonically active and may represent the onshore continuation of gravitational collapses that have been documented offshore along the continental shelf and slope

closed form solution of stress state and stability analysis of wellbore in anisotropic permeable rocks abstract this work aims to develop a closed form solution of stress state around wellbore drilled in the anisotropic permeable rocks taking into account the transient effect of fluid flow for this purpose a simplified method was adopted to solve the anisotropic hydraulic diffusion equation knowing the explicit transient evolution of pore pressure the hydraulic and hydro mechanical potentials were proposed through which stress state around wellbore are determined thanks to the use of the well known complex potential approach the validity of this closed form solution was verified by comparing with the result obtained from the numerical simulation by combining the analytical solution of stresses on well wall with a chosen anisotropic failure criterion of rock mass the stability analysis of wellbore was carried out through the parametric study we elucidated the influence of different factors on the safe mud pressure window notably the anisotropic poro elastic properties the anisotropic strength of the rock masses as well as the anisotropic initial stress state and the inclined bedding plane of formation

late oligocene miocene transgressions along the equatorial and eastern margins of brazil abstract the seaboard of brazil is a highly favorable setting for studying episodes of worldwide sea level rise because of its passive nature previous studies of the equatorial coast of brazil have led to the recognition of extensive oligo miocene and miocene marine influenced deposits but these deposits have been overlooked in other coastal areas of brazil the aim of the present study is to show the origin and evolution of late oligocene and miocene marine influenced strata exposed in more than 5000 km of coastal areas along the wnw trending equatorial and nne trending eastern brazilian continental margins we review the most relevant data concerning the strata of these ages exposed along the equatorial and eastern margins of brazil and combine them with new data based on these occurrences we analyze the history of sea level fluctuations and show that marine influenced strata are more widespread in these margins than originally thought our main results indicate that a marine influence is not only imprinted in the deposits exposed along the equatorial margin but it is also widespread in several areas of the eastern margin for a long geological time interval ranging from the end of the cretaceous or paleogene up to the late oligocene most of the brazilian coast remained a non depositional site exposed to subaerial erosion and lateritic soil development the combination of sea level rise and tectonic subsidence promoted sediment accumulation in the onshore portion of several basins two transgressive episodes occurred one in the oligo miocene and another in the early middle miocene the latter gave rise to deposition of the barreiras formation however during the supposedly middle miocene eustatic sea level highstand in coastal brazil there was a renewed and extensive phase of non deposition and subaerial erosion with lateritic soil formation a process that lasted up to the late quaternary this mismatching of the sedimentary record with other south american and continental areas is discussed herein as in many other areas of south america and of the world the regressive transgressive events recorded along the brazilian coast most likely responded to a combination of eustatic sea level fluctuations and local factors such as tectonic activity intraplate stresses and changes in dynamic topography during the post rift history of the eastern south american plate

effects of water on rock strength in a brittle regime abstract failure strengths of granite and andesite have been measured under various conditions of strain rate and confining pressure both in the dry and wet states constant stress creep tests for granitic samples have been also conducted all specimens show that the failure strength decreased linearly as the logarithm of the strain rate decreased the strain rate dependence of the failure strength is increased at higher confining pressures the strain rate effect is more apparent on the failure strengths of wet samples than dry samples in lower confining pressure ranges in the constant stress creep experiments the creep failure strength decreased as the logarithm of the time to failure increased time dependent failure strength of all rocks observed in the regime of the present study may be interpreted by subcritical crack growth assisted by the stress corrosion mechanism

stress induced borehole elongation a comparison between the four arm dipmeter and the borehole televiewer in the auburn geothermal well the nature and origin of borehole elongation recorded by the four arm dipmeter calipers is studied utilizing information obtained from hydraulic fracturing stress measurements and borehole televiewer data taken in a well located in auburn new york a preferred orientation n10 w s10 e 10 and a less prominant e w orientation of borehole elongation was observed on two runs of the dipmeter comparisons of borehole geometry determined using the televiewer and the dipmeter show that both tools give the same orientation of borehole elongtion provided that the zone of elongation is longer than 30 cm comparisons of dipmeter caliper data with orientation of in situ stress and natural fractures obtained from hydrofracturing tests and televiewer data show that the n10 w s10 e borehole elongations 1 are axisymmetric 2 are aligned with the minimum horizontal stress sh and 3 are not associated with natural fractures intersecting the well these elongations are interpreted as stress induced well bore breakouts the e w elongation direction is characterized by an asymmetric borehole cross section in thinly bedded rocks and is not caused by breakouts this asymmetric geometry can be discriminated from breakouts using the oriented electric measurements provided by the dipmeter this study demonstrates that the dipmeter can be used to determine the orientation of sh by mapping breakouts confirming the results of earlier less detailed studies and provides a firm basis for mapping regional stress patterns using existing dipmeter data

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

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

effect of confining pressure unloading on strength reduction of soft coal in borehole stability analysis underground borehole drilling usually causes instability in the surrounding coal due to in situ stress redistribution including stress concentration and stress release however the mechanisms of unloading induced coal strength reduction are still poorly understood the primary objective of this study is to investigate the effect of confining pressure unloading on soft coal strength reduction for borehole stability analysis a series of mechanical tests were conducted on both the traditionally and newly reconstituted coal samples under two different experimental stress paths including conventional uniaxial triaxial compression and triaxial compression with confining pressure unloading the unloading stress path was obtained by analyzing the stress redistribution around a borehole to capture a more accurate coal mechanical response according to our experimental results plastic deformation generated before failure under the unloading stress path is smaller than that generated under the conventional loading stress path furthermore the cohesion of the traditionally and newly reconstituted samples diminishes approximately by 44 77 and 29 66 respectively with confining pressure unloading indicating that there is a significant reduction in coal strength due to confining pressure unloading the mechanism for unloading induced coal strength reduction comes from confining pressure unloading induced increase in shear stress on the fracture surface and a decrease in shear strength this effect increases the shear slipping potential whose driving force generates tension fractures at both ends of the preexisting fractures

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

a reassessment of the stress and natural fracture orientations from analysis of image logs in the chinese continental scientific drilling program borehole at donghai county jiangsu province china abstract the chinese continental scientific drilling ccsd project has drilled a 5100 m deep research borehole in the sulu ultra high pressure uhp metamorphic belt eastern china the uhp metamorphic belt is thought to be a product of continent continent collision and has consequently experienced very intensive structural deformation based on a more complete well log data set of ccsd borehole we can have a more detailed and reliable study on the structure features of borehole wall and other rock physical properties than a previous study published in 2009 abundant data related to borehole breakouts bos drilling induced tensile fractures ditfs and natural fractures were collected from the image logs the bo and ditf data indicate that the average direction of the maximum horizontal principal stress sh of the ccsd borehole site is about 79 2 which is consistent with the convergent direction e w of the pacific sea plate with respect to the eurasian plate analysis of ditfs indicated that in the case of the ccsd borehole axial drilling induced tensile fractures aditfs occur occasionally in the upper section 0 2300 m of the borehole with low dipping angle 0 10 while transverse drilling induced tensile fractures tditfs occur significantly in the lower section 3800 5000 m with high dipping angle 10 30 the natural fracture distribution at depth in the metamorphic rocks of the ccsd borehole indicates that 1 the failure strength of rocks and borehole depth are two factors that affects natural fracture frequency 2 most of the dip azimuth of natural fractures is consistent with the dip azimuth of foliations observed in the core 3 the development of most of the natural fractures probably was dominated by the development of foliations and both the natural fractures and foliations developed in response to the subduction and exhumation of the sulu terrene

observed stress state for the iodp site c0002 and implication to the stress field of the nankai trough subduction zone abstract conducting a stress state analysis is one of major challenges for evaluating earth crust and formation conditions stress magnitude is especially essential to comprehend stress heterogeneity significant structural anisotropy and or pre existing fracture systems stress state is influenced by rock strength structural properties and near field principal stresses which impact borehole integrity during drilling factors that we can measure through logging while drilling lwd and measure while drilling mwd to better understand borehole conditions include mud weight overburden weight and velocity the lwd resistivity images in holes c0002a and c0002f of the nankai trough seismogenic zone experiment nantroseize indicate that most of the drillings in stable environments however the bottom hole assembly became stuck around 3 km depth in hole c0002p and did not reach the mega thrust fault we evaluated the stress state for the site c0002 by the geomechanical model with the breakout width and rock strength estimation we constrain the stress profiles in the drilled depth our analysis inferred that the instability of hole c0002p had been caused by an overpressure drilling fluid state and low horizontal principal stress furthermore we developed an optimally oriented plane oop model to exam the stress state in the site c0002 from nankai accretionary prism to the nankai trough subduction zone the normal faulting stress regime modeled from oop is consistent with the geomechanical model in site c0002 the horizontal principal stresses magnitude appears to less than the overburden weight above this subduction zone

bonded particle discrete element modeling of mechanical behavior of transversely isotropic rock abstract this paper presents bonded particle discrete element modeling with embedded smooth joints for simulating the mechanical behavior of transversely isotropic rock with systematic verifications and extensions to laboratory and upscaled problems verification was conducted against the analytical solutions in terms of anisotropic elastic and strength behavior the developed model was compared with laboratory observations of three rock types gneiss shale and schist leading to good agreements bonded particle dem modeling successfully captured the failure patterns observed in anisotropic rock in which weak planes play a significant role the model was upscaled to large scale foundation problems subjected to surface line load the study achieved reasonable agreements both for isotropic and anisotropic solutions the results demonstrate that the bonded particle dem with embedded smooth joints can model the equivalent anisotropic medium and it has potential to be applied to problems of larger engineering problems in which anisotropy is important

comparison between analytical and 3d finite element solutions for borehole stresses in anisotropic elastic rock we present a rigorous validation of the analytical amadei solution for the stress concentration around an arbitrarily orientated borehole in general anisotropic elastic media first we revisit the theoretical framework of the amadei solution and present analytical insights that show that the solution does indeed contain all special cases of symmetry contrary to previous understanding provided that the reduced strain coefficients b11 and b55 are not equal it is shown from theoretical considerations and published experimental data that the b11 and b55 are not equal for realistic rocks second we develop a 3d finite element elastic model within a hybrid analytical numerical workflow that circumvents the need to rebuild and remesh the model for every borehole and material orientation third we show that the borehole stresses computed from the numerical model and the analytical solution match almost perfectly for different borehole orientations vertical deviated and horizontal and for several cases involving isotropic transverse isotropic and orthorhombic symmetries it is concluded that the analytical amadei solution is valid with no restriction on the borehole orientation or the symmetry of the elastic anisotropy

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

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

deglacial meltwater pulse 1b and younger dryas sea levels revisited with boreholes at tahiti reconstructing sea level changes during the last deglaciation provides a way of understanding the ice dynamics that can perturb large continental ice sheets the resolution of the few sea level records covering the critical time interval between 14 000 and 9 000 calendar years before the present is still insufficient to draw conclusions about sea level changes associated with the younger dryas cold event and the meltwater pulse 1b mwp 1b we used the uranium thorium method to date shallow living corals from three new cores drilled onshore in the tahiti barrier reef no significant discontinuity can be detected in the sea level rise during the mwp 1b period the new tahiti sea level record shows that the sea level rise slowed down during the younger dryas before accelerating again during the holocene

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

coal strength and young s modulus related to coal rank compressional velocity and maceral composition abstract this paper presents an experimental investigation of coal rank and maceral composition influences on the coal mechanical behaviors the complete stress strain behavior uniaxial compressive strength young s modulus and acoustic compressional velocity were measured and correlated to coal ranks and microstructures the test results show that coal is an elasto brittle geo material and its uniaxial compressive strength and young s modulus increase as coal rank increases this occurs because as vitrinite reflectance or coal rank increases coal has less microporous structure and thus higher uniaxial compressive strength therefore using vitrinite reflectance value instead of vitrinite content is advantageous for correlating coal strength the experimental results also demonstrate that compressive strength and young s modulus have positive exponential correlation even for different types of coal therefore the compressive strength of coal is highly related to its young s modulus the uniaxial compressive strength and acoustic compressional velocity of coal are also correlated but a single correlation does not exist for different coal ranks instead different relationships occur for different types of coal

in situ stress pore pressure and stress dependent permeability in the southern qinshui basin this study focuses on the in situ stress pore pressure and permeability in the southern qinshui basin one of largest coalbed methane basins in china well tests show that permeability in this basin is higher than other coalbed methane reservoirs this is because it is located in an extensional basin where the normal faulting stress regime is dominated this in situ stress regime is advantageous to keep coal cleats open hydraulic fracturing tests indicate that the fracture gradient or minimum horizontal stress is much lower than the shales in the gulf of mexico and other oil basins the minimum horizontal stress model is proposed with consideration of the stress coefficient based on the uniaxial strain method this model provides a fairly good prediction on the minimum stress permeability data show that the effective stress dependent permeability is pronounced in the coalbed methane reservoir this is significant for the dual porosity and dual permeability coal reservoirs which consist of coal porous matrices and cleats the reason is that a rapid increase in effective stress can induce the closure of cleats which may cause a permanent loss of permeability in the cleats this reduces the connectivity between the cleats and coal matrices hence the coal matrices cannot deliver gas pressure to the cleats for supporting the cleat space therefore slowing down the effective stress change during production e g slowing reservoir drawdown can decelerate the permeability reduction this is particularly important for the reservoir in which the pore pressure is not significantly overpressured such that in the southern qinshui basin

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

characterization of pore pressure fracture pressure shear failure and its implications for drilling wellbore stability and completion design a case study from the takouazet field illizi basin algeria abstract this work presents the assessment of overburden stress pore pressure and fracture pressure distribution in the paleozoic stratigraphic units in the takouazet field eastern illizi basin algeria we studied a vertical exploration well where significant hydrocarbon discoveries have been established in devonian and ordovician sandstone reservoirs overburden stress interpreted from density log has an average 1 07 psi ft 24204 04 pa m gradient pore pressure derived from sonic log reveals that hydrostatic conditions prevail in devonian and ordovician units whereas abnormal pore pressure with a maximum 0 66 psi ft 14929 60 pa m gradient was recorded against the marine silurian shale fracture pressure was modeled using effective stress ratio models as well as a poisson s ratio based approach rock property based fracture pressure estimate is more sensitive to lithological variations and hence this estimate was used for determining safe drilling mud window and casing placements the estimated pore pressure and fracture pressure were calibrated with direct downhole formation pressure measurements and leakoff test lot measurements respectively shear failure using mohr coulomb rock failure criterion detected the minimum allowable mud pressure to avoid wellbore instability issues and the result was validated with caliper log observations it indicated that solid production from the reservoir is very likely during initial production from the reservoir and a solid control mechanism has been suggested also widening of breakouts and deteriorating wellbore interpreted from time lapse formation imaging logs strongly suggest against open hole completion based on interpreted pressure gradients and image log analyses optimum drilling mud and casing design and completion policies were recommended for safe and successful operational planning

evolution of stress induced borehole breakout in inherently anisotropic rock insights from discrete element modeling the aim of this study is to better understand the mechanisms controlling the initiation propagation and ultimate pattern of borehole breakouts in shale formation when drilled parallel with and perpendicular to beddings a two dimensional discrete element model is constructed to explicitly represent the microstructure of inherently anisotropic rocks by inserting a series of individual smooth joints into an assembly of bonded rigid discs both isotropic and anisotropic hollow square shaped samples are generated to represent the wellbores drilled perpendicular to and parallel with beddings at reduced scale the isotropic model is validated by comparing the stress distribution around borehole wall and along x axis direction with analytical solutions effects of different factors including the particle size distribution borehole diameter far field stress anisotropy and rock anisotropy are systematically evaluated on the stress distribution and borehole breakout propagation simulation results reveal that wider particle size distribution results in the local stress perturbations which cause localization of cracks reduction of borehole diameter significantly alters the crack failure from tensile to shear and raises the critical pressure rock anisotropy plays an important role on the stress state around wellbore which lead to the formation of preferred cracks under hydrostatic stress far field stress anisotropy plays a dominant role in the shape of borehole breakout when drilled perpendicular to beddings while a secondary role when drilled parallel with beddings results from this study can provide fundamental insights on the underlying particle scale mechanisms for previous findings in laboratory and field on borehole stability in anisotropic rock

a comparative study of the anisotropic dynamic and static elastic moduli of unconventional reservoir shales implication for geomechanical investigations abstractwe obtained the complete set of dynamic elastic stiffnesses for a suite of shales representative of unconventional reservoirs from simultaneously measured p and s wave speeds on single prisms specially machined from cores static linear compressibilities were concurrently obtained using strain gauges attached to the prism regardless of being from static or dynamic measurements the pressure sensitivity varies strongly with the direction of measurement furthermore the static and dynamic linear compressibilities measured parallel to the bedding are nearly the same whereas those perpendicular to the bedding can differ by as much as 100 compliant cracklike porosity seen in scanning electron microscope images controls the elastic properties measured perpendicular to the rock s bedding plane and results in highly nonlinear pressure sensitivity in contrast those properties measured parallel to the bedding are nearly insensitive to stress this anisotropy to the pressure dependency of the stra

investigation of present day in situ stresses and pore pressure in the south cambay basin western india implications for drilling reservoir development and fault reactivation abstract this work presents the assessment of present day stresses and pore pressures in a prolific onshore hydrocarbon bearing field located in the southern cambay basin western india through the development of a comprehensive geomechanical model a c quality wellbore breakouts inferred from the available four arm caliper logs and anisotropy analyses of dipole shear sonic logs indicate a ne n40 e to n50 e azimuth for maximum horizontal stress shmax pore pressure characterisation reveals mild overpressure in the lower eocene shale caused by disequilibrium compaction resulting from high sedimentation rates vertical stress sv was derived from density logs and a poroelastic strain model was employed to quantify the minimum shmin and maximum horizontal stress components shmax being later validated with leak off tests based on the relative magnitudes of three principal in situ stress components post rift strata upper eocene and younger reveal a normal faulting regime sv shmax shmin whereas a normal fault to strike slip faulting shmax sv shmin transition is observed in the syn rift unit lower eocene and older the mohr coulomb rock failure criterion was utilised to depict the shear failure gradient to prevent wellbore instability and results were validated with caliper logs we have also analysed the fault reactivation potential at reservoir level using frictional fault theory and stress polygons the results indicate that a minimum of 31 increase in pore pressure by fluid injection and hydraulic fracturing treatment can cause shear slippage of critically oriented faults resulting in induced seismicity our study has helped to understand the interplay between stress states and pore fluid pressure and their impact on reservoir development drilling and completion design and fault stability analysis in any given oil and gas field

experimental and theoretical study of the anisotropic properties of shale abstract the effect of bedding plane orientations on shear strength and failure mechanisms of shale is investigated with an emphasis on the shear parameters of the bedding planes this study is based on direct shear tests conducted on directional shale samples obtained from formations which exhibit well developed stratification the tested cylindrical specimens were prepared by coring at 30 intervals from the direction of the bedding planes the results indicate that the anisotropy of the shear strength of shale can be readily observed due to the anisotropic effects of bedding plane orientation and normal stress the shear parameters such as strength cohesion and internal friction angle reach their maximum and the minimum values at 60 and 0 respectively where is the angle between the bedding planes and the coring orientation which is also the direction of normal stress during the laboratory tests it is demonstrated that the bedding planes are weak not only with regards to cementing strength but also with regards to friction three different shear failure modes were built up depending on the shearing angle 1 sliding failure across the bedding planes 2 sliding failure along the bedding planes and 3 sliding failure across the bedding planes combined with tensile splitting along the bedding planes an expression of the shear stress concentration factor of the direct shear model is derived to evaluate the anisotropy of shear strength