critically stressed fractures analysis of the shaikan field kurdistan region of iraq the shaikan field is a large producing oil field in the kurdistan region of iraq it consists of multiple fractured reservoirs consisting of limestones calcareous sandstones and mudstones the surrounding tectonic terrane is situated in the seismically active zagros taurus orogenic zone where present day stresses are high the regional stresses are found to impose conditions that satisfy failure along reservoir bound fractures suggesting that a significant proportion of fractures are likely to be critically stressed the in situ maximum principal stress magnitudes are estimated using three methods namely the tensile and compressive strengths of reservoir rock and leak off test lot data stress field orientations are determined from wellbore image log data which are used to interpret wellbore breakouts and the associated induced tensile fractures reservoir pressure has declined since production started and poroelastic responses have been assessed and used to estimate the present day stress state and the criticality of those fractures that are most likely to fail or slip although a conventional approach has been used the present authors argue that a new approach to stress response with changing pore pressure should be taken unlike the previous theory of criticality in which a reduction in pore pressure is considered to lead to a stabilization of the fracture network the present study suggests that a system may remain critically stressed regardless of pressure decline thematic collection this article is part of the the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

analytical and numerical solutions on the response of pore pressure to cyclic atmospheric loading with application to horonobe underground research laboratory japan during the long term stability assessment of underground disposal repositories buried in sedimentary formations one of crucial scientific problems is to determine parameters of site formations the analysis of response of the pore pressure due to atmospheric loading provides a possible way of accurate estimation of formation parameters in this paper an analytical solution of response of the pore pressure to atmospheric loading was derived starting from the biot s poroelasticity and its corresponding numerical solutions were developed by finite element methods for 1d and 2d situations with comparison to the analytical solution the experimental results show that the numerical schemes sound reasonable to implement the assessment of response of the pore pressure in horizontal multilayer sedimentary formations in particular it is effective for a 2d model over large area with consideration of background horizontal groundwater flow a highlight of this work is to construct a reasonable approach to capture responses of the pore pressure to atmospheric loading and at the same time to provide the feasibility of integrated analysis of well monitoring data of pore pressure variance in horonobe underground research laboratory url hokkaido japan

implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern junggar basin china abstract the southern junggar basin hosts abundant coalbed methane cbm resources and is expected to be an important cbm production base in china the pore pressure and in situ stress are key factors that influence the exploration and development of cbm however these factors are poorly understood in this area based on extended leak off test data mathematical models were established for pore pressure and in situ stress prediction using regression analysis subsequently the correlations between the pore pressure in situ stress and permeability were analyzed and the stress ratios were investigated finally the forming mechanism of stress zonation in the study area was discussed the results show that cbm reservoirs are generally underpressured and partly overpressured at depths greater than 800 m in the study area the study area is characterized by notable stress zoning the stress regime varies from reverse to strike slip to normal faulting with increasing depth significantly positive correlations exist between the pore pressure maximum principal stress minimum principal stress vertical stress and principal stress difference the minimum horizontal stress is approximately 70 of the vertical stress in the study area a mud weight window varying from the pore pressure to the most likely breakdown pressure is suggested at depths greater than 600 m a relatively high principal stress difference is observed which indicates that deep coals have a relatively high mechanical strength and their permeability can be enhanced by hydraulic fracturing the depth dependent stress zonation is mainly due to the influence of thrusting on the in situ stress which diminishes with increasing depth the compressional stress regime in the shallow area benefits the sealing of faults and the preservation of cbm resources in the study area the deep cbm resources have a development potential the outcomes of this study contribute to the cbm exploration and development in the study area

montessus de ballore a pioneer of seismology the man and his work abstract ferdinand de montessus de ballore was one of the founders of scientific seismology he was a pioneer in seismology at the same level as perrey mallet milne and omori he became familiar with earthquakes and volcanoes in central america 1881 1885 after his experience in el salvador his interest for understanding earthquakes and volcanoes oriented all of his life back in france he worked out a most complete world catalogue of earthquakes with 170 000 events 1885 1907 and completed his career being the head of the chilean seismological service 1907 1923 many of his ideas were in advance of later discoveries he was an exceptional writer and published more than 30 books and hundreds of papers

biot gassmann theory for velocities of gas hydrate bearing sediments elevated elastic velocities are a distinct physical property of gas hydrate bearing sediments a number of velocity models and equations e g pore filling model cementation model effective medium theories weighted equations and time average equations have been used to describe this effect in particular the weighted equation and effective medium theory predict reasonably well the elastic properties of unconsolidated gas hydrate bearing sediments a weakness of the weighted equation is its use of the empirical relationship of the time average equation as one element of the equation one drawback of the effective medium theory is its prediction of unreasonably higher shear wave velocity at high porosities so that the predicted velocity ratio does not agree well with the observed velocity ratio to overcome these weaknesses a method is proposed based on biot gassmann theories and assuming the formation velocity ratio shear to compressional velocity of an unconsolidated sediment is related to the

seismic anisotropy in sedimentary rocks part 2 laboratory data part one of this paper presents a method for measuring seismic velocities and transverse isotropy in rocks using a single core plug this method saves at least two thirds of the time for preparing core samples and measuring velocities in transversely isotropic ti rocks using this method we have measured velocity and anisotropy of many shale and reservoir rocks from oil and gas fields around the world we present some of the data in this paper which include seismic velocity and anisotropy in 17 brine saturated shale samples 1 gas and brine saturated coal sample 8 brine saturated sands 12 gas saturated sands 32 gas saturated carbonate samples and 25 brine saturated carbonate samples the results show that clays and fine layering in sedimentary rocks are the main causes of seismic anisotropy very little intrinsic anisotropy exists in unfractured reservoir rocks such as sands sandstones and carbonates under reservoir conditions in contrast all shales were found seismically anisotropic anisotr

acoustic measurements in unconsolidated sands at low effective pressure and overpressure detection shallow water flows and over pressured zones are a major hazard in deepwater drilling projects their detection prior to drilling would save millions of dollars in lost drilling costs i have investigated the sensitivity of seismic methods for this purpose using p wave information alone can be ambiguous because a drop in p wave velocity vp can be caused both by overpressure and by presence of gas the ratio of p wave velocity to s wave velocity vp vs which increases with overpressure and decreases with gas saturation can help differentiate between the two cases since p wave velocity in a suspension is slightly below that of the suspending fluid and vs 0 vp vs and poisson s ratio must increase exponentially as a load bearing sediment approaches a state of suspension on the other hand presence of gas will also decrease vp but vs will remain unaffected and vp vs will decrease analyses of ultrasonic p and s wave velocities in sands show that the vp vs ratio especially at low effective pressures

effective stress coefficient in shales and its applicability to eaton s equation pore pressure within formations determines the mud weight required to build a balancing fluid pressure downhole an improper understanding of the subsurface geology and the formation pressures may result in fracturing the formation if the mud weight is too high in contrast if mud weight is too low then formation fluids can flow into the well potentially leading to well blowouts if not controlled complex geological settings make pore pressure prediction difficult and often inaccurate due to uncertainty in pressure generating mechanisms estimation of proper pore pressures is necessary for designing stable holes and an optimized casing program in exploration knowledge of pore pressures can assist in assessing seal effectiveness and in high grading reservoir sweet spots it also provides useful calibration information for basin modeling hence it becomes very important to accurately predict pore pressures

the correlation between low tectonic stress and the appalachian basin quiet zone abstract in order to assess the nature of the appalachian basin quiet zone abqz a region of low seismic activity a minimum horizontal stress profile from sonic logs was calibrated using hydraulic fracture stress measurements in a well from mckean county pa the calibrated minimum horizontal stress profile that best fits with the hydraulic fracture stress data was established using an extended eaton s model that treats rock as a transverse isotropic material with a vertical symmetry axis tiv to achieve a best fit several assumptions are introduced 1 a constant tectonically induced elastic strain of ehmax 3 3 10 4 2 an anisotropic velocity model annie approximation for shales and 3 a bimodal pore pressure distribution with abnormally high pore pressure in the marcellus gas shale other calibration methods assume an isotropic medium a constant tectonic strain gradient corresponding to zero stress at ground surface or constraints from sidewall core data to estimate stiffness constants by testing four calibration methods we find that stress profiles are affected by multiple factors including the ratio of maximum tectonically induced elastic strain over minimum tectonic strain pore pressure in the marcellus biot s coefficients and elastic properties sensitivity analysis was conducted to analyze the influence of these parameters our stress profile is most influenced by pore pressure and tectonically induced elastic strain while other factors had much smaller effects tectonically included strain in the abqz is insufficient to achieve the critical stress necessary to induce frictional slip on favorably oriented faults in the sedimentary cover which presumably speaks to the lack of earthquakes in the basement of the abqz however induced earthquakes from deep disposal wells suggest that basement at the edge of the abqz may be decoupled from the sedimentary cover in which case the cover has relaxed through a time dependent deformation mechanism such as pressure solution creep

thermo hydro mechanical modeling of carbon dioxide injection for enhanced gas recovery co2 egr a benchmarking study for code comparison the objective of this paper was to investigate the thm coupled responses of the storage formation and caprock induced by gas production co2 egr enhanced gas recovery and co2 storage a generic 3d planer model 20 000 3 000 100 m consisting of 1 200 m overburden 100 m caprock 200 m gas reservoir and 1 500 m base rock is adopted for the simulation process using the integrated code tough2 eos7c flac3d and the multi purpose simulator opengeosys both simulators agree that the co2 egr phase under a balanced injection rate 31 500 tons year will cause almost no change in the reservoir pressure the gas recovery rate increases 1 4 in the 5 year co2 egr phase and a better egr effect could be achieved by increasing the distance between injection and production wells e g 5 83 for 5 km distance instead of 1 2 km in this study under the considered conditions there is no evidence of plastic deformation and both reservoir and caprock behave elastically at all operation stages the stress path could be predicted analytically and the results show that the isotropic and extensional stress regime will switch to the compressional stress regime when the pore pressure rises to a specific level both simulators agree regarding modification of the reservoir stress state with further co2 injection tension failure in reservoir could occur but shear failure will never happen under these conditions using tough flac a scenario case is also analyzed with the assumption that the reservoir is naturally fractured the specific analysis shows that the maximal storage pressure is 13 6 mpa which is determined by the penetration criterion of the caprock

carbonaceous megafossils from the precambrian 1800 ma near jixian northern china the upper part of the stratotype section of the changcheng system in the yanshan range 100 km east of beijing peking has yielded small megascopic carbonaceous compressions assignable to tyrasotae

estimation of biot s effective stress coefficient from well logs effective stress governs the mechanical response of rock formations to variations in stress and pore pressure which affect wellbore stability and reservoir integrity during drilling and production biot s coefficient is employed to calculate effective stresses from total stress and pore pressure therefore the measurement of biot s coefficient becomes crucial however the laboratory measurement of biot s coefficient is expensive and laborious this paper presents three methods for computing biot s coefficient using logging data the first method calculates biot s coefficient using the existing empirical correlations between porosity and biot s coefficient the second and third methods calculate biot s coefficient using dynamic rock and solid bulk modulus computed using rock and solid wave velocities respectively however the second and third methods calculate the necessary solid wave velocities in different ways the second method calculates solid shear and compressive velocities vs and vp using a newly developed correlation between the differential pressure porosity and wave velocity of sandstone the third method calculates solid wave velocities based on the significant finding that the vp vs ratio with respect to the s wave velocity is constant for sediments including highly compacted sand case studies were undertaken using logging data from the gulf coast gas wells it was found that biot s coefficient calculated using the first method was highly dependent on the chosen relation while the coefficients calculated using the second and third methods were related to well logs results from the third method show that biot s coefficient deflects to higher values in situations where gamma ray surveys read low api values this is in agreement with the phenomenon that rocks with a smaller api should have lower a clay content and bigger value of biot s coefficient therefore the third method is more reliable and also requires fewer input parameters

3d porosity and mineralogy characterization in tight gas sandstones tight gas reservoirs exhibit storage and flow characteristics that are intimately tied to depositional and diagenetic processes as a result exploitation of these resources requires a comprehensive reservoir description and characterization program to identify properties which control production in particular tight gas reservoirs have significant primary and secondary porosity and pore connectivity dominated by clays and slot like pores this makes them particularly susceptible to the effects of overburden stress and variable water saturation this paper describes an integrated approach to describe a tight gas sandstone at the pore scale in 3d in particular the primary and secondary porosity of a tight gas sandstone are identified and quantified in three dimensions using 3d x ray micro ct imaging and visualization of core material at the pore scale 3d images allow one to map in detail the pore and grain structure and interconnectivity of primary and secondary porosity once the tomographic images ar

slope controlled seismic deformation and tectonic framework of deposition koldaha shale india abstract penecontemporaneous deformations with remarkable lateral continuity at selected stratigraphic levels record frequent seismicity in the meso proterozoic koldaha shale vindhyan supergroup central india their consistent bipolar orientation reflects the master control of basinal tectonics boudinage small scale conjugate faults and bidirectional joint planes indicate east northeast west southwest basinal extension detailed study of a 27 km strike parallel stretch between chorhat and shikarganj against a broader perspective reveals existence of multiple nw se elongated subbasins stratigraphic and sedimentologic criteria point to inheritance and subsequent sustenance of steep western and gentler eastern flanks of the sub basins it is also indicated that the perennial supply of sediment took place from the east while the western slope contributed only during tectonic seiches the relatively large scale faults are correlatable with the steeper and the smaller deformation structures to the gentler flank the nnw oriented half graben structures that were reactivated time and again is elicited previous geophysical studies inferred roughly e w elongated rift valley in the granitic basement and the present observations indicate that the rifting continued at least up to the time of koldaha shale deposition the half graben could then be guided by cross faults resulted from dextral shear it is imperative then the basin interior sedimentation and deformation patterns were more closely controlled by the cross faults rather than by the basin margin faults only a set of slump folds in a 10 m thick interval indicates that the northern margin of the main basin was relatively steeper this is further corroborated by concentration of large fans in mineralogically immature sediments and southerly paleocurrent at the northern fringe of the basin in contrast the vindhyan sediments commonly belong to orthoquartzite carbonate association a regional northerly slope of the vindhyan basin floor towards the relatively steeper flank provides an explanation for the general northwesterly paleocurrent even though the eastern flank of the sub basins sloped wsw this integrated study of the physical aspects of the sedimentary pile against the background information of basement tectonics recognize rifting with a dextral shear component as the overriding factor controlling the early part of evolution of the vindhyan basin

effect of fines content and void ratio on the saturated hydraulic conductivity and undrained shear strength of sand silt mixtures the hydraulic conductivity represents an important indicator parameter in the generation and redistribution of excess pore pressure of sand silt mixture soil deposits during earthquakes this paper aims to determine the relationship between the undrained shear strength liquefaction resistance and the saturated hydraulic conductivity of the sand silt mixtures and how much they are affected by the percentage of low plastic fines finer than 0 074 mm and void ratio of the soil the results of flexible wall permeameter and undrained monotonic triaxial tests carried out on samples reconstituted from chlef river sand with 0 10 20 30 40 and 50 non plastic silt at an effective confining pressure of 100 kpa and two initial relative densities d r 20 91 are presented and discussed it was found that the undrained shear strength liquefaction resistance can be correlated to the fines content intergranular void ratio and saturated hydraulic conductivity the results obtained from this study reveal that the saturated hydraulic conductivity k sat of the sand mixed with 50 low plastic fines can be in average four orders of magnitude smaller than that of the clean sand the results show also that the global void ratio could not be used as a pertinent parameter to explain the undrained shear strength and saturated hydraulic conductivity response of the sand silt mixtures

development of a semi analytical method to calculate depth dependent temperature and stress changes investigation of micro seismicity at unterhaching uha gt 2 geothermal well since october 2007 when the hydrothermal production of geothermal energy from the deep underground started at unterhaching near munich germany several micro seismic events have been observed three of the five strongest events with local magnitudes ml ranging from 2 0 to 2 4 occurred in 2008 and were partially felt by the residents the reasons for these events are not clear these micro seismic events could have been induced by a variety of factors the strongest micro seismic event was observed soon after the injection of cold water pointing to an intrinsic relationship between micro seismic event and cold water injection one of the possible reasons for the observed micro seismic events could be the reinjection of cooled thermal water into the hotter underground inducing stress redistribution and shear failures in the reservoir formations around the injection wellbore uha gt 2 to justify this reason further investigation will be carried out in another simulation study to verify whether the shear failures induced by cold water injection were the major cause for the occurrence of these micro seismic events nevertheless before the simulation of shear failures temperature profiles in both the wellbore and surrounding rocks as well as their corresponding thermal stresses during cold water injection should be established in this study in this paper a new semi analytical simulation method has been developed to calculate simultaneously the temperature profiles in the wellbore and the surrounding rock formations and also determine the corresponding thermal stresses in the surroundings near wellbore field and subsequently verified by analytical solution results show that great tensile thermal stresses in the reservoir were initiated by the injection of the cooled thermal water in the injection wellbore uha gt 2 the results of the simulated thermal stresses indicate a great potential for the ensuing shear failures to affect an extensive area and could be used as input data for the planned shear failure simulations in the future

the identification of fault zones in deep karst aquifer of north china coal mine using parallel directional well logs water inrush from ordovician karst aquifer in north china coal mine has become a key issue on underground coal mining to address this problem it is necessary to identify fault zones because fault zones might connect limestone aquifers and coal seams enabling ordovician karst water to enter the mine in the study area a series of parallel directional holes were drilled along ordovician limestone at depths between 70 and 90 m under ordovician limestone boundary to conveniently detect fault zones and govern mine water disasters a series of natural gamma ray logging while drilling grlwd were undertaken the entire detecting region can be comprehensively covered by several directional borehole groups then fast fourier transform and short time fourier transform approaches were employed on the basis of grlwd data and geological data to extract faults information a segmented identification method for deep fault zones was established in this study this method can be used to markedly improve the identification of fault zones within ordovician limestone or the unitary lithology formation and provide crucial information relevant for deep coal mining safety

microbial fossils from the kheinjua formation middle proterozoic semri group lower vindhyan son valley area central india abstract abundant coccoid and filamentous microfossils are found in petrographic thin sections of stratiform stromatolitic cherts from the 1200 ma old fawn limestone of the kheinjua formation lower vindhyan supergroup central india the assemblages is dominated by coccoid forms of probable chroococcacean and entophysalidacean affinities most filaments closely resemble the oscillatoriacean gunflintia minuta unlike other middle to late proterozoic stromatolitic microbiotas the kheinjua is dominated by microfossils taxonomically the kheinjua microbiota most closely resembles proterozoic microbiotas described from the belcher islands kasegalik formation mcarthur group bitter springs formation and the yudoma suite the following taxa are formally described chroococcaceae myxococcoides minor schopf eosynechococcus isolatus n sp tetraphycus congregatus n sp kheinjuasphaera vulgaris n gen n sp glenobotrydion aenigmatis schopf melasmatosphaera media hofmann entophysalidaceae eoentophysalis belcherensis hofmann e magna n sp oscillatoriaceae cf gunflinta minuta barghoorn eomycetopsis siberiensis lo

early proterozoic microfossils and penecontemporaneous quartz cementation in the sokoman iron formation canada early proterozoic microfossils from the sokoman iron formation northeastern canada are indistinguishable from those of the gunflint formation in both morphology and inferred community structure the contemporaneity of the sokoman assemblage with the bitter springs like cyanobacteria of the belcher supergroup indicates that differences between the two major types of early proterozoic microbiotas are primarily ecological and not temporal evolutionary in nature in arenaceous iron formations microfossils are restricted to peloids and are absent from pore filling silica interpreted as cement cemented arenaceous intraclasts indicate that some of the silica was penecontemporaneous and the abundance of minus cement porosity in arenaceous iron formations demonstrates that early pre compaction cementation was common

elastic wave velocities chemistry and modal mineralogy of crustal rocks sampled by the outokumpu scientific drill hole evidence from lab measurements and modeling the outokumpu scientific deep drill hole intersects a 2500 m deep precambrian crustal section comprising a 1300 m thick biotite gneiss series mica schists at top followed by a 200 m thick meta ophiolite sequence underlain again by biotite gneisses mica schists 500 m thick with intercalations of amphibolite and meta pegmatoids pegmatitic granite from 2000 m downward the dominating rock types are meta pegmatoids pegmatitic granite average isotropic intrinsic p and s wave velocities and densities of rocks were calculated on the basis of the volume fraction of the constituent minerals and their single crystal properties for 29 core samples covering the depth range 198 2491 m the modal composition of the rocks is obtained from bulk rock xrf and mineral chemistry microprobe using least squares fitting laboratory seismic measurements on 13 selected samples representing the main lithologies revealed strong anisotropy of p and s wave velocities and shear wave splitting seismic anisotropy is strongly related to foliation and is in particular an important property of the biotite gneisses which dominate the upper and lower gneiss series at in situ conditions velocity anisotropy is largely caused by oriented microcracks which are not completely closed at the pressures corresponding to the relatively shallow depth drilled by the borehole in addition to crystallographic preferred orientation cpo of the phyllosilicates the contribution of cpo to bulk anisotropy is confirmed by 3d velocity calculations based on neutron diffraction texture measurements for vertical incidence of the wave train the in situ velocities derived from the lab measurements are significantly lower than the measured and calculated intrinsic velocities the experimental results give evidence that the strong reflective nature of the ophiolite derived rock assemblages is largely affected by oriented microcracks and preferred crystallographic orientation of major minerals in addition to the lithologic control c 2009 elsevier b v all rights reserved