Pore Pressure Reduction Research Articles

Reservoir depletion and its effect on wellbore stability evaluation

Abstract Production from hydrocarbon reservoirs can lead to decreases in the pore pressures, of up to 60 MPa, over the lifetime of the field. This depletion results in changes of the horizontal stresses acting in the reservoir. Field data are reviewed and new data presented which define linear decreases of the minimum stress with pore pressure, for reservoirs which have undergone pore pressure reductions of 1.8 MPa to 56 MPa. The depletion responses cannot be accurately predicted using the most commonly used linear elastic relationship, due to either inappropriate boundary condition assumptions, or incorrect input values. Relationships for three different boundary conditions and two material, responses, are presented which can potentially be used to describe the minimum stress-depletion response of reservoirs. The significance of the change in the horizontal stress anisotropy with depletion for different boundary conditions is also discussed. Finally, the impact of the stress-depletion response on wellbore stability is assessed for inclined wells, focussing on openhole production wells, whose stability can significantly decrease with depletion.

Undrained instability of very loose Hostun sand in triaxial compression and extension. Part 1: experimental observations

The undrained behavior of very loose Hostun RF sand in triaxial compression and extension tests is described. The samples are consolidated isotropically or anisotropically along constant effective stress ratio paths. Very loose sand exhibits partial liquefaction, deviator peak stress at relatively low to very low axial strain, gradual post-peak stress reduction to a small residual value at large strains, sharp loss of effective mean pressure due to generation of large pore pressure and overall volume reduction. The instability line of Lade is examined in the case of extension tests and extended for anisotropic samples. It is shown that monotonic and anisotropic consolidation strongly influences the instability concept. A higher positive anisotropic consolidation level produces a greater slope of the instability line in compression and a reverse trend can occur in extension. Complex stress history can develop a fossilized instability line depending on the amount of axial strain attained. Effective strain ratio increment at peak has an asymptotic stabilization effect. © 1997 by John Wiley & Sons, Ltd.

Bottom-heave control of a deep sensitive clay excavation in Ottawa, Canada

For a new Police Headquarters Complex in Ottawa, an excavation was required to a depth of 10 m in medium to stiff sensitive silty clays and clayey silts. The risk of a bottom-heave failure due to uplift caused by significant groundwater pressures in the silty soil a few metres below the bottom of the excavation was recognized, and contract requirements included a wellpoint installation assisted by electro-osmosis. The successful bidder was able to develop an alternative method based on the observational approach and completed the foundation at a lesser cost. A small test excavation was instrumented, and digging proceeded cautiously to full depth. The observed bottom heave and pore pressure changes were encouraging, so the test area was enlarged in steps until a workable size of excavation was safely made, then the structural raft was able to be placed. A backup system of pore pressure relief wells using wick drains in the bottom of the excavation was also developed. A large pore pressure reduction, due to the release of overburden pressure, was expected by the contractor's consultant, and a slow regain of the groundwater pressure was also expected due to the low horizontal permeabilities involved. The test excavation instrumentation and observations confirmed these expectations. Key words: bottom heave, excavation, clay, pore pressure.

Borehole televiewer data analysis of Hole 504B from legs 137 and 140 : A. Krammer, P. Pezard, P. K. H. Harvey & K. Fuchs, in: Proc., scientific results, ODP Leg 137/140, Costa Rica Rift, ed J. Erzinger & others, (ODP, Texas A&M University, College Station), 1995, pp 293–304

Hole 504B, about 200 km south of the Costa Rica Rift in the eastern equatorial Pacific, has been drilled over a succession of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legs to a total depth of 2000.4 meters below seafloor (mbsf). Borehole televiewer (BHTV) measurements were recorded over the intervals 275-497 mbsf on Leg 137, and 14971715 mbsf and 1868-1990 mbsf on Leg 140. Detailed investigation of the BHTV data revealed stress-induced wellbore breakouts in all depth intervals with variable occurrence. The statistical analysis of the wellbore breakouts shows that the orientation of the maximum horizontal stress (SHmax) varies from N53°E ± 28° in the interval 410-490 mbsf, to N21 °E ± 35° in the interval 1497-1715 mbsf, and N82°E ± 38 in the interval 1868-1990 mbsf. Whereas the intermediate depth interval shows a mean azimuth that is consistent with previous observations from borehole breakout analysis between 700 and 1500 mbsf, the upperand lowermost breakout intervals are rotated clockwise by 22° and 61°, respectively. As none of the observed intersecting structures correlates with the observed rotation ofSHmwc, it is assumed that the rotation is affected by non-intersecting structures or structures beyond the wellbore head. The onset of the breakout interval from 410 to 490 mbsf correlates well with the Layer 2A/2B boundary and corresponds to a seismic anisotropy observed in the upper 500 m of the crust. For breakouts to occur in shallow depths, high SHmajShmin ratios are required or rock strengths must locally vary (to values <IOO MPa) to allow the initiation of breakouts. Analytical results also propose a drastic reduction of pore pressure (20%) for breakout initiation, which is, however, far beyond present-day observations. The breakouts below 1497 mbsf occur within the sheeted dikes and close to the sheeted dike/gabbro transition zone (1868-1990 mbsf). The observation of breakouts below 1497 mbsf and the lack of vertical fractures in this depth allow us to estimate 131 MPa for SHmax and 96 MPa for Slmm, which agree with stress estimates deduced with Byerlee's law for a TF/SS regime.

Soil Stabilization by Ambient Pore Pressure and Geomembrane Containment

Substantial atmospheric and pore water pressures exist in soil deposits. These pressures may be used to exert additional confining stresses with the aid of relatively impervious barriers. Any reduction in pore pressure within the impervious boundaries of an isolated soil domain will result in an equal increase in confinement. Confinement that directly increases effective stress will in turn increase shear stiffness and strength of the soil. In situ applications are becoming increasingly feasible in view of the recent advances in construction and installation of geomembrane systems. In this study, the concept of increasing confinement by action of ambient pore pressure is discussed. The effectiveness of this approach when applied to dry and saturated cohesionless soil is evaluated using a series of centrifuge model tests. In these experiments, a major increase of soil resistance to deformations is observed. In the saturated soil models, the potential for liquefaction induced deformations is eliminated. Practical applications and the applicability of retrofit efforts including the required geomembrane characteristics are discussed. Currently, field tests are needed to assess full-scale implementation challenges.

Calibration Chamber Studies of Piezocone Test in Cohesive Soils

The authors have presented a series of piezocone dissipation tests in reconstituted kaolin specimens. From a comparison of the measured radial coefficient of consolidation and the values interpreted from dissipation data, it was concluded that interpretation of the dissipation results to evaluate the radial coefficient of consolidation should be based on the initial dissipation values of excess pore pressure, and not the penetration pore pressure. The discusser will show that the dissipation data presented may be interpreted rationally using the analysis by Teh and Houlsby (1991), which was based on the penetration excess pore-water pressure. Firstly, some features of the measured dissipation data will be examined. It was noted that the tests performed on soil specimen 1 showed a distinctly different behavior from the other tests. The penetration pore pressure measured at the cone tip in soil I was higher than that measured at the base. The opposite was true in other tests, with higher pore pressure at the cone base. This difference was also reflected in the dissipation data presented in Figs. 5-8. A significant drop in tip pore pressure was observed at the early stages, with reduction of 12%, 26(/(., and 16% in specimens 2, 3, and 4, respectively. The initial pore-pressure reduction in specimen I was comparatively smaiL The authors attributed the initial drop in pore pressures to the reduction of normal stresses on stopping penetration. However, any reduction in normal stresses would be reflected in a decrease in cone-tip resistance, and this could be verified by monitoring the tip resistance during dissipation. Teh and Houlsby (1991) have studied cone penetration into an isotropic material using the strain-path method. Although the analysis cannot properly account for the different stress histories of the soil, some features of the analytical results may provide better insights to the interpretation of piezocone dissipation test. The analytical results show that penetration pore-water pressure, is highest at the cone tip and cone face region. This is consistent with the data for soil specimens 2-4 but at variance with data in specimen I. The dissipation rates monitored at the cone tip and the cone base are governed by the spatial distribution of excess pore pressure around the cone. Cavity expansion theories assumed a one-dimensional variation of flu, which decreases logarithmically with distance. However, the pore-pressure distribution around the cone is more complicated and one-dimensional analyses such as cavity expansion theories may not provide an adequate basis for interpreting piezocone data. It should be noted that the theoretical dissipation curves presented by Teh and Houlsby (1991) were based on the penetration excess pore pressure, flu. Hence, it was incorrect to use it to interpret dissipation data normalized by initial

Influence of Pore Pressure on Drilling Response in Hard Shales

Summary Results are presented from laboratory drilling studies of two different hard shales similar to those found in some areas of the North Sea. Two laboratory drilling machines with milled-tooth bits were used. Pore pressure was measured in all the experiments, and in some it was reduced to several megapascals below bottomhole pressure (BHP). Pore-pressure reduction was achieved by allowing the pore fluid to drain out of the shale while external stresses were maintained constant. The influence of BHP was studied for values from 3 to 33 MPa [435 to 4,786 psi]. The differential pressure, which equals BHP minus pore pressure, ranged from near balance to &amp;gt; 10 MPa [&amp;gt; 1,450 psi]. BHP was found to have a strong influence on drilling response, but differential pressure did not, contrary to conventional wisdom. The results are compared with other drilling experiments and single-cutter experiments in the literature.

Frost heave characteristics of undisturbed sensitive Champlain Sea clay

Undisturbed Champlain Sea clay samples were subjected to laboratory freezing tests with pore-pressure measurements in order to determine the freezing characteristics of a structured compressible soil. Step-freezing and ramped-freezing tests with applied back pressure were conducted on 10 cm high samples in open-system conditions. Significant pore-pressure reductions in the unfrozen soil induce important frost-induced consolidation and destructuration of the clay. It was found that the freezing characteristics of Saint-Alban clay are best defined by the segregation potential at the active ice lens, SPℓ, which includes water fluxes generated within the frozen fringe and within the unfrozen soil as excess water is expelled during consolidation, and finally water from an external source. For the Saint-Alban clay, SPℓ values of the intact clay ranged between 450 and 600 × 10−5 mm2/(s °C), whereas those of destructured clay at a lower void ratio were significantly smaller. Back-calculating the segregation potential solely from surface heave measurements in laboratory tests may underestimate considerably the frost susceptibility of compressible structured clays. Segregation potential inferred from instrumented field sites was 430 × 10−5 mm2/(s °C) and is consistent with the laboratory tests results. Key words : freezing, frost heave, structured clay, undisturbed, consolidation.

A model of dynamic confinement during drilling in pressurized boreholes

The rate of penetration achieved in drilling oil and gas wells tends to decrease continuously as the well depth increases. Increased drilling time has important economic consequences in deep wells, because the daily costs of equipment, supplies, and services also increase with depth. It has long been recognized that borehole pressure plays an important role in limiting the drilling rate as the depth increases. Weighted drilling muds are used while drilling oil and gas wells to prevent the influx of formation fluids. The bottomhole mud pressure is typically kept about 10% above the estimated formation pore pressure. This pressure differential can increase rock strength and inhibit the removal of cuttings from the bottom [1, 2]. There is evidence to suggest that the absolute value of pressure in a borehole may have an even greater effect on drilling rate, particularly in low-permeability rocks. In laboratory drilling tests in low-permeability rock at atmospheric pressure, and at a moderate hydrostatic pressure, van Lingen [3] observed that the drilling rate is sensitive to the absolute pressure at high bit rotation speeds, although no effect was observed at low rotation speeds. These observations were attributed to dynamicchip-hold-down pressures that arise as the cutter displaces rock chips from the bottom. Warren and Smith [4] proposed that poroelastic relaxation of low-permeability shales due to the introduction of the borehole can result in confinement pressures equal to the absolute borehole pressure; however, studies of pore pressure invasion ahead of the bit have concluded that pore pressures equilibrate within the first millimeter even in low-permeability shales [5]. Poroelastic relaxation pressures are unlikely to influence drilling with fixed cutter bits since the depth of cut is typically less than 1 nun. We present here a model that can account for the observed nonlinear decline in drilling rate with absolute borehole pressure. The model is based on the dynamic differential pressures that arise from the dilatation of rock during deformation by a shear cutter in a pressurized borehole. Brace and Martin [6] observed that the dilatation of rock during drained triaxial testing at high fluid pressures results in rate-dependent hardening. Dilatation-induced pore pressure reductions cause an effective dynamic confinement pressure that results in dilatancy hardening. Dynamic confinement pressures can also force the transition from brittle fracture to ductile shear (cf. review by Paterson [7]). Cook et al. [8] report rate-dependent dilatancy hardening and ductility enhancement in shales with

Collapse behavior of sand

Loose cohesionless materials can collapse during either static or dynamic loading, resulting in a rapid buildup of pore pressure and associated reduction in shear resistance. As the cohesionless material collapses, it rapidly looses resistance until the acting shear stress decreases to the available residual or steady-state strength. Specially designed stress-path testing has been performed on sand to investigate this collapse process. Results from this test program and previously published data show that a state boundary can be defined when a cohesionless material moves from peak to steady state along a constant void ratio stress path regardless of whether it is loaded drained or undrained. Further, it is demonstrated that the state boundary represents a surface in the effective mean normal stress–deviator stress–void ratio space. Hence, flow slides and liquefaction can be initiated when the stress path followed during either drained or undrained loading attempts to cross this state boundary surface. Key wordy : sand, collapse, liquefaction, stress path, state boundary, triaxial test.

Effect of reservoir depletion and pore pressure drawdown on in situ stress and deformation in the Ekofisk Field, North Sea : Teufel, L W; Rhett, D W; Farrell, H E Rock Mechanics as a Multidisciplinary Science: Proc 32nd US Symposium, Norman, 10–12 July 1991P63–72. Publ Rotterdam: A A Balkema, 1991

Knowledge of in situ stress and how stress changes with reservoir depletion and pore pressure drawdown is important in a multi-disciplinary approach to reservoir characterization, reservoir management, and enhanced oil recovery projects. Over 20 years of petroleum production from the Ekofisk field has resulted in a 21-24 MPa reduction in reservoir pore pressure. The effect of pore pressure drawdown on the minimum horizontal in situ stress in the Ekofisk field has been determined from shut-in pressure data of 32 hydraulic fractures. The effective stresses in the reservoir increase linearly with pore pressure drawdown, but at different rates. The ratio of the change in effective minimum horizontal stress to the change in effective vertical (overburden) stress is approximately 0.20. Laboratory experiments, which simulate the stress path followed by reservoir rock during the production history of the Ekofisk field, clearly indicate that shear failure has occurred during compaction of high porosity chalk as the shear stress increased with pore pressure drawdown. It is suggested that shear failure during primary production has increased fracture density and reduced matrix block dimensions, and has therefore maintained reservoir permeability, which may account for the continued good producibility of the Ekofisk field, in spite of compaction. 9more » refs., 5 figs.« less

Fluid pressure transients on seismogenic normal faults

Fluid inclusions in hydrothermally altered footwall rocks of the Dixie Valley fault, Nevada, and the Wasatch fault, Utah, indicate that pore fluid pressure fluctuated. Minimum entrapment pressures for fluid inclusions consisting of H 2O-CO 2-NaCl ranged from 295 MPa to 60 MPa in the temperature range 350° to 170 ° C on the Wasatch fault, and from 158 to 35 MPa in the temperature range 350° to 200 ° C on the Dixie Valley fault. Scatter in the pressure estimates at constant temperature is interpreted as paleo-fluid pressure transients at depths of up to 11 km on the Wasatch fault and 3 to 5 km on the Dixie Valley fault. Observed pressure transients range from 5 MPa, within the limits of error in pressure determination, to 120 MPa on the Wasatch fault and 7 to 126 MPa on the Dixie Valley fault. The pressure transients are greatest on both faults in the temperature range 270 ° to 310 ° C. The fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures. High fluid pressures may initiate rupture, then dilatancy, pore-pressure reduction, and dilatant hardening may arrest the rupture. However, decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90% which reduces the dilatant hardening effect and may permit ruptures to propagate.

Subsidence associated with the abstraction of fluids

Abstract Subsidence of the ground surface due to the withdrawal of groundwater, oil, gas or brine from sedimentary deposits has ccurred in many parts of the world. The abstraction of groundwater has been the principal cause of subsidence, primarily because more groundwater is abstracted than all the other liquids put together. Subsidences of several metres have been recorded, for example, in California due to the exploitation of oil, as well as groundwater. Such ground movements represent a notable problem in engineering geology. Generally these subsidences take place slowly but the occurrence at the surface of sinkholes as a result of water tables being lowered in limestone terrains is a rapid process. In the case of groundwater, gas or oil abstraction the reduction in pore pressure in the voids due to the decline in head leads to an increase in effective load on the sediments concerned, bringing about consolidation, which is reflected at the surface as subsidence. On the other hand when mineral deposits are worked by solution mining the rock material itself is removed which, if uncontrolled, resultsin subsidence. The removal of fluids from sediments frequently has resulted in the formation of fissures at the surface. Indeed there are cases on record where faults are alleged to have been formed. Such fissures often occur around the periphery of the subsidence trough.

Monitoring Local Subsidence in Areas of Potential Geopressured Fluid Withdrawal, Southwestern Louisiana: ABSTRACT

ABSTRACT Growth faulting, rapid deposition, and deep burial of sediments in south Louisiana have resulted in the abnormal pressurization of pore fluids within some sedimentary units. Fluid production from geopressured sandstones may result in pore-pressure reductions, clay compaction, and land-surface subsidence Subsidence is being monitored at the U.S. Department of Energy's Sweet Lake, Parcperdue, and Rockefeller Refuge geopressured-geothermal prospects in southwestern Louisiana. Reservoir-defining growth faults were extrapolated to the land surface, surface lineations were mapped from aerial photographs, and historical elevation surveys were compared to obtain base-line subsidence rates. Data from the Sweet Lake prospect indicate that no correlations exist between subsidence anomalies and lineations or fault projections. Subsidence here is probably a result of normal sediment compaction, salt domerelated land-surface adjustments, and/or historical subsurface fluid production. Conversely, a good correlation exists between the Parcperdue subsidence profile and the extrapolated updip extensions of mapped subsurface faults. Historical land-surface subsidence north of the Parcperdue prospect may have resulted from down-to-the-coast slumping along these growth faults which define the prospect. Subsidence at the Rockefeller Refuge prospect ranges from virtually zero on the eastern end of the prospect to 50 mm on its western end. The gradation may be due to oil- and gas-associated fluid withdrawals or land management variations. A subsidence analysis near Lake Charles, Louisiana, suggests that a more extensive historical record of leveling data could provide a more qualifying subsidence record. Repeated leveling surveys at the geopressured-geothermal prospects should aid in determining the causes of subsidence before and after depressurization of the geopressured aquifers.

Permafrost Thaw-Subsidence Casing Design

Casing design for arctic wells requires estimates of maximum casing strains resulting from permafrost thaw subsidence. These maximum strains have been determined for the Prudhoe Bay field by sensitivity studies with an analytical thaw-subsidence model that correlates with field test data. Introduction Deep permafrost at Prudhoe Bay consists of permanently frozen sediments about 1,850 ft thick. Thaw subsidence is the soil compaction resulting from the thawing of permafrost by a producing oil well. Thaw subsidence permafrost by a producing oil well. Thaw subsidence should be considered in well design because of the strains induced on well casing by this compaction. Atlantic Richfield Co. and Exxon Co., U.S.A., conducted a field test to assess thaw-subsidence effects. Subsidence was measured by surveying the motion of the wellheads, using subsidence rods and wires to monitor vertical motion in the soils, monitoring the motion of radioactive bullets shot into the formation, and measuring casing axial strains using a multiple-casing collar locator tool. The casing axial strains are most important because they indicate the casing's resistance to soil motion. An analytic thaw-subsidence model developed at Exxon Production Research Co. gives good qualitative explanations for the field test results and good quantitative agreement with the measured casing strains. The analytic subsidence model provides the tool needed for extrapolating the field test results to other areas in the field. This extrapolation was accomplished through a series of sensitivity studies. Two types of parameters influence thaw subsidence and are required in the analytical model. The first type is the amount of thaw, which can be controlled through the completion scheme and the production schedule. The second type involves the permafrost description, which cannot be controlled. The permafrost description is defined by the lithology, mechanical properties, and pore pressure. The lithology consists of the arrangement, pressure. The lithology consists of the arrangement, thickness, and location of the layers of the various soil types. The mechanical properties are the soil compressibility (compactibility) and the resistance to shear. The pore-pressure behavior during thawing is required because the pore-pressure reduction is the main driving force in the thaw-subsidence model. The confidence in determining the values for the input parameters in the analytical model is closely related to the parameters in the analytical model is closely related to the sensitivity of the model to these parameters. If the model is not very sensitive to the lithology, for example, then one does not have to have confidence in determining lithology across the field. Conversely, if the model is sensitive to lithology, then one should be confident in determining the lithology within some reasonable bound. The objective of the sensitivity studies is to determine and, it is hoped, bound the dependence on the various input parameters. The sensitivity studies include the strain dependence onthaw radius,lithology as defined by separation, thickness, and location of the layers, andmechanical properties including the compressibility and shear modulus. In the course of the sensitivity studies, certain ratios quantities were found to be important parameters when investigating layered systems. The layer separation is characterized by the sand/silt thickness ratio, which is the ratio of the thickness of a given sand layer to the thickness of an adjacent silt layer. JPT P. 455

Loading Mechanisms in Thawed Permafrost Around Arctic Wells

Four permafrost thaw-subsidence loading mechanisms are described for inducing casing strain. Phase change contraction in ice rich soil and consolidation with fluid flow are more likely to occur in surface and near surface soils. Stiffness reduction and pore pressure reduction are important in deep permafrost and generate body force-type loads. The equations for stiffness reduction loading are presented in terms of an elastic model. The effects of lithology and thaw variations on casing loads are examined in the context of the four mechanisms.

A Mechanical Model for Permafrost Thaw Subsidence

A mechanical model describing permafrost subsidence has been developed and correlated with a full scale field test performed by the Atlantic Richfield Company and Exxon Company, USA at Prudhoe Bay. The permafrost mechanical response is modeled with a linear stress-strain relation that incorporates pore pressure reduction as the subsidence loading mechanism. The pore pressure reduction is due to the phase change of pore ice upon thaw and was verified by field test measurements. The mechanical response of the well casing is included in the model with no slip assumed between casing and permafrost. The thaw subsidence model explains several features observed in the field test. The pore pressure reduction mechanism produces an upward rebound of the permafrost base, resulting in compressive casing strains above the base and tensile strains below. The pore pressure loading also produces an inward lateral motion of the thawed-frozen interface. The inward motion, together with layers of different soil types, produce the alternating compressive and tensile strains measured in the field test. These alternating strains can be significant, depending on permafrost lithology.

Local sea level changes before and after the Hyuganada, Japan, earthquakes of 1961 and 1968

The two Hyuganada, Japan, earthquakes of 1961 and 1968 (M = 7.0 and 7.5, respectively) each broke 80-km segments of the thrust zone abutting to the southern end of the Nankaido 1946 rupture. Mean annual sea levels (period 1942–1973) at eight tide gage stations along a 400-km segment of the Japanese coast centered in the epicentral area were compared to each other. It was found that mean annual sea level differences between the northernmost and southernmost stations (Kochi and Kagoshima) remained constant over a 16-year period, with a standard deviation from the 16-year mean of 1.2 cm. Compared with these northern and southern reference stations, four central tide gages located within 50 km of the two Hyuganada ruptures showed the following changes: (1) At the station between the two earthquakes (Hosojima), sea level dropped by about 5 cm in 1957, and it rose by about 5 cm after the earthquake in 1968. (2) At two stations north of the ruptures (Tosashimizu and Uwajima), sea level rose by about 4 cm in 1957. (3) At a station close to the southern end of the ruptures (Aburatsu), sea level remained constant with a standard deviation of about 1.1 cm between 1950 and 1966. These sea level changes are interpreted to reflect local crustal uplift at Hosojima and subsidence in the Tosashimizu-Uwajima area in 1957. If one assumes that these vertical movements were precursors to the two Hyuganada earthquakes, the data support the dilatancy-diffusion hypothesis: Near the center of the ruptures, dilatancy may have caused uplift (Hosojima), and beyond the northern end of the aftershock area, subsidence may have occurred due to reduction in crustal pore pressure. Precursor times of these magnitude 7.0 and 7.5 earthquakes may have been approximately 3.5 and 5 years, respectively.

A New Technique for Reduction of Excess Pore Pressure during Pile Driving: Discussion

The authors are to be complimented on an interesting case history presenting an original method of solving the commonly occurring problem of development of excess pore pressures, when driving piles in soft clays. In Fig. 5, the authors present the measured pore pressures and number of piles driven on specific days. This figure indicates that the developed pore pressures were considerably smaller after the installation of the Geodrain on the piles, as compared with the measurements before the drain application. However, as the magnitude of the measured pore pressures is less dependent on the number of piles driven per day and more on the distance from the piles to the piezometers, a different plotting of the results gives a better basis for the conclusions drawn by the authors. In Fig. 1, the writer has plotted £or each driven pile the distance against time to two piezometers, numbers P-1 and P-3. A black dot shows the distance (and date) of a pile relative to piezometer number P-1 and an open dot that of the same pile relative to piezometer number P-3. The lower diagram in Fig. 1 shows the piezometric elevations measured in piezometer numbers P-1 and P-3. A study of the measurements taken before

The effect of oriented cracks on seismic velocities

We have considered the problem of elastic wave velocities in a matrix containing aligned ellipsoidal fluid-filled cracks. This problem is relevant to a variety of geophysical applications, including crustal and mantle seismology and the behavior of stressed and dilatant rock. When the cracks are ellipsoids of revolution, the composite is transversely isotropic and is describable with five elastic constants. For aligned oblate spheroids the major reduction in velocity occurs along the axis of symmetry. The opening of new cracks, the widening of old cracks, or the reduction of pore pressure accompanying crustal dilatancy can be expected to cause a large decrease in compressional velocity and considerable compressional wave anisotropy.