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EUROCK 2006 – Multiphysics Coupling and Long Term Behaviour in Rock Mechanics – Van Cotthem, Charlier, Thimus & Tshibangu (eds) © 2006 Taylor & Francis Group, London, ISBN 0 415 41001 0 The New Geophysics: a new understanding of fluid-rock deformation Stuart Crampin School of GeoSciences, The University of Edinburgh, Scotland, UK ABSTRACT: Recent observations of stress-aligned shear-wave splitting (seismic-birefringence) show that the splitting is controlled by the stress-aligned fluid-saturated inter-granular microcracks and preferentially orientated pores pervasive in most in situ igneous, metamorphic, and sedimentary rocks in the earth’s crust. These fluidsaturated microcracks are the most compliant elements of the rock-mass and control rock deformation. The splitting shows that microcracks are so closely spaced that they verge on fracture-criticality and fracturing, and are critical systems with the butterfly wing’s sensitivity. As a result, the evolution of such fluid-saturated rock can be modelled with anisotropic poro-elasticity (APE), so that behaviour can be monitored with shear-wave splitting, future behaviour calculated (predicted ), and in principle future behaviour controlled by feed back. This is likely to have massive implications for virtually all solid-earth deformation at levels of stress below those at which fracturing takes place. 1 INTRODUCTION Widespread observations of stress-aligned shearwave splitting (seismic birefringence) in hydro-carbon reservoirs, and above small earthquakes, show that almost all in situ rocks are pervaded by distributions of stress-aligned fluid-saturated microcracks (Crampin, 1994, 1999; Alford, 1986). These are grain-boundary cracks in crystalline rocks, and preferentially-aligned pores and pore throats in sedimentary rocks (Figure 1). The degree (%) of observed splitting indicates that microcracks are so closely spaced they verge on fracture criticality, when levels of cracking are so great that shear-strength is lost and fracturing necessarily occurs (Crampin, 1994, 1999). Phenomena verging on criticality are critical systems are said to have self-organised criticality (Jensen, 1998). Critical systems are a New Physics (Bruce & Wallace, 1998), a New Geophysics, with different properties from conventional sub-critical behaviour. Below criticality, individual elements of the system, in our case the fluid-saturated microcracks interact locally with conventional behaviour. At criticality, the elements throughout the region combine and act cooperatively, and the rocks (sometimes over very large volumes) spontaneously fracture leading to deterministic chaos. The behaviour can be calculated but the final position depends critically on minuscule differences in initial conditions. Such behaviour at criticality is characteristic of almost all complex complicated systems which are extremely common. It is said to be “one of the universal miracles of nature” that at critical points whole assemblages of elements “are capable of organising themselves into patterns of cooperative activity” (Davis, 1989). Thus our claim that the cracks in the crust are critical systems merely confirms that the Earth behaves like almost every other complex interactive system: the weather; quantum mechanics; super-fluidity and super-conductivity; life cycle of fruit flies; clustering of traffic; etc. This brief note outlines the evidence and implications of the New Geophysics.The detailed deformation of the crust can be monitored with shear-wave splitting; evolution calculated if the stress changes are known; future behaviour predicted; and, in some circumstances, future behaviour potentially controlled by feedback. 2 SHEAR-WAVE SPLITTING The key phenomenon for observing the internal microcracked structure of rocks is stress-aligned shear-wave splitting, where shear-waves split into two approximately orthogonal polarisations where the faster polarisation is aligned approximately in the direction of the maximum horizontal stress (Figure 1) (Crampin, 1994, 1999). Seismic P-waves are rather insensitive to fluidsaturated microcracks, whereas stress-aligned shearwave splitting is a second-order effect that, by rotating seismograms into preferential orientations, can typically be measured with first-order accuracy. Thus measurements of shear-wave splitting often have much greater resolution than conventional seismics and it is possible to show that the parameters that control shear-wave splitting are directly those that control 539 Figure 1. Schematic illustration of stress-aligned seismic shear-wave splitting in the stress-aligned microcracked crust. Figure 3. The Anisotropic Poro-Elastic (APE) model of rock deformation where the driving mechanism is driven by pore-fluid movement along pressure-gradients between neighboring cracks at different orientations to the stress field. Figure 2. Schematic illustration of observed percentage of shear-wave velocity anisotropy in terms of uniform distributions of penny-shaped cracks. low-level deformation before fracturing occurs (Crampin, 1999). Crack density ε is a dimensionless parameter approximately equal to Na3 /v, where N is the number of cracks of radius a in volume v (Crampin, 1994). As crack density ε is approximately equal to one hundredth of the percentage of the maximum shear-wave velocity anisotropy, it is comparatively easy to estimate crack density from observations of shear-wave splitting. The crack density determines the effect of cracks on the effective elastic tensor of an anisotropic microcracked solid. The observed shear-wave velocity anisotropy, 1.5% to 4.5%, illustrated schematically in Figure 2, indicates a narrow range 0.015 ≤ ε ≤ 0.045 of the inferred crack density in ostensibly intact rock. The values appear to be more-or-less independent of porosity, geology, and tectonic history, and are observed to have similar values in all sedimentary, igneous, and metamorphic rocks, with only a few well-understood exceptions (Crampin, 1994, 1999). Since there is a factor of less than two in average crack radius, between the minimum observed, and the maximum at fracture criticality, almost all in situ rocks are so heavily microcracked that shear strength lost and fracturing occurs in the presence of almost any increase in differential stress. The limited range of crack density means that distributions of cracks in in situ rocks are critical systems verging on criticality and failure and have self-organised criticality (SOC) (Jensen, 1998). The evolution of such fluid-saturated microcracks can be modelled by Anisotropic Poro-Elasticity (APE), where the mechanism for deformation is fluid movement by flow or dispersion along pressure gradients between neighbouring grain-boundary cracks in crystalline rocks, and aligned pores and pore-throats in sedimentary rocks, at different orientations to the stress field (Zatsepin & Crampin, 1997; Crampin & Zatsepin, 1997). APE is a fully three-dimensional model but 3D variations are difficult to plot on paper, so Figure 3 is a schematic image of the APE mechanism on an initially random distribution of vertical cracks for increasing differential stress sH . 3 APE MODEL OF ROCK MASS EVOLUTION APE modelling approximately matches a huge range of different phenomena referring to cracks, stress, and shear-wave splitting in both exploration and earthquake seismology (Crampin, 1999, 2004; Crampin & Chastin, 2003). The match is approximate because the internal microcrack structure of in situ rocks is essential inaccessible. Even drill cores deform rock: drilling cools and de-stresses in situ rock, and disturbs the compliant internal crack geometry by invasion of high-pressure drilling fluids. The most successful in situ calibration to-date (Figure 4) is Angerer et al. (2002) who modelled changes in three-component reflection surveys in a hydrocarbon reservoir following both high-pressure and low-pressure CO2 -injections in a fractured carbonate reservoir at 1280 m-depth. The first five traces in Figure 4(a) show observed three-component record sections rotated into faster 540 These are the best in situ demonstrations of the applicability of APE-modelling. This means that, if the changes to the rock are known, the effects can, at least in some circumstances, be calculated (predicted) by APE. It also means that if a particular result is desired: a particular degree of aligned cracking during a water-flood operation, say, the intended effect could be controlled by adjusting fluid-pressure injections by feedback. 4 Figure 4. (a) Pre-injection waveforms of a multi-component nearly-vertical ray reflection survey near the centre of the Vacuum Field, New Mexico, carbonate reservoir (Angerer et al., 2002). S1, S2, and P are record sections with mutually orthogonal polarisations, where the horizontals S1, and S2, have been rotated into the split shear-wave polarisations parallel and perpendicular to the direction of maximum horizontal stress, respectively. Left-hand (LH) five traces are observed waveforms at recorders 17 m apart, and the right-hand (RH) three traces are synthetic seismograms modelled by APE to match the shear-wave arrivals. Top and bottom of injection zone for shear waves are marked by arrows with time-delays in ms/km. (b) Post-injection waveforms after a high-pressure CO2 -injection. Again, the LH traces are observations and RH traces are synthetic seismograms modelled by APE with structure from (a) and an injection pressure of 2500psi. S1-wave and slower S2-wave horizontal, and vertical P-wave, components showing ∼2% shear-wave velocity anisotropy. The last three traces show theoretical reflections (calculated by ANISEIS, Taylor, 2000) through a model structure incorporating fractures and microcracks. The shear-wave splitting arrivals from the top and bottom of the target zone (arrowed) match observations. Figure 4(b) shows the observed arrivals after a high-pressure CO2 -injection, where the modelled arrivals are the initial cracked model in Figure 4(a) modified by inserting the fluid-injection pressure into APE. The match of observations and APE-model is excellent, as it is for a low-pressure injection (not shown). 90◦ -FLIPS OF SHEAR-WAVE POLARISATIONS IN HIGH-PRESSURE CRACK FLUIDS Figure 4 also shows another confirmation of APEmodelling. In Figure 4(a) the two-way reflection times of the (faster) S1-wave from the top and bottom of the target zone differ by 176ms whereas, as expected, the delay for the (slower) S2-wave is 178 ms. In contrast, in Figure 4(b) after the high-pressure injection, with the same seismogram rotation, the S1-wave delay is 204 ms and the S2-wave delay is 184 ms. Thus the S2-wave is now faster than S1, and there has been a 90◦ -flip in shear-wave polarisations in the presence of critically high pore-fluid pressures (Crampin et al., 2004).These 90◦ -flips, in some circumstances can lead to observations of time-delays between split shearwaves decreasing with distance, and observations of shear-wave polarisations parallel to the minimum horizontal stress instead of the more typically parallel to the maximum horizontal stress. 90◦ -flip phenomena, first modelled theoretically by APE (Crampin & Zatsepin, 1997), have now been observed elsewhere in critically-high pressured hydrocarbon reservoirs, and on all seismically-active fault zones due to the high fluid-pressures necessary to permit movement on lithostatically compressed faults (Crampin et al., 2004). Above large faults, 90◦ -flips are observed at the surface, but on smaller seismicallyactive faults, the 90◦ -flips lead to negative, say, timedelays near the fault, and the positive time-delays along the normally-pressurised remainder of the ray path to the surface. The summation of negative and positive time-delays leads to the large ±80% scatter in observed shear-wave splitting time-delays above all small earthquakes (Crampin et al., 2004). 5 INDIRECT CONFIRMATION OF APE-MODELLING Further indirect confirmation of APE-modelling is that APE indicates that the principal effect of changes of low-level stress is to modify microcrack aspect ratios. In particular, increases of stress before fracturing occurs (pre-fracturing deformation) can be 541 monitored by increases in shear-wave splitting timedelays in particular segments of the solid angle of ray path directions (Crampin, 1999). Observations of such, rather subtle, easy-to-overlook changes, monitor the accumulation of stress before earthquakes and allow estimates of the progress towards fracturecriticality and earthquake occurrence. The duration of the increase allows the magnitude of the impending earthquake to be estimated. Recognised with hindsight before some 17 earthquakes (Volti & Crampin, 2003; Gao & Crampin, 2004). On one occasion the time, magnitude, fault break of an M 5 earthquake in Iceland was successfully stress-forecast three days before the event (Crampin, 1999). 6 CRITICAL SYSTEMS OF FLUID-SATURATED CRACKS IN THE EARTH between adjacent fluid-saturated grain-boundary cracks and pore-throats. This is a quantifiable physical process that can be modelled, monitored, and calculated by APE (Zatsepin & Crampin, 1997; Crampin & Zatsepin, 1997; Crampin & Chastin, 2003). The criticality, where different critical systems have similar statistical behaviour near criticality, despite very different sub-critical physics, is the reason APE-modelling matches a huge range of phenomena (Crampin & Chastin, 2003). This is known as criticalpoint universality (Bruce & Wallis, 1989), and implies the self-similar scaling seen in crack distributions, as well as the well-known Gutenberg-Richter earthquake magnitude frequency relationship, and many other phenomena. 7 The capability of imaging the behaviour before earthquakes ranging from a M 1.7 in Iceland to the Ms 7.7 Chi-Chi earthquake by the highly-constrained APE model and calculate, even predict, the response of fluid-saturated microcracked rock to changing conditions in a highly complicated heterogeneous crust, is remarkable and requires explanation (Crampin & Chastin, 2003). Critical systems are typically sensitive (the "butterfly wing’s" sensitivity) to otherwise negligible variations in initial conditions that can lead to order-of-magnitude differences as the systems evolve (Crampin, 1999; Crampin & Chastin, 2003). This manipulation of fluid-saturated microcracks as monitored by shear-wave splitting indicates that stressaligned fluid-saturated microcracks are remarkably pervasive features with similar parameters in almost all rocks in the crust. The observed calculability is because the fluid-saturated cracks in the crust are so closely spaced that they are critical systems (Bruce & Wallis, 1989). Critical systems involve dynamic interactive processes that below criticality perturb only locally. Once systems reach criticality, all members of the critical system influence all other members (Jensen, 1998; Bruce & Wallis, 1989; Crampin & Chastin, 2003). The transition temperature of equilibrium thermodynamics is the classic critical system, but critical systems are common in an enormous range of phenomena including almost all complex interactive systems in nature and elsewhere. Crampin & Chastin (2003) suggest that the stress-aligned fluid-saturated cracks in the earth’s crust are also interactive critical systems. Similar schemes for the Earth have been suggested previously. The difference here is that we have identified the micro-scale mechanism for deformation as stressinduced fluid-movement along pressure-gradients OBSERVED SENSITIVITY The expected sensitivity was confirmed by an experiment to test a borehole Stress-Monitoring Site (SMS) (Crampin et al, 2003) for stress-forecasting earthquakes. The “SMSITES” SMS near the Húsavík-Flatey transform fault of the Mid-Atlantic Ridge in Iceland recorded seismic travel times over 315 m at 500 m-depth (Figure 5). Repeated pulsing of the Downhole Orbital Vibrator (DOV) borehole source several times a minute over 13 days allowed, with 100-fold stacking, ±0.02 ms time-lapse accuracy in Pwave and split shear-wave (SV and SH ) travel times, and shear-wave anisotropy (SV – SH ). The arrivals showed well-recorded 5%, 2%, 2%, and 10% variations, respectively, which correlated with NS and EW Global Position System (GPS) variations and a one metre decrease in the level of a water well immediately above the Húsavík-Flatey fault. All seven variations coincided with small-scale seismicity, with total energy release equivalent to one M 3.5 event, on a neighbouring transform fault 70 km-away (Crampin et al., 2003). These well-observed variations, show exceptional sensitivity, well beyond that expected in a conventional brittle-elastic crust at several hundred times the conventional source diameter. 8 IMPLICATIONS OF THE NEW GEO-PHYSICS FOR HYDROCARBON PRODUCTION Since liquid-saturated cracks have little effects on P-wave propagation, the New Geophysics is likely to have little effect on conventional exploration seismology, where the principal tool is P-wave arrival times. Consequently, if you are happy with conventional P-wave seismics, and happy with conventional resolution, you can probably ignore this New Geophysics. 542 However, if you wish for higher resolution, and particularly wish to use shear-wave seismics for a better understanding of low-level deformation, you are necessarily committed to the New Geophysics. There are two major effects. The New Geophysics potentially alters the behaviour of seismic wave propagation, and alters the geophysical behaviour of the reservoir. 8.1 Monitoring production with time-lapse seismics in Single-Well Imaging (SWI) The movement of oil/water fronts can be analysed by subtracting record sections before and after some oil field production process in time-lapse seismics. Success depends on successive records shot with identical source-to-receiver geometry. Such detailed sections are expensive offshore, where many major oil fields are situated, and one of the major expenses is laying areal arrays of sea-floor geophones for optimal time-lapse seismics. Figure 5 shows time-lapse signals contaminated by substantial temporal variations in seismic travel times over 315 m correlating with distant small-scale seismicity. The effects of stress accumulation before higher seismicity, or volcanic eruptions, are likely to be seen across the whole tectonic plate (Crampin et al., 2003). Note that the effects of the New Geophysics are non-linear, as in the ±80% scatter of shear-wave timedelays observed above all small earthquakes (Crampin et al., 2004), that cannot be interpreted by conventional sub-critical techniques. Consequently, any in situ measurements necessarily degrade both temporally and spatially as soon they are made, and the longer the ray paths the greater the possible degradation. The only way to avoid such degradation is to make and interpret measurements at the time and place (the producing reservoir) they are required by Single-Well Imaging (SWI) (Crampin, 2004). SWI is where the scattered reflections from a borehole (DOV) source are recorded by three-component recorders behind the casing or behind tubulars in the same well as the source. There are time-lapse techniques for interpreting the scattered signals in terms of changing background: moving oil/water contacts for example (Crampin, 2004). One great advantage of SWI is that it is much cheaper (× 1/100) than conventional 4D reflection surveys. Since SWI tracking of fluid fronts is largely independent of geological structure, SWI could eventually replace costly reflection surveys. 8.2 Slower production for More Oil REcovery (SMORE) Figure 5. Observations at the prototype SMS (Crampin et al., 2003). (a), (b), and (c) are travel times in ms of P-, SV - and SH -waves, and SV -SH anisotropy, respectively, at 500 m-depth between boreholes 315 m-apart. The directions are parallel to and about 100 m South of the Húsavík-Flatey Fault (HFF). Shear-waves are polarised SV, and SH propagating in a vertical symmetry direction of the stress field. Also shown are (d) NS and EW (GPS) displacements in mm; and (e) water-pressure ‘pulse’ measured in bars in a well immediately above the HFF (the ‘pulse’ is ∼1 m deep; the ∼40 cm oscillations are oceanic tides). Observations (a to e), correlate with (f ), a histogram of small scale seismicity on a parallel fault 70 km NNW of the SMS. The total energy of the seismicity is equivalent to one magnitude M 3.5 event. One of the characteristics of critical systems with SOC is that the self-similarity and calculability only occur when the complex interactions are responding to slow changes (Jensen, 1998). Rapid changes and aggressive production strategies are likely to produce chaotic deformation which will not be self- similar or calculable. This suggests that modelling and calculating the response will only be possible for slow recovery rates, hence the hypothesis of Slower production for MOre REcovery (SMORE). The question is how slow is slow and how much more is more? The delay between the initial and second survey in for Figure 4 was approximately two weeks and was clearly sufficient as the results were calculable, but it could have been days or hours. Note that the slow recovery might need to be for the whole production history. Initial aggression might so disturb the rock as to be irreversible. There are to our knowledge no estimates of how much more is more. Currently, oil fields typically recovery less than ∼40% of the oil in the reserve. Thus, even a conservative increase of 2%, say, to 42% (it might be substantially greater) of produced oil over possibly marginally longer recovery would be ∼5% more oil. This would be additional profit on the initial 543 infrastructure cost of oil rigs and pipe lines at the cost of a slower recovery rate. 8.3 Optimising water-flooding by APE-modelling The response of the rock mass can be potentially controlled by matching time-lapse shear-wave splitting in 4D reflection surveys or VSPs to APE-modelling by feedback. This might be important for fluid-injection. If a water-flood operation were initially monitored by appropriate seismic measurements, such as VSPs or SWI, it would be possible to recognise whether cracks had opened in the desired direction to aid fluid sweep operation. If non-optimal orientations were indicated, different injection procedures could be attempted and tested. This could establish the likelihood of success of the water fluid operation within hours or days of the flood and long before production returns indicated success or failure. 9 CONCLUSIONS The New Geophysics is a recent construct and the full implications will take some time to evolve. Why do we need a New Geophysics? After some 80+ years of oil production and vast expenditure of research funds, we still recover, typically, less than ∼40% of the total oil in the reservoir. Why should we be so unsuccessful? Shear-wave splitting (seismic birefringence) shows there are stress-aligned fluid-saturated microcracks in almost all in situ rocks that are so closely spaced they are highly compliant and are critical systems. This means that the reservoir and seismic waves do not behave as expected in conventional geophysics. Figures 4 and particularly Figure 5, for example, show that rocks, including hydrocarbon reservoirs, display sensitivity to minor disturbances, with much more sensitivity than would be expected in a conventional brittle-elastic crust. These and other fundamental anomalies suggest that there is a serious gap in our understanding of how rocks behave. The New Geophysics may not be the whole answer to our lack of success in oil production, but it certainly provides a large part of the answers, where the key observable is stress-aligned shear-wave splitting. 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