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technology Stress from Seismic John K. Davidson, Predrill Stresses International Pty Ltd T he most important variable in the conventional and unconventional gas and oil sectors is the direction and magnitude of the horizontal stresses approaching and at the target horizons. Why rely on sparsly measured data which are insufficient to determine the optimum direction of inclined/horizontal wells, especially for fracture propagation? Mapping Anderson Stress States Deformed seismic horizons are routinely mapped, but do the same horizon and fault files also contain quantitative stresses? The three Anderson stress states of normal, strike slip and reverse faulting can be identified on isopachs (strictly speaking ‘isochores’) from successive pairs of structure maps. The isochores representing periods of compression, contain reverse and normal faults and the ‘thins’ are synchronous growth of anticlines. The three stress states can be sub-divided into seven, coloured pink for reverse, strong compression through to grey for normal, and mapped (see Figure 1). It was found on the North Sea, Valhall 3D survey that the stress isochores at shallow horizons had low stress states and increased to strike slip and higher stress states of reverse faulting with depth (Figure 1). This is observed in all non-orogenic basins, that is, those beyond the horizontal ‘push’ of plate tectonics. Drilling Risk Reduction Each of the stress states is defined in terms of mutually perpendicular compressive stress axes of the Earth. These axes are SV, the weight of the rock (vertical) and the maximum and minimum horizontal stresses, SH max (SH) and Sh min (Sh). The strike slip stress state was Fig. 1. A detailed quantified pressure-depth graph from a 3D stress volume derived from a 3D survey. 34 | PESA News Resources | February / March 2013 written by Anderson as SH>SV>Sh. It, and each stress state, can also be represented by two simultaneous ratios SH/SV and Sh/SV. This allows the numerical representation of gradational stress state boundaries for software incorporation to produce the well planning essential, pressure-depth graphs. These can be made for wells of any direction, and any inclination, avoiding differential stresses and low angle fault approach. No longer is there a dependence on often distant and potentially poorly related ‘offset well’ information. It is all about drilling risk reduction. Orogenic and Non Orogenic Stresses The Valhall example shows the shallow, SHD, dark blue arrow is perpendicular to the blue anticline. That is also the direction of SH at the deeper horizons and is perpendicular technology to the pink reverse faults in the deepest. These are large basin-forming and modifying faults and they cut the crust. In the Andes the World Stress Map shows SHD is east-west perpendicular to the major north-south compressional thrusts and anticlines. Less than 1800 km to the east SHD is north-south and perpendicular to the Amazon basement generated trends. This means there is a gradational boundary from plate tectonic eastwest compressional mountain building orogeny to the non orogenic, multiple SHDs from crust cutting structures in sedimentary basins. In the Amazon and the North Sea graben with the Valhall anticline, the crust must be flexing. Vertical Stresses Such flexing occurs on a grand scale and is due to vertical forces, a third dimension to plate tectonics. Australia has drifted north some 40° of latitude in the last 50 million years. The shape of the oblate spheroidal figure of the Earth imposes a radius change of 21 km for a continent moving from the South Pole to the Equator. The extension is absorbed at northerly oriented oceanic ridges but essentially rigid continental (and older oceanic) crust will ‘feel’ the extension at the base of the crust with Lake Eyre sagging to 12 m below sea level. The surface will actually bend upwards at the eastern highlands and the east side of the Darling Fault, Western Australia, and be locally compressed at crust cutting faults. The 50 million year to Recent sediments in the Cooper Basin are strongly compressed with SHD perpendicular to crust cutting faults. Measured Stress = Seismic Stress Seismic lines in several basins in Australia show the Tertiary compression has not been increasing steadily with the northward drift as would be anticipated. These basins have experienced five compressional peaks: the Mid Eocene, Early Oligocene and the Early and Mid Miocene and Pliocene to Recent. The same events occur in the North Sea yet the global GPS shows Europe headed in the opposite direction with a North Polar component. There must be another vertical force not recognised by 2D plate tectonics. The five Tertiary compressional pulses are not alone. Thirty globally synchronous Fig. 2. An initial regional stress from seismic map using widely spaced 2D seismic. compressional pulses have been documented from the Carboniferous in up to 20 non orogenic continental and oceanic border areas. In all cases the stresses are perpendicular to crust cutting faults. The strike of these faults has not changed and the Earth is experiencing a compressional pulse today. This repetition of globally small but basin scale significant compression is like a heartbeat, they are all the same and predictable. Hence past isochores record today’s SHD’s and SHMagnitudes provided the amount of synchronous crustal flexing has been approximately unchanged in magnitude. Past stresses are the key to the Present. but SH is estimated or modelled from wellbore breakouts. Knowing, or reasonably estimating SV, also gives a quantified value of SH and Sh from seismic. The result is a 3D stress volume comprising quantified stress isochores derived entirely from the interpretation of reflection seismic. Four horizons are shown at Valhall but 12 were interpreted from the 3D survey resulting in detailed pressure-depth graphs. More horizons not only implies more dense 3D stress, but in this instance, quite accurate overpressure can be produced. All can be created pre-drill, from seismic and a known stratigraphy, alone. Scales of Applications Stress Consistent Seismic Interpretation Some of the ‘interpretation’ has now gone from seismic interpretation. The measured SHDs on a field should be equal to the total survey interpretation for ‘stress consistent seismic interpretation’ provided by available software (Pat.). Sh can be determined from mini fracs Not all areas have 3D seismic. A regional stress map of the Permian employing a 2.5 to 25 km 2D line spacing is shown (Figure 2). When the number of lines in the area of the yellow rectangle was trebled to a 2 to 5 km line spacing, the enlarged yellow area map resulted (Figure 3, left). Two almost perpendicular reverse fault trends generate 4 February / March 2013 | PESA News Resources | 35 technology a more detailed stress map of SHDs and stress states. SHDs between crust cutting faults are calculated on the basis of the relative lengths and throws of the faults, not by averaging between diverging measured SHDs. The seismic method can cause SHD to vary up to 90° over distances as small as 2 km which has been measured on at least two fields. Fraccing All components of the stress map SHD = SH/SV, SH, ShD = Sh/SV and Sh can be mapped. Of particular interest is a map of the Earth’s minimum horizontal stress or fracture gradient (ShM = Sh/SV) which equals the fracture strength for soft rocks with essentially zero tensile strength, TO. The ShM map (Figure 3, right) was derived from the yellow area. The red areas indicate Sh/SV>1, a reverse fault stress state in which fracs would propagate horizontally and have low Quantified stress has numerous applications in exploration ... productivity. Vertical fracs will occur in the green and amber areas where Sh/SV<1. Green is the desirable area for fracs and the amber is within 3 % of the red undesirable reverse faults and should be treated with caution. Fault Seal In fault seal analysis, consider the geometry and history of stresses on the fault plane. Extensional normal faulting is well known for creating fault gouge and clay smear, thereby enhancing seal. Intermittent periods of compression may reverse the throw of the fault and considerably increase fault gouge, often sealing sand against sand. Late, steep normal faulting across post migration thief zones or to the surface can leak, yet lower angle normal faults can seal. All these physical aspects of a fault can be handled by stress from seismic software but geological models of timing of migration need to be considered. Quantified stress has numerous applications in exploration for fault seal, in drilling for planning multidirectional and inclined wells and in production for fraccing old fields where initial field stress state can only be achieved accurately with the ‘stress from seismic’ method. Structural history is now more detailed by incorporating the fundamentals and new aspects of global tectonics. Over the past 20 years a wealth of measured stresses, GPS and geoidal data have added to our early 1990s concepts of all ridges pushing when the Atlantic and Indian clearly do not. Fig. 3. Left side, a regional to prospect stress state map using treble the number of 2D lines than in the yellow rectangle in Figure 2. Right side, fracture gradient map derived from the map at left. 36 | PESA News Resources | February / March 2013