Patent Application: US-201313772570-A

Abstract:
refracted energy travel time can help to derive anisotropic parameters in a target layer . these anisotropic parameters allow us to both explore for new reservoirs and to understand stress and fracturing in existing reservoirs . this information can be used to i ) detect oil reservoirs , ii ) spot naturally fractured , hence high production zones , iii ) detect dominant natural stress directions , iv ) better place horizontal wells to optimize production , v ) monitoring man made fractures or induced directional stress changes . the method is demonstrated using synthetic and real data .

Description:
turning now to the detailed description of the preferred arrangement or arrangements of the present invention , it should be understood that the inventive features and concepts may be manifested in other arrangements and that the scope of the invention is not limited to the embodiments described or illustrated . the scope of the invention is intended only to be limited by the scope of the claims that follow . as shown in fig1 , the travel path of reflected and refracted waves from a target horizon ( or reservoir ). upon reaching the target medium the refracted energy travels in the target layer and as a result refracted waves carry much more travel time and amplitude information about the target layer than the reflected wave which just reflects locally off the top of the layer . the main condition to reach the critical angle and form refracted waves is that the p - wave velocity above the target layer is slower than the p - wave velocity in the target layer . in a 3d shooting geometry , the energy from each source will propagate in different azimuthal directions . fig2 shows how refracted energy will travel in two different azimuth directions in a 3d geometrical setting . if the target layer is an hti medium , refracted waves will travel faster along 90 ° azimuth ( fracture strike or maximum horizontal stress direction ), and slower along 0 ° azimuth ( perpendicular to the fast direction ). here the azimuths are with respect to the symmetry axis of an hti medium . the effect of anisotropy recorded from these two azimuthal directions will be captured in the refracted wave travel times . to demonstrate this : fig3 shows a plot of synthetic seismograms for a multi - layered case , where the overburden is isotropic and the target layer is hti . left panel of fig3 shows synthetic p - wave seismograms from the 0 ° azimuth , middle panel shows the synthetic p - wave seismogram for the 90 ° azimuth , and the left panel shows the difference between them . it is clear from this plot that the difference ( signature of anisotropy ) is more for the refracted energy than the reflected energy . this phenomenon is true for any azimuthally anisotropic medium and not affected by the overburden even if it is anisotropic . this is also true for any other mode of waves ( converted , shear , surface wave etc ). a standard work flow of estimating seismic fracture parameters ( seismic anisotropy parameters ) is described below . fig4 , shows a typical vertical component receiver gather seismic data recorded in a 3 component receiver . note that the observed data does not require any special instrumental arrangement , and can be recorded in any standard recording systems ( e . g . single component , multi - component , ocean bottom node etc ). refracted energy is clearly observed in this receiver gather from the target reservoir . some pre - processing is applied before extracting the refracted travel time from the observed signal . pre - processing steps include ( but not limited to ) application of linear move out ( estimated from the slope of the refracted signal or from external information of the velocity of the target layer ), applying frequency - wavenumber ( or other ) filtering to remove dipping signals , extraction of travel time ( using first break picking , or cross correlation or other technique ), and removal of the applied linear move out . this process is repeated for every pair of source and receiver for every azimuth ( where refraction signal is visible ). additionally , preprocessing may be required to remove heterogeneity in the overburden and dip correction for reservoir features . extracted refraction travel time information is used to derive the anisotropic parameters and fracture characterization . amongst many potential methods , we show two different methods that are used to extract anisotropic information from the collected refracted travel time data . in the first method observed refracted travel times are sorted based on one receiver and multiple shot offsets and azimuths ( fig5 ). then using an equation derived by sil and sen ( 2009 ) anisotropic parameter ε ( a measure of fracture strength ) and fracture strike direction is derived for the receiver location . a grid search algorithm is applied in this case to derive these parameters . note that any inversion / optimization method can be used to derive those parameters from the extracted data shown in fig5 . similarly the extraction method is not limited to the equation derived by sil and sen ( 2009 ). any other complicated or simple form of equation describing refracted travel time variation with azimuth for an anisotropic medium ( e . g . landro and tsvankin , 2007 ) can be used for this purpose . extracted fracture strike and ε values at each receiver location are plotted over the reservoir in fig6 . in another embodiment a different sorting technique is used to derive anisotropic ( and fracture ) parameters more precisely . in this sorting or binning technique data is selected from multiple shots and receivers such that the refracted wave passes through a common point in the target reservoir in different directions or azimuths . this point is called common refraction point ( crrp ). fig7 shows a simplified example of the method with only 3 receivers . for each receiver , a line of shots along three different azimuths is selected . the intersection of the three lines indicates the considered crrp location with refraction travel time data from three azimuths . in practice this is actually applied to all the relevant receivers with refractions to get anisotropy information from a large range of azimuths making the inversion for anisotropy strength and direction robust at each crrp location . the advantage of the crrp technique is it sees variation of velocity with azimuth in a considerably smaller or localized area . once data is selected for each crrp the same optimization ( grid search ) technique as in method 1 can be applied to detect anisotropic parameters and displayed in the same manner ( fig8 ). it has been shown that : 1 ) refracted travel time contains hti anisotropy signals from a target anisotropic layer , 2 ) refracted travel times vary with azimuth due to azimuthal anisotropy in the target layer , 3 ) two methods to robustly obtain anisotropic parameter ε and fracture direction from the refracted wave is demonstrated . in closing , it should be noted that the discussion of any reference is not an admission that it is prior art to the present invention , especially any reference that may have a publication date after the priority date of this application . at the same time , each and every claim below is hereby incorporated into this detailed description or specification as a additional embodiments of the present invention . although the systems and processes described herein have been described in detail , it should be understood that various changes , substitutions , and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims . those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein . it is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description , abstract and drawings are not to be used to limit the scope of the invention . the invention is specifically intended to be as broad as the claims below and their equivalents . all of the references cited herein are expressly incorporated by reference . the discussion of any reference is not an admission that it is prior art to the present invention , especially any reference that may have a publication data after the priority date of this application . incorporated references are listed again here for convenience : 1 . u . s . pat . no . 6 , 864 , 890 , us2004041815 , “ method of building and updating an anisotropic velocity model for depth imaging of seismic data ,” meek and anno , conocophillips co . 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( 2011 ) 6 . cn102053260 , “ method for acquiring azimuth velocity of primary wave and method for processing earthquake data ,” tang and ma , china petroleum & amp ; chemical ( 2009 ). 7 . diebold and stoffa , “ the traveltime equation , tau - p mapping , and inversion of common midpoint data ,” geophysics , 46 : 238 - 254 ( 1981 ). 8 . landro , et al ., “ seismic critical - angle reflectometry : a method to characterize azimuthal anisotrophy ?” geophysics , vol . 72 , no . 3 ( mayjune 2007 ) p . d41 - d50 9 . sil and sen , “ seismic critical - angle anisotrophy analysis in the τ - p domain ” geophysics , vol . 74 , no . 4 ( july - august 2009 ) p . a53 - a57 . 10 . zadeh , et al ., “ 4d critical angle analysis using valhall lofs data ” seg expanded abstracts 29 , 4145 ( 2010 ). 11 . hansteen , et al ., “ time - lapse refraction seismic monitoring ” seg expanded abstracts 29 , 4170 ( 2010 ). 72nd eage conference & amp ; exhibition , barcelona , spain , paper b033 12 . hansteen , et al ., “ refraction monitoring shows promise in heavy oil field ,”