10_岩石物理量版用于叠前反演结果的定量解释_英文

Rockphysics Interpretation Chart for Quantitative Interpretation of AVA Simultaneous Inversion: A Seismic Hydrocarbon Detection Case Study from the Eastern China SeaFrank Chen* ZhiLiang Ming* Yang Su* MinQiang Zhang** Lei Zhang** * Fugro-Jason China** CNOOC ShangHaiSummaryIt is challenge to infer petrophysical properties from geophysical data, specifically acoustic and shear impedance data derived from inversion of seismic data. A technique known as the “Rock Physics Interpretation Chart” was developed by Fugro-Jason. The chart provides a bridge between the geophysical parameters and the petrophysical parameters and thus gives a petrophysical basis for the interpretation of seismic inversion results.AVA Simultaneous Inversion was carried on partial stack seismic data and quantitative interpretation of the resulting P-Impedance & Vp/Vs ratio was applied with the Rockphysics Interpretation Chart, which was calibrated to the measured logs of the local well. The Gas-Water Contact was mapped for the structure with one well, and an additional high potential gas reservoir was predicted. IntroductionThe main objective of this project was to map the Gas-Water Contact in a structure penetrated by one well, and predict any other high potential gas reservoirs along three 2D seismic lines. The near borehole CRP gather showed a strong AVO effect at the top of the gas reservoir and was matched by the synthetic AVO gather from the well log (Figure 1).Three 2D lines were reprocessed and common angle stacks were produced for inversion. Simultaneous inversion was applied to the angle stacks to convert the seismic data to rock properties. Quantitative interpretation of the rock properties was required to lower the risk for hydrocarbon detection.Figure 1：Comparison between the CRP gather and synthetic AVO gather from well logs.MethodlogyPetrophysical conditioning of the well log data was performed to remove artifacts in the data created by borehole (or other) problems during the logging run. Formation evaluation was performed to derive volumetric analysis of minerals and fluids for input to a rock physics modeling phase.Using the in situ temperature and pressure to model the properties of the fluids, the Self-consistent model was found to provide a suitable model for this high porosity sandstone case. Porosity was included in the model using ellipsoids with aspect ratios (alphas) of 0.10 for quartz and 0.04 for clay.The first QC step was to make sure that the rock physics modeled data matched the measured logs in the zones of interest (Figure 2). And the cross plot of measured P-Impedance & Vp/Vs was compared with the modeled P-Impedance & Vp/Vs (Figure 3).Figure 2：Fitting the measured data with a suitable rock physics model (red: modeled, blue: measured).Figure 3: The character of the crossplot is similar for the measured and modeled logs.The Rock Physics Interpretation Chart was is created with forward modeling for a series of conditions. And in this example, the fluids are gas and brine (Figure 4).Line 1 is the brine-sand line, which shows the following trend: With decreasing porosity, P-impedance increases and Vp/Vs decreases. The right end of this line is the pure matrix point (quartz in this example). The slope of the line is controlled by the aspect ratio model.Line 11 is the gas-sand line, which shows a very flat Vp/Vs trend versus porosity. Also, the cluster of lines 1 to 11 shows that when gas saturation increases, the Vp/Vs value drops, and the differential value is controlled by the porosity. Clearly, if porosity is low and the differential value of Vp/Vs is small, fluid prediction from the seismic becomes difficult.Lines 12 and 13 represent the trend when the sand is more shaly in steps of 10% of shale.Lines 14 to 20 represent constant porosity trends, which provide quantitative measures of how P-impedance and Vp/Vs change when the porosity is a fixed value and the formation is filled with various mixtures of gas and brine. Since the precision of P-impedance inversion is normally higher than the Vp/Vs ratio, the higher sensitivity of P-impedance to saturation can help in the final interpretation or uncertainty analysis for fluid.For this example, the mixture of brine and gas was modeled, and a 100% oil line was modeled using parameters supplied by the oil company. Line 21 indicates that the oil line is nearly equal to the 40% gas line (line 5). The implication is that fluid prediction is more difficult for oil than for gas.Figure 4: An example of Rock Physics Interpretation Chart that shows P-impedance vs. Vp/Vs vs. Sw vs. PHI.Assumptions & LimitationsBefore discussing applications for the interpretation chart, it is important to consider several assumptions that underlie this work, the methods used to calibrate key parameters, and some inherent limitations.o Long Wave Length Assumption & Fixed Matrix Parameters. Most of the rock physics models we are discussing, including the Self-Consistent and Xu-White models, assume that the sonic wave length is much longer than the size of the matrix mineral and pore space and consequently the composite material (matrix and fluids) can be described as a homogeneous material with a certain modulus. The Long Wave Length Assumption is fundamental to modeling the elastic modulus in low seismic frequencies based on calibration with sonic logs at higher frequencies. When the rock physics model can predict the sonic data very well, the response at seismic frequencies is assumed to be the same as the response at sonic frequencies with the same parameters of matrix points. However, it is well known that velocity is dispersive in the frequency domain. For instance, the P-velocity is often a few percent slower at seismic frequencies than at sonic frequencies. This kind of dispersion effect is not taken into account by the modeling discussed in this document.o Extended Alpha Model. The log samples that can be used to calibrate the rock physics model are limited. For instance, the porosity range is from 6 P.U. to 20 P.U. in our example. As we know, the slope of the brine-sand line is strongly dependent on the alpha model, but the model is calibrated by the well log only in the porosity range of 6-20 P.U. To compute the Rock Physics Interpretation Chart, the alpha model must be extended based on certain assumptions, for instance, a constantalpha for the low porosity range to the matrix point anda porosity-related alpha model for porosities higherthan 20 P.U. Again, part of the chart is not actually calibrated by the well log samples; so care must be taken when using it as a prediction tool.o Fluid Mixing Factor. Brie’s mixing rule is used for determining most of the composite fluid properties.This raises two issues. First, the log samples do not cover the full saturation/porosity range, and the well log is strongly influenced by the drilling mud invasion, especially in areas of light hydrocarbons. The second issue is the difficulty in determining the mixing factor for seismic frequencies. Case studies have shown that some gas reservoirs follow Wood’s law more at seismic frequencies and Voigt’s law more at sonic frequencies.It is difficult to determine the optimum fluid mixing factor for seismic frequencies; the Brie’s factor used for sonic log modeling is generally between 1.5 and 3.5.In short, it is still difficult to quantitatively calculate the water saturation, Sw, at seismic frequencies because the calibration is at log frequencies, unless the fluid contact can be confirmed on the seismic data. Since the gas-sand line is more or less a constant Vp/Vs, the fluid mixing factor is just controlling the gradient of the Sw direction, or in other words, the sensitivity of low saturation, and it is important to keep this limitation in mind when applying the interpretation chart.ApplicationThe Rock Physics Interpretation Chart is designed and calibrated for interpreting the results of full band inversion. For the simultaneous inversion method, the low frequency model is derived from well log interpolation; calibration of the rock physics model is also based on well log interpolation. Therefore, if the inversion results are constrained to lie close to the low frequency model, the scatter of seismic inversion data should follow the same shale trend as the rock physics model.The corner points of any polygon (Figure 5) used for interpreting the results of simultaneous inversion can then be assigned a clear petrophysical meaning in terms of porosity, saturation, and volume of clay. This description can then be used instead of some fluid index for simultaneous inversion interpretation (Figure 6). And it should be applied to quantify reservoirs thicker than half of the tuning thickness from deterministic inversion.Figure 5: Interpretation chart for Simultaneous Inversion result interpretation, the polygon is assigned a clear petrophysical meaning .Figure 6: Gas-Water Contact be mapped with all the samples with property in the polygon of figure 5.Full band inversion is highly dependent on the low frequency model, and it is critical that this model be correct in any prospective structure that has no wells. Band limited inversion is not so strongly dependent on the low frequency model. So the interpretation of the results of band limited inversion may be more reliable, especially for the fluid contact (Figure 7).The Rock Physics Interpretation Chart can also be used for qualitative interpretation of the results from band limited inversion. For instance, from the compaction trend we can predict that the porosity of the sand in Fig. 5 at 1.5 seconds in time is around 25 P.U. and the relative Vp/Vs is -0.18 for the sand above the gas/water contact and -0.05 for the sand below it. It is reasonable to assume that the top shale has a constant Vp/Vs value, and then the fluid contact around 1.51 seconds can be predicted (Figure 8) with solutions based on the interpretation chart:o Above the gas/water contact:Vp/Vs of shale = 1.85, VpVs of gas sand = 1.55,relative Vp/Vs for Gas Sand ~= -0.18o Below the gas/water contact:Vp/Vs of shale = 1.85, VpVs of brine sand = 1.75,relative Vp/Vs of brine sand ~= -0.05.Figure 7: Band passed Vp/Vs Ratio from Simultaneous Inversion.Figure 8: A high potential Gas Reservoir with Gas-Water Contact ConclusionsThe Rock Physics Interpretation Chart is designed for quantitative interpretation of rock properties derived from AVA Simultaneous Inversion. By using the chart it is possible to give a petrophysical meaning for any polygon used for capturing geobodies for interpretation.The chart can help to lower down the risk for seismic hydrocarbon detection with a rock physically meaningful cutoff value, and a high potential gas reservoir was predicted in this case study.AcknowledgmentsThe authors would like to thank the CNOOC ShangHai office for their cooperation in providing the data and support, and the management for permission to publish thiswork.。