3D3C技术在深层致密储层裂缝预测中的应用-e.doc

3D3C Application in Prediction of Fractures of Deep-seated Tight Reservoir-An Example of Gas Exploration in Member 2 of Upper Triassic Xujiahe Formation, Xinchang Gas FieldTang Jianming 1，ion person2，Xu Xiangrong1，ion person2，Li Xiangui1（1.Deyang Branch Institute Exploration and Production Research Institute Southwest Petroleum Branch SINOPEC Deyang Sichuan 618000 2.GX Technology ion Geophysical Houston Texas 2222222)AbstractThe deep-seated gas reservoirs in the realm of continental deposits of the western Sichuan depression, which are super-tight, heterogeneous, fracture-pore gas reservoirs scattering mainly in Members 2 and 4 of Upper Triassic Xujiahe Formation, formed large- and middle-sized gas fields such as Xinchang, Hexingchang, Qiongxi, Zhongba and Bajiaochang. Deep in the continental realm of the western Sichuan depression there were favorable geological conditions for forming large- and middle-sized gas fields, for example, abundant gas sources, developed reservoirs, good preserving conditions and complete structural traps; the formed deep-seated gas reservoirs have the features of super depth, super tightness and low porosity and permeability, superhigh pressure, complex gas-water relations and complex reservoir heterogeneity, creating a great challenge to the prediction and identification of so complex reservoir and fracture systems using conventional seismic exploration technologies. The 3D3C converted wave seismic exploration technology, which is able to acquire at the same time C wave data reflecting rock matrix and anisotropy and P wave data reflecting matrix and fluid characteristics, is helpful to the exploration of deep-seated, tight, fractured gas reservoirs. Through the processing, inversion, multi-wave attributes analysis and comprehensive interpretation of 3D3C converted wave data, this technology has solved such problems about the deep-seated, tight, fractured gas reservoirs in western Sichuan as identification of high-quality reservoirs and detection of fractures, and has great potentials in determining fluid characters.Key Words: 3D3C, Fractured Reservoir, Multi-wave Attribute, Reservoir Identification, Fracture DetectionWith an area of about 180,000km2, the Sichuan Basin, a diamond-like basin, is located in the large deposit zone of Upper Yangzi. The equally developed tow major sedimentary structures, which are of marine and continental facies, have a total thickness up to 6,000-12,00m; from the Upper Sinian Series to the Jurassic System, both had oil generating abilities to different degrees and favorable conditions for oil and gas migration, storage and preservation. Gas reservoirs have been found in Sinian, Ordovician, Carboniferous, Permian, Triassic, Jurassic and Cretaceous sedimentary deposits; the total volume of gas resources is estimated at 6.01012m3, where that in the middle section of the western Sichuan depression is 1.231012m31.1. Basic Characteristics of Member 2 Reservoirs, Xujiahe Formation1.1 Reservoir types and characteristicsFigure 1-1 shows the pattern of reservoirs in the western Sichuan depression. Generally 5,000m in depth, the reservoirs in Member 2 of Xujiahe Formation scatters mainly in the sand bar at the front edge of the delta and partially in the river course of the delta plain.Figure 1-1 The Pattern of Reservoirs in Xinchang Gas FieldResults obtained through such means as core analysis, well logging and microscopical identification of reservoir thin sections show that there are three types of effective reservoirs in Member 2 of Xujiahe Formation: porous reservoirs, fractured-porous reservoirs and fractured reservoirs. Study reveals that fractured and fractured-porous reservoirs have a matrix porosity generally below 4% and that most samples have a porosity below 3% and their matrix permeability is less than 0.110-3mm2 (see Figure 1-2). The porosity of porous reservoirs is generally higher than 4%. In Xujiahe Formation there still are some reservoirs with a porosity above 6%, which are considered as relatively good ones in a tight environment. Figure 1-2 Porosity and Permeability Distribution of Member 2 of Xujiahe FormationActual drilling reveals that fractured-porous reservoirs prevail in Member 2 of Xujiahe Formation, with coexistence of fractured reservoirs and porous ones (for example, the high porosity/permeability gas reservoir at 4987-4994.5m deep in Well CG561, with an open flow capacity of 21104m3/d).1.2 Reservoir log response characteristics(1) Fractured reservoirsAn example is Well CX560. At 5161.6-5171.8m deep, the logging reveals a gas reservoir and FMI images show quite developed reservoir fractures (Figure 1-3), but the matrix porosity is very low, making the reservoir rather tight.Figure 1-3 FMI (left) and Log Curve (right) of Well CX560 Reservoir in Member 2 of Xujiahe Formation(2) Porous reservoirsAn example is Well CX565. At 4942.0-4960.6m deep, the log curve (Figure 1-4) shows good gas bearing, but the FMI data reveals no obvious fractures, indicating that reservoir is a porous one.(3) Fractured-porous reservoirs (prolific reservoirs)Examples are Wells X851, X856 and X2 (all the three wells have acquired an industrial high-yielding gas flow above 50104m3/d). At 4831-4836m and 4842-4846m deep in Well X851 (Figure 1-5), both conventional and full wave-train log reflect fracture characteristics of reservoirs, with fracture sections featuring spiny decrease in difference of positive amplitudes of lateral curve, fingerlike increase or jumping of sound wave, and obvious attenuation in longitudinal waves of full wave-train curve. The reservoirs are considered overall as fractured-porous ones.Figure 1-4 Log Curve at 4942.0-4960.6m of Well CX565 Reservoir in Member 2 of Xujiahe FormationFigure 1-5 Characters of Well X851 Fractured-Porous Reservoirs in Member 2 of Xujiahe FormationAt 4820-4825m deep in Well X856, both ARI and FMI images (Figure 1-6) clearly reflect the developments of effective fractures. FMI images provide all fractures in borehole wall, effective and ineffective, while ARI images provide only fractures with radial extensions greater than 2m. The comparison of both enables the estimation of radial extensions of factures and the identification their effectiveness. Figure 1-7 shows quite developed vertical and netlike fractures in reservoir sections.Figure 1-6 ARI (left) and FMI (right) Images of Fractures in Well X856Figure 1-7 Core from Well X856 (Effective fracture is clearly visible)1.3 Physical characteristics of reservoir rockThe determination of physical characteristics of rock is conducted under simulated high-temperature and highpressure underground conditions. Physical characteristics of rock show obvious regular changes under water- and gas-saturated conditions (Figure 1-8). When sandstone reservoirs in Member 2 of Xujiahe Formation of Xinchang contain gas, the Lame constant (l) reflecting rock compressibility decreases; the shear modulus (m) reflecting rock matrix basically remains unchanged; the ratio of P-wave to S-wave (Vp/Vs) decreases; and Poissons ratio (s) decreases to below 0.27.Figure 1-8 Physical Characteristics of Rock of Reservoirs in Member 2 of Xujiahe Formation of Xinchang RegionThe statistical analysis on log data corrected with physical characteristics of rock provides the distribution rules of different lithologies in Vp/Vs, P wave velocity, P-wave impedance and density. Figure 1-9 shows: P-wave velocity and impedance generally are high in gas-bearing sandstones and low in water-bearing and tight sandstones but have relatively obvious overlaps, making it difficult to distinguish gas-bearing sandstones through the P wave impedance zone; the density of gas-bearing sandstones is on the low side but has obvious overlaps with water-bearing and tight sandstones too; the distribution of Vp/Vs makes it easier to distinguish gas-bearing, water-bearing and tight sandstones and mudstones. Gas-bearing sandstones have a Vp/Vs ratio on the low side overall, at 1.55-1.65, while that of mudstones is at 1.7-1.8. In reservoir prediction, therefore, P- and S-wave information can be used to improve the precision of quality gas-bearing reservoir prediction.Figure 1-9 Physical Characteristic Analysis on Logging of Reservoirs in Member 2 of Xujiahe Formation of Xinchange RegionThe primary geological and geophysical research shows that good physical matrix conditions serve as the foundation of forming deep-seated, tight or supertight continental reservoirs of western Sichuan while fractures, especially developed high-angle netlike ones, are necessary conditions to acquire high yields.As far as seismic exploration is concerned, the complexity in deep-seated seismic geological conditions poses a great challenge the exploration of gas reservoirs there using conventional seismic exploration technology. The 3D3C converted wave seismic exploration technology is helpful to the exploration of the deep-seated, super fractured gas reservoirs of western Sichuan. Through the processing, inversion, multi-wave attributes analysis and comprehensive interpretation of 3D3C converted wave data, this technology can solve such problems as identification of high-quality reservoirs and detection of fractures and has great potentials in in determining fluid characters.2. 3D3C Converted Wave Exploration Approach2.1 3D3C converted wave data acquisitionThe recording geometry and primary acquisition parameters used for 3D3C exploration in Xinchange are as follows:Recording geometry: 12L16S264T2R66FBin size: 25m25m Coverage: 11(v)6(h)66Receiving points: 3168Receiving point spacing 50mShot point spacing: 50mReceiving line spacing: 400m Perpendicular offsetAspect ratio: 0.7Inline array: 6575-25-50-25-6575A rose diagram of distribution of 3D3C acquisition array and offset azimuth in Xinchang is shown in Figure 2-1. It is a bunchy broad-azimuth 3D3C recording geometry.Figure 2-1 Array and Characteristic of 3D3C Recording 2.2 3D3C converted wave data processingThe processing of 3D3C converted wave data involves the following key techniques:- Three-component redirection of converted wave- Coordinate rotation- Static correction of converted wave- Noise elimination- Surface consistency processing of converted wave- Pre-stack time migration of converted waveXinchang is different from other regions in processing thought and flow of 3D3C converted wave. Figure 2-2 gives the processing flow.Figure 2-2 3D3C Data Processing Flow of XinchangThe thought on 3D3C converted wave data processing of Xinchange is given below:(1) Process Z, R and T components in a combined way, and use the high reliability of P wave through the combination of the three components to solve statics of R and T components, eliminate surface wave noise and provide g0 etc.(2) Use the isotropy and anisotropy processing flows to make 3D3C converted wave data processing meet at the same time the requirements of reservoir prediction, gas-bearing detection and fracture detection.(3) Use the anisotropy processing flow to acquire data on synchronous pre-stack inversion of P wave, and joint or dual pre-stack inversion of P and S wave.(4) Process by azimuth to enable equally partitioned pre-stack time migration of azimuth gathers of Z, R and T components, providing basic data necessary for anisotropic fracture detection of P wave and for split fracture detection f S wave.(5) Use azimuth-specific processing results on R and T components to provide imaging profiles of fast and slow shear waves by means of Alford rotation, layer stripping or linear transformation, for the purpose of fracture density calculation and reservoir prediction research.2.3 3D3C converted wave data interpretationTechniques for converted wave data interpretation are very important in the 3D3C converted wave exploration technology, whose development however has not been mature yet. An usual flow of interpretation is shown in Figure 2-3.Figure 2-3 3D3C Converted Wave Interpretation FlowKey techniques for 3D3C converted wave interpretation include:- Joint calibration of P and S wave- Match of P and S wave- Joint inversion of P and S wave- Full-wave attribute analysis- Fracture detectionThe joint interpretation of P and S wave provides more and more precise information than single P or S wave exploration.- More accurate structural imaging. In the case of “gas cloud”, P wave cannot be imaged very well but converted wave can.- Good lithologic discrimination. Sandstones and mudstones can be well distinguished using Vp/Vs or Lame constant (l) and shear - More reliable prediction of quality reservoirs. The use of elastic impedance (EI), ratio of P-wave to S-wave (Vp/Vs), Poissons ratio (s), density (r) is helpful to the prediction of quality reservoirs.- Certain fluid identification capability. On the basis of petrophysics and well log interpretation, the use of such parameters as ratio of P-wave to S-wave (Vp/Vs) and Poissons ratio (s).- Plentiful and reliable fracture detection means. Broad-azimuth 3D3C seismic data of converted wave makes it possible to use all present fracture prediction techniques, including methods based on structural cause of formation, seismic attributes, azimuthal anisotropy of P wave, S-wave splitting and fracture modeling. These have obviously improved the reliability of fracture prediction.3. Joint Fracture Detection of P and S Waves3.1 Fracture prediction with multi-wave seismic attributes(1) 3D coherent bodyUp to the present the coherent body technology has developed to C4. Usually C3 algorithm can achieve large-sized fault and fracture systems and offer high reliability. Figure 3-1 is a 3D3C P wave diagram of coherent section from Member 2 of Xujiahe Formation of Xinchang, which clearly provides, in addition to the distribution of main fractures, small faults and fractures of rock formations near faults difficult to find using conventional structural interpretation. High-yielding wells are located near fractures or in coherent zones.Figure 3-1 T51 Coherent Section of 3D3C P Wave of XinchangeFigure 3-2 T6 Dip Body Section of 3D3C P Wave of Xinchange(2) 3D dip bodyThe 3D dip body is a byproduct of C3 calculation, which can describe soundly the existence of fracture systems because of its reflection of dip angles in space of similar seismic wave trains. Figure 3-2 is a 3D dip body secton of 3D3C P wave of Xinchange, showing clear fracture systems.(3) 3D curvature bodyChoose a sub-volume centering on a point in the 3D P wave data volume, automatically pick up peak values or zero crossing point at the central point, estimate using the least square or fit with other method a camber, and solve the second derivative of the camber to obtain the curvature of the central point 7. Figure 3-3 is a visualized diagram of 3D3C P-wave curvature body of Xinchang, with high curvature indicating clearly the fracture network.Figure 3-3 Visualized Diagram of 3D3C P-wave Curvature of Xinchange3.2 P-wave azimuthal anisotropy fracture detection technologyThe P-wave azimuthal anisotropy fracture detection technology uses the changes of seismic waves with azimuth in amplitude, velocity, propagation time and AVO attributes when relaying in anisotropic media, to detect development azimuth and density of fractures (especially vertical or high-angle ones). Its prediction results are linked more closely to microscopic features of fracture development zones. (1) AVAZ fracture detectionThe AVAZ fracture detection technique implements fracture detection on the basis of P-wave amplitude changes with azimuth. Figure 3-4 is a diagram of 3D3C P-wave T512 fracture detection and coherent low-value zone superposition about Xinchang, on which high-yielding wells are all in zones with high fracture density. In the diagram X851 and X856 are in the zone with the highest predicted fracture density, which is related to developed netlike fractures of the zone. Theoretically it is difficult to reflect truly the fracture development in the case of developed netlike fractures.(2) VVAZ fracture detectionThough the azimuthal anisotropy of P-wave propagation velocity has not a higher axial resolution than AVAZ or AVOAZ, it is an important method for P-wave azimuthal anisotropy fracture prediction thanks to its reliable reflection of overall macro anisotropic features of fracture development. Figure 3-5 shows 3D3C P-wave VVAZ analysis results about Xinchang. In the figure, both Vf and Vf-Vs are low near Wells X851 and X856, revealing developed netlike fractures. Around Well X851, however, the red parts have high Vf and Vf-Vs, showing developed single-group fractures; blue zones with longer line segments are ones without developed fractures.Figure 3-4 P-wave Anisotropy Fracture Detection and Coherent Superposition of T512 in 3D3C Data of XinchangeFigure 3-5 Diagram of 3D3C P-wave VVAZ Fracture Detection of Xinchang3.3 S-wave splitting fracture detectionS-wave splitting fracture detection is an important value of P- and S-wave joint exploration. S wave splits when passing through anisotropic media (fracture); when particles vibrate along a fracture, their propagation is fast, while it is slow when vibrating vertically to a fracture, and fast S wave and slow S wave appear when passing through a fracture system, also known as S-wave birefringence. As long as the direction of fast S wave is identified, the strike of fracture development is precisely located, while the interlayer time difference indicates the density of fracture development.There is substantive evidence proving the existence of fracture development in 3D3C C wave records of Xinchang. Figure 3-6 is the azimuthal gather after azimuthal superposition of compone