Xian Qiang*, Liu Yonglei, Lv Dong, An Haiting, He Xiaosong, Li Haiyin, Xiao Yong, Zhou Chenguang, Xu Jianyang, Dong Lei,and Mao Xianyu,BGP,CNPC Summary Although high density, FAZ(Full azimuth) seismic acquisition have been tried at Tabei and Tazhong area, NAZ(Narrow-azimuth) seismic data is the mainly type of Tarim basin in present time. Due to high costs of multiwave and high density, FAZ seismic acquisition, it is difficult to expand in a large scale. Multi-wave and multicomponent seismic explorations are also at the starting stage. So, how to use seismic data to detect fracture has realistic meaning. In this paper, some areas of Tazhong and Tabei are used as cases to analysis the applicability and application of Pre-stack fracture detection based on P-wave azimuthal anisotropy. We contrast the different ways of fracture detection by using NAZ, WAZ and FAZ seismic data. Due to limitation of NAZ and WAZ seismic data, we especially introduce the prospect of fracture prediction by using FAZ seismic data. The result of fracture prediction is analyzed and used to divide the carbonate fracture-cave systems. The result provide basis for further exploration. Introduction There are two problems need to be solved in fractured reservoirs exploration: One is the orientation of fractures, the other is fracture density. The existence of fracic. Seismic plays an important role in detecting fractured reservoirs, for seismic data contains the information of fractures. The basis is the theory of Azimuthal anisotropy. Since 1990s, the use of P-wave Azimuthal anisotropy to detect the fracture was widespread. Mean square amplitude, mean square velocity, azimuthal AVO/FVO gradient and azimuth frequency are sensitive properties that are used to detect fracture. Azimuthal AVO (AVOZ) describes the change in AVO as azimuth varies. AVOZ reflects the change of rock hardness. AVO gradient is smaller along the direction of fractures, and the direction of maximum gradient is normal direction of fractures. Fracture density is proportional to the ratio of normal and cross direction [2]. So fracture density can be calibrated out by using the sensitive properties mentioned before [3]. In all, fracture can be quantitatively detected. The significance of the research The carbonate reservoirs are buried more than 6000 meters in Tarim Basin, and they belong to deeply buried reservoirs. Primary porosity is not developed due to compaction. Effective porosity is secondary dissolved pores and holes. Fractures are main channel to connect reservoirs. The seismic response of reservoirs is Beadlike Reflection(BR). The production of a single BR is limited. The big fracturecave system formed by several BRs, which are connected by fractures, has higher yield. No matter the extension fractures formed by early tectonic or shear fractures created by later strike-slip faults were contributed to reservoirs reformation, oil migration. Fractures play very important role during the whole process of oil accumulation. Therefore, accurate description of fractures is significant for exploration and development. After 2002, NAZ 3D seismic data acquisition is a main type in Tarim Basin. The study indicates this kind of data is hard to use to research complex geologic target. Since 2008, WAZ 3D seismic data acquisition was adopted to improve seismic data quality. In recent years, high density and FAZ seismic data acquisition was tried in TaZhong and TaBei areas. Seismic data quality is greatly improved. The quality of seismic data in TaZhong coved by desert has a significant improvement. Multi-wave and multi-component seismic data acquisition was tried in LunGu area. The hydrocarbon detection of carbonate also moves forward by using multi-wave seismic data. We contrast the applicability and application of Pre-stack fracture prediction based on NAZ, WAZ and FAZ seismic data. Frist, fractures prediction based on NAZ seismic data will be introduced. Fracture detection based on NAZ seismic data tures cause the change in physical properties of the medium as azimuth varies [1]. It is called Azimuth anisotropy in seismthe azimuthal AVO forward model experiment was conducted at TZ86 well. We divide azimuthal angle 0-90 into 9 equal sections and make 9 different AVO forward gathers. The experimental results show that: amplitude increase with the increases of incidence angle; due to strong anisotropy caused by the fracture, there is great difference of amplitude in the different azimuthal AVO forward model; the axial ratio, fracture density, is 1.079. Figure 1 (Yellow Oval) is an elliptic fitted graphic of amplitude of the result of forward in maximum offset distance of each azimuth gathers data at a given time. This yellow ellipse is also equivalent to the graph which is transferred from the values of amplitude,and these values are based on the data of different azimuths gathers (Figs 1) when the angle of incidence is 33. Theoretically, in Figs 1 the direction of fracture is north, and the azimuth is 90 ; while the normal direction of fracture is east, and the azimuth is 0. According to the Figs 1, the horizontal coordinate is long axis of ellipse and the normal direction of fracture, and the vertical coordinate is the short axis of
elliptic and the direction of fracture. The above parameters obtained from forward model can guide the actual data. Fracture density can be used as a threshold to indicate the intense of fracture. The forward model also can determine the fracture orientation in this area. We believe that the fracture of this region develop well based on previous study, and we can predict the fracture based on azimuthal anisotropic. result of fracture prediction coincide with the structural shape; The Fracture density of warm region, near the faults zone and high-steep parts of structure, is relatively large; The fracture density is relatively small in flatten area. According to AVOZ forward modeling, we know that the short axis of fractures is the direction of fractures. TO3s Figure 1: Azimuthal AVO forward model at TZ86 well. Sub-azimuth plan and fracture detection The study area is located in the middle of Tarim basin. The Acquisition has been finished in 2006. The bin size is 25X25m, the fold number is 64 times, the maximum offset is 6575m, and the aspect ratio is 0.29. This is a typical NAZ 3D seismic data. When we use this kind of data to detect fractures, we need to pay attention to three factors: First, the energy of different azimuthal seismic data should be in accordance. Second, the fold number should reach 20 times to ensure the imaging quality. Third, we need at least more than 3 different azimuthal data to fit the ellipse which can be used to describe fracture density and direction. In order to solve the problems mentioned before, we need to use the offset from 0 to 1850m, and divide into 4 azimuthal data(0-45,45-90,90-135,135-180 ). Then, the single bin is 25m and the fold number become 4 times. It is difficult to image. Therefore, we need to use the super bin stack to improve the imaging quality. In order to ensure the accurate imaging of the seismic data, we use the 3x3 super bin stack. The super bin size is 75X75m and the fold number is 20-25 times. Due to the effect of fracture anisotropy, the amplitude, travel time, impedance and frequency properties show differences in different azimuth. We finally selected frequency attenuation as the sensitive parameter to predict fracture. There are mainly two types of faults develop in this area: one is NW trending thrust faults, which is mainly developed in the northeast area. It is called the Tazhong No.1 slope break belt; the other is NE trending strike slip fault which cut the early thrust faults. Figs 2 shows that the Figure 2: The plan of pre-stack fracture prediction (NAZ) We amplify and compare the fractures direction of prediction nearby wells with drilling data. Then we found that: their similarity is lower. This has occurred for a number of reasons: First, due to the use of super bin, the seismic resolution is decrease. Second, in order to ensure the agreement of energy about different azimuth seismic data, we only use the near offset seismic data which have a weak anisotropy, and give up the middle-far offset seismic data which have a strong anisotropy and high SNR. Eventually these lead to a better macroscopic law in the density of fracture prediction and the poor effect in micro fracture direction prediction. We analyze the coincidence rate of 8 wells. The results show that 3 Wells match and the coincidence rate reaches 37.5%. Now we need to think how much the aspect ratio reach, it can satisfy the need of fracture prediction? Fracture detection based on WAZ seismic data Rawapu district is located at the south of the Halahatang uplift in Tabei. The data was acquired in 2010.The bin size is 25x25m, the maximum longitudinal offset is 6775 m, the maximum horizontal offset is 3600 m, the aspect ratio is 0.54, and the fold is 91 times. This kind of data belongs to the wide azimuth seismic data (wide azimuth is the aspect ratio > 0.5). Compared with Tazhong area, Rawapu area has wider azimuth, longer offset,higher folds and smaller stack bin.
Sub-azimuth plan and Pre-stack fracture detection For the stack of sub-azimuth, we consider the directions of fractures development. The fracture directions in this area are mainly in two groups: one is the northeast direction, the other group is the northwest direction. Both of them are strike-slip faults. We use the azimuthal angle from -45 to 45 and divided into four directions. The four directions include two groups of fracture direction. Secondly, the use of the offset range in 500-6500m, the seismic data of near offset is affected by multiple-waves, the far offset is easily affected by the anisotropy. The imaging effect is better. After limited azimuthal stack, the fold of single bin is about 15 to 20 times. It can meet the conditions of the imaging and doesn t need super-bin stack. Due to the fold is different, four azimuthal seismic data regularization is needed to ensure the data energy at the same level. After we analysis and comparative some properties of seismic data, relative impedance is considered as most sensitive parameter of cracks response. The main fractures of this area are two groups of strike-slip faults, which are oriented northeast and northwest. The northwest direction fracture cut the fracture of northeast direction. The fracture develop in the work area which is similar to the channel is a large igneous fault from south to north across the whole work area (figure 3). Predicted fracture density and the fault development get a better match rate, and the predicted fracture direction is accurate beside a single set of fracture. According to comparative analysis of nine wells in the work area, four wells forecast the right direction and the coincidence rate 44%. But at the intersection point of the two set of fractures, it has the poor effect. extending bin. But the limited azimuthal angle can lead to a stable fracture density prediction, and the accuracy of fracture orientation prediction is not high. Fracture detection based on high density and full azimuth seismic data High density and full azimuth 3D seismic acquisition has been used in Ha7 area. The smaller Bin is 15 x 15m, and the aspect ratio is 1. The fold number has significantly increased. It has been achieved good results in improving the carbonate reservoir imaging and later stage fracture prediction and hydrocarbon detection. Post stack fracture detection can qualitatively reflect fracture development. For pre-stack fracture prediction, conventional method is to divide the seismic data azimuth angle 0-180 degree (360 symmetry) into six parts in the work area. We use the entire offset to stack six different azimuthal seismic data, and analysis sensitive attribute of the six post-stack seismic data to predict fractures. The result shows: the energy in the northern part of the area is relatively strong. Compared seismic predicted fracture direction to azimuth logging fracture among the 15 wells which have data, 10 wells coincide well. The coincidence rate is 68%. Compared to wide-azimuth and narrow-azimuth seismic data, full azimuth data are more accurate in pre-stack fracture prediction. Figure 3: The plan of pre-stack fracture prediction (WAZ) The NAZ 3D seismic data has less information to detect fracture. If we just use near-offset, it will reduce the accuracy of fracture orientation and density.so we need to extend the stack bin to improve the SNR. For the WAZ 3D seismic data, more information can be used without Figure 4: The plan of pre-stack fracture prediction (FAZ) Detection of pre-stack fracture based on OVT domain Because of the conventional processing technique instead of the targeted wide azimuth processing technology, the
data potential is not fully realized. The wide-azimuth processing technology mainly refers to OVT (offset vector tile) in foreign, which is usually translated as "offset vector piece ".It is proposed firstly by Vermeer [4] at the study of the minimum data set in acquisition. OVT is the natural extension of the cross-aligned gathers and a subset of it conceptually. The number of cross spread lines equals to the number of the intersections of the shot lines and the detector lines; many small rectangles are obtained by dividing into equal distance according to the source line interval and the receiver line interval in a cross-aligned gather. Each rectangle is an OVT offset vector piece [5]. The corresponding OVT extracted from all the cross-aligned gathers constitutes the OVT gathers, which is often called the spiral gathers, including offset and azimuth information. Those kinds of gathers retain the azimuthal anisotropy information, which are commonly used to the fractures prediction in different orientations and the azimuthal AVO (AVOZ) inversion. It can improve the seismic imaging based on stacking the OVT gathers, which are removed the remaining time difference by the azimuthally anisotropic shifting. We use the data remaining the azimuthally anisotropy to detect fractures. Compared with the measured results, the coincidence rate of the pre-stack fractures detection based on the OVT gathers reaches 80%, 12 wells fit in 15 wells with information. Two wells in the un-fit wells develop many groups of fractures Conclusions 1. The direction of the fractures can be determined by the forward modeling. 2. The density and the direction of fractures can be detected by the NAZ seismic data. When the aspect ratio is 0.3, the direction of fracture detection is poor. When the aspect ratio is greater than 0.5, a single set of fractures density and orientation can be predicted. 3. The density and direction of a single set of fractures are better predicted by high-density and FAZ seismic data. Using the OVT gathers can further improve the accuracy of fracture prediction. 4. For the precise description of the multi-directional fractures, not only do we need wide azimuth data as well as more complex and accurate algorithm! Revelation Currently fracture prediction method for predicting a single set of fractures, the response of the seismic velocity anisotropy is a standard oval to a single set of fractures, but it is a nonstandard round to more than two different directions of fractures, which can t be predicted by conventional techniques, and need wide azimuth seismic data and more sophisticated algorithms to fit the fractures in multiple directions. High-density, FAZ seismic data provided a solid foundation. Multi-directional fractures prediction algorithm is the direction of further research, the quantitative research of multiple sets of fractures will be achieved soon! Figure 5: The plan of pre-stack fracture prediction (OVT) Different with regular data, when we use OVT gathers to predict fractures, we can interactively select the offset and azimuth to carry on the anisotropic analysis. In the properties choice, you can select the attenuation properties of the azimuthal AVO for quantitatively anisotropic analysis. The more information the different azimuthal data provide, which are far more than 6, the more stable the orientation and density of the fractures predicted are.
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