(China Petroleum Exploration and Development Research Institute, Beijing 100083)

Carbonate reservoirs in Tahe Oilfield are buried deep, generally below 5000 meters. The reservoirs are mainly carbonate fractures and dissolved fracture-cave systems, which have strong vertical and lateral heterogeneity and are difficult to predict laterally. This paper introduces the basic theory of wavelet transform, and introduces the absorption attenuation analysis technology in the localization analysis of wavelet transform. Aiming at the Ordovician carbonate reservoir in Tahe Oilfield, a theoretical model close to the actual depth is designed, and the forward simulation results of the model are analyzed to verify its feasibility. Finally, the actual data of Tahe Oilfield are processed, and the oil and gas prediction results are very consistent with the actual drilling situation, which proves the effectiveness of this method.

Absorption attenuation wavelet transform; Prediction of carbonate reservoir; Tahe oilfield

Application of Absorption Attenuation in Reservoir Prediction in Tahe Oilfield

Yang Liqiang, Dong Ning

(Petroleum Exploration and Development Research Institute, Beijing 100083)

Carbonate reservoirs in Tahe Oilfield are deeply buried, and the normal buried depth is over 5300m. Carbonate fractures and dissolved pore systems are dominant, and the horizontal and vertical heterogeneity is serious, which makes it difficult to predict the lateral distribution of reservoirs. This paper expounds the basic theory and principle of wavelet transform, and introduces the absorption and attenuation technology of wavelet transform. According to the results of geological analysis, a theoretical model close to the actual depth is established, which proves the feasibility of absorption and attenuation. Finally, the technology is applied to the actual seismic data in Tahe Oilfield, and the oil and gas prediction results are consistent with the drilling results.

Keywords carbonate reservoir prediction by absorption attenuation wavelet transform in Tahe Oilfield

The attenuation of seismic waves reflects the inherent properties of the propagation medium, so the attenuation information of seismic waves in a specific stratum includes the lithology and fluid-bearing information of the stratum. Many researchers have expounded the examples of earthquake monitoring [1] or lithology prediction [2], saturation analysis [3], fluid property analysis [4] and even permeability analysis [5] by using the attenuation of seismic waves.

There are many factors that cause seismic wave attenuation, which can be roughly divided into two categories. One kind is attenuation related to seismic wave propagation characteristics, such as spherical diffusion, scattering caused by medium inhomogeneity related to seismic wave wavelength [6] and seismic wave attenuation caused by layered structure strata [7] (referred to as impedance filtering or effective quality factor of impedance filtering for short). The other is the inherent attenuation of formation (called the inherent quality factor of formation), which reflects the inherent characteristics of medium. The intrinsic quality factor of formation is of great significance, which reflects the lithology, fluid-bearing type, fluid saturation, pressure and permeability of formation.

At present, Tahe Oilfield is the main block for Sinopec to increase and stabilize production in the west. Ordovician carbonate weathered fractured-vuggy reservoir has become an important field of oil and gas field development in China Petrochemical Group. The Ordovician reservoirs in Tahe Oilfield are mainly pores, fractures and caves, which appear in different combinations. Its distribution is controlled by structural conditions, diagenetic environment, development degree of primary caves, karst landforms and paleohydrological conditions. Its distribution law is very complicated, its heterogeneity is strong, its burial depth is below 5300m, and its seismic exploration resolution is low, which brings great difficulties to reservoir prediction. In this paper, the popular wavelet transform method is introduced to calculate the attenuation gradient coefficient of seismic wave amplitude, which avoids the influence of Fourier transform time window, and the oil and gas potential of Ordovician carbonate reservoir in Tahe Oilfield is predicted by using seismic wave attenuation technology.

1 principle

1. 1 wavelet transform theory

Wavelet transform is a new mathematical method developed rapidly in recent years. It is considered to be a major breakthrough in tools and methods in recent years, a perfect crystallization of functional analysis and harmonic analysis, and has important theoretical and practical application value. Wavelet transform inherits and develops the localization idea of window Fourier transform, which can localize the signal in time domain and frequency domain, and can automatically adjust the shape of the window with the change of frequency components to meet the required requirements. Therefore, it has a broad application prospect in seismic data processing [8 ~ 1 1]. The basic principle of wavelet [12 ~ 14] is as follows:

Let a=, j∈Z, a can be discretized. If j=0, then ψj, b(t)=ψ(t-b). The simplest way to discretize B is to sample B uniformly. For example, if b=kb0, b0 should be selected to ensure that x(t) can be recovered from WTx(j, k). When j≠0, A is amplified by a0 times. At this time, the center frequency of wavelet ψj, k(t) is a0 times lower than ψj- 1, k(t), and the bandwidth is a0 times lower. Therefore, at this time, the interval of sampling b can be correspondingly enlarged by a0 times. It can be seen that when the scale A takes a0,,, … respectively, the sampling interval of B can take a0b0,,, … Therefore, the results of discretization of A and B are as follows:

Oil and gas accumulation theory and exploration and development technology

The typical values of a0 and b0 are: a0=2, b0= 1, and the wavelet telescopic translation system is obtained:

Oil and gas accumulation theory and exploration and development technology

Because wavelet transform has a series of outstanding advantages, such as constant Q value and automatic adjustment of time width/bandwidth of signal analysis, it is called "mathematical microscope" of signal analysis.

1.2 absorption and attenuation technology

When the local seismic wave propagates in the underground rock stratum, the elastic energy of the seismic wave is irreversibly converted into heat energy, resulting in amplitude attenuation and high frequency loss. Due to the different physical properties of rocks and fluids, the amplitude attenuation of elastic waves is also different. When the rock contains oil, especially natural gas, the amplitude attenuation of elastic waves increases significantly, so the amplitude attenuation of elastic waves in the rock can sensitively reflect whether there are oil and gas reservoirs underground. Therefore, we can determine the lateral changes of lithology and oil-bearing property by studying the absorption of seismic waves by formation media, delineate the scope of oil-gas reservoirs, and predict reservoirs by combining other parameters.

Figure 1 is a schematic diagram of the principle of calculating the amplitude attenuation gradient factor by wavelet time-frequency analysis. After wavelet transform, the amplitude energy attenuation of each sampling point is analyzed in frequency domain. Firstly, the detected maximum energy frequency is taken as the initial attenuation frequency; Then the frequencies corresponding to 65% and 85% seismic wave energy are calculated respectively; Finally, in this frequency range, according to the energy value corresponding to the frequency, the attenuation gradient of energy and frequency is fitted to get the amplitude attenuation gradient factor. When processing actual data, the correct frequency range for calculating amplitude attenuation gradient can be adjusted according to the quality of seismic data and research purpose.

Figure 1 principle of calculating amplitude attenuation gradient factor by wavelet time-frequency analysis.

Analysis of absorption and attenuation characteristics of forward model

The Ordovician reservoirs in Tahe Oilfield take holes, caves and fractures as their storage spaces and appear in different combinations. Fissures and caves are developed in carbonate rocks. After being filled with materials with oil, gas, water or lithological differences, seismic waves form a phenomenon of "rapid energy attenuation" during propagation, and high-frequency component energy rapidly attenuates or even disappears when passing through the fracture-cave system, that is, the phenomenon of "high-frequency absorption". As can be seen from the above, the fractured-vuggy system has great energy attenuation and high-frequency absorption for seismic waves.

According to the reservoir type, development scale, buried depth, velocity, density and porosity tested by previous petrophysical analysis, combined with the actual drilling data, a geological model similar to the actual geological structure of Tahe Oilfield is designed (Figure 2), and the model parameters are shown in Table 1. In this study, wave equation forward modeling (finite difference method) is adopted, which not only clarifies the seismic identification mode of carbonate reservoirs, but also emphatically verifies the applicability of absorption attenuation technology in this area.

Fig. 2 Different karst cave development models

① ~ ⑥ are hole numbers.

Table 1 cave model parameters

Fig. 3 shows the forward simulation results (a) and absorption reduction calculation results (b) of different cave models. From the forward simulation results, it can be seen that holes ①, ③ and ⑥ are inside holes, and the seismic response is that the bottom boundary of the hole is a wave peak, and the whole hole is a beaded reflection; Cave ② is located near the weathering crust, and the fracture zone near Cave ⑥ is weakly reflected at the top of the cave and the fracture zone, and randomly reflected below; Cave ④ is three vertically superimposed caves with typical beaded reflection characteristics. It can be seen from the absorption-reduction diagram of the forward modeling results that the internal caves can be clearly identified on the absorption-reduction profile, which belongs to the characteristics of strong absorption-reduction. The part with strong absorption and attenuation in the picture is basically in the real position of karst cave development, which can better reflect the development of karst cave than the earthquake. In addition, compared with earthquakes, absorption attenuation makes energy more concentrated and boundaries clearer.

Forward simulation results of different karst cave development models and their absorption rate reduction characteristics.

(a) forecast results; (b) The absorption rate minus the calculation result ① ~ ⑥ is the number of caves.

3 example analysis

Using the above technology, the area 4 of Tahe Oilfield is calculated and analyzed, as shown in Figure 4, which is a three-dimensional view of the profile of the well with decreasing absorption rate of 0 ~ 0~ 120ms in the virtual environment. According to the analysis, the oil and gas production of wells with strong absorption reduction is higher, such as TK427, T429, TK430H, T40 1, S48 and TK407, in which TK429 produced 360m3/d after acid fracturing and drainage at 5442 ~ 546 1.5m; The oil-producing zone of Well TK427 is a thick oil-bearing zone in the middle of Ordovician, with an average oil production of more than 500t/d. At present, the cumulative oil production is 23× 104t, which has produced good economic benefits. Well TK430H is the first horizontal well in Ordovician in Tahe Oilfield to obtain high oil and gas production, with a daily oil production of 420 t .. while the oil and gas production of wells with relatively weak absorption reduction is low, or even no oil and gas production, such as T4 14 and other wells.

Fig. 5 is a perspective view of absorption and reduction of the lower reservoir in Area 4 of Tahe Oilfield. As can be seen from the figure, wells S48, TK4 12, TK408 and other high-yield wells are in strong absorption zones with high coincidence rate, indicating the effectiveness of absorption attenuation in this area. The analysis shows that the strong absorption attenuation of well TK427-—S48-—TK407 in Area 4 is obvious, while the gully area on the axis of well S64-T414 in the east of Area 4 shows weak absorption area, which basically corresponds to the distribution zone of current productivity wells. It can be seen that the high attenuation distribution area corresponding to the local high point is a favorable part for the development of cracks and caves. The decrease of absorption rate can provide a relatively developed position for delineating favorable reservoir zones in local areas.

Oil and gas accumulation theory and exploration and development technology

Oil and gas accumulation theory and exploration and development technology

4 conclusion

In this paper, wavelet transform is introduced into the calculation of absorption attenuation. By using the excellent time-frequency localization characteristics of wavelet transform, the influence of the time window size of conventional Fourier transform on the calculation results is overcome and the local characteristics of seismic signals are enhanced. This technology has been applied to the prediction of Ordovician carbonate reservoirs in Tahe Oilfield, and good results have been achieved. Compared with actual drilling, the coincidence rate is very high, which provides a more effective solution to the worldwide problem of carbonate reservoir prediction in Tahe Oilfield.

refer to

[1] Huang Zhongyu, Wang Yujing, Su Yongchang. A new seismic wave attenuation analysis method-an effective tool for predicting oil and gas anomalies [J]. Petroleum Geophysical Exploration, 2000,35 (6): 768 ~ 773.

[2]Batzle M, Han D H, Castagna J. Attenuation and velocity dispersion in seismic frequency [J]. Extended summary of Mtg in Section 66, 1996.

Jones M. Influence of saturation on ultrasonic velocity and attenuation in sandstone [J]. Extended summary of Mtg in section 67, 1997.

Klinmentost, McCann C. Relationship among P-wave attenuation, porosity, clay content and permeability in sandstone [J]. Geophysics,1990,55 (8): 998 ~1914.

[5]Klimentos T, McCann C wave and S wave attenuation as a method to distinguish gas and condensate from oil and water [J]. Geophysics,1995,60 (2): 447 ~ 458.

Wang Baoshan, Sun Daoyuan, Li Shengjie, et al. Influence of rock heterogeneity on ultrasonic attenuation and its correction [J]. China Earthquake, 200 1, 17 (1): 1 ~ 7.

Fortman. W.I. Dispersive body waves [J]. Journal of Geophysical Research, 1962, 67:5279~529 1.

Guo Gangming, Shi, Gao, et al. Effect analysis of wavelet transform in seismic data processing [J]. Petroleum Geophysical Exploration, 2003,42 (2): 237 ~ 239.

Zhu Guangming, Gao Jinghuan, Wang Yugui. Wavelet transform for one-dimensional filtering [J]. Petroleum geophysical exploration,1993,32 (1):1~10.

Wang Yong, Li Jun, Zhang Kuifeng. Seismic signal denoising based on wavelet transform [J]. Petroleum Geophysical Exploration,1998,37 (3): 72 ~ 76.

Zhang Ke, Liu Guizhong. De-noising of seismic signals by time-frequency separation with dyadic wavelet transform [J]. Journal of Geophysics,1996,39 (2): 265 ~ 270.

[12] Cui Jintai. Introduction to wavelet analysis [M]. Xi An: Xi Jiaotong University Press, 1995.

[13] Peng Yuhua. Wavelet transform and engineering application [M]. Beijing: Science Press, 1999.

[14] Qin, Practical wavelet analysis [M]. Xi 'an: xidian university Publishing House, 1994.