多波联合反演相对优质储层预测-e.doc

Forecast of Relative Quality Reservoir
with Multiwave Joint Inversion
——Take Chuanxi Deep Compact Fragmentary Rocks for Instance
Ye Tairan1 ion person2，Xu Xiangrong1， ion person2，Tang Jianming1
（1.Deyang Branch Institute Exploration and Production Research Institute Southwest Petroleum Branch SINOPEC Deyang Sichuan 618000
2. GX Technology ion Geophysical Houston Texas 2222222)
Abstract: The sandstone gas reservoir in the delta of Section 4 of Xujiahe Formation at the deep level of Chuanxi Depression are developed with a large sand body depth, where the relative quality gas-bearing reservoir has good geophysical properties, but has a small thickness, and belongs to a low-permeability compact reservoir. The wave impedance of the reservoir against the compact sandstones reduces to a certain extent, but is relatively close to the wave impedances of compact sandstones and mudstones as the adjacent rocks. The geophysical response characters are not very obvious in the conventional P wave data, and the conventional seismic reservoir forecast method based on the P wave data suffers great difficulty in forecasting the distribution of such reservoirs and the bad forecast precision. The joint inversion of the primary wave and the shear wave has been implemented in Xinchang Region of Chuanxi Depression, capitalizing on the advantages of the 3D3C prospecting. Based on the sensitivity analysis of the physical parameters of the reservoir rocks, the forecast concept of “looking for sand in sand” is introduced, such multiwave properties as Poisson’s ratio, impedance of P wave and VPVS are selected, and the multi-body converging means is adopted to eliminate mudstones and compact sandstones and forecast the distribution of relative quality reservoirs.
Key words: Chuanxi deep horizon, compact reservoir, multiwave joint inversion, quality reservoir forecast
The upper Triassic Xujiahe Formation gas reservoir is currently the hot destination of the natural gas prospecting in Chuanxin Depression, and also the main section layer for reserve growth. Xujiahe Formation, with a burying depth of 3000~6000m, is home to Xujiahe #2 Gas Reservoir and Xujiahe #4 Gas Reservoir, the backbone gas reservoirs. These reservoirs are developed in the deltaic sandstones with the porosity below 5% in general and the permeability rate below 0.1MD in most cases. The quality reservoirs with a porosity of 5-10% are developed in the compact background, leading to very strong heterogeneity, and belong to the typical compact fragmental sandstone type. From the middle of the last century till now, Zhongba and Bajiaochang Gas Fields have been discovered in the north section of Chuanxi Depression where the burying depth is relatively shallow and the geophysical properties are relative good, Pingluoba and Qiongxi Gas Fields have been discovered in the south section, and Hexingchang, Xinchang and other upper Triassic Xujiahe Formation gas fields have been discovered in the middle section of Chuanxi Depression where the burying depth is relatively large. In 2000, X851 Well, boasting an open flow of 1.514 million cubic meters per day and a stable yield of 380,000 cubic meters per day, was successfully drilled in the upper Triassic Xujiahe Formation in Xinchang Gas Field, presenting the vast prospecting and development outlook for the deep-level compact sandstone gas reservoirs in the middle section of Chuanxi Depression. However, prospecting of the gas reservoirs involves great difficulty. CX560, CX565, CL562 and other deep wells have been deployed in the same tectonic zone, but failed to achieved a very ideal effect[1].
The multiwave seismic prospecting, as a very potential resort for prospecting hidden oil and gas reservoirs, has been closely followed by many oil and geographical companies at home and abroad. When spreading underground, the elastic primary wave will not only generate the reflected P-wave but also the converted shear wave when crossing the stratum interfaces with differential geographical properties. The primary wave serves as an integrated reflection of the underground stratum skeleton, porosity, pore fluids, earth temperature and pressure character, while the shear wave is only associated with the skeleton, porosity, temperature and pressure of the stratums, and unrelated to the nature of the pore fluids. Therefore, it is more effective to integrate the information on the converted shear wave and the primary wave than merely use the primary wave information when forecasting the reservoirs, gas-bearing reservoirs in particular. The scope of its application mainly includes the gas cloud shielding, the rock character shielding imaging, the fissure forecast, the sand shale reservoir forecast, and the fluid identification[2].
Considering the superiority of the multiwave prospecting in the compact reservoir forecast and the fissure forecast, the 3D3C multiwave seismic prospecting was implemented in the main position of Xinchang Gas Field in 2005 with a view to speeding up the prospecting and development of the continental deep Xujiahe Formation gas reservoirs and promoting the prospecting and development of the Xujiahe Formation gas reservoirs in Xinchang Region by leveraging the advantages of the multiwave seismic data in solving complex geological problems about the gas reservoirs.
I. Geological Properties of Reservoirs
Xujiahe 3 Section and Xujiahe 5 Section of the upper Triassic Xujiahe Formation in Chuanxi Depression constitute the hydrocarbon source development sections, while Xujiahe 2 Section and Xujiahe 4 Section are the main reservoir development sections. The burying depth of Xujiahe 4 Section is usually greater than 3000m, the rocks are chiefly debris quartz sandstones, calcium-rich debris sandstones, shale and conglomerates, and the delta front underwater distributary channels and estuary dams constitute the majority, accompanied by some delta plain distributary channels.
This paper will take Xujiahe 4 Section Reservoir for instance. Judging from the porosity and permeability data obtained from a total of 999 samples collected from 12 wells in Xinchang Region, the value of the porosity of Xujiahe 4 Section ranges from 0.47% to 12.71%, while the permeability ratio varies from 0.001×10-3µｍ to 0.86×10-3µｍ2. The upper subsystem sandstones in Xujiahe 4 Section have the best physical properties with the porosity being 12.71% at maximum and 6.21% on average, followed by the lower subsystem with an average porosity of 3.27%, and the middle subsystem, the most compact with an average porosity of less than 2%. The permeability of Xujiahe 4 Reservoir presents a rising trend against the increase in the porosity, but the porosity-permeability relationship is ordinary. If the fact is not considered that the fissures lead to a large permeability value but an excessively low porosity, the porosity-permeability relationship is Xujiahe 4 Section is relatively good. In conclusion, Xinchang Xujiahe 4 Reservoir belongs to a typical low-porosity low-permeability reservoir due to bad physical properties.
According to the survey on the domestic and overseas prospecting experience with respect to various compact sandstone gas reservoirs, the research on the rule of distribution of quality reservoirs has always taken an important position. The term “quality reservoir” is a relative concept, and in the compact sandstone field, it means the effective reservoir with relatively good physical properties that is developed in the low-porosity low-permeability reservoirs. The porosity and permeability of a quality reservoir are not subject to a fixed scope, but determined according to the oil & gas-bearing property and the oil & gas yield condition in the research area. The relative quality reservoir in the compact fragmental rocks is called the “sweetheart” at abroad. In Sulige Temple Gas Field in Erdos City, a quality reservoir in general means a reservoir with a porosity of more than 6% and a permeability value of more than 1.0×10-3μm2; in the assessment of Xujiahe Formation Reservoir in Chuanzhong Region of Sichuan Basin, a quality reservoir is defined as a reservoir with with a porosity of more than 8% and a permeability value of more than 0.5×10-3μm2; while in Tongchang 6 Reservoir in north Shaaxi Province, a quality reservoir is defined as a reservoir with a porosity of more than 10% and a permeability value of more than 1.0×10-3μm2.Compared to its counterparts at home and abroad, the deep quality reservoir in Chuanxi Depression has the worst porosity and permeability conditions, and the physical property standard for the quality reservoir varies from one horizon to another. In general, a quality reservoir means one with a porosity of more than 4% and a permeability value of more than 0.06×10-3μm2 in Xujiahe 2 Section, and one with a porosity of more than 6% and a permeability value of more than 0.06×10-3μm2 in Xujiahe 4 Section. Only when these reservoirs are drilled, can industrial capacities be obtained.
II. Geographical Properties of Reservoir
The spread velocity of the seismic waves in rocks and the rock density determine the size of the wave impedance, while the wave impedance properties of the reservoir are determined by the difference between the wave impedance of the reservoir and that of the adjacent rocks. Figure 1 analyzes the properties of the wave impedance of the sandstone reservoir in Xujiahe 4 Formation using Xinchang 851 as an example. The method is to perform the GR-AC, GR-AC and GR wave impedance converging analysis on all the sandstone sections with a thickness of more than 10m and the typical sandstone sections in Xujiahe 4 Section of the well. According to the result, the AC values of low-velocity sandstones, medium-velocity sandstones and low-velocity sandstones in Xujiahe 4 Section have clear boundaries, which respectively correspond to the 47.5~57.5, 57.5~75 and 75~85us/ft ranges. The AC value of the medium-velocity sandstones has an equivalent AC value to the adjacent rocks, the DEN values don’t have clear boundaries but with a large range, and the wave impedance properties are also obviously divided into three types, which similar to the AC value. In reality, the aforesaid statistical regularity not only exists to Xinchang 851 Well in the Xinchang Structure but also exists in Xujiahe 4 Section of a few areas such as Luojiang and Hexingchang.
Mudstone
Sandstone
Type III
Type II

Type

Figure 1 GR, AC, DEN and Wave Impedance Convergence Diagram on Xujiahe 4 Section Reservoir in Xinchang 851 Well and Adjacent Rocks
The wave impedance is a major factor that determines the seismic response properties of the reservoir, and the size of the difference between the reservoir and the wave impedance of the adjacent rocks determine the seismic reflection strength. The wave impedances in the research area can be divided into three types according to the available well logging data, the size of the difference between the reservoir and the wave impedance of the adjacent rocks and the aforesaid statistical regularity and based on the AVO classification concept:
Type I: high-impedance reservoir, with the wave impedance obviously higher than that of adjacent rocks; Type II, medium-impedance reservoir, with the wave impedance close to that of adjacent rocks; and Type III, low-impedance reservoir, with the wave impedance obviously lower than that of adjacent rocks.
Judging from the oil and gas show of the already known wells in Xujiahe 4 Section, in Xinchang Region, the relative quality reservoirs with the best physical properties and the best oil and gas show often present the feature of the medium-to-low wave impedance (Types II or III), while the partially effective reservoirs with poor physical properties and relatively poor show of oil and gas and the super-compact sandstones bearing no gas often have the feature of high wave impedance (Type 1). Therefore, the wave impedance can be used for the qualitative identification of relative quality reservoirs. However, the low wave impedance property does not always indicate quality reserves, since mudstones also have a medium-to-low wave impedance. The prior research has proved that it is very difficult to exactly forecast the horizontal and the transverse distribution of the reservoirs with such wave impedance property using such traditional methods as impedance inversion, GR simulated sound wave inversion, and seismic multiple property analysis.
III. Quality Reservoir Forecast with PP/PS Joint Inversion
1. Inversion Flow
The most effective and economical natural gas reservoir prospecting technology is the hydrocarbon direct detection technology. So far, the foreign countries have developed and put in use such geographical prospecting technologies as “highlight”, "AVO” and “multiwave multi-component prospecting” technology[2]. The AVO prestack simultaneous inversion uses the AVO properties of the primary wave to generate the shear wave information through the Aki-Richard approximation, and thus approximate and tentative. The reservoir gas-bearing property forecast represents the advantage of the multiwave multi-component prospecting, and directly acquires the shear wave information. The result of the poststack joint inversion using PP wave and PS wave is more authentic and reliable.
By the poststack joint inversion of PP wave and PS wave, it means performing the poststack inversion on the primary wave and the shear wave with the conventional poststack inversion methods respectively to acquire the wave impedance of PP wave and that of PS wave, and then using the wave impedance of PP wave and that of PS wave to calculate the Poisson's ratio, the shear-primary velocity ratio and other reservoir parameters. The basic principle and the flow of the poststack joint inversion of PP wave and PS wave are the same as those of the conventional poststack methods (Figure 2), and its realization process is basically the same as the normal impedance inversion process: respectively calibrate the sections of PP wave and PS wave, extract separate wavelets, separately conduct the inversion of the PP wave impedance and the PS wave impedance to generate the gamma volume (the seismic section compression factor), and use the gamma volume to compress the PS impedance to the temporal section corresponding to the PP impedance to PS impedance body (Figure 3) that can match the interpretation.
After the primary wave impedance body and the shear wave impedance body are acquired through the inversion, and the QC tool is used to compare the inversion result against the well logging result. The data distribution range and the deviation of the inversion result and the well logging result are basically the same, reflecting the inversion result is relatively reliable.
In addition to generating the primary wave impedance and the shear wave impedance, the inversion can simultaneously calculate and obt