(1. School of Earth Science and Technology, Youshi University, China, Qingdao, Shandong 266555; 2. Geological Logging Company of Shengli Petroleum Administration Bureau of China Petrochemical Company, Dongying, Shandong 257064; 3. Logging Institute, China Petrochemical Petroleum Engineering Technology Research Institute, Beijing 100 10 1)
Fund Project: National Natural Science Foundation "Structural Strain and Structural Heterogeneity of Sandstone Diagenesis" (No.4 1002034).
About the author: Xu, female, senior engineer, now engaged in the research of logging technology, email: slljxxq @163.com.
Abstract: It is one of the main tasks of natural gas exploration to identify gas reservoirs in time. Tight sandstone gas in Jiyang depression is mainly distributed in Dongying depression and Gubei-Bonan area, and there are two types of oil-type gas and coal-type gas. Because the reservoir has the characteristics of poor physical properties, strong heterogeneity and complex genesis, drilling identification is much more difficult than conventional sandstone gas reservoirs. Based on the geological characteristics of main tight sandstone gas producing areas, the response characteristics and identification methods of gas chromatography logging and tank top gas light hydrocarbon chromatography logging data in different types of gas reservoirs are analyzed and studied. The results show that gas chromatography logging and tank top gas light hydrocarbon chromatography logging have a good response to tight sandstone gas, and they complement each other, which are the two most effective geochemical logging methods for directly identifying gas reservoirs while drilling. When an obvious gas layer is drilled, the total hydrocarbon content in gas chromatography is obviously higher than the background value, the total hydrocarbon contrast coefficient is generally greater than 3, the light hydrocarbon components in the top gas of the tank are rich, and the abundance of C 1-C4 light hydrocarbon compounds is generally greater than 1000%. With the increase of evolution degree, the methane content in gas components gradually increases and the heavy hydrocarbon content gradually decreases. On the Pixler hydrocarbon composition diagram, there are dry gas zone, wet gas zone, coal-formed gas zone and condensed gas zone from top to bottom. Coal-type gas is mostly located in the middle and upper part of the triangle diagram of C5 -C7 aliphatic hydrocarbon group, and oil-type gas is located in its lower part, which can be used to distinguish oil-type gas from coal-type gas. Using the above method to identify tight sandstone gas reservoirs in Jiyang Depression, the coincidence rate reaches 9 1.6%, which improves the accuracy of identification while drilling.
Key words: tight sandstone gas; Identification while drilling; Mud gas; Tank top gas; accuracy rate
Identification method of tight sandstone gas while drilling —— Taking Jiyang Depression as an example
Xu Xiaoqiong 1, 2 Wang Zhizhan 3 Ci Xinghua 2 Liu Caixia 2 Niu Qiang 2
(1. School of Earth Sciences, China Shiyou University, Qingdao, Shandong 266555; 2. Geological Logging Company of China Petrochemical Li Sheng Petroleum Administration Bureau, Dongying, Shandong 257064; 3. Logging Technology Department of China Petrochemical Petroleum Engineering Research Institute, Beijing 100 10 1, China)
Abstract: gas identification while drilling is one of the main tasks of natural gas exploration. Tight sandstone gas in Jiyang Depression is distributed in Dongying Depression and Gubei-Bonan area, including oil-type gas and coal-type gas. Tight sandstone gas reservoirs are more difficult to identify than ordinary sandstone gas reservoirs because of their poor physical properties, strong anisotropy and complex genesis. According to the geological characteristics of tight sandstone gas in Jiyang depression, the logging response characteristics of mud gas and top gas of several different types of tight sandstone gas are deeply analyzed, and the evaluation standards and identification methods while drilling are summarized. It is considered that mud gas logging logging and top air logging have obvious responsiveness and complementarity, and they are two effective geochemical logging methods while drilling. When drilling a typical gas layer, the total gas content is much higher than the background value, and the contrast coefficient is greater than 3. Generally, the composition of light hydrocarbons is very rich, and the content of C 1-C4 light hydrocarbons is more than 1000%. With the increase of evolution degree, the composition of methane gas gradually increases and the heavy hydrocarbons gradually decrease. In the Pixler diagram, the hydrocarbon components of natural gas are dry gas, wet gas, coal-formed gas and condensate oil from top to bottom. On the triangle diagram of C5-C7 aliphatic hydrocarbon group composition of overhead air, coal-type gas is in the upper part, oil-type gas is in the bottom part, and the lateral distribution is wide, which can be used to identify oil-type gas or coal-type gas. Using the above method to identify tight sandstone gas reservoirs, the accuracy rate reaches 965438 0.6%, thus improving the accuracy rate of tight sandstone gas identification while drilling in Jiyang Depression.
Key words: tight sandstone gas; Identification while drilling; Mud gas; Head air; accuracy rate
Tight sandstone gas refers to sandstone layers with low porosity (< 12%), low permeability (< 1× 10-3 μ m2), low gas saturation (< 60%), high water saturation (> 40%) and slow natural gas flow [65438]. There are rich types of tight sandstone gas in Jiyang Depression, which can be divided into oil-type gas and coal-derived gas according to different parent material types, and can be divided into thermal degradation gas and high-temperature cracking gas according to the evolution degree of organic matter. According to the test results, there are mainly three phase types: dry gas, wet gas and condensed gas [5 ~ 7]. Due to the deep burial depth (generally below 4000m), the reservoir has strong diagenetic evolution, poor physical properties, complicated reservoir-forming mechanism [8 ~ 1 1], and the fluorescence display of cuttings is weak or not, which is far less easy to detect than that of oil-bearing cuttings. According to the analysis of geological characteristics and response characteristics of tight sandstone gas producing areas in Jiyang Depression, it is difficult to identify while drilling [1]
1 Geological characteristics of the study area
Figure 1 Structural Outline of Jiyang Depression
The tight sandstone gas in Jiyang Depression is mainly distributed in the northern belt of Dongying Depression, the fourth member of Shahejie Formation in Bonan Depression and the Carboniferous-Permian system in Gubei area [7,8, 15 ~ 17] (Figure 1). Dongying sag and Bonan sag are salt lake-freshwater lake facies deposits, and the gas source rocks are mainly dark mudstone and gypsum-bearing mudstone, with good organic matter types and high abundance. The source rocks in the middle part of the depression are in the mature-high mature evolution stage. The reservoirs are mainly nearshore subaqueous fan, fan delta front sand body and shore shallow lake beach bar sand body. The reservoir space is mainly intergranular pores, and the formation pressure is low-positive pressure. Carboniferous-Permian in Gubei area is a set of coal measures strata, with black coal, carbonaceous mudstone and dark gray and gray-black mudstone, rich in organic matter, mainly type III parent material, and with high evolution degree, which is the main gas source rock in this area. The reservoir is delta and fluvial sedimentary sandstone, the pore type is mainly secondary dissolved pore, and the formation pressure is low-weakly high (Table 1, Figure 2).
Table 1 Geological characteristics of main producing areas of tight sandstone gas in Jiyang Depression
Fig. 2 Comprehensive histogram of tight sandstone gas producing areas
From the reservoir physical properties, all unconventional reservoir cementation is dense-very dense, and the reservoir physical properties are evaluated as good-medium reservoir [18]. The northern part of Dongying sag is dominated by oil-type gas, and the geochemical characteristics of deep natural gas in Gubei-Bonan area change regularly, and gradually change from oil-type gas to coal-derived gas from west to east [16, 17].
2 Response characteristics of logging data of tight sandstone gas reservoir
The main task of natural gas logging is to identify gas reservoirs in time during drilling. When drilling the formation, the oil and gas in the formation enters the wellbore through two ways: one is brought in by the cuttings mechanically broken by the drill bit; Second, the oil and gas in the drilled formation directly enters in the form of seepage or diffusion under the action of pressure difference. Gas chromatography logging mainly detects gas (mud gas) existing in free state (bubbles) and dissolved state (dissolved in water or oil) in drilling fluid, which is the most effective geochemical logging method for directly identifying oil and gas reservoirs while drilling. Chromatographic logging of light hydrocarbon in tank top gas is used to detect light hydrocarbon naturally desorbed from cuttings or cores (compounds with molecular carbon number of C1-C7 [19,20]). The formation and evolution of light hydrocarbon are closely related to natural gas, and it is an important index for gas source identification and gas source correlation [16,65438+]. They complement each other and can comprehensively detect underground gas reservoirs while drilling.
2. 1 gas chromatographic data characteristics
The tight sandstone gas in the study area has a strong response to gas chromatography data. The total hydrocarbon content (TGAS/%) is obviously higher than the background value when drilling an obvious gas layer, so the gas layer display while drilling can be detected. Whether it is oil-type gas or coal-type gas, methane is absolutely dominant in gas composition, with a content of 64.0% ~ 96.0%. Among the heavy hydrocarbon gases, ethane and propane are the most common, and the content of hydrocarbons with carbon number greater than 4 is low (Table 2). Methane in oil-type gas has a large distribution range, generally from condensate gas-wet gas-dry gas. With the increase of evolution stage, the relative content of methane (C 1/%) and the drying coefficient η increased gradually. For the same type of gas, due to the difference of geological conditions, the gas composition characteristics are also different. The gas composition of coal-based gas is relatively complete, and it has similar display characteristics with water and dry gas in oil-based gas, so it is difficult to identify gas layer types only by gas composition content.
Table 2 Gas and hydrocarbon composition characteristics of typical tight sandstone in Jiyang Depression
2.2 Characteristics of chromatographic data of light hydrocarbons in tank top gas
As can be seen from Table 3, tight sandstone gas is rich in light hydrocarbon components. Except that dry gas methane is absolutely dominant and C6-C7 light hydrocarbon compounds are poor, other types of gas light hydrocarbons are widely distributed. The abundance of C 1-C4 light hydrocarbon compounds is generally greater than 1000, and the number of components is between 6 and 27, but in isopentane/n-pentane (iC5/nC5).
Table 3 Light hydrocarbon composition of typical tight sandstone gas in Jiyang Depression
3 tight sandstone gas reservoir logging identification method
3. Qualitative identification of1gas reservoir
The formation pressure in the study area is relatively consistent. Under similar drilling conditions, the higher the oil-bearing gas content, the better the formation physical properties, the more oil and gas drilled into the drilling fluid through the unit volume of oil and gas reservoir, and the total hydrocarbon content in gas chromatography is obviously higher than the background value. Total hydrocarbon contrast coefficient (abnormal value/background value) is often used to measure the amplitude of abnormal display. Accordingly, the abundance of light hydrocarbons in the top gas is higher. In the same horizon, the apparent amplitude of gas layer is higher than that of gas-bearing water layer and dry layer. According to the gas composition, the abundance and composition characteristics of light hydrocarbons, gas layers can be identified qualitatively and quickly (Table 4).
Table 4 Evaluation criteria for logging parameters of tight sandstone gas reservoirs in Jiyang Depression
3.2 Identification of gas reservoir types
Fig. 3 Pixler hydrocarbon composition ratio diagram of natural gas in Jiyang Depression
Because the characteristics of gas composition and light hydrocarbon composition vary with the type of organic parent material and the evolution degree of hydrocarbon generation, it can be used for the classification of genetic types of natural gas, gas source correlation and maturity evaluation [15 ~ 17]. With the increase of evolution degree, the methane content in gas components gradually increased, while the heavy hydrocarbon content gradually decreased [16,19,20], and the component ratios were C 1/C2, C 1/C3, C 1/C4 and c/respectively. Coal-formed gas falls at the intersection of wet gas zone and condensed gas zone, but the trend of broken line is obviously different from that of oil-type gas. The ratios of C 1/C3, C 1/C4 and C 1/C5 decrease gradually, which can be well distinguished.
Light hydrocarbons with different structures (n-alkanes, isoparaffins and cycloalkanes) have different contents in different types of parent materials. Light hydrocarbons in sapropelic parent materials are rich in n-alkanes and cycloalkanes, while light hydrocarbons in humic parent materials are rich in isoparaffins [15 ~ 17]. The composition of C5, C6 and C7 aliphatic hydrocarbon groups in tight sandstone gas in Jiyang Depression obviously shows the above characteristics (Figure 4). Coal-type gas is mostly located in the middle and upper part of the triangle, and oil-type gas is located in the lower part, and its lateral distribution is wide, which can be used to distinguish oil-type gas from coal-type gas.
The above method was applied to identify 54 gas-bearing layers in Jiyang Depression 16 exploratory wells, and the coincidence rate reached 9 1.6%, which proved the feasibility of this method.
Fig. 4 Triangle diagram of C5-C7 aliphatic hydrocarbon group composition of natural gas in Jiyang Depression
4 conclusion
Gas chromatography logging and tank top gas light hydrocarbon chromatography logging technology are effective analysis methods for rapid detection of tight sandstone gas reservoirs by logging while drilling. According to the gas composition of natural gas and the abundance and distribution characteristics of light hydrocarbons, gas reservoirs can be qualitatively identified and classified. However, any analysis method will inevitably be affected by complex drilling conditions and geological conditions, so there are always some shortcomings in the identification method. In the practical application process, under the premise of fully understanding the geological characteristics, we should comprehensively use various methods to learn from each other and verify each other to improve the identification accuracy.
refer to
Jiang, Lin, Pang, et al. Comparative study of two types of tight sandstone gas reservoirs [J]. Petroleum Experimental Geology, 2008,32 (5): 35 ~ 40.
Gong Xiumei, Zeng Sishui, Qiu Nansheng, et al. Characteristics of deep tight sandstone gas accumulation in Weibei Depression [J]. Natural gas geochemistry, 2008,25 (6): 7 ~10.
Zhang Xiong, Pan Heping, Luo Miao, et al. Summary of logging interpretation methods for tight sandstone gas reservoirs [J]. Journal of Engineering Geophysics, 2005,2 (5): 431~ 435.
Spencer ·C·W, Master ·R·F, et al. Geology of tight gas reservoirs. AAPG Geology Research 24, 1986:(299).
Kang Zhu Lin. Deep gas exploration prospect in China [J]. Natural gas industry, 2000,20 (5), 1 ~ 4.
Dou lirong. Introduction to reservoir geology [M] Petroleum Industry Press, 200 1, 57 ~ 59.
Hu Zongquan, Zhou Xinke, Zhang Yulan. Paleogene oil and gas exploration prospect in Jiyang Depression [J]. Petroleum and Natural Gas Geology, 2005,26 (5): 655 ~ 660.
Gong Xiumei, Jin Zhijun, Zeng Sicui, et al. Characteristics and controlling factors of deep hydrocarbon accumulation in Bonan Depression [J]. Petroleum and Natural Gas Geology, 2005,26 (4): 473 ~ 479.
Ginger. Characteristics and conditions of natural gas accumulation in Bohai Bay Basin [J]. Journal of Petroleum University: Natural Science Edition,1999,23 (5): 9 ~13.
[10] Peng Chuansheng. Conditions and characteristics of coal-formed gas accumulation in Gubei low buried hill of Jiyang Depression [J]. Journal of China Ocean University, 2005,35 (4) 670 ~ 676.
[1 1], Zhang,. Formation mechanism of tight sandstone and its geological significance —— Taking Yingnan Well 2 in Tarim Basin as an example [J]. Offshore Petroleum Geology, 2005,10 (1): 31~ 36.
Li Yunsheng, Zeng, Tian Jianbo, et al. Study on tight sandstone gas reservoir identification method [J]. Journal of Southwest Petroleum Institute, 2003,25 (1): 25 ~ 28.
Yang. New logging evaluation technology for tight sandstone gas reservoirs in Ordos Basin [J]. Natural Gas Industry, 2005,25 (9): 45 ~ 47.
Dai Jinxing, Qi Houfa, Song Yan. On several indexes to distinguish coal-formed gas from oil-type gas [J]. acta petrolei sinica, 1985, (4), 3 1 ~ 38.
Li Hongmei. Analysis on conditions of coal-formed gas accumulation in Gubei slope belt [J]. Natural gas industry, 2006,26 (2): 23 ~ 25.
Wang Li, Jin Qiang, Wan, et al. Genetic types and controlling factors of deep natural gas in Gubei-Bonan area of Zhanhua sag [J]. Journal of Youshi University: Natural Science Edition, 2008,32 (5): 35 ~ 40.
Wang Li, Jin Qiang, Lin, et al. Geochemical characteristics and gas source analysis of 4 buried hills in Gubei-Bonan area of Jiyang Depression [J]. Natural Gas Geoscience, 2007, 18 (5): 7 15 ~ 749.
[18] Guan Deshi, Niu Jiayu. Unconventional petroleum geology in China [M]. Beijing: Petroleum Industry Press, 1995, 60 ~ 85.
[19] Sharla Cheung Lu Shuangfang. Natural gas geochemistry [M] Petroleum Industry Press, 2008, 148 ~ 16 1.
Dai Jinxing, Pei Xigu, Qi Houfa. Geology of natural gas in China (Volume I) [M]. Beijing: Petroleum Industry Press, 1992.
Dai Jinxing, Song Yan. Identification index of coal-formed gas [a]. Geological study on coal-formed gas [c]. Beijing: Petroleum Industry Press, 1987, 156 ~ 170.
[22]Leythaeuser D, Schaefer R G, Corford C, et al. Generation and migration of light hydrocarbons (C2 -C7) in sedimentary basins [J]. Organic geochemistry, 1979,1(4):19/kloc-.
[23]Snowdon L R, Powell T G. Modification of hydrocarbon generation model of terrestrial organic matter by immature oil and condensate oil [J].AAPG,1982,66 (6): 775 ~ 788.