Petroleum Exploration and Development

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Unconventional hydrocarbon resources in China and the prospect of exploration and development

Jia Chengzao; Zheng Min and Zhang Yongfeng

, 2012, 39(2): 2016-0.

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By analysis of the characteristics of unconventional hydrocarbon resources, this paper assesses the potential for unconventional hydrocarbons in China, summarizes the key technical progress in exploration and development, and discusses the prospects and developing strategies of unconventional hydrocarbons. The resources of unconventional oil and gas in China are abundant. The recoverable tight gas is 8.8×1012-12.1×1012 m3, the recoverable shale gas is 15×1012-25×1012 m3, the recoverable coalbed methane is 10.9×1012 m3, the recoverable tight oil is 13×108-14×108 t, and the recoverable shale oil is 160×108 t. There is also some resource potential for oil sand. Such key techniques as the full-digital seismic exploration technique, low permeability and low resistivity reservoir identification technique have been developed and their applications in oil and gas fields obtained good results. Tight gas and tight oil are the most realistic to develop in China, and the development and utilization of coalbed methane and shale gas just begins. In the next ten or twenty years, the production of unconventional hydrocarbons in China will increase remarkably and play an important role in national hydrocarbon resources.

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Types of subtle buried-hill oil reservoirs and their accumulation simulation in Jizhong Depression, Bohai Bay Basin

Zhao Xianzheng; Jin Fengming; Cui Zhouqi; Han Chunyuan; Zeng Jianhui; Wang Quan and Guo Kai

, 2012, 39(2): 2017-0.

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The types of subtle buried-hill resevoirs and their accumulation characteristics are analyzed based on the exploration progression of subtle buried-hill reservoirs in the Jizhong Depression in recent years. These reservoirs are classified into easily discovered buried-hill head reservoirs, and difficultly discovered buried-hill slope reservoirs and buried-hill internal reservoirs. Effective seal is the key factor for the formation of buried-hill internal reservoirs, and mountain controlling faults are the important hydrocarbon migration pathways. As for buried-hill slope reservoirs, two sealing conditions must be satisfied, including the argillaceous caprocks at the top of the unconformities and the buried-hill internal barriers at the flank, and the unconformities act as the primary migration pathways. Simulation experiments on petroleum migration and accumulation by a 2D experimental model show that the preferential charging location of hydrocarbon is determined by the charging force and the permeability ratio of transport pathways (faults or unconformities) to buried-hill reservoirs. When faults or unconformities are high-efficient transport pathways, hydrocarbon will charge the top of the buried hills preferably, thus forming buried-hill head reservoirs. If faults or unconformities are poor in permeability or they are characterized by heterogeneity, hydrocarbon will charge the internal layers or slopes of buried hills, forming buried-hill internal reserovirs or buried-hill slope reservoirs when there are high-permeability reservoirs at the internal layers or slopes of the buried hills.

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Reservoir-forming conditions of the Ordovician weathering crust in the Maigaiti slope, Tarim Basin, NW China

Wu Guanghui; Li Honghui; Zhang Liping; Wang Chenglin and Zhou Bo

, 2012, 39(2): 2018-0.

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Based on the seismic and drilling data, the characteristics and reservoir controlling factors, as well as oil and gas distribution law and exploration direction, are analyzed in the Ordovician weathered crust in the Maigaiti slope and its periphery with paleo-uplift background in the Tarim Basin. Structure mapping and analysis reveal that the largest carbonate weathered crust is developed around the Maigaiti slope which forms a favorable reservoir-seal assemblage with the overlying Silurian/Carboniferous mudstone. The Ordovician carbonates experienced multi-stages of karstification and are dominated by small-size fracture-vug reservoirs which are controlled by the karst palaeo-geomorphology, tectonism and lithology. Drilled wells are located mainly in the northern karst depression which has weak karstification and heavy filling. Palaeo-geomorphology reconstruction and seismic reservoir prediction suggest that reservoirs be well-developed in the southern karst slope. The comprehensive analysis of structure revolution and hydrocarbon generation history demonstrates that the hydrocarbon migration and accumulation is controlled by the evolution and migration of the palaeo-uplift. There are two charging stages: large scale oil accumulation in the late Hercynian period and adjustment and re-accumulation of oil cracked gas in the late Himalayan period. The Manan and Maixi slopes in the eastern and western sides of the palaeo-uplift are the favorable areas for hydrocarbon accumulation and adjustment over a long period of time, have the conditions of forming large-scale carbonate fracture-vug type pools, and are important strategic relay exploration areas in the Tarim Basin.

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Evolution of the Ordovician top boundary and its relationship to reservoirs’ development, Ordos Basin

Ran Xinquan; Fu Jinhua; Wei Xinshan; Ren Junfeng; Sun Liuyi and Bao Hongping

, 2012, 39(2): 2019-0.

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The control of structural evolution on the development of different types of Ordovician reservoirs in the Ordos Basin is discussed by studying the evolution of the Ordovician top boundary in geological times, and the mechanisms of formation and evolution of carbonate reservoirs are examined. The top boundary of the Ordovician is an important sequence boundary in the Ordos Basin. It represents not only the depositional change between lower marine carbonate rocks and upper clastic rocks, but also a depositional break caused by weathering and corrosion from the Late Calidonian to the Early Hercynian. It experienced three great structural changes in the geological history, which influenced the formation and conservation of the Ordovician reservoirs. Lithology difference of the Ordovician top sequence between the east and west of the basin controlled the type of reservoir space and caused the formation of different reservoirs of corrosive pores. However, the later reversal of the sequence boundary causes the filling of early formed pore space in the eastern basin, and affects the space distribution of effective reservoirs.

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固相流动对垂直气井井筒压降影响的数值模拟

Adekomaya Olufemi A

, 2012, 39(2): 2020-0.

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为了准确估算垂直气井井底压力，更精确地预测单井产量，在总结前人有关垂直井筒单相管流井筒压降计算方法的基础上，建立了固相流动对垂直气井井筒压降影响的数值模型。该模型考虑了直井中气相湍流、固相颗粒流动及Moody摩擦系数对井筒压降的影响，扩展了Adekomaya等（2011年）及Sukkar & Cornell提出的单相垂直管流井筒压降计算方法。模型计算结果表明，固相流动对井筒压降影响显著，而前人提出的计算模型未能考虑其影响；相同条件下，当有固相颗粒伴随流体产出时，井筒压降高于无固相颗粒产出时的井筒压降；固相含量越高、固相相对密度越大，井筒压降越大，压力随深度的增加降低越快。图5参13

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Genesis and hydrocarbon significance of vesicular ignimbrite: A case study from Fengcheng Formation, Wu-Xia area, Junggar Basin, NW China

Zhu Shifa; Zhu Xiaomin; Liu Jishan; Yao Yuan; Xian Benzhong; Niu Huapeng and Zhao Changyong

, 2012, 39(2): 2021-0.

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On the basis of a large number of cores and thin-sections observations, the petrologic and pore evolution characteristics of ignimbrite in the Fengcheng Formation in the Wu-Xia area, Junggar Basin, are studied, and the genesis of vesicular ignimbrite and formation mechanism of vesicles are analyzed. Volcanic action in the Fengcheng Formation is characterized by welded pyroclastic rocks. There are predominant plastic shards with rounded edges and obvious devitrification in the rocks. Plastic debris is deformed and wrinkled, and crystal fragments are usually corroded. A lot of lithophysae occur in the ignimbrite, and welded texture is obscured. Vesicles are the cavities of lithophysae, which are formed by solidification contraction. They have little or no late filling cement because the volatile constituent and water vapour in the bubbles have escaped with the help of devitrification. Lithophysae distribution within a cooling unit obviously has the characteristic of zonation in vertical: increasing upwards and then decreasing gradually. The cavities of lithophysae greatly improve the reservoir capability of ignimbrite. Influenced by intensive welded process, lithophysae are not subject to collapse or damage. In the study area, deeply buried vesicular ignimbrite is an important hydrocarbon reservoir rock, and the vesicular ignimbrite has good petroliferous property.

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Hydrocarbon origin and reservoir forming model of the Lower Yanchang Formation, Ordos Basin

Li Xiangbo; Liu Xianyang; Zhou Shixin; Liu Huaqing; Chen Qilin; Wang Jing; Liao Jianbo and Huang Junping

, 2012, 39(2): 2022-0.

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According to the comparison of biomarkers in source rocks and crude oil, fluid inclusion analysis, and basin modeling, this paper discusses the oil source, hydrocarbon accumulation period and reservoir forming model of the Chang 9 and Chang 10 oil-bearing formations, Yanchang Formation, Ordos Basin. The crude oil of Chang 9 in the Longdong and Jiyuan areas can be divided into two types, type Ⅰcrude oil originated from the source rocks within Chang 7, while type Ⅱ crude oil came from the source rocks within Chang 9. The crude oil of Chang 10 in Northern Shaanxi originated mainly from the source rocks of Chang 9. The Chang 9 oil reservoirs in both the Longdong and Jiyuan areas experienced two periods of hydrocarbon injection. The former reached the peak period of hydrocarbon injection in the first period (the depositional period of Middle Jurassic Zhiluo Formation), while the latter in the second period (the depositional period of Lower Cretaceous Zhidan Formation). There are two periods of continuous hydrocarbon injection in Chang 10 of Northern Shaanxi, generally from the early depositional period of the Middle Jurassic Zhiluo Formation to the middle-late depositional period of the Cretaceous Zhidan Formation. There are three types of hydrocarbon accumulation models in Chang 9 and Chang 10, i.e. “upper generation and lower storage”, “adjacent generation and lateral storage” and “self-generation and self-storage”.

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Resource potential of water-soluble gas in the Palaeogene Huizhou Sag, Pearl River Mouth Basin

Xu Sihuang; Li Songfeng and Yuan Caiping

, 2012, 39(2): 2023-0.

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Based on the formation mechanism of water-soluble gas, combined with the geological conditions of the Palaeogene Huizhou Sag, the resource potential and utilization of water-soluble gas in the Palaeogene source rock in the Huizhou Sag are analyzed quantitatively using grid calculation method. The Palaeogene Wenchang and Enping Formations of the Huizhou Sag are major source rocks that generate significant quantities of gas and the sandstone reservoirs are weakly overpressured to overpressured on the whole. Calculated according to the thickness, sandstone content, porosity, water saturation, strata pressure, temperature, and mineralization of the Wenchang and Enping Formations, the resource volume of water-soluble gas is about 1 791.49′108 m3 and 2 688.5′108 m3, 1.19 and 3.46 times that of the conventional gas, respectively. If the strata pressure is relieved to normal pressure, the degasification volume is 80.84′108 m3 and 97.04′108 m3. The water-soluble gas is an unconventional resource with great potential, and can provide substantial gas source for conventional pools through degasification.

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Indication of natural gas migration and accumulation by three-dimensional fluorescence characteristics of gas field water: A case of gas field water in the Permian-Triassic in the central-western Sichuan Basin

Li Wei

, 2012, 39(2): 2024-0.

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The Permian-Triassic natural gas migration paths and accumulation patterns in the central-western Sichuan region are studied based on the three-dimensional shape, exciting light intensity and fingerprint characteristics of aromatic hydrocarbons fluorescence spectra of gas field water. The Permian-Triassic gas field water in the central-western Sichuan Basin has (condensate) gas reservoir impact characteristics, and has three subclasses (base peak, double peak and odd peak, mainly the stable type of base peak subclass) and seven types of three-dimensional fluorescence characteristics. Three-dimensional fluorescence characteristics of the water of the Permian to Middle and Lower Triassic differ from that of the Upper Triassic Xujiahe Formation. The former is mainly influenced by hydrocarbons from high to over mature Lower Paleozoic marine shale and Upper Permian Longtan Formation coal, while the latter is mainly by its own source rock maturity differences and organic matter type difference. Research on the three-dimensional fluorescence characteristics indicates that the Upper Permian-Middle and Lower Triassic reservoirs in the Moxi structure mainly accumulate gas from vertical migration; gas in the reservoirs below the Leikoupo Formation originates mainly from the Longtan Formation coal, while the Leikoupo Formation gas reservoir includes gas from gypsum-salt rock section; gas in the Xujiahe gas reservoir in the Pingluoba structure is mainly migrated from central-southern faults and the reservoirs have strong heterogeneity on plan.

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Dolomitization mechanism of Cambrian carbonates in the Bachu area, Tarim Basin, NW China

Zhou Bo; Li Ming; Duan Shufu; Zhu Changjian; Wu Guanghui; Zeng Changmin and Gao Li

, 2012, 39(2): 2025-0.

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Geochemical characteristics of the Cambrian carbonates are studied through systematic sampling in the Bachu area of the Tarim Basin. The dolomitization mechanisms are compared and analyzed of the dolomites above and below salt beds and limestone between the salt beds. The d 18O values of dolomites in the limestone between salt beds are larger than -6‰, the d13C values are normally larger than -1‰, the 87Sr/86Sr values are smaller than ancient sea water plus Mn and Na element enrichment. All these features suggest that the dolomites are developed due to early sabakha dolomitization. For the dominantly medium- and coarse-grained dolomites above the uppper salt bed, the d13C values of samples are lower than zero, the d 18O values are normally –9‰–0, the 87Sr/86Sr values are close to ancient sea water. These features show that the dolomites are developed in a shallow burial environment. For the dominantly powdered and fine dolomites below the lower salt bed, the d13C values of samples are larger than zero, the d18O values are normally lower than zero, the 87Sr/86Sr values are larger than the above mentioned dolomites, plus Fe and Sr element enrichment. These features indicate that the dolomites are developed within an evaporate environment on a restricted platform, and they are of penetration reflux dolomitization and affected by the deep burial hyperthermal environment later.

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Transport mechanism of desorbed gas in coalbed methane reservoirs

Li Xiangfang; Shi Juntai; Du Xiyao; Hu Aimei; Chen Dong and Zhang Dongling

, 2012, 39(2): 2026-0.

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The gas-liquid two-phase flow and mass transfer principle shows that the diffusion caused by concentration difference only happens in a single-phase fluid; gas-liquid two-phase diffluent solution happens in the way of dissolution; and gas-liquid two-phase insoluble or semi- soluble solution flows under differential pressure driving. These facts demonstrate that the transport of desorbed gas through matrix pores is the flow, and it doesn’t conform to Fick law. The dissolution, diffusion, nucleation and bubble processes of desorbed gas through depressurization are studied, and the nonlinear flow model of free gas from matrix pores to the cleat and fracture system is established based on force analyses of the gas bubble and the gas column. Research shows that a small amount of desorbed gas diffuses by dissolution; most of them becomes nucleation and bubble, and then flows to the coal cleat and fracture system under the pressure difference driving; considering the existence of the pressure difference between the matrix pores and cleats, the pressure in coal matrix will reduce more slowly, the investigated radius will be shorter, and the outflow lag phenomenon of desorbed gas will appear. The dynamic reserve should be calculated not by using cleat pressure but by the pressure in coal matrix. The mechanism of enhanced methane recovery by CO2 injection is not replacement but displacement. Improved methane recovery can be obtained by optimizing the production pressure difference, it is not reasonable that the lower formation pressure gives higher methane recovery.

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Segmentally variable density perforation optimization model for horizontal wells in heterogeneous reservoirs

Pang Wei; Chen Dechun; Zhang Zhongping; Jiang Lifu; Li Changheng; Zhao Xu and Wang Bing

, 2012, 39(2): 2027-0.

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A 3D three-phase segmentally variable density perforation optimization model of horizontal wells is built by coupling reservoir fluid filtration, near wellbore inflow and wellbore conduit flow based on reservoir numerical simulation. The effects of 8 factors (filtration difference between heel/toe and middle intervals, imperforated interval, permeability heterogeneity, oil layer thickness heterogeneity, porosity heterogeneity, wellbore pressure drawdown, maximum perforation density, and perforation optimization principles) on perforation density and inflow profile, and that of fluid viscosity, casing diameter and pipe wall coarseness on well bore pressure drawdown, are analyzed for segmentally variable density perforation of horizontal wells. Results show that filtration difference between heel/toe and middle intervals, imperforated interval, permeability heterogeneity and oil layer thickness heterogeneity have significant effects on segmentally variable density perforation. In particular, different perforation density optimizations may occur when filtration difference between heel/toe and middle interval is not considered; imperforated interval may affect inflow profile; well bore pressure drawdown can be ruled out for segmentally variable density perforation of most horizontal wells onshore in China. The contrast between predicted and actual production of Well AT9-7H in the Tahe Oilfield indicates that the model is highly accurate.

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Inflow performance for highly deviated wells in anisotropic reservoirs

Wang Haijing; Xue Shifeng; Gao Cunfa and Tong Xinghua

, 2012, 39(2): 2028-0.

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Based on source function method and coordinate transformation, an inflow model for a highly deviated well in an anisotropic reservoir is developed according to reservoir-wellbore coupling. On this basis, the inflow performance of a highly deviated well in an infinite slab anisotropic reservoir with impermeable top and bottom boundaries are studied under both constant rate and constant bottom-hole pressure. For a highly deviated well producing at a constant rate in an infinite slab reservoir of impermeable top and bottom boundaries, when the equivalent permeability and other parameters are constant, the smaller the anisotropy ratio, the shorter the duration of early-time radial flow and intermediate-time transition flow, and the earlier the appearance of late-time pseudo radial flow is. With the increase of deviation angle, the influence of vertical permeability increases, while the influence of horizontal permeability decreases. When the anisotropy is weak, the influence of deviation angle on the inflow performance is smaller. With the increase of anisotropy, the influence of deviation angle increases significantly.

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Attenuation law of MWD pulses in aerated drilling

Li Hongtao; Li Gao; Meng Yingfeng; Shu Gang; Zhu Kuanliang and Xu Xiaofeng

, 2012, 39(2): 2029-0.

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A mathematical model for predicting the attenuation of measurement while drilling (MWD) pressure pulses in aerated drilling was proposed using the two-phase flow model and considering the momentum and energy exchange at the phase interface, gravity of each phase, viscous pipe shear and other closing conditions. The small perturbation theory was used to solve the mathematical model; then, a model was established to compute the velocity and attenuation coefficient of mud pulse signal, and the simulated result of the model is in line with the measured attenuation in field test. With the mathematical model, the influencing factors for mud pulse attenuation were simulated and analyzed. The results reveal that the gas holdup, system pressure and pulse frequency are the major influencing factors. In case of low gas holdup (0–5%), as the gas holdup increases, the pulse velocity decreases drastically, and as it further increases, the pulse velocity tends to be flattened; whereas the attenuation coefficient increases gradually with the increase of gas holdup and begins to decline when it reaches maximum. At the same gas holdup, the mud pulse signal shows higher velocity under a high system pressure, and the attenuation coefficient increases faster with increase of the gas holdup under a low system pressure. Both pulse velocity and attenuation coefficient increase gradually with the increase of pulse frequency, and tend to be a stable high value when the frequency exceeds 100 Hz.

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Major corrosion factors in the CO2 and H2S coexistent environment and the anti- corrosion method: Taking Tazhong I gas field, Tarim Basin, as an example

Qiu Zhichao; Xiong Chunming; Chang Zeliang; Zhao Zhihong; Zhao Chun and Ye Zhengrong

, 2012, 39(2): 2030-0.

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Anti-corrosion techniques are studied to deal with the CO2 and H2S coexistent environment of the TazhongⅠgas field. Orthogonal experiment with five different factors and levels is carried out to compare the influence of the factors on the corrosion rate of the sulfur resistance tubing. These five factors are pressure of CO2, pressure of H2S, temperature, content of Cl-, and water cut. It is found that CO2 pressure is the major corrosion factor. The combination of sulfur resistance tubing and corrosion inhibitor is selected as the anti-corrosion method and the proper corrosion inhibitor named YU-4 is designed according to the result of orthogonal experiment. This anti-corrosion method was used in 12 wells of the Tazhong I gas field, achieving good results. The average corrosion rate of well TZ83 decreased from 1.23 mm/a to 0.025 mm/a and the corrosion rate of well TZ623 decreased from 0.370 mm/a to 0.016 mm/a.

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Computational intelligent methods for predicting complex lithologies and multiphase fluids

Li Xiongyan; Zhou Jinyu; Li Hongqi; Zhang Shaohua and Chen Yihan

, 2012, 39(2): 2031-0.

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On the basis of the basic principles of optimization algorithms and classification algorithms, the Self-Organizing feature Map neural network (SOM) is applied to establish the predictive model of lithology for the K-Means optimized data set including core data, logging data and well tests data. Additionally, the decision tree and support vector machine are used to build the predictive model of fluid on the basis of the lithology identification. The optimization algorithms, including genetic, grid and quadratic, are adopted to optimize the important parameters of C-SVC and υ-SVC, such as C, υ and γ, so as to accurately identify the complex lithologies and multiphase fluids of complicated reservoirs. The SOM model and the decision tree and support vector machine are utilized to process four new wells in the complicated Carboniferous reservoirs of the Wucaiwan Sag, eastern Junggar Basin. The accuracy of lithology identification is 91.30%, and the accuracy of fluid identification is 95.65%. The lithologic complexity is not the main factor leading to the differences of fluids in the reservoirs. Because the complexity and nonlinearity of data set are not strong enough, the accuracy of the decision tree model is better than that of the support vector machine. Their accuracy rates are 94.31% and 86.97%, respectively. The performance of linear polynomial function is better than that of the radial basis function RBF and the neural function Sigmoid. The classification performance and generalization ability of C-SVC are stronger than that of the υ-SVC.

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Classification and evaluation criteria of shale oil and gas resources: Discussion and application

Lu Shuangfang; Huang Wenbiao; Chen Fangwen; Li Jijun; Li Jijun; Xue Haitao; Wang Weiming and Cai Xiyuan

, 2012, 39(2): 2032-0.

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Research on the evaluation criteria of shale oil/gas is conducted so as to accurately assess the resources potential of shale oil and gas. After making statistic analysis of the geochemical index of hydrocarbon source rock layers in five areas such as Songliao, Hailaer, Jiyang, based on the characteristic of triple-division between the oil content and TOC of source rock, this paper classifies shale oil/gas into three levels of resources in terms of enrichment degree: scattered (ineffective) resources, low efficient resources and enriched resources. The mature stage, at which organic matter generates oil/gas in large amounts, corresponds to shale oil/gas enrichment window. Furthermore, recoverable index is defined and its computation formula is proposed to provide a quantitative index for recoverability evaluation, considering the brittle mineral content, thickness and depth of shale, etc. In practice, TOC variable values in well profile obtained by TOC-logging correspondence can be used to draw the TOC isopach map and calculate the resources amount of different levels. Then combined with the evaluation criteria, TOC and Ro isopach maps are superimposed to identify the favorable shale oil/gas areas.

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, Volume 39 Issue 2