Petroleum Exploration and Development

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Progress of Paleozoic coal-derived gas exploration in Ordos Basin, West China

Yang Hua and Liu Xinshe

, 2014, 41(2): 129-137.

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Based on practices of gas exploration in the Ordos Basin, this paper analyzes the controlling factors of large-scale accumulation conditions and distribution characteristics of coal-derived gas, and concludes the exploration progress of coal-derived gas according to the present exploration situations. There are 5.24×1012 m3 proven coal-derived gas reserves in the Ordos Basin. Twelve coal-derived gas fields have been found, mainly distributed in the Upper Paleozoic Carboniferous-Permian clastic rocks and Lower Paleozoic Ordovician marine carbonate reservoirs. Tight sandstone gas reservoirs are developed in the Upper Paleozoic. The widely covered hydrocarbon source of coal stacks each other with large-scale sandstone reservoirs, the reservoirs tightened firstly and natural gas accumulated later, and natural gas migrated shortly and accumulated in a large area. The Sulige large gas field has been found with proven coal-derived gas reserves of 3.49×1012 m3, and two large-scale reserve replacements have been determined in the eastern part of the basin and the Longdong area. Marine carbonate gas pools are developed in the Lower Paleozoic. The gas generated from the overlying coal source rocks migrated downward and accumulated in the Ordovician weathering crust karst and dolomite reservoirs. The Jingbian gas field has been found with proven coal-derived gas reserves of 7 000×108 m3, meanwhile, several gas-rich regions have also been found.

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Gravity flow sedimentation and tight oil exploration in lower first member of Shahejie Formation in slope area of Qikou Sag, Bohai Bay Basin

Pu Xiugang; Zhou Lihong; Han Wenzhong; Chen Changwei; Yuan Xuanjun; Lin Changmei; Liu Sa; Han Guomeng; Zhang Wei and Jiang Wenya

, 2014, 41(2): 138-149.

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Based on observation of cores and thin slices and analysis of drilling, log and test data, the gravity flow deposits characteristics and formation conditions and characteristics of tight oil in lower first member of Shahejie Formation (Es1x) were systematically analyzed in Qikou Sag, Huanghua depression, Bohai Bay Basin. There mainly developed sandy debris flow, muddy debris flow and classical turbidity flow in the study area, and sandstone with boulder-clay is mostly sandy debris flow origin, muddy debris flow deposits are mudstone or silty mudstone with sandy lumps, the normal graded bedding is a reliable sign of classical turbidite. Affected by multi-source supply, slope break, different trigger mechanism and fluctuation of lake level, multi-gravity flows formed multi-level fans in mid-low slope to main sag, which can be divided into five micro-facies including submerged main channel, submerged distributary channel, lateral channel margin, mud between channels and turbidite sheet sand. Multi-level and multi-stage gravity flow sediments provide material foundations for tight oil and gas in the deep mud and sand-rich lake, where wide spread tight reservoirs and high quality source rocks contact closely vertically, forming multiple sets of sandwich-like source-reservoir-seal assemblages. Exploration shows that the submerged distributary channel and turbidite sheet sand of sandy debris flow are two main exploration facies of gravity flow in Es1x and important area of scale reserve increase in slope zones of Qikou Sag.

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Internal structure and sealing properties of the volcanic fault zones in Xujiaweizi Fault Depression, Songliao Basin, China

Meng Lingdong; Fu Xiaofei; Wang Yachun; Zhang Xiaoling; Lü Yanfang; Jiang Yibo and Yang Hongsong

, 2014, 41(2): 150-157.

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Through outcrop observation of the internal structure of volcanic fault zone and in combination with fault sealing mechanism research and lithology juxtaposition evaluation, this paper analyzes the internal structure and lateral fault sealing of a volcanic fault zone of the Xujiaweizi Fault Depression. The “binary structure” of damage zone and fault core is developed inside the volcanic fault zone. The fault does not have the sealing property basically at the juxtaposition of volcanic rock. Lithologic juxtaposition sealing is formed at the juxtaposition of the volcanic rock reservoir stratum and the overlying conglomerate bed and mudstone bed. By improving the traditional Knipe diagram (single-well triangle diagram), a dual-well triangle map is drawn to indicate the influence of strike-slip fault on the lithology juxtaposition. Modeling is established with the dual-well triangle diagram and in combination with the fault lithology juxtaposition diagram to evaluate the fault sealing property. Research showed that the gas pools in the Xujiaweizi Depression are almost in the upside of faults; in the longitudinal, gas-water contract is almost at the most shallow position of volcanic juxtaposition and the horizontal distribution of gas pools around faults is just like the curve “sine” as the result of the “dolphin effect” of the Xuzhong strike fault.

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Characterization and evaluation of ultra-deep fracture-pore tight sandstone reservoirs: A case study of Cretaceous Bashijiqike Formation in Kelasu tectonic zone in Kuqa foreland basin, Tarim, NW China

Zhang Huiliang; Zhang Ronghu; Yang Haijun; Shou Jianfeng; Wang Junpeng; Liu Chun and Chen Ge

, 2014, 41(2): 158-167.

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Taking the Cretaceous Basijiqike Formation sandstone reservoirs in the Kuqa foreland basin as an example, this paper studies the characterization and evaluation methods of fracture-pore (fracture-primary pore and fracture-dissolution pore) ultra-deep reservoirs under the double effects of diagenetic compaction and tectonic compression. Buried over 6 000 m deep, the sandstone reservoirs of Basijiqike Formation are ultra-deep reservoirs, aimed at which, an ultra-deep fracture tight sandstone reservoir characterization technique is built, which integrates macroscopic microfacies-lithofacies identification, centimeter-micron scale fracture description, micron pore depiction and nano-throat characterization. The research indicates that the effective reservoir space consists of structural fractures, micron pores and nano-pore throats. The main radius of matrix pores is 2-100 μm, that of matrix throats is 10-500 nm and the main area of fracture opening degree is 100-300 μm. Effective reservoirs are mainly controlled by microfacies-lithofacies, tectonic compression and erosion. Relatively high-quality reservoir spaces consist of fractures, residual intergranular pores and dissolution pores, developed in the stacked area of weak compacted structure zone and underwater distributary channel zone. Favourable reservoirs over 7 000 m deep can be in continuous band distribution, there still could be effective reservoirs in formations over 8 000 m deep.

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Coupling relationship between sandstone reservoir densification and hydrocarbon accumulation: A case from the Yanchang Formation of the Xifeng and Ansai areas, Ordos Basin

Liu Mingjie; Liu Zhen; Liu Jingjing; Zhu Wenqi; Huang Yanhui and Yao Xing

, 2014, 41(2): 168-175.

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The Chang 8 member of the Xifeng Area and the Chang 6 member of the Ansai Area are taken as examples to define the relationship between the sandstone densification and accumulation from fluid inclusion identification, simulation of sandstone porosity evolution, and analysis of oil charge critical experiment. The fluid inclusion fluorescence shows that there is quartz overgrowth containing the residual asphalt and hydrocarbon inclusions existing in the sandstone reservoir of Yanchang Formation. The simulation of porosity evolution shows that the sandstone reservoir was not tight sandstone during the key accumulation period for the porosity was greater than 10%. The oil charge critical experiment indicates that the oil cannot charge into the sandstone if the porosity is less than 10% in Xifeng and Ansai, even under pressurized conditions. The results demonstrate that the Yanchang Formation of Xifeng Area and Ansai Area in the Ordos Basin is a typical “postforming” tight sandstone hydrocarbon reservoir in which the sandstone undergoes densification after oil accumulation. The compaction and cementation following the oil charging lead to the densification of sandstone.

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Logging optimization processing method for fractured-vuggy reservoirs

Feng Zhou; Li Ning; Wu Hongliang; Liu Yingming; Xia Shouji and Yuan Ye

, 2014, 41(2): 176-181.

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In the traditional logging optimization processing methods, logging response equations are built based on the homogeneous, layered and isotropic formation model, they are not suitable for the heterogeneous and anisotropic fractured-vuggy reservoirs. A new optimization processing method aimed at the fractured-vuggy reservoir was presented and its accuracy was confirmed through simulation calculation and actual logging data processing. The logging response equations and objective function for logging optimization problem were built based on the heterogeneous fractured-vuggy reservoir model. Then, the solving algorithm combining penalty function method with Levenberg-Marquardt method was implemented, which had less calculation and high efficiency. The result of simulation calculation shows that the solving algorithm is less affected by the initial value, and the global optimal solution can be gained with random initial values, which meets the needs of logging evaluation. Finally, the proposed approach was applied to the real data processing. Comparison results show that the new method is more accurate than the traditional ones, and it also has a good consistency with the result of core analysis.

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Evaluation for cluster platform controlling traps exploration: Taking the X block in Oriente Basin, Ecuador as an example

Ma Zhongzhen; Xie Yinfu; Zhang Zhiwei; Zhou Yubing; Han Bin; Yang Xiaofa; Wang Dandan; Liu Yaming and Zhao Yongbin

, 2014, 41(2): 182-189.

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Aimed at the small size, large quantity of traps and high environmental requirements of resource countries on slope belt of South America foreland basins, a new evaluation method is proposed for trap exploration on slope by taking the area controlled by a cluster platform as one evaluation unit. The evaluation process is: (1) trap geological risk analysis: including key oil & gas accumulation element analysis such as source, reservoir, seal, trap, migration and preservation; (2) trap resources calculation: including trap resources calculated with volumetric method, trap accumulation probability evaluation by geological risk analysis, and finally the risk trap resources by multiplying the above two values; (3) platform and drilling target selection: count the total risk trap resources and average risk trap resource of different platform and rank the drilling platforms according to the calculated resources; and rank the trap drilling sequence according to the risk trap resources. The method was used in ranking undrilled traps in the M1 layer of Block X, Oriente Basin, and the platform construction and trap drilling order was recommended accordingly which shows good results in practice.

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Improving the accuracy of geological model by using seismic forward and inversion techniques

Hu Yong; Yu Xinghe; Li Shengli; Chen Gongyang; Zhou Yanli and Gao Zhaopu

, 2014, 41(2): 190-197.

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Sequential Gaussian simulation method is taken as an example to analyze the characteristics and defects of stochastic simulation methods, and a modeling strategy is proposed that is based on the results of seismic inversion to improve modeling accuracy. Stochastic simulation can only achieve “mathematical reality” by recovery of parameters macro statistical regularity through examples, and seismic forward model can verify whether the simulation results deviate from the “geological reality”. Seismic forward modeling can verify the reliability of the geological model, and the results of geological modeling constrained by seismic inversion can improve the accuracy of the model to achieve the “geological reality”. For braided river delta, a modeling strategy constrained by seismic inversion results in the condition of “multi-levels” and “multi-conditions” is put forward. For lithofacies modeling, division of lithofacies in single well acts as the first variable; in the stage of exploration and development, lithology probability and lithology body act as the second variable respectively, and then establish facies models. For property modeling, take lithofacies model and the seismic impedance as the first variable and the second variable respectively to create property model, in the condition of horizontal and vertical impedance constraints. The proposed modeling strategy maintains the statistical regularities of input data, keep a better consistency with seismic data, and match with dynamic field production.

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Discussion on effective development techniques for continental tight oil in China

Du Jinhu; Liu He; Ma Desheng; Fu Jinhua; Wang Yuhua and Zhou Tiyao

, 2014, 41(2): 198-205.

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Based on the main geological features and technical breakthroughs made in tight oil exploration, the major challenges facing tight oil development are analyzed, and the key technical trend for tight oil development is discussed in this paper. Mainly found in continental deposits, tight oil reservoirs in China feature small area, poor physical properties, big differences in geological characteristics between different basins, but low porosity, low permeability and pressure in general. In contrast to marine tight oil, tight oil in continental deposits faces such challenges as low production and recovery, and poor economics. Through nearly three years of research and pilot test, an integrated development mode with repeated fracturing of horizontal wells as the principal technique has been proposed, which includes integrated design, platform long horizontal well drilling, massive volume fracturing, re-fracturing stimulation, controlled production, factory-like operation, concentrated surface construction etc. It is recommended that study be strengthened on basic tight oil development theory, practical development technologies, and economic evaluation of tight oil development over the whole life cycle.

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Complex relationship between porosity and permeability of carbonate reservoirs and its controlling factors: A case of platform facies in Pre-Caspian Basin

He Ling; Zhao Lun; Li Jianxin; Ma Ji; Liu Ruilin; Wang Shuqin and Zhao Wenqi

, 2014, 41(2): 206-214.

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Based on a large amount of core analysis data in eastern Pre-Caspian Basin, the relationship between permeability and porosity and its influencing factors are studied. The sedimentary environments of the Carboniferous System in eastern Pre-Caspian Basin include open platform, restricted platform and evaporate platform. For dolomite reservoirs there are three main combination patterns of pores, namely, inter-crystalline and solution pores, inter-crystalline micro-pores, inter-crystalline micro-pores and intra-crystalline pores, among which the first combination are highest in porosity and permeability. For limestone reservoirs, the main combinations of pores are inter-particle pores, inter-particle and intra-particle pores and moldic pores. Lacking connecting pore throats, moldic pores are poor in permeability. In the dolomite reservoirs, fractures and vugs are well-developed and increase the permeability significantly. Under similar porosity, the permeability is obviously determined by the size of pore throat and the percentage of middle to large size throats in different type of reservoirs. In limestone reservoirs, fractures are principally micro-fractures, percentage of connecting pores to total pores is positively correlated with permeability; while in dolomite reservoirs, there is not such an obvious relationship between the connected pore percentage and permeability. Sedimentary environment controlling the distribution features of reservoir types, is the cause of the complex relationship between permeability and porosity.

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Numerical simulation of mechanism of high-pressure air injection (HPAI) in light oil reservoirs

Jia Hu; Zhao Jinzhou; Yang Huaijun; Li Yueyang and Pu Wanfen

, 2014, 41(2): 215-222.

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Based on the research progress of the crude oil oxidation-combustion model proposed by predecessors, this paper established a new multi-reaction model for high-pressure air injection (HPAI) process to compare the HPAI performance in reservoirs without water injection and high water cut reservoirs by numerical simulation. Results show that gas override is universal in air injection in light oil reservoirs before and after waterflooding, and is more apparent in strongly heterogeneous reservoirs. However, the produced oxygen concentration is very low all the time. For reservoirs that haven’t gone through water injection, at high air injection rate oil is very likely to auto-ignite and keep high peak temperature and long time stable combustion front. However, there exists a critical value for the influence of air injection rate on the combustion front moving speed, above the critical value, oil recover factor increment is not obvious with the increase of air injection rate. At the same air injection rate, high water cut reservoirs are 6.3% lower in recovery factor than reservoirs without water injection, with obvious increase in GOR and high water recovery degree during HPAI process. It is suggested that polymer gel/ and or foam assisted air injection be taken to reduce GOR/ and or water cut in production wells.

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Conversion in equivalent carbon number between interfacial tensions of chain/cyclic hydrocarbon-CO2 system

Ji Zemin; Qin Jishun; Chen Xinglong; Liu Xiaolei; Han Haishui and Lian Liming

, 2014, 41(2): 223-226.

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A great number of published data and our experimental results of interfacial tensions between hydrocarbon compounds and CO2 were collected and screened. Based on these experimental data, the changing laws of interfacial tensions between different kinds of hydrocarbon compounds and CO2 were obtained, interfacial tensions between different kinds of saturated chain hydrocarbons and CO2 were compared with each other, and interfacial tensions of three different binary system (monocyclic hydrocarbon compounds-CO2 system, hydrocarbon compounds combined both of monocyclic and chain structure-CO2 system and dicyclic hydrocarbon compounds-CO2 system) were compared with those of saturated chain hydrocarbon compounds-CO2 system. It is found that molecular structure is the main factor to affect the sizes of interfacial tensions between hydrocarbon compounds and CO2; when carbon numbers of different kinds of hydrocarbon compounds are equal, their proper order from big to small in interfacial tension is: polycyclic hydrocarbon, hydrocarbon combined both monocyclic and chain structure, monocyclic hydrocarbon, saturated chain hydrocarbon. The comparison results at different pressure conditions were respectively used to establish the conversion relations in equivalent carbon number between interfacial tensions of cyclic hydrocarbon-CO2 systems and those of saturated chain hydrocarbon-CO2 systems.

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Study of meso-damage characteristics of shale hydration based on CT scanning technology

Ma Tianshou and Chen Ping

, 2014, 41(2): 227-233.

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In view of the shale hydration problem in wellbore stability analysis during drilling in shale formation, a quantitative evaluation method based on CT scanning technology was proposed to research the meso-damage characteristics of shale hydration, and the CT scanning tests for shale samples in stages of shale hydration were conducted. The analysis results of CT images and their gray-level histograms show that the early period of soaking is the main period of meso-damage in shale samples, and as the soaking time increases, the meso-damage propagates slowly; the mark of meso-damage increasing dramatically is that the gray-level histogram of the CT image changes from unimodal style to bimodal style. The visual resolutions of CT images can be improved by the pseudocolor enhancement technique, and the integrity and damage degrees of CT images provide a convenient way for quantitative analysis. The relationship between damage variable and soaking time was obtained based on the segmentations of CT images, which shows that the meso-damage of shale hydration mainly occurs in the early stage of soaking, which is the initial stage and rapid evolution stage of meso-damage, from then on, as the damage variable increases continuously and slowly, the macro-damage of shale sample occurs.

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Design of structure casing soaking time in deepwater drilling

Zhou Bo; Yang Jin; Liu Zhengli; Luo Junfeng; Ye Jihua; Chen Bin; Liu Shujie and Zhou Jianliang

, 2014, 41(2): 234-238.

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To decrease the risk in deep water drilling and shorten the drilling period, a structure casing soaking time design method for deep-water jetting was provided and verified by oil field application. Based on the field simulation experiment of soil-pile interaction, the relationship between soaking time and bearing capacity was achieved, and a real-time bearing capacity calculation model of structure casing was established. The mechanical properties of structure casing in two conditions were analyzed, including Cam Activated Drilling Ahead (CADA) tool releasing stage and surface casing cementing stage, and the soaking time windows of the two stages were designed. Field application results show that: This method can ensure the safety in deep water drilling, optimize driving depth of structure casing, and also improve the drilling efficiency. More than 12 hours per well on average in CADA tool releasing stage and 24 hours per well on average in surface casing cementing stage were saved.

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An efficient finite difference model for multiphase flow in fractured reservoirs

Zhou Fangqi; Shi Anfeng and Wang Xiaohong

, 2014, 41(2): 239-243.

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Considering the discontinuities of flow phase saturations at the matrix-fracture interfaces, a new difference model is proposed to simulate the multi-phase flow in reservoir with conductive fractures. Structure grids are applied in the simulation to divide reservoir regions, fracture grids are embedded in matrix grids, and reference nodes are chosen at both sides of the fracture to define the exchanges between fracture and two sides of matrix. During solving the matrix equations, the discontinuities of phase saturations also lead to difficulties for defining the physical quantities on the matrix grids covering fracture, and to avoid this difficulty, inner boundary is used to replace these matrix grids in the computations. This model is used to simulate the reservoirs with straight and inclined fractures, and compared with discrete fracture model and embedded fracture model. The comparison shows that: no matter in fracture or matrix, the results from the new model are in good agreement with the discrete fracture model, but embedded fracture model has some error; the new fracture model can significantly improve the efficiency of simulation.

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Reserve growth characteristics and potential of the discovered giant oil fields in the Middle East

Bian Haiguang; Tian Zuoji; Wu Yiping; Pan Xiaohua; Tong Xiaoguang and Zhang Xiaosu

, 2014, 41(2): 244-247.

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Giant oilfields in the Middle East were taken to analyze the characteristics of reserve growth by comparing the estimated additions of recoverable reserves during different periods. The reserve growth potential of the discovered giant oil fields in the Middle East was predicted through statistics of annual reserve growth factors and building annual reserve-growth function by the least square method. The additions of recoverable reserves are comprised of reserves in newly discovered oilfields and incremental reserves of the giant oil fields discovered previously. The study shows that the incremental reserves in the giant oilfields discovered previously have increased sharply, but the reserves in newly discovered oilfields have decreased slightly over the past several decades. The incremental reserves in the giant oilfields discovered previously are the major contributor to the newly increased recoverable reserves, and their proportion increased from 71% in the 1980 s to 96% in the 2000 s. According to the established function of reserve growth, it is estimated that there will be about 4 366×108 tons of reserve growth in the giant oil fields discovered previously in the Middle East, suggesting great potential of reserve growth.

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Effect of pore structure on methane sorption capacity of shales

Hou Yuguang; He Sheng; Yi Jizheng; Zhang Baiqiao; Chen Xuehui; Wang Yi; Zhang Jiankun and Cheng Chunyang

, 2014, 41(2): 248-256.

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Samples from the lower Paleozoic marine shale and Mesozoic continental shale in the west of middle Yangtze region were taken to investigate the effect of pore structure characteristics on the methane sorption of shale using field emission scanning electron microscope (FE-SEM), low pressure N2 isotherm analysis and high pressure methane sorption analysis. A mass of organic matter pores (mostly with pore diameter less than 50 nm) have been found in the high thermal evolution marine shale rich in organic matter. The positive correlation between TOC, N2 BET surface areas and sorption capacity in shale indicate that micro-porosity associated with organic matter is the key factor controlling methane sorption capacity of high thermal maturity shale rich in organic matter. The development of organic matter pores was limited in the organically lean marine shale and continental shale due to the lower TOC content and lower thermal maturity respectively, and their reservoir space is composed of inorganic matter pores in 30 nm to 4.5 μm diameter. Providing bigger specific surface area for methane sorption, the pores within or between clay particles is an important factor to affect methane sorption capacity. With the increase of thermal maturity, the major pore system of shale reservoir changes from inorganic matter pores to organic matter pores, favorable for the improvement of methane sorption capacity of shale.

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23 April 2014, Volume 41 Issue 2