Petroleum Exploration and Development

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Formation and geological characteristics of deep giant gas provinces in the Kuqa foreland thrust belt, Tarim Basin, NW China

Du Jinhu; Wang Zhaoming; Hu Suyun; Wang Qinghua and Xie Huiwen

, 2012, 39(4): 2049-0.

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A series of seismic exploration technologies, such as the “wide line + big combination” mountainous seismic acquisition, highly steep structure pre-stack depth migration processing, and deep tectonic modeling, were used to explore Kuqa deep structures. Pre-salt deep structures were preliminarily confirmed and a group of large structural traps were found. The research shows that deep strata of the Kuqa foreland thrust belt are favorable for forming giant gas accumulations. Large-scale imbricated thrust structures prepare traps for large-scale oil and gas accumulation; excellent hydrocarbon source conditions and later intense charge of gas provide sufficient hydrocarbon sources; deep mass effective reservoir sandstone works as good reservoir space for giant gas accumulation; thick gypsum serves as good cap. The geological characteristics of giant gas accumulations in deep pre-salt structures are as follows: Hydrocarbon distribution is controlled by structural traps and structural gas reservoirs are dominant; reservoir properties of deep thrust belt are generally poor, with fractures being the important controlling factor for hydrocarbon enrichment; oil and gas distribution is characterized by joint control of source and cap, stacking of multi-layer beds, and overall beds containing gas. Reserves are large, and single well productivity is high.

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Formation fluid characteristics and hydrocarbon accumulation in the Dongying Sag, Shengli Oilfield

Zhang Shanwen; Zhang Linye; Bao Youshu; Li Xiaoyan; Liu Qing; Li Juyuan; Yin Yan; Zhu Rifang and Zhang Shouchun

, 2012, 39(4): 2050-0.

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Based on the analysis of geochemical characteristics and distribution of different formation waters, a new origin identification standard was established for the formation water in the Paleogene Shahejie Formation in the Dongying Sag. The migration paths of the connate water expelled from source rocks are similar to those of the hydrocarbons coming from the same rocks, so the geochemical characteristics of the connate water coming from the source rocks can be used as auxiliary indexes to study hydrocarbon migration. Considering fluid pressure and formation water distribution, the Paleogene fluids are divided into three fluid systems: normal-pressure open fluid system, high-pressure closed fluid system, and transition fluid system. The differences in hydrocarbon-bearing fluid characteristics, hydrocarbon migration dynamic, hydrocarbon-bearing fluid flow type between these fluid systems were studied. According to characteristics of the hydrocarbon-bearing fluid charging into traps, and formation water drainage pattern, three typical oil accumulation models were classified. In the high-pressure closed fluid system, the main oil accumulation mode is high oil saturation fluid entering traps and displacing formation water, and “continuous” reservoirs are prone to form, and fluid oil saturation, fluid charging pressure and fluid seeping conditions affect reservoirs’ oil accumulation extent. In the normal-pressure open fluid system, the main oil accumulation modes include low oil saturation fluids enter a trap while formation water is overflowing out of the trap, and low oil saturation fluids enter a trap while formation water is seeping out of the trap. The amount of hydrocarbon-bearing fluid, fluid oil saturation, trap height, and caprock sealing ability affect reservoir’s oil accumulation extent.

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Tectonic evolution of the Leshan-Longnüsi paleo-uplift and its control on gas accumulation in the Sinian strata, Sichuan Basin

Xu Hailong; Wei Guoqi; Jia Chengzao; Yang Wei; Zhou Tianwei; Xie Wuren; Li Chuanxin and Luo Beiwei

, 2012, 39(4): 2051-0.

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Based on structural interpretation of the recent seismic data in central Sichuan Basin, combined with data of drilling and outcrops around the basin, this paper discusses the evolutional characteristics of unconformities at various stages in the Leshan-Longnüsi paleo-uplift, and studies the evolution of the paleo-uplift and its control on Sinian gas reservoir accumulation. Sinian, Paleozoic, Mesozoic and other major regional unconformities were developed in the Leshan-Longnüsi paleo-uplift. According to characteristics and denudation thickness of the unconformities, the paleo-uplift tectonic evolution is divided into four stages, namely, embryonic, developmental, stable burial, and finalization. The analysis of the effects of the tectonic evolution on the Sinian gas accumulation show that: (1) the unconformities controlled the development of ancient karst reservoirs relevant to paleo-weathering crust; (2) the tectonic evolution of the paleo-uplift controlled evolution of organic matter and hydrocarbon migration and accumulation; (3) morphological changes of the paleo-uplift controlled the formation and the mergence of structural traps.

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Hydrocarbon accumulation mechanism of low permeable tight lithologic oil reservoirs in the Yanchang Formation, Ordos Basin, China

Guo Yanru; Liu Junbang; Yang Hua; Liu Zhen; Fu Jinhua; Yao Jingli; Xu Wanglin and Zhang Yanling

, 2012, 39(4): 2052-0.

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By analyzing the hydrocarbon accumulation conditions under the guideline of sequence stratigraphy, the hydrocarbon accumulation mechanism in the low permeability, tight reservoirs of the Yanchang Formation in the Ordos Basin is studied from the perspective of pore evolution, and the hydrocarbon accumulation evolution models are concluded. The low permeability, tight reservoirs of the Yanchang Formation are distributed within the range of good hydrocarbon source rocks. The reservoir sand-bodies are dominated by lacustrine gravity flow deposits and delta front deposits. In the early period when hydrocarbons are accumulated in the Yanchang Formation, the reservoirs are characterized by high porosity and high permeability. The oil and gas generated in the period are accumulated normally under the effect of buoyancy. In the middle-late period, the reservoirs have experienced continuous compression. The petroleum generated in the late period, the major hydrocarbon generation period, migrates and accumulates by infiltration along the previously remained paths under the effect of buoyancy and pressure difference between the source rocks and the reservoirs. When the reservoirs become tight, oil and gas stop migration. From the Late Jurassic to the middle period of the Early Cretaceous, oil and gas widely charge the high porosity sandstones near the source rocks when they migrate from the center of the lacustrine basin to the borders. In the late period of the Early Cretaceous, the diagenesis of sandstone reservoirs is enhanced, making the reservoirs tight. After the Late Cretaceous, the present low permeability tight lithologic reservoirs are formed due to the compressing and uplifing movement of the structure which dips down in the west and tilts up in the east.

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Evolution of continental formation pressure in the middle part of the Western Sichuan Depression and its significance for hydrocarbon accumulation

Guo Yingchun; Pang Xiongqi; Chen Dongxia; Leng Jigao and Tian jun

, 2012, 39(4): 2054-0.

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Based on the distribution features of present formation pressure, analysis on logging data, and genetic mechanisms of abnormal pressure, the overpressure evolution was reconstructed using numerical basin simulation and other quantitative analysis methods. The phase differences of pressure increasing mechanisms, enhancement differences among different mechanisms, and the control of geopressure evolution on gas migration and accumulation were discussed. The causes of overpressure in the Xujiahe Formation include under-compaction (from early Late Triassic to the end of the Jurassic), hydrocarbon generation (from end of the Early Jurassic to the Neogene) and tectonic compression (since the Cretaceous). It is suggested that tectonic compression and hydrocarbon generation are the principal factors of the present overpressure in the Xujiahe Formation. Overpressure transmission is the main cause of overpressure in the Jurassic strata which occurred during the intense tectonic activity periods since the Cretaceous. The overpressure is the main force for hydrocarbon migration and overpressure-related microfractures are the major pathways for gas migration. Fault transport and overpressure driving caused the formation of Jurassic secondary gas reservoirs with distant sources. The areas with high pressure coefficient in the Jurassic and high-overpressure zones in the Xujiahe Formation are optimum targets for exploration.

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Control of oolitic beaches sedimentation and diagenesis on reservoirs in Feixianguan Formation, northeastern Sichuan Basin

He Yunlan; Fu Xiaoyue; Liu Bo; Zhou Minghui; Zhang Xuefeng; Gao Jixian; Yang Yunkun and Shi Kaibo

, 2012, 39(4): 2055-0.

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Based on well correlation, division of sedimentary facies and microfacies, core observation, and analysis of rock thin sections, the authors comparatively analyzed the sedimentation and diagenesis of the oolitic beaches in the Puguang and Yuanba areas of northeastern Sichuan Basin, and expounded their control on reservoirs. In the Puguang area the oolitic beach is thick and lateral migration distance is short, intervals overlaid one another, while in the Yuanba area the oolitic beach is thin and lateral migration distance is long. The two areas both experienced various types of diagenesis. There happened intense early dissolution and wide-spread dolomitization in the oolitic beach of the Puguang area, where type-I reservoirs of high porosity and permeability are developed. In the Yuanba area, strong compaction and pressure dissolution, and late burial cementation led to the development of type-III reservoirs of low porosity and permeability. The main factors that control the sedimentation and diagenesis are paleotopography and changes in sea level. Sedimentation controls the distribution and thickness of reservoir, and diagenesis determines the porosity formation and evolution and ultimately the reservoir quality.

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Structural characteristics of intra-platform shoal in the Leikoupo Formation (T2)in the transitional zone of the central and southern Sichuan Basin

Ding Xiong; Chen Jingshan; Tan Xiucheng; Luo Bing; Wu Jiajie and Du Benqiang

, 2012, 39(4): 2056-0.

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Through comprehensive research on drilling, logging and core data of T2l11 sub-member, which is the major pay of the Leikoupo Formation in the transitional zone of the central and southern Sichuan Basin, the structural characteristics, distribution regularity and its major controlling factors of intra-platform shoal are studied. The major rocks of intra-platform shoal in T2l11 sub-member include oosparite, residual oolitic dolostone, sparrenite, residual doloarenite and sparry doloarenite, sparry bioclastic limestone and residual bioclastic dolostone, which constitute oolitic shoal, arene shoal and bioclastic shoal. The intra-platform shoal is characterized vertically by shallowing-upward sequences, and consists of three sedimentary units, namely bank base, bank core and bank cover. Besides, three microfacies can be distinguished laterally, bank core, bank edge and interbank sea. The variation of water depth and hydrodynamic conditions caused by sea level change control the characteristics and superposed pattern of shoal body. Meanwhile, the palaeogeomorphology with alternating uplifts and depressions defines the differential distribution and lateral combination pattern of intra-platform shoal body.

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Phase change of formation water and the formation of deep basin gas: A case from the Upper Paleozoic, Ordos Basin

Zhou Xinke; Xu Huazheng and Li Lili

, 2012, 39(4): 2057-0.

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The forming process and conditions of Upper Paleozoic gas reservoir in Ordos Basin was analyzed by comparing the water producing characteristics of abnormally high temperature gas reservoir in the Qianmiqiao buried hill with the simulated experiment results of gas/water phases under sealed conditions. The test results of vitrinite reflectance, apatite fission track and inclusion homogenization temperature indicate that the Upper Paleozoic in the Ordos Basin had an abnormally high geothermal field during the Late Jurassic to Early Cretaceous. Driven by the abnormally high temperature, the organic matter matured rapidly to generate a lot of natural gas; meanwhile, the formation water vaporized, with intersoluble gas and vapor to accumulate and generate abnormally high pressure. The gas (vapor) phase fluid migrated towards the upper formation within the compartment under abnormally high pressure, which lowered the pressure in the lower abnormally high temperature formation, sped up the vaporization of formation water and resulted in the accumulation of pressure for a new round of migration. In this circular way, the gas (vapor) phase fluid diffused to every part of the compartment, with inner temperature and pressure being balanced. The high temperature and high pressure gas reservoir at basin level was formed. The uplift and erosion during the Late Cretaceous-Paleogene led to the drop of the temperature and pressure of the Upper Paleozoic. The liquefaction of vapor lowered the vapor and gas concentration. Thus the basin level low-pressure gas reservoir was formed.

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Well spacing optimization for Sulige tight sand gas field, NW China

He Dongbo; Wang Lijuan; Ji Guang; Wei Yunsheng and Jia Chengye

, 2012, 39(4): 2058-0.

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Reasonable well spacing has been studied and assessed to increase the development reserves and improve the recovery factor of the Sulige gas field, where the reserves controlled by individual wells are small. An evaluation methodology for optimizing well spacing in a tight sandstone gas field was established from five aspects including geologic model, drainage radius, interference testing, numerical simulation and economic benefits. The well spacing has been optimized using this method. Through geological modeling, the rational inter-well distance is estimated to be less than 600 m and rational well array distance should be less than 1 000 m. Results of drainage areas estimation reveal that the rational inter-well distance should be ranged from 400 m to 600 m. Analysis of interference well test shows that the low limit for inter-well distance is 400 m and the favorable well array distance is 600 m. Numerical simulation and economic evaluation shows that an inter-well distance of 500 m and well array distance of 700 m are favorable. It is concluded from the above five aspects that, in current economic and technological conditions, the well pattern with inter-well distance at 500 m and well array distance at 700 m is good in the Sulige gas field, and it can improve the recovery factor to 45%.

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Rock fracture kinetics of the fracture mesh system in shale gas reservoirs

Zhao Haifeng; Chen Mian; Jin Yan; Ding Yunhong and Wang Yonghui

, 2012, 39(4): 2059-0.

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The formation mechanism of fracture mesh and the mechanism of activating natural closed fractures during shale fracturing are studied based on rock fracture kinetics. It is found that steering conditions should be satisfied at both left and right ends of a natural fracture to form intensive fracture mesh after the hydraulic fracture reaches the natural fracture; there exists a minimum critical pump rate (critical pump rate) to form intensive fracture mesh. The critical pump rate increases as the inclination of natural fracture increases and reaches a maximum value (constant) when the inclination of natural fracture is 90°. When the inclination of natural fracture is less than 90°, the critical pump rate first decreases and then increases as the angle between horizontal wellbore and natural fracture increases; the critical pump rate reaches a minimum value for the natural fractures perpendicular to the wellbore. The critical pump rate increases as natural fracture length increases and rock elastic modulus decreases. It is also found that natural fracture surface is rough and not fitting, so when hydraulic fractures reopen existing natural fractures, the release of the original shear stress in the fracture surface would lead to the fracture surface slippage, which would greatly enhance the flow conductivity of the natural fracture. Very small or very large inclination of natural fracture is not conducive to fracture activation, best activating result can be reached at 30°-60° inclination. Rise in elastic modulus inhibits fracture surface slippage and Poisson’s ratio has little effect on fracture surface slippage.

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Prediction of water breakthrough time for oil wells in low-permeability bottom water reservoirs with barrier

Zhao Xinzhi and Zhu Shengju

, 2012, 39(4): 2060-0.

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Based on water coning and non-Darcy flow theory, a water breakthrough time prediction equation for low-permeability bottom water reservoirs with barrier is derived by material balance principle, which takes the hemispherical radial flow below barrier and plane radial flow above the barrier of water coning into consideration. The calculated water breakthrough time of an oil producer in the sandstone bottom water reservoir in the Ordos Basin by this equation is close to the real water breakthrough time, with a relative error of only 7.85%. In addition, the water breakthrough time of six wells with natural barrier in c3 reservoir, Block hu152, Ordos Basin, were calculated, among which the predicted water breakthrough time of five wells has a less than 10% relative error with the real water breakthrough time. For low-permeability bottom water reservoirs with barrier, the calculated water breakthrough time considering start-up pressure gradient is smaller and more accurate than that without considering start-up pressure gradient.

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A fluid-solid-chemistry coupling model for shale wellbore stability

Wang Qian; Zhou Yingcao; Wang Gang; Jiang Hongwei and Liu Yushi

, 2012, 39(4): 2061-0.

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A fluid-solid-chemistry coupling model is built considering fluid flow and ion transmission induced by shale-drilling fluid system electrochemical potential osmosis, nonlinearity of flow and solute diffusion in shale-drilling fluid system, and solid deformation resulted from fluid flow and ion transmission. The model is used to compute the time-varying pore pressure, stress and damage index in formations around the wellbore of a well. The result shows that the pore pressure and stress are lower without consideration of the diffusion potential induced by ion selective character of shale membrane. The linear model overestimates the pore pressure and stress fields around the sidewall. The damage index reveals that collapse first occurs in formations near the wellbore, the caving pressure increases and changes obviously with time, the fracture pressure reduces but doesn’t change significantly with time, and the drilling fluid density window narrows down with time.

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Key parameters forecast model of injector wellbores during the dual-well sagd process

Wu Yongbin; Li Xiuluan; Sun Xin’ge; Ma Desheng and Wang Hongzhuang

, 2012, 39(4): 2062-0.

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Based on parameters forecast model of conventional horizontal injector wellbore, combined with coupling calculation of dual-tubing steam mass flow, the mass conservation equation, energy conservation equation and momentum conservation equation of steam flow within injector wellbore under different tubing combinations in preheating and production phases are formulated and the key parameters calculation model of dual-tubing injector wellbore is established. The temperature and pressure along the injector wellbore during the steam-assisted gravity drainage (SAGD) preheating phase of a SAGD well are calculated and compared with the downhole monitored data, which shows good match and verifies the accuracy of the model. Meanwhile, the model calculates that the minimal steam injection rate during SAGD preheating phase is 60 t/d, the longest horizontal length is 564 m under current configuration. Considering the current configuration has disadvantages of two sections of steam imbibition that may result in risk of section/point steam breakthrough at Point A during SAGD production phase, the combination of long tubing and short tubing is optimized: the short tubing is relocated 150 m from Point A and long tubing is located at Point B. Consequently, three sections of steam imbibition could be realized, and the risk of section/point steam breakthrough at Point A is effectively reduced.

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Comparision experiments on steel and non-steel slotted screen pipes used in coalbed methane wells

Huang Zhongwei; Li Gensheng; Yan Xiangzhen; Wang Kailong; Shen Ruichen and Jin Lei

, 2012, 39(4): 2063-0.

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To reduce the current high completion cost for steel slotted screen pipes in coalbed methane wells and to develop low-intensity non-metal completion pipes to perform low-cost completion, this paper adopts electric survey to measure the collapsing strength and bending strength of J55 steel screen pipe and PVC non-metal screen pipe, and makes a comparative analysis of the effect of slot width and slot density on the strength of both kinds of screen pipes. The research results indicate that: the collapse of steel screen pipes mainly results from stiffness reduction or radial unstableness, showing the whole pipe deformation or the slot becoming wide or narrow at the internal or external wall along the short axis, accompanied with the tearing at the slot end-point; while the PVC screen damage mainly results from the material itself. For both steel and PVC screen pipes, the pipe’s collapsing/bending strength declines with increase of the slot width and density, but the PVC screen pipes do not show such an obvious trend, making it possible to apply wider and denser slots on PVC pipes to increase the passing area.

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A model of calculating the circulating pressure loss in coiled tubing ultra-short radius radial drilling

Ma Dongjun; Li Gensheng; Huang Zhongwei; Niu Jilei; Hou Cheng; Liu Mingjuan and Li Jingbin

, 2012, 39(4): 2064-0.

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By analyzing the relationship between measured values of pressure loss in a high-pressure hose and calculated values of metal tubing pressure loss formula under the same condition, a calculation formula of pressure loss in a high-pressure hose is revised on the basis of the existing formula, and a model of calculating the circulating pressure loss in coiled tubing ultra-short radius radial drilling technology is established by theoretical derivation. The effects of tubing diameter, tubing length, pump delivery and fluid dynamic viscosity on each part of pressure loss in circulation system are studied by the pressure loss calculating model. The pressure loss in coiled and straight parts of 0.025 4 m (1 in) coiled tubing is about 8-10 times bigger than the pressure loss of 0.038 1 m (1.5 in) coiled tubing, and the pressure loss in high-pressure hose accounts for a large proportion in circulation system, about 86% in this study; and with drag reducer added in clear water, the pressure loss is about half of clear water pressure loss under the same condition.

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A model for estimating the drilling and completion investment in offshore oilfields in West Africa and the Asia-Pacific region

Wang Dongjin; Li Xiusheng; Zhang Haiying and Wang Zhen

, 2012, 39(4): 2065-0.

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A BP neural network model for estimating the drilling and completion investment is built based on the BP neural network method with 86 representative offshore oilfields in West Africa and Asia-Pacific as samples. The model uses five factors, including oil price, water depth, well number, well depth and geologic condition, as the input parameters, and outputs the drilling and completion investment parameters. Comparison of the model with a regression analysis model shows that the established model is reasonable and valuable because the BP neural network is an active learning process, able to effectively describe the non-linear relationship between variables and solve complicated problems. The established BP neural network model has high fitting accuracy and the errors of most samples are within 30%, satisfying the requirements for engineering development, and are much smaller than that of regression analysis.

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Methane production pathway in biogas reservoirs via molecular biological technique

Wei Xiaofang; Qin Jishun; Shuai Yanhua; Liu Keyu; Luo Yijing; Shi Yanyao and Zhang Shuguang

, 2012, 39(4): 2066-0.

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To investigate the microbes in formation water samples from Well Du 6-3 and Ao-7 in the Alaxin and Aonan gas fields, Songliao Basin, 16S rDNA was amplified using the Polymerase Chain Reaction (PCR) technique with templates from the extracted DNA and primers of bacteria and archaea. The PCR products were also inserted into vectors and then ligated with competent cells for gene sequencing. Based on the obtained gene sequences, bacterial and archaeal libraries were constructed and microbiological phylogeny graphs were obtained for the samples to understand the structure of microbial community and deduce the possible pathways of methanogenesis. The result indicates that there are only a few types of microbes in the samples of the two wells. Pseudomonas and Methanobacterium are the dominant species in the microbial and archaeal libraries, respectively. Methane generation via indigenous microbial metabolism may be ultimately realized using the right method under favourable conditions in the two fields. It is concluded that the indigenous microbial community in Well Du 6-3 mainly generated methane by CO2 reduction, but methanogenesis of acetic acid could not be completely excluded. The methane from Well Ao-7 is shown to have been definitely converted from CO2.

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, Volume 39 Issue 4