23 June 2015, Volume 42 Issue 3

    

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油气勘探
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  Tectonic features of Gaoshiti-Moxi paleo-uplift and its controls on the formation of a giant gas field, Sichuan Basin, SW China

  Wei Guoqi; Yang Wei; Du Jinhu; Xu Chunchun; Zou Caineng; Xie Wuren; Wu Saijun and Zeng Fuying

  . 2015, 42(3): 257-265.

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  Based on the latest geological, seismic, drilling and outcrop data in the Sichuan Basin, the geological structure and evolution history of Gaoshiti-Moxi paleo-uplift was studied to find out controlling factors of the Sinian-Cambrian Anyue giant gas field. The Gaoshiti-Moxi paleo-uplift is an N-S trending syndepositional paleo-uplift related to the Tongwan movement. The top of Sinian Dengying Formation and adjacent strata in the central part of the paleo-uplift, namely the Gaoshiti-Moxi Area, has remained in the relatively high position since the Sinian, and a giant trap structure is developed independently in the area, which is different from Caledonian Leshan-Longnüsi paleo-uplift in development time, geological structure and evolution history. The Gaoshiti-Moxi paleo-uplift controls the development and distribution of the Sinian-Cambrian Anyue giant gas field, which is shown in the following aspects: (1) The western part of the paleo-uplift is adjacent to the source rock center of Cambrian Maidiping and Qiongzhusi Formations in Mianzhu-Changning intracratonic taphrogenic trough, and the source rocks of Sinian Dengying Formation and Cambrian Qiongzhusi Formation also occur in the Gaoshiti-Moxi paleo-uplift itself; (2) the paleo-uplift controls the formation and distribution of the high-quality reservoirs of the fourth and second members of Sinian Dengying Formation and Cambrian Longwangmiao Formation; and (3) there develop 3 sets of reservoir-seal assemblages which provide favorable conditions for the formation of the giant gas pool from in-situ cracking of paleo-oil reservoirs in the Sinian-Cambrian.
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  Bedrock gas reservoirs in Dongping area of Qaidam Basin, NW China

  Ma Feng; Yan Cunfeng; Ma Dade; Le Xingfu; Huang Chenggang; Shi Yajun; Zhang Yongshu and Xie Mei

  . 2015, 42(3): 266-273.

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  The geological characteristics of bedrock gas reservoirs and the reason for the enrichment and high production of gas in the Dongping area of the Qaidam Basin are studied based on logging data, image log data, core observation, thin section analysis, reservoir microscopic study and cap rock condition evaluation. The main lithology of the bedrock reservoirs in the Dongping area is granite and granite gneiss. The reservoir space mainly consists of fractures, dissolution pores and micro-pores, among which massive matrix micro-pores and dissolution pores are the key factors for the high and stable gas production in the study area. Due to the Tertiary salty environment, the fractures and pores 0 to 18 meters from the bedrock top are filled with gypsum and calcite, forming good “top-sealing” cap rock, this special reservoir-cap rock combination in wide distribution results in the high production of these gas reservoirs. There are two types of gas reservoirs: one is fracture-pore reservoirs at the top of the bedrock, mainly distributed 20-50 m below the “top sealing” cap rock, strongly controlled by tectonic background, and high and stable in gas production; the other is fracture-pore reservoirs inside the bedrock, large in gas-bearing depth, great gas-bearing differences, abrupt change in lateral direction, and high but not stable in production.
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  Paleofluid restoration and its application in studies of reservoir forming: A case study of the Ordovician in Tarim Basin, NW China

  Shan Xiuqin; Zhang Baomin; Zhang Jing; Zhang Liping; Jia Jinhua and Liu Jingjiang

  . 2015, 42(3): 274-282.

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  Karst reservoir paleofluid types of the Ordovician formation in the Tarim Basin are restored based on the analysis of element boron of filled mud, the test of fluid inclusions homogenization temperature and salinity, liquid anion and hydroxyl isotope of filled calcite in vug-fractures, and regional geologic background. The origin of the karst reservoirs are analyzed on this basis. The element boron contents of mud filled in vug-fractures are less than 80 μg/g generally; fluid inclusions have different homogenization temperatures in different regions, and the salinities are in a large range. The HCO3- contents are high, and the Cl- and SO42- contents are dispersive in the liquid component of the fluid inclusions. The hydroxyl isotope contents are relatively dispersive, with relatively negative δD value and positive δ18O value. This evidence shows that the paleofluid of the Ordovician was from supergene atmospheric freshwater, buried fresh-brackish mixed water, seawater or concentrated seawater, and buried brine from underlying dolomite or evaporate rock of the Cambrian formation. The main constructive diagenesis for the formation of vug-cave type reservoirs is erosion and dissolution caused by atmospheric freshwater, and the reservoirs have been subjected to thermal fluid reformation from underlying evaporate rock of the Cambrian during burial stage.
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  Restoration of paleokarst geomorphology of Sinian Dengying Formation in Sichuan Basin and its significance, SW China

  Liu Hong; Luo Sicong; Tan Xiucheng; Li Ling; Lian Chengbo; Zeng Wei; Luo Bing and Shan Shujiao

  . 2015, 42(3): 283-293.

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  Based on newly drilled well data in the Gaoshiti area in the central Sichuan Basin, profile data of more than 150 field outcrops of the regional geological survey, and stratigraphic division and correlation of more than 30 wells in the Sichuan Basin and its adjacent areas, combined with regional seismic data, moldic methods are comprehensively used to restore the karst paleogeomorphy of the Dengying Formation, and thus studying the paleogeographic pattern and the significance of oil and gas exploration. The Sichuan Basin was surrounded by paleo-lands/underwater highlands in the late Sininan Dengying period, including Kangdian paleo-land in the west, Songpan paleo-land in the northwest, Hannan paleo-land in the north, Qianjiang-Zheng’an, Zhenba and Wuxi-Jianshi underwater highlands in the southeast and northeast. The Sichuan Basin was adjacent to Jiangnan Basin southeastwards and Qinling paleo-ocean northeastwards respectively. Affected by the separation of NS-striking Zitong-Junlian Aulacogen, NE-striking Langzhong-Tongjiang and Chongqing- Kaixian depressions in this basin, the Sichuan Basin presents the NS-trending framework of “three uplifts (Zhenba, Chuanzhong and Qianjiang-Zheng’an) and two depressions (Langzhong-Tongjiang and Chongqing-Kaixian)”, and is divided into two relatively isolated EW-trending paleo-uplift systems (NS-striking Mianyang-Leshan-Xichang paleo-uplift and nearly NE-striking Chuanzhong paleo-uplift). Controlled by karst paleogeomorphy of the Sinian Dengying Formation, the pattern of karst landscape consists of five secondary geomorphic units, such as karst highland, karst platform, karst slope, karst depression and karst basin, of which the karst platform and karst slope are the favorable zones for the development of karst reservoirs, providing advantages for the formation of large gas fields.
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  Micro-occurrence of Cretaceous tight oil in southern Songliao Basin, NE China

  Gong Yanjie; Liu Shaobo; Zhu Rukai; Liu Keyu; Tang Zhenxing and Jiang Lin

  . 2015, 42(3): 294-299.

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  Combined experiment method of Environmental Scanning Electron Microscope (ESEM) and Energy Dispersive Spectroscopy (EDS) was proposed to detect tight oil occurrence in micro- and nano-pores of tight sands in Member 4 of Cretaceous Quantou Formation. Observation and analysis of 168 measurement points and seventeen samples from seven typical wells shows that the tight oil occurrences in micro- and nano-pores have two main forms: oil film and oil droplet, oil film is dominant. Intra-granular pores, inter-granular pores and micro-fractures are three kinds of micro storage space, mainly inter-granular pores. The oil films in the intra-granular pores and micro-fractures are irregular and adhesive, and the size is about (1-5 μm) × (1-5 μm), the carbon mass percentage of oil films are mainly 40%-90%. The size of oil droplets is (0.2-1.0 μm) × (0.2-1.0 μm), with relatively small occurrence space. The carbon mass percentage of oil droplets are mainly 15%-30%. Reservoir types and distribution of pores control tight oil occurrence. The carbon contents and thicknesses of oil film decrease from type I to type III reservoirs. The carbon mass percentage and thicknesses are controlled by the median pore throat radius and reservoir quality coefficient.
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  Quantitative evaluation of fault-caprock sealing capacity: A case from Dabei-Kelasu structural belt in Kuqa Depression, Tarim Basin, NW China

  Fu Xiaofei; Jia Ru; Wang Haixue; Wu Tong; Meng Lingdong and Sun Yonghe

  . 2015, 42(3): 300-309.

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  Based on the tests of rock mechanics characteristics and the anatomy of field outcrop, the paper analyzed the deformation mechanism of faults in anhydrite-salt caprock and glutenite reservoir in Dabei-Kelasu structural belt in Kuqa Depression, Tarim Basin, and studied the sealing mechanism of fault as well as established the comprehensive quantitative evaluation chart of gas preservation conditions. With the increase of depth, anhydrite-salt rock transforms from brittle stage to brittle-ductile or ductile stage, the depth for brittle to brittle-ductile transition of anhydrite-salt rock in Kuqa depression is about 1 740 m, brittle-ductile to ductile transition is about 3 400 m. Faults in brittle anhydrite-salt rock deform and form through-going faults, using caprock juxtaposition thickness by fault to characterize vertical sealing ability of the faults; faults deformed in brittle-ductile anhydrite-salt caprock form smear, the smear factor (SSF) of 3.5 is the critical value of anhydrite-salt smear from continuous to discontinuous status, which becomes a standard of determining vertical sealing of the faults in brittle-ductile anhydrite-salt rock; faults are generally hard to cut through ductile anhydrite-salt rock. Fault forms a fault zone of fault breccia type after deformation in dense glutenite, has no sealing capacity, the amount of gas gathered in the fault traps is controlled by the lithology juxtaposition amplitude of both walls of fault, the minimum juxtaposition amplitude within the trap determines the gas-water contact and gas column height of the fault trap. Comprehensively considering the four factors, the brittle-ductile transition depth of the anhydrite-salt rock, critical juxtaposition thickness of fault and SSF value as well as the minimum juxtaposition amplitude within the trap, the comprehensive quantitative evaluation chart of gas preservation conditions in Dabei-Kelasu structural belt, Kuqa depression was established, which provided a reasonable basis for fault trap risk assessment.
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  Influence of overpressure on reservoir compaction and cementation: A case from northwestern subsag, Bozhong sag, Bohai Bay Basin, East China

  Shi Liang; Jin Zhenkui; Yan Wei; Zhu Xiao’er; Xu Xinming and Peng Biao

  . 2015, 42(3): 310-318.

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  Based on data from core analysis and thin section, the influence of overpressure on the compaction and cementation of the Paleogene Dongying Formation reservoir is studied quantitatively in the northwestern subsag of the Bozhong sag, Bohai Bay Basin. Reservoir compaction is inhibited obviously by overpressure because the compaction strength of sandstones in overpressure setting is weaker than that of its overlying sandstones in normal setting. The primary porosity of sandstones is preserved about 1.1% as pore pressure is above hydrostatic pressure every 4 MPa in overpressure setting. Moreover, reservoir cementation is affected by overpressure: cementation strength is strong in overpressure setting and the adjacent inner-pressure setting, while it declines rapidly in outer-pressure setting far away from the overpressure. The thickness of zone with strong carbonate strength is thinner than that with strong authigenic clay strength. Differential carbonate cementation presents “build-up effect in fine grain”, which means that carbonate is prior to generate in sandstones of fine size, and causes that physical properties of sandy reservoir are different in the same overpressure setting. The results show that outer-pressure setting is the most favorable zone for preserving primary porosity, overpressure setting is the secondary, and iner-pressure setting is relatively poor.
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  Depositional models in Cretaceous rift stage of Fula sag, Muglad Basin, Sudan

  Wu Dong; Zhu Xiaomin; Li Zhi; Su Yongdi; Liu Yinghui; Zhang Mengyu; Song Jianfeng; Liu Aixiang; Chen Xiangyi and Zhao Dongna

  . 2015, 42(3): 319-327.

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  According to the study of petrology, log-phase, seismic facies, and sandstone distribution, the distribution of the Cretaceous rift sedimentary system in Fula sag of the Muglad Basin have been predicted, and the sedimentary models in two rift periods have been established. There developed eight 3rd-order sequences in the two rift periods in the Cretaceous of Fula sag, which have 5 types of sedimentary facies, namely, braided river delta, meandering river delta, fan delta, lacustrine, and turbidite fan. There mainly developed fan delta-meandering river delta facies in the west steep slope, braided river delta-meandering river delta in the east step-fault, and lacustrine and turbidite fan facies in the center of the sag respectively in the Fula sag. It is found from comparison of the depositional models in the two rift periods that, in the depositional stage of Abu Gabra Formation in early Cretaceous, the rifting activity was strong, the distribution of sedimentary system was mainly affected by syn-sedimentary faults, and there were preferential channels and areas of sediments transportation and accumulation; in the depositional stage of Darfur Group in late Cretaceous, the rifting activity weakened, the distribution of sedimentary system was less affected by syn-sedimentary faults, and the sediments were in wide distribution.
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  Three-dimensional carbonate reservoir geomodeling based on the digital outcrop model

  Qiao Zhanfeng; Shen Anjiang; Zheng Jianfeng; Chang Shaoying and Chen Yana

  . 2015, 42(3): 328-337.

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  To better know the spatial distribution and architecture of carbonate reservoirs, three-dimensional carbonate reservoir geologic modeling based on the digital outcrop model (DOM) is proposed. Based on the traditional geologic study of outcrops, combined with digitizing the outcrop walls by utilizing the advanced instrument (LIDAR, RTK-GPS, GPR, Gigapan, etc), DOM is built, from which geological information based on measured sections and samples (litho-facies, porosity, permeability, sonic velocity) is extracted and used to build the 3-D outcrop reservoir geologic model by modeling software. Eventually the 3-D reservoir geologic model of outcrop is used to guide the subsurface research. The DOM-based 3-D reservoir geologic model for oolitic reservoirs of Triassic Feixianguan Formation in Yudongliang outcrop, NW Sichuan Basin, reveals more realistic spatial distribution of litho-facies, porosity and permeability, and their relationship, consequently providing more reliable evidence for seismic data interpretation and reservoir prediction of subsurface reservoirs with similar geological conditions.
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  An efficient multi-scale waveform inversion method in Laplace-Fourier domain

  Hu Ying; Zhang Dong; Yuan Jianzheng; Huang Shaojian; Yao Di; Xu Ling; Zhang Cai and Qin Qianqing

  . 2015, 42(3): 338-346.

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  Aiming at the problem that large computational resources and long computation time are required for the conventional Laplace-Fourier domain waveform inversion, an efficient multi-scale grid algorithm with variable computed area is proposed, and used in inversion modeling of the Marmousi and Overthrust model. This algorithm can choose a proper grid spacing automatically according to the different frequency, and adjust the depth of computing area according to the Laplace damping constant. This algorithm not only improves inversion efficiency significantly without the loss of inversion precision, but also improves the stability due to the decrease of grid number. Inversion results of the Marmousi and Overthrust model demonstrate the validity of the algorithm. In addition, the inversion results by the algorithm still can be approximate to the real model when low frequency information is missing.
油气田开发
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  Development features and affecting factors of natural depletion of sandstone reservoirs in Sudan

  Mu Longxin; Wang Ruifeng and Wu Xianghong

  . 2015, 42(3): 347-351.

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  Most reservoirs in Sudan are medium to high porosity and permeability sandstone reservoirs, these reservoirs have been developed by natural depletion since put into production, and the development is characterized by sparse wells of high production, big pressure differential, delayed infill drilling and water flooding, and rapid investment recovery. H field, FN field and P field are bottom water drive light oil field, bottom water drive heavy oil field and stratified high pour point oil field respectively, and they are representative fields in Sudan. The production performance of the three oil fields features sparse well spacing and high plateau rate, short stable production period, rapid water cut increase and fast production decline. Commingled production results in poor inter-layer development and complicated residual oil distribution. On the basis of the above analysis, major affecting factors of Sudan sandstone reservoirs natural drive have been identified through lab experiments, field development plan and field monitoring. The high off-take rate is conducive to the increase of the contract period recovery and recovery factor; sparse well spacing based on crude mobility range and determining infill well production cutoff considering contract terms can be helpful for cost-effective development; barriers and inter-layers can be made use of to detain bottom water coning and to enhance the development effect of bottom water oilfields; and delayed water injection in stratified high pour point reservoirs has no effect on recovery factor during contract period.
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  Effects of oil recovery rate on water-flooding of homogeneous reservoirs of different oil viscosity

  Zhao Lun; Chen Xi; Chen Li; Cao Renyi; Zhang Xiangzhong; Liu Jia and Shan Fachao

  . 2015, 42(3): 352-357.

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  Based on physical simulation of water-flooding homogeneous reservoirs, the water-flooding characteristics of homogeneous reservoirs with different oil viscosity are examined at different oil recovery rate. Reservoirs with low-viscosity (<5 mPa×s) oil can be evenly swept, with thick streamline. With increasing oil recovery rate, water rush weakens along the reservoir bottom and sweeps the reservoir more evenly in the vertical direction; and the sweep efficiency difference between top and bottom of the reservoir decreases. In high-rate development of the low-viscosity oil reservoir, the water-free recovery percent is significantly higher than that in low-rate development, and the rising velocity of water cut is lower than that under low-rate development, which proved that such reservoirs are suitable for high-recovery-rate development. For reservoirs with medium-high viscosity (5-50 mPa×s) oil, the injected water fingers significantly in the water-flooding process, with thin streamline, the coverage is not swept completely, especially in area between streamlines, the sweep efficiency difference between top and bottom is great. As the oil recovery rate increases, the streamline becomes thinner, the coverage becomes more incomplete, and the sweep efficiency of top and bottom both decreases. Medium to high-viscosity oil reservoirs developed at high rate have a short water breakthrough time, and the recovery percent in the water-free period is much lower than that in low rate development, and the rising velocity of water cut is higher than that under low-rate development, so high-rate development is not adaptable for medium-high viscosity reservoirs.
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  A new screening method of low permeability reservoirs suitable for CO2 flooding

  Wang Gaofeng; Zheng Xiongjie; Zhang Yu; Lü Wenfeng; Wang Fang and Yin Lina

  . 2015, 42(3): 358-363.

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  Since the existing gas flooding reservoir screening criteria lack economic indexes reflecting future dynamic production performances, indexes reflecting economic profits are added into the exiting criteria (related single well production indexes) to form a new method of selecting low permeability reservoirs suitable for CO2 flooding. Reservoir engineering methods of single well peak oil production rate (OPR) prediction for gas drive tight reservoirs is given by employing the concept of “OPR multiplier due to gas flooding”. Based on the technical economics principles, the method calculating the economical limit OPR of CO2 flooding is also presented. On this basis, a new screening criterion of reservoirs suitable for CO2 flooding is proposed: if the peak OPR predicted by reservoir engineering method is higher than the economic limit OPR, the target reservoir is suitable for CO2 flooding. Furthermore, a four-step reservoirs screening method is advanced: technical screening, economic screening, feasibility evaluation, recommendation of optimal gas flooding blocks. The new screening criteria were applied to evaluate the CO2 flooding potential of seventeen blocks in an oilfield, which ended up with only 32.4% of the geologic reserves from conventional method suitable for CO2 flooding. It is recommended blocks suitable for CO2 flooding be selected according to the new criterion to ensure economic success.
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  Intercalation pattern and its impact on development of braided river reservoirs: A case of Fengcheng Oilfield, Junggar Basin, NW China

  Li Haiyan; Gao Yang; Wang Yanjie; Sun Xinge; Yang Zhi and Zhao Rui

  . 2015, 42(3): 364-373.

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  Through comprehensive analysis of outcrops, core data and well logs, the type, formation mechanism and distribution pattern of intercalations in the braided river reservoirs of Fengcheng Oilfield, Junggar Basin are studied based on the reservoir architecture, and the inter-well distribution is predicted. The intercalations in Fengcheng Oilfield are divided into four types: intercalation in a bar, intercalation between bars, channel-bar transitional intercalation and chute. The intercalations include two lithologic types: argillaceous sandstone and fine grains. The intercalations are recognized in each individual well by core scale logging. Inter-well prediction is achieved by training image of intercalations established by multi-point statistics. The result indicates that the intercalations are not the uniform size and are scattered with lentoid distribution of various thicknesses. The horizontal wells in test area B are designed by the guidance of intercalation 3-D model, and practice indicates that the intercalation prediction is of great importance for improving oil production.
石油工程
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  Solid expandable tubular patching technique for high-temperature and high-pressure casing damaged wells

  Li Tao

  . 2015, 42(3): 374-378.

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  In view of the issue that casing patching by solid expandable tubular (SET) has a low success ratio in high temperature (HT) thermal recovery wells or high pressure (HP) water injection wells, four key techniques of SET were studied theoretically and experimentally, and the casing patching SET tool for the HT/HP wells was designed. The expandable tubular with post expansion mechanical properties reaching API N80 steel grade was developed. The loading plane angle of expandable connecting thread was optimized as -9°. The sealing piece of inlaid welded copper and the cone coated with tungsten carbide were developed. Based on these researches, a prototype SET patching tool for HT/HP wells has been manufactured. The SET patching tool for HT/HP wells was manufactured. Laboratory experiments demonstrated that the expansion force of the patching tool was between 25 MPa and 32 MPa, the pressure resistance in three periods of alternating temperature load was over 15 MPa, the sealing capacity exceeded 35 MPa, all were up to the designed standard. The field tests in 45 wells in the Liaohe and Tuha oil fields demonstrated that this technique has good adaptability in casing patching in high temperature thermal recovery wells or high pressure water injection wells. After the patching operation, the water pressure 15 MPa was maintained for 30 minutes to test the tool’s sealing performance, the pressure drop was less than 0.2 MPa, and the success ratio of one-time construction was 100%. After the casing patching, the oil wells production increase significantly and remarkable economic benefits are achieved.
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  Calculation of micro-annulus size in casing-cement sheath-formation system under continuous internal casing pressure change

  Chu Wei; Shen Jiyun; Yang Yunfei; Li Yong and Gao Deli

  . 2015, 42(3): 379-385.

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  The change of internal casing pressure could result in the failure of the cement sheath or the occurrence of the micro-annulus at the interface between the cement and the casing, or at the interface between the cement and the formation due to plastic deformation of the cement. In a the hope to estimate the size of the micro-annulus, a theoretical model based on Mohr-Coulomb yield criterion which considers the interaction among casing, cement sheath and formation is built. The plastic behavior of the cement sheath and the bonding strength at two interfaces are taken into account. Particularly, the initiation and the development of micro-annulus at two interfaces (casing-cement sheath interface and cement sheath-formation interface) are analyzed. This model is further used to simulate Jackson and Murphey’s[1] experiment, which studied the gas channeling in the annular space. Good agreement is found in general. Results show that both loading and unloading processes contribute to the initiation of the micro-annulus. Loading process may lead the cement sheath into plasticity. The radial stress at the interfaces turns into tensile stress during the unloading process. Micro-annulus could appear at both interfaces if the tensile stress exceeds the corresponding bonding strengths. If the bonding strengths at two interfaces are of similar magnitude, the micro-annulus would be more likely to appear at the casing-cement sheath interface due to its higher tensile stress. This model can be used to evaluate the risk of cement sealing failure, especially during the hydraulic fracturing, and lower the risk of zonal isolation failure.
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  Mechanism of pressure management by injecting nitrogen in casing annulus of deepwater wells

  Zhou Bo; Yang Jin; Liu Zhengli; Luo Junfeng; Huang Xiaolong; Zhou Rongxin and Song Yu

  . 2015, 42(3): 386-389.

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  The influence factors of annular pressure buildup are analyzed, and the temperature and pressure characteristics of annulus trap medium are simulated for deepwater wells. The method of pressure management by injecting nitrogen in casing annulus is proposed based the experiment result. Limited by the well structure and subsea system,?long casing annulus exists between?technical casing and production casing and it is filled with water-based, synthetic based or oil-based drilling fluid.?In the process of oil/gas test and production, the temperature of the trapped fluid rises significantly under the influence of the well-bore fluid and the trap pressure buildup appears because of liquid heat expansion. Experiments show that isothermal compressibility coefficient and thermal expansion coefficient are the key influencing factors for annulus pressure buildup. Trap pressure is very sensitive to the type of trapped medium (liquid, gas). Injecting 5%-20% volume fraction of nitrogen into the annulus can effectively control the annulus pressure build-up, and avoid casing collapse. Field practice shows that the method, convenient and highly reliable, can ensure the borehole safety during testing and production of deepwater oil and gas.
综合研究
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  A prediction of borehole stability while drilling preliminary prospecting wells based on seismic impedance

  Wu Chao; Liu Jianhua; Zhang Dongqing; Chen Xiaofeng and Zhao Weijie

  . 2015, 42(3): 390-395.

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  When the conventional method of predicting borehole stability is applied to the preliminary prospecting wells, the precision of prediction is often affected severely by the lack of data, limit of applying conditions, error of calculated parameters and complexity of operating courses. This paper presents a method of predicting borehole stability while drilling preliminary prospecting wells and establishes a nonlinear model including wave impedance and borehole stability mechanical parameters by investigating the quantitative relationships between wave impedance and pore pressure, in-situ stress as well as rock strength. Based on the nonlinear model, neural network algorithm is used to identify the relationship between the wave impedance and the three formation pressures (pore pressure, fracture pressure and collapse pressure). Through the establishment of the model by layered neural network and the timely analysis of logging data, the wellbore stability before the bit can be predicted while drilling by using the data of seismic wave impedance. Field application in preliminary prospecting wells shows that the new method has higher adaptability, faster calculation speed, and simpler operational procedure than conventional methods, and its prediction accuracy can meet the requirement of engineering.
学术讨论
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  Tight oil occurrence space of Triassic Chang 7 Member in Northern Shaanxi Area, Ordos Basin, NW China

  Li Haibo; Guo Hekun; Yang Zhengming and Wang Xuewu

  . 2015, 42(3): 396-400.

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  Low-temperature adsorption experiment, high-speed centrifugal gas displacing water experiment and nuclear magnetic resonance (NMR) experiment were conducted on?sealed?coring samples taken from Triassic Yanchang Formation Chang 7 tight oil?reservoir of Northern Shaanxi Area, Odos?Basin, to analyze the tight oil occurrence space quantitatively. Micro-capillary bound water T2 (transversal relaxation time) spectra after 2.76 MPa centrifugation and distribution of micro-pores less than 50 nm obtained from low-temperature adsorption experiment both reflect less than 50 nm pore throats consistently, and conversion coefficient C of T2 and pore radius can be computed from comparing their distribution. The conversion coefficient of 15 cores in the study area is 5.80 nm/ms on average. Using C in oil phase T2 spectrum of sealed coring samples, the maximum pore radius of oil occurrence in the study area is 363- 8 587 nm, 3 195 nm on average, and average pore radius of oil occurrence 50-316 nm, 166 nm on average, and main pore radius of oil occurrence is 97-535 nm, 288 nm on average. Tight oil mainly exists in nanometer pores.