Petroleum Exploration and Development

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Two problems on prediction and evaluation of deep basin gas

LI Mingcheng; LI Xianqi and SHANG Erjie

, 2001, 28(2): 85-0.

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This article points out two problems on prediction and evaluation of deep basin gas in Erdos basin. The first problem is the tight reservoir.The effective reservoir of deep basin gas must have densified before the mass generation and expulsion of hydrocarbons from source rock, so do the “sweet points”.It is difficult to form the deep gas in the whole basin due to the strong heterogeneity of the tight reservoir in continental facies basin. Both the formation of tight reservoir and its “sweet points” should be done “timing research” in prediction and evaluation of deep basin gas. The second is the pressure of deep basin gas. The pressure of deep basin gas evolves from initial normal hydrostatic pressure to abnormal high pressure, then to abnormal low pressure, at last back to normal hydrostatic pressure, it indicates the geological process of deep basin gas, from generation to vanish. The lower the underpressure of deep basin gas, the nearer it vanishes in the time and space. There are many factors for underground fluid to form abnormal low pressure. It is some venturous to predict and evaluate the deep basin gas using equal height gas column to substitute water column, since either there are no more actual measure points of underpressure to control, or there is no guarantee of the continuous expansion of the tight reservoir.

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The reservoir-forming process and model in the Kuqa petroleum system

ZHOU Xingxi

, 2001, 28(2): 86-0.

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According to the difference of reservoir-forming process, the Kuqa petroleum system, belonging to the super-charged system, can be divided into two subsystems, southern and northern subsystems. The forming mechanism and the key factors controlling reservoir-forming process and model of both subsystems are very different, three kinds of the reservoir-forming models—Kelasu Model, Dawanqi Model and Yaha Model can been built up, and the Kelasu Model will be the useful kind in searching large gas pool in the north subsystem which is near the source rocks, its Tertiary cap rock well developed and the Cretaceous reservoir is huge in thickness, good in properties and much in trap number.

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Oil accumulation models and their controlling factors in two-river area of Songliao basin

FU Guang; CHEN Zhangming; FU Xiaofei and YANG Mian

, 2001, 28(2): 87-0.

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By analyzing the type of oil reservoirs and the spatial correlation between reservoir and oil source rock and migration way,the authors put forward four oil accumulation models, they are anticlinal and structure-lithological oil reservoirs formed by the matching between high structural place and sandbody, sand lens oil reservoirs formed by oil primary migration from source rock, fault-lithological oil reservoirs formed through the link between source rock and uncontinueous banding and lenicular sandbodies and the fractured oil reservoirs in mudstone formed by the matching between structural stress and abnormal pore fluid pressure. Based on this, the authors hold that the formation and distribution of oil reservoirs in this aera were mainly controlled by the factors such as mature area of source rock, distribution area of good cap rock, development degree of faults and migration way of hydrocarbon.

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Prospective study on coalbed gas resources in Carboniferous-Permian, West Henan

SU Fuyi; NING Zhengwei and GUO You

, 2001, 28(2): 88-0.

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On the basis of rich data available, the authors systemetically studied the deposition characteristices, gas content potential and permeability of reservior in the Carboniferous and the Permian, West Henan. It is regarded that material conditions to generate coalbed gas and geological conditions are existed. Geological factors that affect coalbed gas exploration and development are emphasized and gas content potential and permeability of reservoir are stated in detial. It is put forward that the main geological factors that affect gas content potential of coal seams are diagenetic metamorphism of coal seams, effective thickness of overburden layer and overlying cover conditions, and permeability of coal seam is mainly affected by coal structure, cleat development and sliding structure. The key geological factors that affect coalbed gas exploration and development in the Carboniferous and the Permian, West Henan are not the abundance of coalbed gas resources and resource volume but reservoir conditions of coal seam. Combined with the permeability of coal seam and geological conditions for coalbed gas storage, the fairway, general area and bad area are divided based on coalbed gas exploration and development prospect. Coalbed gas exploration and pilot development shall be conducted firstly in coal-bearing area in Pingdingshan.

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Formation models of buried hill type pools in Dongpu sag

YUAN Xianchun; ZHAO Xinguo; LIU Xiaohong; SUN Qing and ZHAO Yunfeng

, 2001, 28(2): 89-0.

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Buried hills in Dongpu sag consist of many oil bearing strata, which distribute in the Archaeozoic, Lower Palaeozoic, Upper Palaeozoic and Mesozoic. There are two sets of source rock in the Palaeogene and the Upper Palaeozoic. According to the different ways of hydrocarbon supplying, formation models of beried hill type pools can be divided into four types: accumulated in the Palaeozoic with a Cenozoic origin, generated and accumulated in the Palaeozoic, accumulated in the Palaeozoic with a multisource origin, and accumulated in the Mesonic with a multisource origin. Buried hills can be found in a large scope, and better oil and gas shows have been acquired in all kinds of burried hills through many years exploration in the past,which shows a good prospect in explorating buried hill type pools in Dongpu sag.However,up to now any great discovery has not been achieved.The authors hold that the reason why so far the buried hill reservoirs have not been discovered is that there is no a complete set of drilling engineering technologies,except for geological conditions.

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Geochemical characteristics of crude oil in Dayangshu basin

FENG Zihui; ZONG Rui and WANG Xue

, 2001, 28(2): 90-0.

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According to analyses of carbon number distribution of n-alkane, component carbon isotope, monomer hydrocarbon carbon isotope, biological marker and so on,the geochemical characteristics and origin of crude oil of Well Yang-D1 in Dayangshu basin was studied. The results show that content of saturated hydrocarbon is higher; both contents of aromatic hydrocarbon and non-hydrocarbon are lower in the crude oil component. Carbon number distribution of n-alkane shows that the crude oil of Well Yang-D1 is mature oil. Component carbon isotope of the oil is higher, and both component carbon isotopes of aromatic hydrocarbon and non-hydrocarbon are 5% higher than that of the crude oil of Songliao basin. Monomer hydrocarbon carbon isotope of the crude oil is generally－29‰. The distribution of monomer hydrocarbon carbon isotope shows that the component with carbon number less than 21 are mainly derived from bacteria and algae, and the component with carbon number more than 21 are mainly derived from higher plants. Biological marks of the crude oil contain long lateral chain of tricyclic terpane, and the content of gammacerane is low. The mature parameters of C2920S/20 (S+R) and C29ββ/ (αα+ββ) are respectively 0.46 and 0.49, showing features of mature oil. Synthetic analytic result shows that oil of Well Yang-D1 is mature oil that was generated by humic type organic matter, and has obvious migration effect. From geology and geochemical characteristics of the basin, the authors hold that oil of Well Yang-D1 is probably derived from Jiufengshan Formation of Upper Jurassic.

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Application of EASY%Ro model to the studies of thermal history for Mesozoic basins, western China.

WEI Zhibin; ZHANG Dajiang; XU Huaixian; WANG Feng and ZHANG Jiazheng

, 2001, 28(2): 91-0.

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The vitrinite reflectance (Ro) depends on the temperature and time organic matter undergone in the geological history. Thermal history of organic matter, therefore, can be reconstructed from Ro value. The conventional models and methods to restore thermal history exit many limitations in the early exploration of a basin. On the contrary, the chemical kinetic model of EASY%Ro may exactly reflect the authentic information of thermal history. It is, especially, suitable for the medium to high stages of thermal evolution (Ro>0.9%) , while the paleogeothermal gradients will be underestimated in the low level of thermal stage. For this reason, the paleo-heat histories organic matter experienced in the three wells, such as Well YN-2 of Kuqa depression, Tarim basin, Well YC-1 of Yabulai basin and Well Long-101 of Jiuxi basin, were simulated with EASY%Ro, combining the data of vitrinite reflectance. The results of modeling suggest that, the paleogeothermal gradients take the trend of decrease since Mesozoic in Kuqa depression, Tarim basin, and this relatively remarkable temperature drop conforms to the rapid burial in late Tertiary. Also, it is shown that, compared with Well YN-2, the paleogeothermal gradients of Well YC-1 and Well Long-101 with a little variation tend to be lower, which is unfavorable to the hydrocarbon generation. The paleogeothermal gradients of Well YN-2 are higher than the previous results, which may be resulted from the measurement difference of vitrinite reflectance.

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The application of logging geology analysis to the selection of exploration objectives

ZHANG Longhai; ZHANG Zhiwei; HUANG Hongcai and LIU Zongcheng

, 2001, 28(2): 92-0.

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By using modern log interpretation technology on logging datum about old areas, which have been drilled, great breakthrough may be made in the important petroleum geology issues. Well Moushen is located in Menggangji depression, south of Dongpu seg. It was first drilled in the beginning of 80's. The evaluation of the formation led to the conclusion that Members 3 and 4 of Shahejie Formation are thin and the oil and gas resource are spare in south area of Dongpu seg. Since that time, so little was input in the exploration of it. Well Moushen is still the deepest testing well in this area, having 9 logging curves produced by 3600 log series. During the preresearch on the testing well location selection in the south of western depression, three kinds of process and research work have been done according to the log datum on Well Moushen: (1)Formation analysis and formation pressure evaluation which verdicts that formation under 5198.5m is not of Trias, whereas is still in Permian, and abnormal formation pressure phenomenon exists in Members 1 and 3-4 of Shahejie Formation; (2)Re-interpretation of oil and gas formation and evaluation of reserve formation; (3)Evaluation of oil source rock. Members 1 and 3-4 source rocks of Shahejie Formation have good capability of oil generation. On the basis of the conclusion made on Well Moushen logging evaluation, combined with seismic datum process, comprehensive oil geology evaluation about this area was made. As a result, exploration area increased 1000km2, oil resource volume was forecasted to 1.2×108t, and 8 favorable structures were proposed. Fangliji was selected preferentially and Well Moushen-8 was successfully drilled.

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Calculating the concentration of hydrocarbon gases in rock by mud logging.

CAI Kaiping and WANG Yingrong

, 2001, 28(2): 93-0.

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The concentration of hydrocarbon gases in rock (CHGR) is an important parameter in oil and gas exploration. The theory and method were established herein to calculate CHGR. Hydrocarbon gases in the subsurface exit in free form, by absorbing on organic matter and mineral particles, or by dissolving in formation water. As the drill bit breaks the rock into small cuttings, the gas present is released into the mud stream. As the mud comes out of the hole, a portion is run into a mud-gas separator. The hydrocarbon gas volume (HGV) in the mud depleted to drill 1m layers is calculated from the gas logging data, drilling time, mud displacement, hole size, quantitative gas analysis and instrument's parameters. With C standing for CHGR (cubic meter gas / cubic meter rock ), the HGV in the rock drilled 1m footage is equal to the C multiplied by the penetrated rock volume. C is calculated from the equality between the HGV in the rock and the one in the mud. The variation of CHGR with depth is available from the calibration of the porosity, gas saturation of electric log interpretation of the reservoir and hydrocarbon gases expansion coefficient. The calculating 5-well CHGR of Baimamiao area in Western Sichuan basin supplied the important geological basis for hydrocarbon generation, migration, accumulation and exploration decision.

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The calculatoin of stable production capability of vetically fractured well

JIANG Tingxue; SHAN Wenwen and YANG Yanli

, 2001, 28(2): 94-0.

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The seepage flowing pattern of an oil well with vertical fractures is transferred to a simple single direction flowing one by using the method of conformal transformation. After that, a secondary pressure differential equation of seepage flowing with a character of changeable quality in fracture is derived on the basis of seepage flowing analysis of microelement; and of material balance theory as well as Darcy's law. What's more, the theory of pressure coupling (the pressure is equivalent at the joint of the fracture and the matrix) is also used. Therefore, an equation of estimating oil rate of a hydraulic fracturing well is available which has a various fracture length and conductivity (finite and infinite). Compared with some classical charts and the real postfracturing performance, excellent agreements can be abstained. Furthermore, it is not only simple but also feasible, so that it can be put into field application conveniently.

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A research for the development technology of lithologic oil reservoir in Wellblock Ken-622 of Kenxi oil feild

BI Qiujun; ZHANG Benhua and YANG Xiaomin

, 2001, 28(2): 95-0.

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The central Es3 oil reservoir in Wellblock Ken-622, Kenxi Slope area, is a lithologico-structural composite reservoir that is rather complicated in geologic conditions, which is much difficult to be exploited for many factors constrained. In order to exploit it rationally and reasonably, the authors take advantages of the technology of wave impedance inversion, modern method of well testing and numerical simulation to trace and describe sandbody, reason out the characteristics of oil reservoir and perform research of economic and reasonable well pattern arrangement, suitable maintaining level of formation pressure, selection of water injection and optimization of initial oil production rate etc. The technology the authors mentioned in this paper would help the engineers to make scientific policy decision in development program design. After this method being carried out in practical field trial, there appear somewhat perfect effects-each development well of total 8 reaches its design requirement without exception, and compared with the predictive thickness of sandbody, the average thickness error would be 6%. Remarkable economic benefit has been achieved since this development program was put into effect one year ago, the real production results very much tallies with that from research, now the daily oil production of the wellblock has been reached 255t/d, which shows that it can be considered as a reference for developing similar lithologic oil reservoirs.

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Using interwell tracer test to determine the remaining oil saturation

LI Shuxia; CHEN Yueming; FENG Qihong and ZHANG Qingzhen

, 2001, 28(2): 96-0.

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Interwell tracer test hasn't been widely used for many years because only average parameters from injector to producer can be determined from interwell tracer test data. In order to get more useful information, a three-dimension two-phase (oil, water) four-component (oil, water, partitioning tracer and non-partitioning tracer) mathematical model has been established and the tracer numerical interpretation model has been developed. Based on this model, the pressure, saturation and tracer concentration can be obtained, then the distribution of remaining oil saturation can be obtained by matching the tracer production curve numerically, and this is very important for the later development of the oil field. This paper presents the basic theory of how to get the information of remaining oil saturation from the data of interwell tracer test. Tracer test data of 76-30 well group of the Guan-104 fault block in Dagang oil field have been analyzed by this model. The distribution of oil saturation from this model is consistent with the result of reservoir performance and geological analysis. So it is concluded that this model is accurate and reliable. With the development of the interpretation method, interwell tracer technique will play more and more important role in waterflooding oil fields.

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A new method for determination of wettability of reservoir rocks

YAN Jienian

, 2001, 28(2): 97-0.

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On the basis of previous work, a new method for the determination of wettability of reservoir rocks, which can be designated as the modified method of spontaneous imbibition, has been established. The method is provided based on the principle that the wettability of rock surfaces is a key factor affecting the spontaneous imbibition rate as the rock contacts fluids. The influence of oil/water interfacial tension, liquid viscosity, pore geometry, as well as the shape and size of core samples, can be eliminated by introducing a dimensionless time, tD, instead of the imbibition time. The wettability index of rock surfaces under a given condition, WR, was determined quantitatively from the pseudo capillary pressure curve measured in the process of imbibition. The larger the value of WR is, the stronger the water wetness will be. The magnitude of WR depends on the rate of spontaneous imbibition and the total volume of the imbibition liquid. The imbibition tests were conducted by suspending a core from an electronic balance and immersing it in brine. The constitution of brine used in this test should be the same as the brine used for establishing the initial water saturation. The change in weight of the suspended core versus time was monitored for the period of time from core immersing to the weight of the core keeping constant (usually 72h). The oil recovery by spontaneous imbibition, Rim, at a given time was calculated and the curve of Rim versus tD, was plotted from the change in core weight and the density difference between oil and brine. The pseudo capillary pressures, pc,ps, and water saturation, Sw, corresponding to various values of tD were calculated and the curve of pc,ps versus Sw was plotted. Finally, the wettability index, WR, for each core sample was determined from the area under this curve.The experimental results indicate that the wettability indices determined using this method is considerably comparable with the wettability indices, Iw-o， determined using the traditional Amott method. Compared with the Amott method, the accuracy of the results determined using the new method is higher, and the experimental procedures are simpler. It is therefore convenient for applying more widely. However, the method is only suitable for the determination of wettability ranging from neutral wet to strongly water-wet condition. In general, the resolving power of WR is higher for the strongly water-wet rocks, while the resolving power of Iw-o is higher for the weakly water-wet rocks.

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A technique for modeling asphaltene precipitation in oil.

LI Min; SUN Lei; LI Shilun; LIU Bin and WANG Jianhui

, 2001, 28(2): 98-0.

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Phase equilibrium in oil and gas system is very complex but a research topic of practical application. In the past, research on phase equilibrium in oil and gas system was restricted to two-phase equilibrium of gas and liquid. However, there are a large amount of organic solids in the oil and gas. In order to describe phase equilibrium in oil and gas system accurately, research on three-phase equilibrium has to be done. Up to now, study of three-phase equilibrium has been done little. Especially research on phase equilibrium of gas-liquid-asphaltene has remained in qualitative description lever. In the light of the precipitation mechanism of asphaltene that asphtene precipitates with resin absorbed on the asphtene surface simultaneously, the paper put forward a method for modeling asphaltene precipitation with EOS. With three phase flash equation and algorithm of three phase flash which take asphaltene precipitation into consideration, it is able to model asphaltene precipitation quantitatively with a small number of parameters.

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The determination of the single well economic limit water-cut

YU Le-xiang and WANG Xing

, 2001, 28(2): 99-0.

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Shengli oil field now is at high and special high water-cut oil production stage. From economic benefit, on the basis of specific production data and on the principle of the profit and loss balance, and taking factors such as the growth rate of tax and cost into account, the authors develop the formula of the economic limit water-cut. Mature region of Shengli oil field is divided into two kinds of reservoir, high permeability reservoirs and low permeability reservoirs, and their economic limit water-cuts are predicted and tested under the conventional cost and lowest cost circumstances. For medium-high permeability reservoir, if oil price is at RMB￥900/t and liquid production of single well is 60-420t/d, ecnomic limit water cut increases from 93.39% to 97.79%, and shut-in limit is between 3.97t/d and 9.27t/d under conventional cost circumstance, while ecnomic limit water cut increases from 96.41% to 98.23% and shut-in limit is between 2.15t/d and 7.45t/d under the lowest cost circumstance; For low-permeability reservoir, if oil price is at RMB ￥900/t and liquid production of single well ranges from 10t/d to 50t/d, ecnomic limit water-cut increases from 75.39% to 91.58% and shut-in limit is between 2.46t/d and 4.21t/d under conventional cost circumstance, while ecnomic limit water-cut increases from 88.95% to 94.29% and shut-in limit is between 1.1t/d and 2.86t/d under the lowest cost circumstance. The results of the study may provide the reliable and scientific basis for development policy decision and production.

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Hydrocarbon prospective evaluation of Albert rift basin—An example of evaluation about least explored basins.

ZHANG Xing and TONG Xiaoguang

, 2001, 28(2): 100-0.

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Based on geological survey and evaluation of Albert basin, authors analyzed controlling role of the tectonic transfer zones to the development of deposits and the formation and evolution of traps in the basin, preliminarily predicts the models of oil and gas pooling of that basin and put forwards the evaluation method of least explored basins like Albert. In addition, the authors hold that the East African tectonic transfer zones classification of Morley should be suplimented with the concept of “countercurrent-overlapping and cocurrent pulling apart”. The breakthrough areas, which are the structures and traps adjoining the depocenter and source rock, are pointed out as the favorable targets for future exploration, including the tectonic transfer zones of southern Kiburu, middle Kibiroand and northern Bulisa in the basin.

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Characteristics of deep clastic reservoir in Banqiao-Beidagang area of Huanghua depression and their causing factors

GAO Yong and ZHANG Lianxue

, 2001, 28(2): 522-0.

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The Es2-3 Member with buried depth more than 3500m in Banqiao-Beidagang area, consists of clastic rocks, develops a sedimentary system mainly containing deep-water turbidite fan,fan delta and alluvial digitate sand bar. On the basis of research results of the sedimentary feature, the authors mainly studied lithologic features, diagenesis and rock evolution stages by means of rock thin section, cathodology, SEM, X-ray diffraction, as well as vitrinite reflactance. Meanwhile, we have made both the deep sandstone reserving ability evaluation and the causing factors analysis, and found out that the pore type of the reservoir in Banqiao-Beidagang area is dominated by secondary pores that make up two secondary pore zones. This study also indicates that it is the abnormal high-pressure zone and the organic matter that caused formation of the secondary pores directly.

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A study on the origin of the carbonate cements within reservoirs in the external metamorphic belt of the Bohai Bay oil-gas bearing region

WANG Darui and ZHANG Yinghong

, 2001, 28(2): 523-0.

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The igneous rocks are widely distributed within the typical rift zone basin of Bohai Bay. Twenty-seven commercial oil traps of igneous rocks have been found in this region up to now, in which twelve are external metamorphic belt reservoirs. The carbonate cement within this kind reservoir has been analyzed isotopically and the results are as δ13C:-6.8‰ to -3.3‰;δ18O:-17.1‰ to -12.5‰, which suggest that the carbonates are deposited by thermally metamorphic dissolution of hydrocarbon generated in the geologic thermal events. The carbon isotopic compositions of the analyzed samples suggest that thermal events would accelerate the generating of hydrocarbon in this area. From this study, the valuable geologic information about filling and special developing history of this kind reservoir, and modeling of the reservoir spaces in this kind of trap should be obtained.

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Pilot study on seismic anomaly body and its geological characteristics in Tarim basin

LI Honghui; ZHOU Dongyan and CONG Zhuan

, 2001, 28(2): 526-0.

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In Tarim basin there are many seismic anomaly bodies in the Ordovician, Carboniferous and Permian which are mostly caused by sedimentary and structure process. Some of them have been discovered and proved by drilling well and most of them are not. Based on the seismic reflection characteristics and inside reflection structure these seismic anomaly bodies could be classified into 9 kinds as downward cut, wedge, mound diagonal forest, mushroom, arborization, pillow, dome, down concave and lenticular bodies. Depending on the study of sedimentary environment, seismic velocity analysis and synthetic sonic logs proceeding, most of them are confirmed as nonmarine paleo-channel bar or marine reef and organic bank with good reservoir condition. They will become exploration targets if nearby there have sufficient hydrocarbon source. To a favorable seismic anomaly body research work of geophysics, geology and logging must be united and the comprehensive study should go deeply to reduce the exploration risk and get a good result.

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Identifying formation lithology using formation microscanner images

CHEN Ganghua; WU Wensheng and MAO Keyu

, 2001, 28(2): 527-0.

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Formation microscanner images (FMI) can show directly subtle changes of sidewall formation and provide abundant formations information. The authors, combining logging with geology, interpretes in detail FMI images of more than ten wells from the Shengli oil field and Changqing oil field through cores scale, and identification modes of all kinds of lithology for clastic rock, carbonate rock and magmatic rock have been finished, their results are very perfect.

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The effect of growth fracture of Abei Andesite reservoir on oilfield development

LIANG Guanzhong; WANG Qunchao; XIA Xin; ZHANG Xin and LI Jihong

, 2001, 28(2): 530-0.

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Abei Andesite reservoir is located in the central Arshan structure of Anan depression, Erlian basin. Its reservoir rocks are of fracture-porous type which are superimposed by many sets of andesitic strata (rock flowage or karst deposits). The fractures (especially secondary fractures) are the main seepage path; effective reservoirs are mainly located in the top of Andesite strata with growth fractures. The production of Abei Andesite reservoir declines quickly after water injection. The effect of the fracture on oilfield development is mainly characterized by: (1) At early stage of development, the thicker the fractured zone is, the better developed the fracture is, the higher the production of single well will be; (2) At water injection stage, these high and stable production wells are mainly near the faults or in the high position of the structure; (3) Because of the difference of their fracture development, the pattern of the increase of water cut of oil production wells during water flooding can be divided into 4 types: inefficient water flooding, effective for microfractures, water flooding in fractures and early water-flooded. In the early field life, wells that are located in the area where the fractures are developed have good response in the fracture treatment; while the wells with low pressure and low production have no good response after water flooding in the fracture treatment and have no response with zonal injection, zonal recovery and water plugging, but they have good results from cyclic water flooding combined with chemical profile modification agent. The research on the distribution and the growth law of fractures as well as their effects on oilfield development will be able to provide scientific basis for producing the crude oil remaining in microfractures between big fractures in the medium and the late phases of water flooding development.

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Estimating maximum output of loose sandstone oil reservoir without controlling sand production at Minqiao oil field

LUO Ying; WEN Guangyao; AO Lide and SUN Dongsheng

, 2001, 28(2): 531-0.

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Minnan fault block of Minqiao oil field belongs to lithologico-structural oil researvoir, where there are many faults and its geologic structure is complex, which causes difficulty in development. The evaluation of exploration and development for this oilfield was done again in 1997, the shallow buried depth of the fault block, the unconsolidation of the formation and sand production easily cause difficulty in development. Under this circomstance, the research for controlling sand production was done at initial period of oil production. By utilizing the logging data and the principle of Mohr stress analysis, the maximal pressure difference of production and the maximal limit value of production of sand production caused by different formation pressure has been calculated. After oil wells are putting into production and the indexes are controlled within limit value, periods of checking pump can be prolonged and sand control can also be achieved. But it has to be pointed out that the limit value only stands for limit flow and if the flow in a well is highter than the limit, a sanding problem will be resulted in. In this case this limit is not the maximal production of well.

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Laboratory experiment of dry gas injection in super low-permeability carbonate fractured reservoir

GUO Ping; LI Shilun; ZHANG Siyong; WU Ying; TANG Xianyi and ZHAO Junsheng

, 2001, 28(2): 534-0.

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Daanzhai is a low permeability carbonate fractured reservoir, the recovery factor is only 3-5% when it is exploited by natural energy. Seeking after good way to EOR is the key point. The autrors use matrix cores to make fractures by a manual method to replace real rock in our experiments. Based on the harmonic average principle the authors connect seventeen single cores to form a long core, and use it to study hydrocarbon injection. From comparing the different flooding methods and pressures it is concluded that for the low-permeability reservoir the recovery factor is low with depletion drive recovery, and it can cause water channeling and gas channelling in either way of water injection or gas injection; the recovery factor can be properly improved by water injection alone, but have no high sweep efficiency. Even though hydrocarbon gas injection isn't easy to form miscible, the recovery factor will be high with the increasing of the injection pressure. Under existing formation pressure of Daanzhai oil reservoir, hydrocarbon gas injection is very effective, its recovery factor is 6.21% higher than that of the natural depletion recovery, 3.91% higher than that of water injection.

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Calculating the relative permeability from displacement experiments based on percolation theory

WANG Jinxun; LIU Qingjie; YANG Puhua and SHEN Pingping

, 2001, 28(2): 535-0.

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JBN method is often used to calculate relative permeability curves from displacement data. But this technique requires restrictive assumptions to obtain a solution. In this paper, a history-matching method, which combines pore-level network model and Buckley-Leverett equation, relating microscopic flow mechanism in porous medium to macroscopic simulator, is used to obtain two-phase relative permeability curves from displacement experiments. The pore-space topology is approximated by Bethe network. Functions relating the volume and hydraulic conductivity of a pore-throat segment to its radius are determined through matching the measured final oil saturation and water relative permeability. The coordination number of Beth network and throat radius distribution can be adjusted to fit the cumulative oil produced. Case study suggests that the technique presented in this paper is promising.

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On the stimulation with “exploding in fractures” in low permeability reservoirs

DING Yan-sheng; CHEN Li; XEI Xei; ZHANG Shengzong; LIU Xiangui; LIU Xiao; YANG Yemin and YU Jia

, 2001, 28(2): 540-0.

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A basic idea was brought forward about the stimulation with “exploding in fractures” in low permeability reservoirs on the basis of review of the stimulation with exploding, including well shooting, nuclear explosion and high energy fracturing. Explosives are pressed into the fratures in oil layers with the technology of hydraulic fractuing and lit up without any damages to wells. After exploding, fractured zones are generated around the original main hydraulic fractures. And then, the recoveries are enhanced and more oils are produced. At least a group of basic ingredients of the special explosives and powders have been found. The processes of injection, lighting up and deflagration have been realized in the laboratory simulation experiments of the scale of 200mm. That means the basic idea is practical. A one-dimension model of compressible fluid with the chemical reaction of the explosives and the powders in thin fractures has been proposed, which takes into consideration the heat conduction in the fluid, heat loss on boundaries and damps. The computational results are reasonable on the assumption of steady propagation and simplification of incompressible fluid. It demonstrates in theory that it is possible for the deflagration of the explosives and powders in fractures. It is safe in principle to treat with the liquid from the wells after“exploding in fractures”. It is difficult for the residual explosives that were not ignited in rock fractures to come out to the ground during the production stages. The concentration of the residual explosives is less than 1% in oil gathering systems and these residual explosives can be separated with centrifugal methods. Micro mass of the explosives after separation will be decomposed in heating furnaces of 400℃. Such a thermal decomposition cannot induce explosion. It is a strategic, advanced and initiation research of the stimulation with “exploding in fractures” in low permeability reservoirs. It is expected that the increased production due to “exploding in fractures” is much more than that due to hydraulic fracturing. The technology of “exploding in fractures” may make the extraction in low permeability reservoirs much improved and change the low permeability resources impossible to produce at present to the extractable resources in future. The future research should be focused on large-scale laboratory tests and related theories, technology of safety, techniques and equipment for the tests in-situ, and in-situ tests.

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The combined action mechanism of the pressure and pressure differential in casing damage

LIU Jianzhong; ZHOU Shengmin; LI Ziping and WANG Xinsheng

, 2001, 28(2): 541-0.

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The central unit of Saertu oil reservoir in Daqing oil field is an area of frequent casing damage, the casing damage mainly occurs near the datum bed, and group by group. Through the stress measured at the field site and core tested in laboratory and compared with the stress, pressure and pressure differential of the formations where casing damage occurred, it is concluded that: the casing damage occurred in recent years mainly in the strong pressure differential areas, which is mainly caused by the combination of stress, pressure and pressure differential of neighbor blocks. When the formation pressure is longer than the original critical pressure, casing damage still occurred even without pressure differential action. While the formation pressure is smaller than the original critical pressure casing damage can be induced when there exists pressure differential action. To avoid the occurrence of the casing damage it has to stabilize the formation pressure to control the water production as well as to ensure the injection-production balance so as to prevent the strong pressure differential between neighbouring blocks. This theory has been applied to Daqing oil field and obvious achievement has been obtained in casing damage treatment overthere.

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Strengthening petrophysical study to raise exploration benefit

OUYANG Jian

, 2001, 28(2): 565-0.

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In recent years, the China's oil and gas exploration focuses on low-relief structure, low-porosity, low-permeability and complex lithology reservoirs. The log technology has to face low-resistivity and complicated reservoirs and some other disadvantageous conditions. This article introduces some petrophysical study achievements of oil/gas-bearing formation log interpretation in recent 10 years:(1) The formation of low-resistivity pay zones in Bohai Bay is due to the fact that sea-water drilling mud invaded into oil/gas-bearing formation; Complex dual pore texture of pay zones with high average water saturation and additional conductivity of the clay minerals. (2)The reasons of low testing successful ratio of low-porosity, low-permeable pay zones are drilling-mud invaded into oil-water transitional zone of low-relief structure reservoir, leading to high invaded section and so laterolog resistivity being much high. For porosity bigger than 12%, influence of the pore texture on the resistivity is much big. So using electric log to interprete pay zone or not must be based on the pore texture partition.(3)The resistivity of matrix is quite different from that of the drilling-mud-invaded fracture, and dual laterolog can recognize and quantitatively evaluate the high angle fractured sandstone oil zones. Dual laterolog log, neutron and density log can interprete fractured reservoirs. This article summarizes the electric log technology applying to low-resistivity, low-porosity, low-permeable pay zones and fractured reservoirs. To study the influence of drilling-mud invasion and pore texture to oil zone resistivity and its electric log response rule, “in-time log” and pressure gradient log should beapplied. Though the logging-based saturation interpretation technology becomes mature, the high precision numerical control log, more subtle “core calibration log” interpretation method are needed to meet much more demands. Finally the author gives some suggestions on making feasible petrophysical technology policies.

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The geological conditions of deep-basin gas in western depression of Sichuan basin

SONG Yan and HONG Feng

, 2001, 28(2): 576-0.

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Deep-basin gas accumulation contains huge resources of natural gas, and will be an important target of natural gas exploration. Two aspects decide whether the deep-basin gas can exist. One is the geological conditions for deep-basin gas, the other is the characteristics of deep-basin gas accumulation, and there is a cause-effect relationship between them. The structure and deposition of western Sichuan depression lead to favorable conditions for deep-basin gas: (1)Upper Triassic coal-measure source rock with high gas generating strength and present gas supply provide huge natural gas for deep-basin gas; (2)Good tectonic conditions exist in western Sichuan basin, which has the characteristics of foreland basin and gas generating center overlapping on depression center; (3)The low-porosity and low-permeability sandstone distributing continuously and turning better upward slope is favorable for deep-basin gas formation. Because of an assemblage of the above-mentioned geological features, Upper Triassic gas accumulation has these characteristics such as bearing gas in large area, uncontrolled by structure, gas-water inversion and abnormally high pressure.

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New Opinions on oil and gas generation and exploration in Jiuxi basin (Ⅱ)—Determination of process of hydrocarbon generating and the principal oil source rock.

CHEN Jianping; CHEN Jianjun; ZHANG Liping and ZHONG Ningning

, 2001, 28(2): 577-0.

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The threshold of oil generating is distinctly different in several sags of Jiuxi basin. Maturity of Lower Cretaceous source rock, researched by the authors, is closely relative to the deposition thickness of the Tertiary and the Quaternary. It shows that source rock at the depth of 4,000-4,400m is low mature, and 4,400-5,200m is in the peak of oil generating. Correspondingly, the source rock of the Zhonggou Formation is immature; the Upper Xiagou Formation is low mature; and the Lower Xiagou Formation and the Upper Chijinpu Formation are in the peak of oil-generating; and the Lower Chijinpu Formation is at the stage of generation of condensate-wet gas with high maturity. There is no source rock reaching the overmature stage of dry gas at present. In the Shibei sag, the threshold is about 3,000m, which is apparently different from that of the Qingnan sag. Only deeply buried source rock of Chijinpu Formation reaches the mature stage. Revealed by the burial and thermal evolution history of the source rock, the low Cretaceous source rock was shallowly buried at the end of Cretaceous period. The main oil generating stage is in late of the Neogene period. There is only one time of hydrocarbon generating and expulsing in geological history of the basin. The characters of oil samples from different oil fields are very similar in Jiuxi basin. Aquatic organisms of early Cretaceous are the principal precursors of oil generating, disclosed by the stable carbon isotopic ratio and character of biomarkers. The Chijinpu Formation is the major source rock, and relatively, the Xiagou Formation is subordinate.

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Geological characteristics and hydrocarbon accumulation pattern of Penglai 19-3 oil field

GUO Taixian; LIU Chuncheng; LU Hongzhi; JIANG Peihai; XU Hongling; LI Xiufen and SUN Yingtao

, 2001, 28(2): 601-0.

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Penglai 19-3 oil field is located in the northeast of middle Bonan Uplift in Bohai Sea and on the east branch of Tancheng-Lujiang deep faulted zone. The structure of the field is a faulted anticline developed on a palaeohigh and complicated by the faults. The structural area is 65km2. The main pay zone of the field exists in Guantao Formation and Lower Minghuazhen Formation of Neogene. The lithology of the reservoir is fluvial terrigenous clastic rock. According to the drilling data of 7 wells, the thickness of the pay zone in the field is 63-151m, the oil-bearing area is more than 50km2 and the OOIP is hundreds of million tons. The shallow structure of this field is broken due to the well-developed faults, which results in the complicated hydrocarbon occurrence and pressure systems. This oil field is a structural layered reservoir that consists of many faulted blocks and has multiple oil-water systems in both vertical and lateral directions.Penglai 19-3 oil field is characterized by crude oil with high surface density,high viscosity and low pour point, high oil saturation pressure, little pressure difference between formation and saturation pressures, moderate dissolved gas-oil ratio and distinct biodegradation and water washing. According to the oil-source correlation analysis, the hydrocarbon of the field mainly comes from source rock of Lower Dongying Formation and Shahejie Formation of Palaeogene in the surrounding sags.According to the study of tectonic history, the development and formation of the structure and the reservoir of Penglai 19-3 oil field is closely relative to the long-term and strong tectonic movements of Tancheng-Lujiang deep faulted zone, especially many tectonic movements happened in Neogene period. Consequently developed many faults and numerous microcracks in Cenozoic formation that are good hydrocarbon migration pathways from sags to uplifts, and play an important role in finally forming the reservoir in Penglai 19-3 structure. The discovery of Penglai 19-3 oil field is a new breakthrough in hydrocarbon exploration history of Bohai Sea, having great significance for steady and sustainable development of offshore petroleum industry in China.

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, Volume 28 Issue 2