Lithologic Reservoirs

The Upper Permian Xuanwei Formation in Junlian area has attracted more attention,which is considered as a potential coalbed methane development zone in the southern margin of the Sichuan Basin. In order to research the distribution and evolution of the coal beds in the Upper Permian Xuanwei Formation,based on the data of drilling,logging and regional geology,combined with the theories and methods of sedimentology and regional geology,this paper divided the sedimentary facies of single well,compared the well- tie section,compiled the map of sedimentary facies,and discussed the controlling factors of coal accumulation. The results show that the Upper Permian Xuanwei Formation mainly developed meandering river facies and tidal flat facies,and a small amount of diamictic platform facies. The coal beds were mainly developed in the swamp microfacies. The warm and humid palaeo-environment prepared good condition for plants blooming. The distribution and development of coal beds in the Upper Permian Xuanwei formation are controlled by tectonic-palaeogeography and palaeoenvironment. This research is significant for understanding coupling relationship of the depositional environment and coal accumulating effect,and favorable for coalbed methane exploration in Junlian area.

New geological understanding of tight sandstone gas

WEI Xinshan, HU Aiping, ZHAO Huitao, KANG Rui, SHI Xiaoying, LIU Xiaopeng

2017, Vol.29(1): 11–20 Abstract ( 393 ) PDF (7122 KB) ( 272 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.002

With the exploration and development practice of tight sandstone gas, the traditional geological understanding exposed many limitations and is inconsistent with practical data. Based on practical data from the exploration and development and experimental data from nuclear magnetic resonance, gas phase permeation analysis, isothermal adsorption experiment and gas reservoir paleo-pressure recovery of tight sandstone gas reservoirs in Ordos Basin, the knowledge of the geological characteristics of tight sandstone gas was supplemented and improved. we get some new geological understandings of tight sandstone gas：(1)the tight sandstone gas field composes of multiple pressure systems reservoir, which determines the need of dense well pattern rather than that of conventional gas reservoir dilute well wide agent and dilute well development;(2)the change of water saturation in tight sandstone reservoirs is great, and there also exists the phenomenon of super low water saturation, which is easy to form water block damage, so it demands to do best in reservoir damage protection;(3)water production is serious in the late development stage of tight sandstone gas reservoirs, so how to control the water production is the key problem in the phase of gas development;(4)there is a certain amount of adsorbed gas in tight sandstone gas reservoirs, which is important for the description of tight sandstone gas reservoir characteristics and seepage law, the prediction of single well productivity, the precise calculation of tight sandstone gas resource and reserves evaluation.

Organic geochemical characteristics of Niutitang Formation in Ningqiang Bay,the northwestern margin of Upper Yangtze platform

CHENG Jun, XU Xiaofei, ZHANG Wenfeng, MI Mawangjiu, GUO Jun

2017, Vol.29(1): 21–26 Abstract ( 306 ) PDF (1370 KB) ( 489 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.003

The organic-rich shales of Niutitang Formation are well developed in Ningqiang Bay,the northwestern margin of Upper Yangtze platform,and they have good oil and gas generation conditions, nevertheless,no investigation and evaluation for shale gas has been carried out in this area. Based on field geological survey and sample test,the thickness,distribution characteristics,depositional settings and organic geochemical characteristics of shales of Niutitang Formation were investigated. The results show that the cumulative thickness of organicrich shales is up to 108.5 m;the mass fraction of total organic carbon is 0.08%-7.54% with an average value of 1.97%;kerogen is type Ⅰ;the Tmax ranges from 419 ℃ to 496 ℃ with an average value of 448 ℃;and the average Ro is 2.36%. These data show that the source rocks have entered into mature to high mature stage. The source rocks are influenced by complex palaeogeography in Ningqiang Bay,and the source rocks on the west side are better than that on the east side,with good prospects for shale gas exploration.

Lithofacies types and sedimentary facies of Pinghu Formation in Xihu Depression

WU Jiapeng, WAN Lifen, ZHANG Lan, WANG Yingmin, ZHAO Qianhui, LI Kun, WANG Siqi

2017, Vol.29(1): 27–34 Abstract ( 367 ) PDF (36290 KB) ( 225 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.004

The research on sedimentary facies of Pinghu Formation in Xihu Depression is relatively limited,and the types of sedimentary facies,distribution characteristics and sedimentary model are unclear. Based on grain size and sedimentary structures of cores from 46 wells,the lithofacies of Pinghu Formation was identified. Then according to genetic types of lithofacies,lithofacies assemblage characteristics and well logging features,the sedimentary facies of Pinghu Formation was identified. The lithofacies was divided into three types：conglomerate facies,sandstone facies and fine- grained facies. The main sedimentary facies are tidal delta,tidal flat and confined shallow marine,which can be divided into several subfacies including delta plain,delta front, prodelta, supratidal zone,intertidal zone,subtidal zone,and shelf sand ridge etc. Considering the influencing factors such as the regional tectonic movement,palaeogeomorphology,provenance supply at the different Pinghu stage,the provenance-sedimentary model both controlled by half graben and tidal action of Pinghu Formation in Xihu Depression was established,which provides the base for further reservoir research.

Origin and distribution of sandbody in slope break zone：a case study of the Lower Cretaceous Qingshuihe Formation in Chepaizi area,Junggar Basin

LI Xiao, LIU Hailei, WANG Xueyong, BIAN Baoli, YANG Fan, CHEN Gangqiang

2017, Vol.29(1): 35–42 Abstract ( 368 ) PDF (30084 KB) ( 317 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.005

The slope break zone in hydrocarbon-abundant depression is an important exploration area for stratigraphic reservoirs. Stratigraphic overlap controlled by four different levels of slope break zone in six stages is developed in Qingshuihe Formation in Chepaizi area,Junggar Basin. In order to investigate the controlling factors of petroleum accumulation and distribution,the sedimentary facies was studied by the data of drilling,core and seismic attributes,combined with the technologies of stratigraphy and paleogeomorphology restoration,the origin and controlling factors of reservoir were discussed,and the favorable reservoir and potential exploration prospects were defined. The results show that Qingshuihe Formation developed meandering river delta and shore-shallow lacustrine sedimentary facies,and can be characterized by“mud-rich and sand-poor”and limited high quality reservoir distributions. The favorable reservoir distribution was controlled by the“dual”(thrust squeeze-stress relief)systems tract formed by episodic squeeze,sedimentary facies,valley topography and slope break zone. The delta front facies developed at the under-part of the slope break zone is favorable for high-quality reservoir enrichment and potential for petroleum prospecting.

Lithofacies characteristics and sedimentary model of braided delta：a case study of He 8 member in the southern Sulige,Ordos Basin

LI Zhihua, HUANG Wenhui

2017, Vol.29(1): 43–50 Abstract ( 388 ) PDF (9491 KB) ( 383 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.006

The He 8 member(the eighth member of Shihezi Formation)in southern Sulige is important reservoir of Sulige Oilfiled,Ordos Basin. In order to make clear the lithofacies characteristics and sedimentary models of the target strata in the study area,there is an important guiding effect for the oil and gas exploration. The typical lithofacies of Shihezi Formation was divided into eight types through core observation of 22 wells,and the lithofacies assemblies were divided into four types. According to the shape characteristics and contact types of logging curves of 167 wells,the logging curves were divided into four types. Combined with lithofacies and logging curves,the sedimentary microfacies were divided into eight types. The distribution of the lithofacies assemblages and sedimentary model were summarized. The assembly A developed in braided delta plain in the north,the assembly B developed in the transition area between the braided delta plain and braided delta front in the north,the assembly C developed in channel of braided delta front in the south,and the assembly D developed in deep lake of braided delta front in the southwest.

Characteristics and influencing factors of Chang 8 tight sandstone reservoir of Triassic Yanchang Formation in Zhoujiawan area, Ordos Basin

WANG Weibin, ZHU Jing, MA Wenzhong, FENG Shunyan, LIU Yanni, ZHAO Jing

2017, Vol.29(1): 51–58 Abstract ( 340 ) PDF (28115 KB) ( 176 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.007

The characteristics and controlling factors of Chang 8 tight sandstone reservoir of Triassic Yanchang Formation in Ordos Basin is unclear,which restricts the hydrocarbon exploration and development in this area. Based on core observation and data of cast thin sections,scanning electronic microscope,mercury intrusion and conventional physical properties,the characteristics and influencing factors of the Chang 8 tight sandstone reservoir were analyzed. The results show that the Chang 8 reservoir rocks are mainly composed of lithic arkose and feldspathic lithic sandstones,with high content of matrix. The porosity of Chang 81 and Chang 82 sublayers are 8.03% and 8.17%,respectively,and the permeability are 0.17 mD and 0.25 mD,respectively, showing the poor properties and tight sandstone reservoirs. The pores are dominated by the remaining primary intergranular pores and secondary feldspar dissolved pores,together with small throat,which characterizes the Chang 8 reservoir with high expulsion pressure,high median pressure and low ejection efficiency. Under the control of deposition and diagenesis,the favorable reservoirs are mainly developed in distributary channel and subaqueous distributary channel. Compaction,carbonate and siliceous cementation make the reservoir properties poor,and dissolution improve the reservoir porosity and permeability.

Influencing factors of tight reservoirs and favorable diagenetic facies： a case study of Chang 8 reservoir of the Upper Triassic Yanchang Formation in Maling Oilfield,Ordos Basin

WANG Meng, ZENG Ming, CHEN Hong'ao, YANG Gang, XU Jifang, QIU Haibin, GUO Hongguang, WANG Jiamin

2017, Vol.29(1): 59–70 Abstract ( 387 ) PDF (41976 KB) ( 169 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.008

The Chang 8 reservoir of the Upper Triassic Yanchang Formation in Maling Oilfield is characterized by tight sandstone of typical low porosity and low permeability. To analyze the influences of diagenesis on reservoir densification and evaluate favorable diagenetic facies, based on the data of core observation, cast thin section, mercury injection test, scanning electron microscopy and X-ray diffraction test, the reservoir petrology, pore structure and physical properties of Chang 8 reservoir were studied, the influence of different diagenesis on reservoir quality was discussed, a standard of diagenetic facies division was established, and the distribution of favorable diagenetic facies area was evaluated. The Chang 8 sandstone is characterized by strong microscopic heterogeneity and striped- shaped or lentoid, which have a good matching with subaqueous distributary channel and mouth bar sand bodies. The sandstone experienced medium to strong compaction, medium to strong cementation, and weak dissolution. The compaction, siliceous cementation and carbonate cementation led to reservoir densification synthetically. The presence of illite resulted in permeability reduce; chlorite coating, hydrocarbon emplacement, feldspar and debris dissolution contribute to reservoir development; kaolinite indicates the formation of secondary porosity, while it would cause permeability damage. The sandstones were divided into six types of diagenetic facies, among these diagenetic facies, chlorite- authigenic quartz cementation facies and dissolution facies are favorable diagenetic facies, which developed in the upper and middle of large scale Chang 81 and small scale Chang 82 reservoir.

Reservoir characteristics and pore evolution of the second member of the Lower Cretaceous Bayingebi Formation in Chagan Depression

WU-CHEN Bingjie, ZHU Xiaomin, WEI Wei, JIANG Feihu, TAN Mingxuan, PAN Rong

2017, Vol.29(1): 71–80 Abstract ( 321 ) PDF (24649 KB) ( 147 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.009

In order to study the characteristics of clastic reservoir and the effects of diagenesis on porosity of the second member of the Lower Cretaceous Bayingebi Formation(K1 b2)in Chagan Depression,a pore evolution model was established based on the data of core,thin section observation,X-ray diffraction and SEM. The reservoir rock of K1 b2 is mainly composed of feldspar lithic sandstone. The reservoir properties are poor,with ultralow porosity and ultra-low permeability. The porosity ranges from 0.17% to 15.32% with an average of 5.7%, and the permeability ranges from 0.003 7 mD to 162.300 0 mD,with an average of 3.4 mD. Intergranular dissolved pores are the main reservoir space. The reservoir of K1 b2 underwent mainly mechanical compaction,early carbonate cementation,carbonate dissolution and late carbonate cementation. Dissolution of carbonate cements, feldspar and rock debris created a secondary pore zone among 2 100-2 800 m,which improves the reservoir properties. The K1 b2 reservoir is mainly at the middle diagenetic A2 stage. The results show that the original porosity of K1 b2 is 30.5% in the Wuliji tectonic zone,compaction reduced the porosity of 19.8%,while the cementation reduced porosity of 9.7%,and the dissolution increased porosity by 3.7%,which improves the reservoir properties( the current porosity is approximately 4.8%). This result matches well with actual porosity,which has a contribution to cognize pore evolution process and predict high quality reservoirs.

Accumulation and enrichment factors of granite reservoirs

LI Fuheng, HOU Lianhua, SHI Lei, XIAO Kunye, SHI Feng

2017, Vol.29(1): 81–89 Abstract ( 365 ) PDF (23511 KB) ( 400 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.010

In recent years, reservoir in granite rock has important discoveries for hydrocarbon exploration. Based on the characteristics of the typical granite reservoirs, according to the origin of hydrocarbon, reservoir formation mechanism, trap types, main producing formation and water/oil/gas system, the granite reservoirs were classified, and the accumulation and enrichment factors of granite reservoirs were summarized based on the data of cores, thin sections and out crops. The results show that the accumulation and enrichment factors of granite reservoirs include the following aspects:(1)high quality source rocks and thick cap rocks, with strong hydrocarbon generation and sealing ability;(2)widely distributed granite reservoirs, with a large area of granite rock, fault, weathering and underground hydrothermal dissolution playing an important role in granite reservoir development and distribution;( 3)different types of trap formed in the granite rock buried hill hilltop, hillside and hill-inside;(4)the good matching of effective source rock, migration pathway, favorable trap and regional cap rock is favorable for the enrichment of granite reservoir, which distributes mostly together with the overlying reservoir from sedimentary rock symbiotically. This study could provide guidance in theory and case reference for granite reservoir exploration.

Characteristics and evolution of oil reservoir zones of Buqu Formation in the southern Qiangtang Depression

XIA Guoqing, YI Haisheng, LI Gaojie, JIN Feng, JI Changjun, LI Qilai

2017, Vol.29(1): 90–96 Abstract ( 257 ) PDF (38958 KB) ( 154 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.011

Oil reservoir zones of Middle Jurassic Buqu Formation are widely exposed in southern Qiangtang Depression. Based on the geological occurrence,stratigraphic sequence,sedimentary facies and space distribution of oil reservoir zones,combined with paleogeography framework,the subsurface characteristics and evolution of the oil reservoir zones were studied. The results show that the oil reservoir zones developed in three blocks：Long' eni,Angdaercuo and Sairen,respectively,and they are characterized by east-west stable linear distribution and good stratification. A large number of reef bank facies residual structures can be seen in the reservoir,and they are associated with coquinite,bioclastic limestone,oolitic limestone and calcarenite. These oil reservoir zones are controlled by platform margin facies and the sea-level changes. At the early Buqu stage,the reef and shoal facies deposited along platform margin from Sairento Angbacundong. At the middle Buqu stage,with the rise of sea level,a series of parallel and linear platform margin facies formed by retrograding northward in Xiaogaxiaona-Rigaerbao, Zharen-Donglangpa and Riagen-Aozong. At the late Buqu stage,the progressive reef and bank facies advanced southward gradually along the platform margin with the drop of sea-level,and widely developed in Long' eni block.

The status and analysis of SEC evaluation results for new-added proved reserves in Shengli Oilfield

TIAN Min

2017, Vol.29(1): 97–103 Abstract ( 376 ) PDF (1755 KB) ( 567 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.012

The SEC reserve evaluation results are important indicators to measure the development potential of oil companies. In order to decrease the difference between domestic new- added proved reserves and SEC reserves, and increase the proportion of new-added proved reserves to the total reserves,the status and difference between domestic reserves and SEC reserves in the past years in Shengli Oilfield were analyzed. It is considered that the SEC rules change,the low quality of reserves and unfavorable offset resulted the stepped down tendency of SEC results for domestic new-added proved reserves. The parameter differences were illustrated by using evaluation examples of past years,including static parameters(oil-bearing area,net pay and recovery factor)and dynamic parameters(initial rate,decline rate and drilling plan). Two countermeasures：management mode optimization for domestic reserves and normalization of evaluation data,were put forward to reduce the differences. This research has some reference significance for the reserves task completion of domestic oil companies and the integration of SEC reserves evaluation systems.

Key techniques for surface array microseismic monitoring

DIAO Rui, WU Guochen, CUI Qinghui, SHANG Xinmin, RUI Yongjun

2017, Vol.29(1): 104–109 Abstract ( 348 ) PDF (2493 KB) ( 560 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.013

Based on shot points imaging of reservoir hydraulic fracturing, the microseismic monitoring technology can be used for hydraulic fracturing monitoring and well network layout optimization. The signal of surface microseismic monitoring is weaker than borehole, and the noise is widely distributed, which leads to locating errors and interpretation uncertainty. According to the characteristics of surface microseismic data, key techniques can be used to provide a basis of reservoir hydraulic fracturing. The microseismic joint denoising technique is used for noise attenuation, the energy focusing location technique can be used to improve the location precision, and the fracture integrated interpretation technique is conducive to improve the certainty of fracture prediction. In field application of microseismic monitoring, monitoring results are identical with reservoir development data of hydraulic fracturing wells, which confirms the validity and practicability of these key techniques. The surface array microseismic monitoring techniques have broad application prospects in tight reservoir exploration and development.

Image processing method of LWD azimuthal gamma data

TANG Haiquan

2017, Vol.29(1): 110–115 Abstract ( 462 ) PDF (3150 KB) ( 798 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.014

Using LWD(Logging While Drilling)azimuthal gamma curves and gamma images can more accurately determine the location of reservoir boundaries,and can guidance geosteering construction of horizontal wells better. This paper first introduced the structure of two sectors and basic principles of LWD azimuthal gamma measurement instrument. During drilling,the gamma tool is responsed by rotary speed,and generally can only measure up to 16 azimuthal sectors data. The circle borehole measuring range is limited,and the real-time data uploading to surface,due to the influence of downhole signal transmission rate,uploading up to no more than eight sectors. At the same time,the data correlation between different sectors is relatively high,and mutual influence is relatively large. In use of the periodic cubic spline interpolation process,a continuous variation gamma data along the borehole was obtained. Through applying RGB and HSI color space conversion method and specific chromatography static and dynamic color mapping method,the gamma image is more in line with the characteristics of the geological image. Through LWD data processing,better gamma images can be obtained,and the observation and identification of stratigraphic characteristics are clearer. It directly shows the wellbore formation lithology changes in different directions,and reduces the risk penetrated reservoir.

Quantitative evaluation of favorable reservoir in ultra-low permeable reservoir based on“petrophysical facies-flow unit”log response： a case study of Chang 6 oil reservoir set in Yanchang Oilfield

ZHOU You, LI Zhiping, JING Cheng, GU Xiaoyu, SUN Wei, LI Xiao

2017, Vol.29(1): 116–123 Abstract ( 372 ) PDF (1882 KB) ( 326 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.015

Petrophysical facies and flow unit can both represent the heterogeneity of the reservoir from different angles. The ultra-low permeability reservoir is characterized by strong heterogeneity in the eastern Ordos Basin, so there will be some deviations in reservoir evaluation by using petrophysical facies or flow unit. Combined with qualitative identification and quantitative classification, a comprehensive assessment target system of“petro physical facies- flow unit”was established to quantitatively evaluate the high quality reservoirs. Meanwhile,it utilized grey system theory to integrate multiple information and established relationship between limited core data and logging data,achieving to use well log information to quantitatively evaluate and divide“petrophysical facies- flow unit”. The results show that the first-class and the second-class“petrophysical facies-flow unit”possess relatively preponderant reservoir properties and seepage structure,by which 49 high quality reservoirs was optimized for developing,and the distribution and extension direction of oiliness zones were determined. This method can effectively reduce the error caused by the use of petrophysical facies or flow unit only to optimize high quality reservoirs. At the same time,the field application effect is good,and it also can provide favorable well site and district for enhancing reserve and productivity in the study area.

Application of integrated reservoir classification method to the quantitative characterization of heterogeneity reservoir

YANG Xipu, YANG Xiaoli, LIU Jun, FANG Lei, ZOU Jingyun

2017, Vol.29(1): 124–129 Abstract ( 280 ) PDF (14652 KB) ( 335 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.016

In order to carry out the fine classification and 3D quantitative characterization of strong heterogeneity reservoir in H oilfield which lies in Albert Basin,the East African Rift,the heterogeneity reservoir classification was studied based on 36.5 m cores and assay data of 399 samples,by analyzing physical properties,thin section, scanning electron microscope and capillary pressure. The result shows that reservoirs were divided into four types according to lithology,properties,pore structure and seepage capacity. The key parameters of shale content and density were selected from logging data based on the reservoir classification result and the relationship of lithology- electrical log. The reservoir rock types were classified on the wells with the parameters. Then the reservoir geologic model was established by the parameters of flow zone indicator and pore throat radius. And finally the quantitative reservoir characterization was carried out,which lays the foundation for development programs.

Fracturing technology of Chang 6 tight sandstone reservoir in Ordos Basin

HAN Jingjing, LIU Jian, WU Long

2017, Vol.29(1): 130–134 Abstract ( 293 ) PDF (8597 KB) ( 188 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.017

In order to improve single-well production, simplify workflow and reduce costs, through the analysis of the geological conditions of the tight sandstone reservoir in S area,combined with the foreign experience of successful development of tight oil, it is considered that the reservoir has the characteristics of poor reservoir properties, well developed fractures and high brittleness index, and volume fracturing reconstruction with large displacement, large volume and low sand ratio is suitable for the study area. The thickened water fracturing fluid system has the characteristics of high sand carrying capacity, low wellbore friction, simple liquid preparation and low comprehensive cost. The thickened water fracturing fluid system was used in 22 wells, and good results have been achieved. A set of matching technology was formed for tight sandstone reservoir fracturing in Ordos Basin, which could effectively increase the production and has a good value of application.

Optimization of reasonable production pressure difference of fractured horizontal well in low permeability tight reservoirs

DENG Xuefeng

2017, Vol.29(1): 135–139 Abstract ( 348 ) PDF (1433 KB) ( 484 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.018

In order to establish effective numerical simulation method to calculate the reasonable production pressure difference of fractured horizontal well, the threshold pressure gradient and pressure-sensitive effect in tight and fractured reservoirs of Yanchang Formation in HH oilfield, Ordos Basin. The permeability was modified by introducing dynamic permeability theory and pressure- sensitive effect into numerical simulation calculation, since the theory could effectively simulate the characteristics of nonlinear flow in low permeability reservoirs. Therefore, a new method for productivity of fractured horizontal well in low permeability reservoir was established, which takes into consideration the effects of threshold pressure gradient together with pressure-sensitive effect on the productivity of fractured horizontal well. The results indicate that the calculation result by this new numerical simulation method accord well with field production characteristics of fractured horizontal well in low permeability reservoirs, and there are apparent effects of threshold pressure gradient and pressure-sensitive effect on productivity of fractured horizontal well. In addition, the range of reasonable production pressure difference of fractured horizontal well in HH oilfield was optimized.

Productivity simulation for fracturing horizontal gas wells considering overdisplacing operation

YAN Xiangyang, WANG Tengfei, HE Shuangxi, SHEN Beibei, XU Yonghui, CHEN Lin

2017, Vol.29(1): 140–146 Abstract ( 306 ) PDF (1431 KB) ( 323 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.019

Overdisplacing operation often occurs in fracturing treatment in order to meet the safety requirements of tools or treatments, especially in multi-stage fracturing horizontal wells with drillable bridging plug. However, overdisplacing may affect post- frac production due to decreasing fracture conductivity, hardly any models for studying this effect. The calculation model of overdisplacing sweep region was established based on the fractureseam features and Harrington formula; the conductivity model of unpropped hydraulic fracture was established according to the fracture closure, Carman-Kozeny formula and Walsh model; moreover, the point source theory, the unstable gas seepage formula and the potential superposition theory were adopted to establish the productivity calculation model of fracturing horizontal gas wells. From the above models, a new simulation model was built to predicting the production of fractured horizontal gas wells considering overdisplacing, and a calculation example and single factor analysis were performed. The calculation results show that the larger the rock grain sizes, the higher the fracture conductivity and the smaller the overdisplacing liquid volume, the lower the production reduction. The fracture conductivity of overdisplacing sweep region plays an important role in productivity performance. Overdisplacing, once formed, will have a great effect on the productivity, however, the productivity of horizontal gas wells gradually goes down with overdisplacing volume increment, and downward margin decreases too. Thus, the models proposed could not only provide basis for a research idea for productivity of fracturing horizontal gas wells considering overdisplacing operation, but also supply guidance for the optimization design of a multi-stage fracturing horizontal wells.

The research progress, significance and relationship of astrostratigraphy with cyclostratigraphy

YAN Jianping, YAN Yu, PENG Jun, LI Zunzhi, GENG Bin, LAI Fuqiang

2017, Vol.29(1): 147–156 Abstract ( 521 ) PDF (2201 KB) ( 456 )

doi: https://doi.org/10.3969/j.issn.1673-8926.2017.01.020

Cyclostratigraphy has made great progress in explaining many problems in the field of geology,and provides a new way for stratigraphic classification of Ma scale and subscale. The cyclostratigraphy does not highlight the influence of earth orbit parameters on the stratigraphic cyclicity,and it cannot be distinguished from formation cycles resulted by other driving force,which produces some confusion. So,scholars put forward“astrostratigraphy” to solve these problems. On the basis of a large number of literature research,the connotation and development process of astrostratigraphy and its relationship with cyclostratigraphy were summarized. In addition, it is pointed out that the methods such as logging time-frequency analysis,spectrum analysis and digital filter have obvious advantages in astrostratigraphy research. These methods can be used to effectively extract stratigraphic response information related to earth orbit parameters,and then determine the stratigraphic age of Ma scale and subscale. With the development of the unconventional hydrocarbon exploration and development,the astrostratigraphy will further lead to the development of sub-Milankovitch cycle with a higher precision,so as to meet the needs of geological stratification in thick shale oil and gas exploration and development.