Long-standing and increasing undeveloped natural gas reserve is one of the characteristics in exploration and production of natural gas in China.Analysis on its development process and its distribution in different basins is a necessary precondition for increasing reserve producing rate and keeping enhancement of production.Undeveloped geological reserve of natural gas in China in 2013 was about 5.62 trillion cubic meters with its undeveloped rate of 42.3%,while undeveloped recoverable reserve is 0.535 4 trillion cubic meters with its undeveloped rate of 23.7%.According to the analysis of the distribution of basins and companies of undeveloped reserves and different oilfields,some issues are raised under the condition of new ideas and technologies:(1) undeveloped reserves are the realistic regions for enhancement|(2) insight into present situation and dynamic state of proved reserves|(3) the key is the application of proper technologies.Issues,including specialty of each undeveloped reserve,new round deliberate seismic work,moderate lead-in of drilling and reservoir reconstruction,advanced methods in developing middle-small gas fields and more attention on development of dissolved gas,should be emphasized.Three national oil companies should concentrate on making contribution to natural gas production enhancement,narrowing block area,enhancing recovery so as to achieve return on capital and economic benefits.
Paleokarst reservoir is an important target for hydrocarbon exploration and development.Distinguishing of paleokarst development period is always a difficulty facing modeling of paleokarst reservoir.The existing methods for identifying paleokarst period are mostly based on exposure features.
But paleokarstification involves various forms,with some lacking of exposure features,such as hydrothermal flow paleokarstification.Thus,we put forward a comprehensive method involving carbon and oxygen isotopic characteristics of paleokarstic fillings and fluid inclusions characteristics.Based on abundant outcrop data of Tarim Basin,we then analyze the multiple inclusions,and get an average uniform temperature distribution chart based on division of diagenesis sequence.Contrasting discriminant chart of carbon and oxygen isotopes and inclusions homogenization temperature diagram can be used for judging the paleokarst development time.The δ13C-δ18O crossplot,according to the combination of core and cutting relation,was divided into different filling period time chart that are included in the distribution range of values.We apply these charts in period distinguishing of Ordovician paleokarst in Halahatang area and get a well matched result with those obtained based on regional geological background and seismic data interpretation.
FMI (Formation Micro-Scanner Image) is widely used to analyze reservoir properties with the advantages of high resolution,visible and quantified.In this study,11 FMI facies are classified based on the combination of borehole images,cores,thin sections and open-hole logs.It includes reservoir types like carves and tectonic fractures,sedimentary interlayer types like high resistivity facies and two compound facies of fractures and holes.Besides description of karst systems in borehole images,anomalies in GR curve attracting the researchers are analyzed by FMI images.It shows that compound facies of fractures and holes could be effective reservoir instead of caves with high GR value because oil/gas-bearing holes are connected by fractures.Two suites of karst system were formed in the OY1 while small scale caves and holes were formed in the OY2 because of lithological and structure locations.The results of this study demonstrated that,uncomformity and tectonic geomorphology control the differences of the paleokarst system.
The hydrocarbon charging history of Hongliuquan oilfield,Gasikule oilfield and Yingdong oilfield is studied in this paper by synthetically adopting fluid inclusion petrography,fluorescence spectrum,infrared spectrum,microthermometry analysis and burial history modelling.The results illustrate that grain with oil inclusions is well developed in the southwestern Qaidam Basin with the index of grain with oil inclusion (GOI) value between 5%-90%.According to the characteristics of fluorescence color,occurrence,host mineral of oil inclusion,two fluid inclusion assemblages can be classified as yellow fluorescence inclusion and blue fluorescence inclusion.The peak of fluorescence spectrum and methyl content of infrared spectrum both reflect that the maturity of blue fluorescence inclusion is higher than that of yellow fluorescence inclusion.Combined with homogenization temperature of fluid inclusions and thermal evolution analysis,it is confirmed that there were mainly two episodes of hydrocarbon charging in the southwestern Qaidam Baisn.The charging time of Gasikule oilfield and Hongliuquan oilfield is 26Ma and 10Ma,respectively.The charging time of Yingdong oilfield is separately 10Ma and 5Ma.The later hydrocarbon charging process and tectonic evolution control the preservation condition and hydrocarbon enrichment.
To analyze the controlling influence of original sedimentary environment on diagenesis,and highlight the distinctiveness of the diagenesis of the red-bed reservoir,based on various analyses such as thin section identification,scanning electron microscope,core analysis,X-ray diffraction mineral analysis,etc.,alluvial plain of Wangjiagang and beach-bar of Boxing were selected for a comparative research between red-bed sandstone and grey-bed sandstone.The paper qualitatively analyzed the diagenetic characteristics and quantitatively calculated the diagenetic products,evolution process of diagenetic porosity with two types of sandbody.The results show that,grey-bed beach sandstones have characteristics of late ferrous-bearing carbonate cements,strong compaction,strong denudation|Red-bed reservoirs have characteristics of early carbonate cementation,weak compaction,and weak denudation.The two kinds of sandbodies experienced diagenesis of different intensity,resulting in the obvious difference between the diagenetic porosity evolution and pore types of reservoir,red-bed reservoir is mainly primary pore,reservoir of beach-bar sandstone is dominated by secondary pores.
There was extensive evaporative fractionation effect in Xihu Depression,Eastern Sea Basin.However,most of the previous study on the gas washing is qualitative description.Taking Kongqueting region in Pinghu structural belt as an example,based on the geochemical characteristics and fracture characteristic,this paper discussed geologic conditions and main controlling factors of gas washing and analyzed effect of hydrocarbon reservoir by fluid inclusion system analysis technology and n-alkanes testing method.These findings suggest that coal measures which can generate gas continuously have abundant organic matter and provide good source rocks.Numerous faults and good match between oil and gas filling time result in the occurrence of gas washing in the Kongqueting region.Quantitative calculation of gas washing fractionation effect show that n-alkane loss rate of crude oil in the wells in the study area is 35.3% to 97.1% and break number is 18 to 27.Source distance and vertical depth control strength of gas washing fractionation effect.Besides,the former is more powerful controlling function than the latter.Gas washing fractionation effect not only makes gas layers and ratio of condensate gas layers decrease along the direction of the gas invasion,but also leads to the pattern of oil reservoir above gas reservoir and formation of condensate with low wax content and condensate gas with high wax content.
Shale gas reservoirs can get industrial gas flow now due to the technical progress of horizontal well drilling and hydraulic fracturing.Based on the special flow mechanism and desorption phenomenon of shale gas reservoir,this paper presents a transient analysis model of multi-stage fractured horizontal well with the consideration of wellbore storage,skin effect and hydraulic fractures parameters of shale gas reservoirs.Accurate solution to this flow model was obtained by the use of source function theory,Laplace transform,three-dimensional eigenvalue method and orthogonal transformation.Production decline type curves were plotted by using the Stehfest algorithm.Seven different flow regimes were identified.The effects of influence factors such as skin coefficient,absorption index,fractures number,horizontal well length and fracture permeability were discussed.This research could be used to interpret the production decline behavior more accurately and effectively for shale gas reservoirs.
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