Evaluation technology and practice of continental shale oil development in China

This paper analyzes the differences in geological and development characteristics between continental shale oil in China and marine shale oil in North America, reviews the evaluation methods and technological progress of the continental shale oil development in China, and points out the existing problems and development directions of the continental shale oil development. The research progress of development evaluation technologies such as favorable lithofacies identification, reservoir characterization, mobility evaluation, fracability evaluation, productivity evaluation and geological-mathematical modeling integration are introduced. The efficient exploration and development of continental shale oil in China are faced with many problems, such as weak basic theoretical research, imperfect exploration and development technology system, big gap in engineering technology between China and other countries, and high development cost. Three key research issues must be studied in the future:(1) forming differentiated development technologies of continental shale oil through geological and engineering integrated research;(2) strengthening the application of big data and artificial intelligence to improve the accuracy of development evaluation;(3) tackling enhanced shale oil recovery technology and research effective development method, so as to improve the development effect and benefit.

Key exploration & development technologies and engineering practice of continental shale oil: A case study of Member 2 of Paleogene Kongdian Formation in Cangdong Sag, Bohai Bay Basin, East China

Based on detailed core description and systematic joint test data,enrichment laws of continental shale oil have been examined deeply.Key technologies such as the identification and quantitative evaluation method for sweet spot,precise design and tracking of horizontal well trajectory,and the low-cost horizontal well volume fracturing technology of the whole process"slick water+quartz sand"for continental shale oil have been formed.The research results show that the enrichment of pure continental shale oil of the Paleogene Kong 2 Member in Cangdong Sag is controlled by predominant fabric facies and cross-over effect of retained hydrocarbons jointly;and there are four modes of shale oil enrichment,i.e.laminar felsic,laminar mixed,thin-layer limy dolomitic,and thick-layer limy dolomitic shales.The identification and evaluation method for shale oil sweet spots can predict sweet spots accurately.The precise trajectory design for sweet spot layer and tracking-trajectory optimization while drilling by considering geological and engineering factors have been proved effective by field application,with drilling rate of sweet spots reaching 100%and drilling rate of type I sweet spots reaching over 75%.The whole process"slick water+quartz sand"low cost volume fracturing has been proved effective in creating multi-stage fracture network in the horizontal section,and improved productivity greatly.It can lower the comprehensive engineering cost by 26.4%.

Heterogeneous geological conditions and differential enrichment of medium and high maturity continental shale oil in China

Based on the comparison of basic geological conditions and enrichment characteristics of shale oil plays, the heterogeneity of source and reservoir conditions and differential enrichment of medium-high maturity continental shale oil plays in China have been confirmed.(1) Compared with the homogeneous geological settings and wide distribution of marine shale oil strata in North America, the continental medium and high maturity shale oil plays in China are significantly different in geological conditions generally;continental multi-cyclic tectonic evolution forms multiple types of lake basins in multi-stages, providing sites for large-scale development of continental shale oil, and giving rise to large scale high-quality source rocks, multiple types of reservoirs, and diverse source-reservoir combinations with significant heterogeneity.(2) The differences in sedimentary water environments lead to the heterogeneity in lithology, lithofacies, and organic material types of source rocks;the differences in material source supply and sedimentary facies belt result in reservoirs of different lithologies, including argillaceous and transition rocks, and tight siltstone, and complex source-reservoir combination types.(3) The heterogeneity of the source rock controls the differentiation of hydrocarbon generation and expulsion, the diverse reservoir types make reservoir performance different and the source-reservoir configurations complex, and these two factors ultimately make the shale oil enrichment patterns different. Among them, the hydrocarbon generation and expulsion capacity of high-quality source rocks affect the degree of shale oil enrichment. Freshwater hydrocarbon source rocks with TOC larger than 2.5% and saline hydrocarbon source rocks with TOC of 2% to 10% have a high content of retained hydrocarbons and are favorable.(4) High-abundance organic shale is the basis for the enrichment of shale oil inside the source. In addition to being retained in shale, liquid hydrocarbons migrate along laminae, diagenetic fractures, and thin sandy layers, and then accumulate in laminae of argillaceous siltstone, siltstone, and argillaceous dolomite, and dolomitic siltstone suites, etc. with low organic matter abundance in the shale strata, resulting in differences in enrichment pattern.

Microscale comprehensive evaluation of continental shale oil recoverability

This paper targets the shale oil reservoirs of middle to high maturity in four major basins of China,including the Permian Lucaogou Formation of the Jimsar Sag in the Junggar Basin,the Chang 73 Member of the Triassic Yanchang Formation in the Longdong area of the Ordos Basin,the Kong 2 Member of the Paleogene Kongdian Formation in Cangdong Sag of the Bohai Bay Basin,and the Qing 1 Member of the Cretaceous Qingshankou Formation in Changling Sag of the Songliao Basin.The key parameters of the shale oil reservoirs in the four basins,such as reservoirs effectiveness,oil content,crude oil movability,and fracability,have been revealed under identical experimental conditions using the same evaluation technical system,on the basis of technique development and integrated application of multi-scale spatial distribution depiction,effective connectivity calculation,movable oil assessment based on the charging effect,and simulation of fracture propagation during reservoir stimulation.This research overcomes insufficient resolutions of conventional analysis approaches and difficulties in quantitative evaluation,develops the evaluation method for resource recoverability of different types of shale oil,and gains insights into different types of shale oil via comparison.The results of experiments and comparative analysis show that there are significant differences in the endowment of continental shale oil resources in the four major basins in China.Among them,the Lucaogou Formation in the Junggar Basin has more effective shale reservoirs,the Chang 73 sub-member of the Ordos Basin has a comparatively good proportion of movable oil and the Kong 2 Member of the Bohai Bay Basin has the best fracability.These results can provide references and basis for choosing development plans and engineering techniques.

A review of commercial development of continental shale oil in China

Continental shale oil is an important strategic supplement to the sustainable development of petroleum industry in China.Based on the thermal maturity of organic matter,most shale oil in China is mediumhigh or medium-low mature oil with relatively high density that is usually stored in thin layers and difficult to extract due to strong fluid flow resistance.Most petroliferous basins in China host shale oil but the abundance and distribution are not favorable for operators looking for a commercial exploitation.This study systematically investigates the recent progress of shale oil exploration and development in China.The result shows that the country is facing the following challenges while trying to explore its shale oil resources:complex geological conditions;a lack of established effective development mode;high development cost;and an absence of substantial breakthroughs in key technologies.Given these,the study suggests from research and policy-making perspectives to strengthen the research on basic theories and key technologies,establish demonstration zones for development of different types of shale oil,pursue breakthroughs in a stepwise approach,innovate the management mode to reduce the development cost,set shale oil as an independent mineral type,and give special policy incentives.

Shale oil development techniques and application based on ternary-element storage and flow concept in Jiyang Depression,Bohai Bay Basin,East China

The ternary-element storage and flow concept for shale oil reservoirs in Jiyang Depression of Bohai Bay Basin,East China,was proposed based on the data of more than 10000 m cores and the production of more than 60 horizontal wells.The synergy of three elements(storage,fracture and pressure)contributes to the enrichment and high production of shale oil in Jiyang Depression.The storage element controls the enrichment of shale oil;specifically,the presence of inorganic pores and fractures,as well as laminae of lime-mud rocks,in the saline lake basin,is conducive to the storage of shale oil,and the high hydrocarbon generating capacity and free hydrocarbon content are the material basis for high production.The fracture element controls the shale oil flow;specifically,natural fractures act as flow channels for shale oil to migrate and accumulate,and induced fractures communicate natural fractures to form complex fracture network,which is fundamental to high production.The pressure element controls the high and stable production of shale oil;specifically,the high formation pressure provides the drive force for the migration and accumulation of hydrocarbons,and fracturing stimulation significantly increases the elastic energy of rock and fluid,improves the imbibition replacement of oil in the pores/fractures,and reduces the stress sensitivity,guaranteeing the stable production of shale oil for a long time.Based on the ternary-element storage and flow concept,a 3D development technology was formed,with the core techniques of 3D well pattern optimization,3D balanced fracturing,and full-cycle optimization of adjustment and control.This technology effectively guides the production and provides a support to the large-scale beneficial development of shale oil in Jiyang Depression.

Enrichment factors of movable hydrocarbons in lacustrine shale oil and exploration potential of shale oil in Gulong Sag,Songliao Basin,NE China

The geological characteristics and production practices of the major middle-and high-maturity shale oil exploration areas in China are analyzed.Combined with laboratory results,it is clear that three essential conditions,i.e.economic initial production,commercial cumulative oil production of single well,and large-scale recoverable reserves confirmed by the testing production,determine whether the continental shale oil can be put into large-scale commercial development.The quantity and quality of movable hydrocarbons are confirmed to be crucial to economic development of shale oil,and focuses in evaluation of shale oil enrichment area/interval.The evaluation indexes of movable hydrocarbon enrichment include:(1)the material basis for forming retained hydrocarbon,including TOC>2%(preferentially 3%-4%),and typeⅠ-Ⅱkerogens;(2)the mobility of retained hydrocarbon,which is closely related to the hydrocarbon composition and flow behaviors of light/heavy components,and can be evaluated from the perspectives of thermal maturity(Ro),gas-oil ratio(GOR),crude oil density,quality of hydrocarbon components,preservation conditions;and(3)the reservoir characteristics associated with the engineering reconstruction,including the main pore throat distribution zone,reservoir physical properties(including fractures),lamellation feature and diagenetic stage,etc.Accordingly,13 evaluation indexes in three categories and their reference values are established.The evaluation indicates that the light shale oil zones in the Gulong Sag of Songliao Basin have the most favorable enrichment conditions of movable hydrocarbons,followed by light oil and black oil zones,containing 20.8×10^(8) t light oil resources in reservoirs with R_(0)>1.2%,pressure coefficient greater than 1.4,effective porosity greater than 6%,crude oil density less than 0.82 g/cm^(3),and GOR>100 m/m^(3).The shale oil in the Gulong Sag can be explored and developed separately by the categories(resource sweet spot,engineering sweet spot,and tight oil sweet spot)depending on shale oil flowability.The Gulong Sag is the most promising area to achieve large-scale breakthrough and production of continental shale oil in China.

Key theoretical and technical issues and countermeasures for effective development of Gulong shale oil, Daqing Oilfield, NE China

Aiming at the four issues of underground storage state,exploitation mechanism,crude oil flow and efficient recovery,the key theoretical and technical issues and countermeasures for effective development of Gulong shale oil are put forward.Through key exploration and research on fluid occurrence,fluid phase change,exploitation mechanism,oil start-up mechanism,flow regime/pattern,exploitation mode and enhanced oil recovery(EOR)of shale reservoirs with different storage spaces,multi-scale occurrence states of shale oil and phase behavior of fluid in nano confined space were provided,the multi-phase,multi-scale flow mode and production mechanism with hydraulic fracture-shale bedding fracture-matrix infiltration as the core was clarified,and a multi-scale flow mathematical model and recoverable reserves evaluation method were preliminarily established.The feasibility of development mode with early energy replenishment and recovery factor of 3o%was discussed.Based on these,the researches of key theories and technologies for effective development of Gulong shale oil are proposed to focus on:(1)in-situ sampling and non-destructive testing of core and fluid;(2)high-temperature,high-pressure,nano-scale laboratory simulation experiment;(3)fusion of multi-scale multi-flow regime numerical simulation technology and large-scale application software;(4)waterless(CO_(2))fracturing technique and the fracturing technique for increasing the vertical fracture height;(5)early energy replenishment to enhance oil recovery;(6)lifecycle technical and economic evaluation.Moreover,a series of exploitation tests should be performed on site as soon as possible to verify the theoretical understanding,optimize the exploitation mode,form supporting technologies,and provide a generalizable development model,thereby supporting and guiding the effective development and production of Gulong shale oil.

Practice and development suggestions of hydraulic fracturing technology in the Gulong shale oil reservoirs of Songliao Basin, NE China

This paper reviews the multiple rounds of upgrades of the hydraulic fracturing technology used in the Gulong shale oil reservoirs and gives suggestions about stimulation technology development in relation to the production performance of Gulong shale oil wells.Under the control of high-density bedding fractures,fracturing in the Gulong shale results in a complex fracture morphology,yet with highly suppressed fracture height and length.Hydraulic fracturing fails to generate artificial fractures with sufficient lengths and heights,which is a main restraint on the effective stimulation in the Gulong shale oil reservoirs.In this regard,the fracturing design shall follow the strategy of"controlling near-wellbore complex fractures and maximizing the extension of main fractures"Increasing the proportions of guar gum fracturing fluids,reducing perforation clusters within one fracturing stage,raising pump rates and appropriately exploiting stress interference are conducive to fracture propagation and lead to a considerably expanded stimulated reservoir volume(SRV).The upgraded main hydraulic fracturing technology is much more applicable to the Gulong shale oil reservoirs.It accelerates the oil production with a low flowback rate and lifts oil cut during the initial production of well groups,which both help to improve well production.It is suggested to optimize the hydraulic fracturing technology in six aspects,namely,suppressing propagation of near-wellbore microfractures,improving the pumping scheme of CO_(2),managing the perforating density,enhancing multi-proppant combination,reviewing well pattern/spacing,and discreetly applying fiber-assisted injection,so as to improve the SRv,the distal fracture complexity and the long-term fracture conductivity.

Organic matter transformation ratio, hydrocarbon expulsion efficiency and shale oil enrichment type in Chang 7_(3) shale of Upper Triassic Yanchang Formation in Ordos Basin, NW China

The major enrichment type of shale oil in the Chang 7_(3) shale of Upper Triassic Yanchang Formation in the Ordos Basin is unknown.This paper analyzes the organic matter transformation ratio,hydrocarbon expulsion efficiency and roof/floor sealing conditions of the Chang 7_(3) shale,and evaluates the major enrichment type of shale oil in this interval.The average organic matter transformation ratio of the Chang 7_(3) shale is about 45%;in other words,more than 50%of the organic matters have not transformed to hydrocarbons,and the lower the maturity,the greater the proportion of untransformed organic matters.The cumulative hydrocarbon expulsion efficiency of the transformed hydrocarbon is 27.5% on average,and the total proportion of untransformed organic matters plus retained hydrocarbons is greater than 70%.The relative hydrocarbon expulsion efficiency of the Chang 7_(3) shale is 60%on average,that is,about 40% of hydrocarbons retain in the shale.The Chang 7_(3) shale corresponds to Chang 7_(1+2) and Chang 8 sandstones as the roof and floor,respectively,and is further overlaid by Chang 6 shale,where extensive low porosity and low permeability–tight oil reservoirs have formed in the parts with relatively good porosity and permeability.Moreover,the Chang 7_(3) shale is tested to be in a negative pressure system(the pressure coefficient of 0.80–0.85).Therefore,the roof/floor sealing conditions of the Chang 7_(3) shale are poor.The retained hydrocarbons appear mostly in absorbed status,with low mobility.It is concluded that the medium–high mature shale oil is not the major enrichment type of shale oil in the Chang 7_(3) shale,but there may be enrichment opportunity for shale oil with good mobility in the areas where the sealing conditions are good without faults and fractures and oil reservoirs are formed off Chang 7_(1+2),Chang 6 and Chang 8.Furthermore,low–medium mature shale oil is believed to have great potential and is the major enrichment type of shale oil in the Chang 7_(3) shale.It is recommended to prepare relevant in-situ conversion technologies by pilot test and figure out the resource availability and distribution.

Characteristics,Types,and Prospects of Geological Sweet Sections in Giant Continental Shale Oil Provinces in China

“Sweet sections”in giant shale oil provinces are preferential fields that primarily support China to increase the reserves and production of continental shale oil.Based on the study of the geological conditions of shale oil in the continental basins in China,it was found that the shale stratum in major oil generation windows generally has higher degrees of oil and gas accumulation,and mostly contains oil.Hydrocarbon generation and reservoir capacities are the two key parameters for evaluating and optimizing favorable shale oil provinces.The evaluation index(volume of shale stratum multiplied by total organic carbon(TOC)multiplied by total porosity)for the giant continental shale oil provinces is also proposed.It is optimized that the Upper Triassic Chang 7 Member in the southcentral Ordos Basin,Lower Cretaceous Qing 1 Member in the Gulong-Changling Sag in the Songliao Basin,Middle-Lower Permian in the Junggar Basin,Da’anzhai Member of the Ziliujing Formation of Lower Jurassic in the central and northern Sichuan Basin,and Paleogene oil-rich sag in the Bohai Bay Basin are the five giant continental shale oil provinces.The word“geological sweet sections”in continental shale oil provinces of China refers to favorable shale intervals which are relatively rich in oil,with superior physical properties,and more easily modified and developed commercially under applicable economic and technological conditions.After evaluation,there are mainly two types of“geological sweet sections”of giant continental shale oil developed onshore in China.One type of“geological sweet sections”is generally mudstone with optimal physical properties or a thin tight reservoir,to which the shale oil migrates a short distance.They are medium-to-high-mature zones with a thin sandy shale stratum in the Chang 7 Member in the Ordos Basin,mixed shale stratum in the mediummature Lucaogou Formation in the Jimsar Sag,and multi-layered mixed Paleogene shale stratum in the Bohai Bay Basin.The other type of“geological sweet sections”is generally shale oil residing in various shale reservoir spaces.This type was developed in the Qing 1 Member in the Gulong Sag and Da’anzhai Member in the north-central Sichuan Basin.Free shale oil mainly occurs in shale,sandycarbonate lamina,micro-lamella structure,and micro-fractures.Layers with lamina,lamination,and micro-fractures are generally shale oil“geological sweet sections.”Starting from field tests and the construction of the“geological sweet sections”in giant continental shale oil provinces,the shale oil industry has been rapidly developing and will become an important supplement to domestic oil production in China.

An analysis of major scientific problems and research paths of Gulong shale oil in Daqing Oilfield, NE China

After the preliminary basic research on the problems encountered during the production period of Gulong shale oil in the Songliao Basin, NE China, and the scientific exploration, the special characteristics of Gulong shale oil in terms of reservoir space, phase distribution, flow pattern, and mineral evolution are proposed. The main results are as follows :(1) The source of organic matter, mechanism of hydrocarbon generation and expulsion, and key factors affecting shale oil abundance;(2) The types and structural characteristics of the reservoir and their contribution to porosity and permeability;(3) The mineral origin and evolution of minerals and their influence on reservoir availability, sensitivity, and compressibility;(4) The rock mechanical characteristics and fracture propagation law of Gulong shale;(5) The shale oil products, phase change law and main control factors of adsorption and desorption conversion of Gulong shale oil;(6) The mechanism of shale oil-liquid, solid-liquid gas interaction and enhanced oil recovery. Three key research suggestions are proposed to realize the large-scale economic utilization of the Gulong shale oil as follows:(1) Deepen research on the mechanism of oil and gas generation and discharge, storage and transportation, to guide the selection of geological sweet spots of shale oil;(2) Deepen research on the compressibility and fracture initiation mechanism to support the selection of engineering sweet spots and optimization of engineering design;(3) Deepen research on the fluid interaction mechanism under reservoir conditions, os us to guide the optimization of development schemes and the selection of EOR technologies. A successful development of Gulong shale oil requires global experts and scholars to contribute multidisciplinary innovative ideas and technical ideas to solve production problems.

Geological theory and exploration & development practice of hydrocarbon accumulation inside continental source kitchens

Continental shale strata in China are rich in petroleum resources and are an important area to strengthen domestic oil exploration and development.Based on the latest progress in geological research and exploration and development of petroleum inside continental source kitchens of China National Petroleum Corporation exploration areas in recent two years,we have achieved the following results:(1)The geological connotations of continental hydrocarbon accumulation inside source kitchen and"sweet spot"have been proposed.The intra-source petroleum accumulation refers to the accumulation of liquid-rich hydrocarbons retained or captured in the continental organic-rich shale strata,and"sweet spot"refers to the favorable reservoir with higher oil content,better physical properties,easier to stimulate and higher in commercial development value in the overall oil-bearing continental source rock series,they can be divided into three types,interlayer,hybrid sediment and shale.(2)High-quality shale formations in both salt water and freshwater lacustrine basins can generate hydrocarbons on a large scale,shale strata have multiple types of favorable reservoirs with large-scale storage capacity,the intra-source shale strata are overall oil-bearing and large in resource scale,and there are multiple favorable shale series for development.The exploration and development practice is propelling the formation of a series of exploration and development key technologies with"sweet spot exploration"and"volume development"as the core.Some pilot tests of these technologies have provided an important scientific basis for the economic and effective development of hydrocarbon accumulation inside source kitchen,and popularization of these technologies have achieved encouraging results preliminarily.(3)Two types of continental intra-source petroleum resources in China have great potential,including medium-high maturity with liquid-rich hydrocarbons and medium-low maturity with organic-rich matter.The Ordos,Songliao,Bohai Bay and Junggar basins are the main areas of these resources.By addressing the theoretical and technical challenges in the exploration and development,the two types of resources inside continental source kitchens will become the realistic and major strategic replacement oil resources respectively in the future.

Multi-source genesis of continental carbonate-rich fine-grained sedimentary rocks and hydrocarbon sweet spots

This paper systematically discusses the multiple source characteristics and formation mechanisms of carbonate-rich fine-grained sedimentary rocks through the analysis of material source and rock formation.The hydrocarbon accumulation characteristics of carbonate-rich fine-grained sedimentary rocks are also summarized.The results show that the main reason for the enrichment of fine-grained carbonate materials in rift lake basins was the supply of multiple material sources,including terrestrial material input,formation of intrabasinal authigenic carbonate,volcanic-hydrothermal material feeding and mixed source.The development of carbonate bedrock in the provenance area controlled the filling scale of carbonate materials in rift lake basins.The volcanic-hydrothermal activity might provide an alkaline fluid to the lake basins to strengthen the material supply for the formation of carbonate crystals.Authigenic carbonate crystals induced by biological processes were the main source of long-term accumulation of fine-grained carbonate materials in the lake basins.Carbonate-rich fine-grained sedimentary rocks with multiple features were formed through the interaction of physical,biochemical and chemical processes during the deposition and post-deposition stages.The source and sedimentary origin of the fine-grained carbonate rock controlled the hydrocarbon accumulation in it.In the multi-source system,the types of"sweet spots"of continental shale oil and gas include endogenous type,terrigenous type,volcanic-hydrothermal type and mixed source type.