Sub-cirtical Water Extraction Of Organic Matter From Oil Shale Lumps

Oil shale is an organic-rich fine-grained sedimentary rock which can generateshale oil after retorting. It is a kind of unconventional oil and gas resource. Oil shaleresources are abundant in the earth. Estimates of global deposits, converted into shaleoil, are3times higher than the proved oil reserves. The resource of shale oil in Chinais second in the world next after the United States. It has a very great developmentprospects.There is no need to mining out the oil shale by the in-situ retorting technology.The in-situ technology can exploit the deep and high thickness oil shale resourceswith several advantages as high conversion rate, few occupied space and lowenvironmental pollution. Accordingly, many countries which are rich in oil shale andsome large energy companies are actively developing the in-situ retortingtechnologies of oil shale. However, the traditional in-situ retorting technology is notapplied to Chinese oil shale for the low oil content level and high bed depth resources.Concentrating on the characteristics of the oil shale resources in our country, weput forward the concept of using sub-critical water as a heat transfer medium andextractant to the in-situ conversion of oil shale. In this paper, high temperature andhigh pressure reactor was used to simulate the sub-critical water extraction process ofoil shale underground. The impacts of the extraction conditions of sub-critical, thesize, texture and level of the oil shale sample on the organics extraction were studied.The composition of the extracts, the extraction and mass transfer mechanism ofsub-critical water were analyzed. The extraction experiments proved the feasibility ofextracting organic matter from the large-sized oil shale lumps by sub-critical waterand provided some experimental and theoretical basis for the in-situ conversionconcept.The main research contents and results are shown as follows:(I) Sub-critical water extraction of the natural bitumen in oil shale was performed at a low temperature,260°C. The results showed that the increase of system pressurecould enlarge the infiltration of sub-critical water at this temperature and enhance theextraction of bitumen.The composition of the extracted bitumen was very complex. The bitumen wasconsisted of hydrocarbons, ketones, indanones, thiophenes, phenol derivatives andother heteroatomic and aromatic compounds. The content of heteroatomic compoundsin the bitumen was significantly higher than that in the NMP-CS2extracted bitumenfrom oil shale.(II) The extract ability of sub-critical water improved with the increase oftemperature and more small molecular weight organics could be extracted compounds.It showed that some larger molecules were more likely to decomposed in sub-criticalwater at a higher temperature. When the temperature was higher than330°C, thekerogen matters in oil shale began to be decomposed in sub-critical water. And whenthe temperature was increase to350°C, the extraction efficiency of sub-critical waterwould have a significantly improvement. The maximum extract yield ofkerogen-decomposed bitumen at350°C was higher than that at365°C.(III) The results of the GC-MS analysis of the kerogen-decomposed bitumenextracted by sub-ctitical water at350°C showed that the major components of thebitumen were n-alkanes,1-alkene, isoprenoid alkanes, alk-2-one and n-alkanoic acids.The content of aromatics was low in the extracts while it was high in the remainingbitumen which was survived in the oil shale residue. It indicated a selectivity ofsub-critical water that sub-critical water was apt to extract aliphatics relative toaromatics although it was capable of releasing the two kinds of compounds fromkerogen.(IV) Three sizes of oil shale lumps (<1cm,2-4cm and6–10cm in diameter)were extracted by sub-critical water at350°C. The results showed that, the extractyield would not be affected by the size of oil shale when the extraction time washigher than20h. It was because that the oil shale lumps were fractured alone the shaletexture in direction parallel to bedding under the action of sub-critical water and thefractures were considered to be due to the tensile stress generated by expansion associated with kerogen conversion to bitumen. The results of the SEM and BETanalysis of shale residue showed that many micro-and macro-pores appeared on thesurface and interior of the samples. And the pore phenomenon would be more andmore significant with the increase in extraction time.The fractures and the pores greatly improved the extraction ofkerogen-decomposed bitumen by sub-critical water. And the feasibility of extractinglarge-sized oil shale lumps by sub-critical water would provide some reference for theconception of oil shale in-situ extraction by sub-critical water.(V) According to the changing actions of sub-critical water on kerogen, thewhole extraction and mass transfer process was divided into six stages:①Inactionof water,②Permeating of sub-critical water,③Swelling and decomposition ofkerogen,④Effusion of asphaltene and pre-asphaltene,⑤Dissolution of bitumenand⑥Self-separation of oil and water.The formation rate of kerogen-decomposed bitumen was also affected by theactings changes of sub-critical water. At first, before2h, few kerogen wasdecomposed in oil shale because the sub-critical water was undergoing the permeatingprocess. Following that, during2-10h, weak-binding kerogen fragments had beenreleased from oil shale and it was the fastest formation phase of the bitumen products.After that, in10-50h, the tight-linked organic fragments were gradually releasedfrom kerogen. After50h extraction in sub-critical water, most of the organics had beenbroken down from kerogen and the new forming bitumen was very less. In addition,the released compounds would be converted into gaseous compounds in sub-criticalwater.The chemical reactions of kerogen were also changed with the extension ofextraction. Small molecular weight cyclic hydrocarbons, aromatics and isoprenoidswere firstly released from kerogen. After that, the straight chain compounds such asn-alkanes,1-alkenes, alk-2-ones and n-alkanoic acids were cracked from the kerogenmacromolecule. Finally, some other aromatic compounds, which mighe be the corepart of the sub-units of kerogen, were released from kerogen. Moreover, with thecracking of kerogen in sub-critical water, there are also many reactions happen on the extracts, such as the secondary cracking, hydration, oxidation and rearrangementreactions. Through these reactions, the kerogen in oil shale were decomposed andextracted out by sub-critical water.(VI) The oil shale lumps from Fushun, Nong’an and Fuyu were extracted bysub-critical water. The extract yields of the kerogen-decomposed bitumen from thesesamples were high. In which, the extract yields from Fushun and Nong’an oil shalewere higher than the oil yield from the Fischer analysis assay respectively and theextract yield of bitumen from Fuyu oil shale was also more than90%of the Fischeroil yield.The compositions of the bituem from different oil shale samples were similar.The major components were n-alkanes, isohydrocarbons and aromatics with a certainamount of cyclic alkanes, n-alkenes, alcohols, ketones and phenols. The contents ofisohydrocarbons and aromatics in the bitumen from Fushun, Nongan and Fuyu oilshale were much higher and the flowability of these bitumen products were muchbetter than the Huadian oil shale bitumen.