Catalytic Pyrolysis And Kinetics Of Oil-based Drilling Cuttings In Shale Gas Exploitation

As an inevitable product of shale gas exploitation,oil based drilling cuttings（OBDCs）production is increasing along with the large-scale extraction of shale gas.OBDCs contain toxic substances such as petroleum hydrocarbons and heavy metals.If not well treated or long-term stockpiling in the well field cause leakage,will cause serious harm to the surrounding environment.This study conducted catalytic pyrolysis of OBDCs to recover petroleum hydrocarbon for realizing the resource utilization.Based on the analysis of the basic physicochemical properties and leaching toxicity of OBDCs,the pyrolysis characteristics of drilling chips were studied by TG-DSC The effects of different heating rates for the pyrolysis process were investigated and kinetic parameters were calculated.The effects of temperature and 4 catalysts（HZSM-5,Fe₂O₃,Al₂O₃ and 5A zeolite）on the pyrolysis product distribution and pyrolysis oil composition were analyzed.The Immobilization of heavy metals was investigated by comparing the contents of heavy metals in the solid phase before and after pyrolysis.The primary pollutant of OBDCs is petroleum hydrocarbons.with a content of108482 mg/kg,which is of great recovering value.The components of heavy metals are relatively low,and did not exceed the standard（GB4284-2018）.The concentration of toluene in the leaching solution is high,and 9 kinds of EPA monitoring PAHs can be detected:naphthalene,phenanthrene,fluorene,pyrene,Nene,[1,2,3-cd]pyrene,benzo[b]fluoranthene,benzo[a]anthracene,benzo[g,h,i]perylene and fluoranthene.The pyrolysis process consists of three steps,i.e.,25¹00℃,100³00℃and300⁶00℃.The last two main stages are volatilization of light oil component and heavy oil cracking.TG curves moved toward higher temperature zone caused by thermal hysteresis with the increase of temperature rising rate.The Coats-Redfern method gets higher correlation coefficient,and the activation energy（E）is average,so the single mechanism function can not accurately describe the pyrolysis reaction.The experimental data can be fitted very well by 3 kinds of iso-conversion rate methods（FWO,Vyazovkin and Friedman method）In the second stage,E changed a little with conversion,had almost the same results as CR method（14.39²0.08 kJ?mol-1）.For reactions of heavy oil cracking with mixed mechanism,E rose gradually with the increase of reaction time.Results of FWO method were similar to the Vyazovkin method,and the Friedman method is more accurate.E was given as 155.74⁵61.10kJ?mol^-1,141.06⁵24.96 kJ?mol^-1 and 74.37⁶05.10 kJ?mol^-1,respectively.The pyrolysis temperature has a great influence on the yield of the product,the pyrolysis liquid content reached the maximum at 500 C.The addition of catalyst can increase the yield of the pyrolytic liquid,in which the HZSM-5 and 5A zeolite have the most obvious influence.GC-MS analysis of fatty hydrocarbon and aromatic hydrocarbon components of pyrolytic oil showed that the distribution of carbon number were mainly concentrated in C₁₂-C₂₁ and C_7.6^C_15.5,HZSM-5 and 5A zeolite promoted the formation of fatty hydrocarbons in low carbon segments and reduced fatty hydrocarbons in high carbon segments.After pyrolysis,heavy metals were increased and more concentrated in the residue.Pyrolysis is a good method for immobilization of heavy metals.