An Experimental Study On Thermal Cracking Of Crude Oil In Reservoir

The kinetics of hydrocarbon generation and carbon isotopic fractionation combined with various organic geochemical technologies and methods(such as kinetics simulation experiment of hydrocarbon generation, gas chromatography, and gas chromatography-isotope ratio mass spectrometry) are used in this study to investigate the geochemical characteristics of gaseous products and solid bitumen during the process of crude oil cracking in reservoir environments. A confined gold tube-autoclave system and a heating mode of constant temperature(365°C) and constant pressure(50MPa) were used to simulate the crude oil cracking in different reservoir media. In addition, a series of simulation experiments on the oil cracking under the adding of different sulfates and determination of the yields and the stable carbon isotopes of generated hydrocarbon gases and non-hydrocarbon gases were used to further study the mechanism of thermochemical sulfate reduction(TSR).The pyrolysates of oil cracking, including gaseous hydrocarbons, C6-C12 light hydrocarbon and solid bitumen, were quantitatively analyzed using a series of the pyrolysis simulation experiments at a constant heating rate(20℃/h, 2℃/h). Based on the pyrolysis data and kinetic calculation, a correlation of gaseous hydrocarbons and solid bitumen during the process of oil cracking has been established and extrapolated to geological condition. Combined with the geological background and related parameters of Feixianguan formation in NE Sichuan basin, an evaluation method based on the solid bitumen content was suggested and used to estimate the lower limit content of reservoir solid bitumen in a large gas field. It can provide scienfic evidences to identify hydrocarbon sources, delineate the range of paleo-reservoirs, and assess the size of paleo-oil reservoirs. The main conclusions are as follows:1. Water and minerals can promote the crude oil cracking and lead to the significant increase of gas yields, for example, the yield of alkane gases increases 2 times about and the yields of H2 and CO2 also increase significantly.2. The adding of magnesium sulfate can make the TSR happen in this reaction system of oil cracking and the production of hydrocarbon gas and non-hydrocarbon gas such as H2 S increase significantly. At the same time, TSR makes hydrocarbon gas become dryer. The result of carbon isotopes shows that TSR makes the carbon isotopes of gaseous hydrocarbons become heavier. Sodium chloride solution can promote the TSR, which makes the production of hydrocarbon gas and non-hydrocarbon gas increase and makes the carbon isotopes of gaseous hydrocarbon become heavier, especially for ethane, resulting in a positive difference of up to 4‰. So the influences of TSR should be taken into consideration when the carbon isotopes of gaseous hydrocarbons are used to the gas-source rock correlation in some place.3. The elemental composition of Solid bitumen in the Feixianguan formation of NE, Sichuan Basin is characterized by high S/C ratio and low H/C ratio. The Changxing formation is considered as primary hydrocarbon source rock for the solid bitumen according to the comparative analysis data of solid carbon isotope composition. Biomarker parameters and bitumen reflectance indicate that the solid bitumen have experienced a relatively high thermal maturity and high temperature deposition process, which can provide a geological condition for the Thermochemical Sulfate Reduction in Feixianguan formation.4. The formation of solid bitumen begins in the condensate and wet gas stages(Easy Ro=1.0%~2.1%) and mainly generated in the dry stage(Easy Ro >2.1%), indicating that solid bitumen is the oil pyrolysates at highly mature stages. The quantitative results show that a good correlation occurs between the yields of methane and solid bitumen during the process of oil cracking. A conversion parameter(CR) between methane and solid bitumen yields can be established based on the above relationship. Preliminary results show that the solid bitumen content of 1.22% is likely to be an important parameter to delineate areas of paleo-oil accumulation. Then this parameter can extrapolate to geological conditions and can provide some experimental evidences and new ideas for the resource assessment of natural gas in the high- and over- mature marine carbonate source rock areas in China.