Research On Shale Oil Reservoir,Occurrence And Movability Using Nuclear Magnetic Resonance(NMR)

Shale oil reservoir,occurrence and movability are the core and difficult issues of the shale oil geology.Nuclear magnetic resonance(NMR)is an effective experimental technique to unify the characterization and establish interrelation of these three issues.In this paper,shale samples were collected from the Shahejie Formation in the Dongying Sag,Bohai Bay Basin.Based on the NMR technique,experimental work such as XRD,TOC and rock pyrolysis,low temperature nitrogen gas adsorption-desorption(NGA),scanning electron microscopy(SEM),mercury intrusion capillary pressure(MICP),X-ray CT,thermogravimetric analysis,centrifugal analysis and seepage simulation,were carried out on the shale cores.Shale reservoir characteristics,such as the pore systems,pore structures,porosity and permeability,as well as the contents,occurrence and the interconversion of shale oil at different states were studied and analyzed.Moreover,the movable amount and the flow law of shale oil were also revealed.And the favorable lithofacies for shale oil was identified.The application of NMR technology in the research of the shale pore systems,pore structures,porosity and permeability was systematically analyzed,and the NMR quantitative evaluation system for the shale reservoir was established.According to the characteristics of the shale reservoirs,the optimal parameters for shale NMR tests were calibrated,and the NMR test method for shale(taking the dry sample as the base of the saturated oil sample)was established to accurately evaluate the reservoir characteristics.Then,the shale porosity and permeability were obtained using NMR.An NMR T₂ spectrum identification method for shale pore system was established based on the T₂ spectrum peak distributions of oil-saturated shales.The shale pore system was divided into micropore(<100 nm),mesopore(100 nm?1000 nm)and macropore(>1000 nm).The“segment joint”calibration method for shale T₂ spectral pore size distribution was established using the pore size distributions of NGA measured micropores and SEM tested mesopores,and the pore size distributions of shales were quantitatively evaluated.According to the shale T₂ peak distributions,a quantitative T₂ spectrum evaluation method for grading shale reservoirs was established,and the shale reservoirs were divided into four categories.Then,the T₂ spectral multi-fractal characteristics of various shale reservoirs were calculated,and the pore structure complexity and heterogeneity were quantitatively evaluated.The NMR T₁-T₂ plate for the shale main hydrogen nuclei identification was obtained by analyzing the NMR characteristics of shale,clay minerals and kerogen in different states,and the shale oil occurrence states were revealed.Shale adsorption/free oil quantitative evaluation methods were established by thermogravimetry-NMR and centrifugal-NMR measurements.Then,adsorption/free oil amount and distributions at different temperatures were revealed to establish and calibrate shale adsorption/free oil amount evaluation models.The occurrence pore sizes,conversion lows and NMR T₁-T₂ relaxation characteristics of adsorption and free(irreducible and movable)oil were also revealed.Finally,the shale oil occurrence mode was established.The shale movable oil contents were obtained using cterifugal-NMR method.The results showed that shale oil had lower momentum and poor flowability,and the movable oil amount increased with the increasing of free oil.Based on the physics simulation experiments,the shale oil seepage law was revealed,and the effects of the effective stress and boundary layer effects on shale oil seepage were analyzed,which are the key factors in controlling shale oil seepage.The shale oil seepage evaluation model controlled by the effective stress and the boundary layer effects was obtained.The distribution characteristic of porosity,permeability,adsorption/free oil,movable oil and seepage lows of shales samples with different lithofacies were analyzed,and the favorable lithofacies for shale oil were revealed.Namely,lamellar organic-rich calcareous shale and massive organic siliceous mudstone are the most favorable shale oil reservoirs,which is most beneficial to shale oil accumulation,occurrence and flow.