Critical Factors Controlling The Tight Oils Movability Of Continental Lacustrine Basins

The in-situ formation conditions,together with the tight pore and fracture space,contribute mostly to the tight oils movability,and further determine the tight oils accumulation process and the recovery resources.Until now,four issues aroused concerns on the tight oils movability as follows:?1?the dynamic propagation of the nature fracture net and the fracture flow principle during the rock rapture phase;?2?the influence of tight pore-throat space on the movable fluid and the relative permeability;?3?the influence of water-laid tuff on the tight oil generation-migration-storage process;?4?the influence of gas-oil-ratio?GOR?on the tight oils movability in situ.To solve these four issues,we proposed our research in four sections respectively as follows:To study the influence of gas-oil-ratio?GOR?on the tight oils movability in situ,equations of modified total capillary force for the liquid phase are proposed to indicate the tight oil mobility in situ,which are derived based on the Kelvin capillary force equation,the Peng-Robinson,and Van-Laar equations.In addition,relationships between the vapor-liquid state,phase flow state for production and gas-to-oil production ratio?GOR?are established in correspondence with the production data of the North American and China?Yanchang?tight oil plays.To study the dynamic propagation of the nature fracture net and the fracture flow principle during the rock rapture phase,we propose our research as follows:?1?we designed some experiments to study the LF propagation process,considering the controlling factors in terms of mineral component,the lamina angle and the petrophysical faces;?2?we modify the Tien-Kuo?T-K?criterion using four critical parameters(i.e.,the maximum principal stress?s₁?,the minimum principal stress?s₃?,the lamina angle???and the lamina friction coefficient(?_lamina)),and compare it with the other bedding failure criterion to make a rational finite element simulation controlled by those four variables;and?3?we propose an LF permeability model composed of a variable of radius strain???and some critical parameters,i.e.,the‘lamina angle???',model is based on the Palmer and Mansoori's fracture permeability model?i.e.,the P–M model?,the modified Tien and Kuo's LF failure criteria?i.e.,the T–R criteria?,and the redefined rock compressibility(i.e.,the porosity change rate with the radius strain,_?e^?ff).The simulation results agree well with the testing data.The middle lamina angle always induce fractures with higher flow conductivity.To study the influence of tight pore-throat space on the movable fluid and the relative permeability,we selected five samples of tight sandstone,and carried out a multiscale comprehensive study on the pore structure of tight sandstone reservoirs by scanning electron microscopy?SEM?,casting thin sections?CTS?,high-pressure mercury injection?HPMI?,and constant-rate mercury injection?CMI?tests.The combined nuclear magnetic resonance?NMR?and HPMI test were performed to calculate the connectivity ratio of different samples and evaluate the connectivity of tight sandstone reservoirs.Furthermore,the combined NMR and CMI test were performed to recalculate the pore throat radius of the sample,from quantitative characterization to qualitative analysis.The reservoir throat radius distribution and throat number together control the tight oils movability.To study the influence of water-laid tuff on the tight oil generation-migration-storage process,studied the element migration principle in tufaceous shale and its influence on the hydrocarbon generation using geochemistry methods.Samples are from a complete interval of Upper Triassic Yanchang Formation?Fm.?in Ordos Basin.Three diagenetic minerals trajectories related to the OM generation and evolution were proposed,i.e.,the Fe-S cycle,the devitrification,and the apatite nodules process.In addition,five tuffaceous shale samples are made from underground cores.Wettability angle is tested for those five samples for the water and n-Decane.The SI experiments,using the fluid of n-Decane,are conducted to study the tight oils movability in tuffaceous reservoirs.The residue hydrocarbon for tuffaceous shale layers is tested using the Rock-Eval method.We also proposed a modified scaling model of the SI data,considering two factors especially for the tuffaceous shale samples,i.e.,the factor I of the tuff-hosed-pores indicating tuff components,and factor II of the organic matter?OM?-covered-pores indicating the residue hydrocarbon.Influence of tight oils movability is discussed from the perspective of‘hydrocarbon generation',‘hydrocarbon migration'and‘reservoir wettability'.Significance of the tuff layers are emphasized for the further tight oils exploration.