Kinetic Mechanism And Application On The Lower Limit Of Effective Reservoir Physical Properties Of Tight Oil

As an important contributor to unconventional oil and gas resources,tight oil has huge resource potential and expected to play a more important role in future.The exploration and development of tight oil are relatively late in our country,and researches on the lower limit of effective reservoir physical properties are insufficient.Exploration practices show that the lower limit of effective reservoir physical properties significantly varies with depths,and the reservoirs are dry layers when the physical properties reach a certain small value.There are three questions that need to be answered: Why do the effective reservoirs physical properties change with depth? What are the main controlling factors? How can we identify the lower limit of effective reservoir physical properties? Answers of these questions are critical to predict the distribution of tight oil reservoir,calculate tight oil reserves and guide the tight oil exploration.Based on the analysis of tight oil accumulation conditions and characteristics of Lucaogou Formation in Jimusar Sag,Junggar Basin,the lower limit of effective reservoir physical properties are obtained by the distribution function curve method,which demonstrates that the lower limit decreases with the depth.Physical simulation experiments were conducted to analyze the key factors influencing the lower limit of effective reservoir physical properties.The charging pressure experiments are designed to simulate the charging pressure in the tight oil reservoir,and artificial cores are made to simulate the tight reservoir.Based on the study of critical conditions of oil charging pressure,the influence of reservoir petrology characteristics on the lower limit of effective reservoir physical properties can be obtained.Results show that coarser grains,better sorting,and higher contents of quartz generally lead to the greater possibility for oil to seepage,and thus contribute to the smaller value of the lower limit of effective reservoir physical properties.Through the analysis of tight oil reservoir accumulation process,the geological model reflecting the process that oil was charged into the tight reservoir was established.It is identified that the overpressure caused by hydrocarbon generation is the driving force,and the capillary pressure is the major resistance for seepage.According to the percolation theory,the quantitative expression of hydrocarbon-generation overpressure was built and the method and process of the discriminant of lower limit of effective reservoir physical properties were established.On this basis,the influence of hydrocarbon expulsion intensity,the thickness of source rocks and the formation temperature were discussed.Results show that the lower limit of effective reservoir physical properties become smaller corresponding to the greater hydrocarbon expulsion intensity,the bigger thickness of source rocks,and the higher formation temperature.Based on above analyses,the lower limits of effective reservoir physical properties corresponding to the depth intervals of 2000~2500m,2500~3000m,3000~3500m,3500~4000m,and 4000~4500m are respectively 0.352?m,0.287?m,0.230?m,0.187?m,and 0.147?m in terms of the critical pore throat radius,6.9%,5.7%,4.4%,3.2%,and 1.8% in terms of the critical porosity,and 0.249×10-3?m~2,0.148×10-3?m~2,0.085×10-3?m~2,0.051×10-3?m~2,and 0.028×10-3?m~2 in terms of the critical permeability.Comparative studies show that results of the accumulation dynamics method and the distribution function curve method are reliable.Effective reservoir has been evaluated based on the results of the lower limit of effective reservoir physical properties,and tight oil reserves in Junggar Basin's Jimusar Sag have been calculated as 21.6×10~8t by using the volume method.