| Tight oil has become a new central issue of oil and gas exploration as the proportion of tight oil is higher and higher in the added oil reserves with the continuous development of petroleum exploration.Core analysis is an important means to understand the characteristic of the tight oil reservoirs.Nuclear magnetic resonance?NMR?core analysis has many advantages,such as fast detection,no pollution,low cost,and so on.The method for measuring characteristic parameters of tight oil reservoir rock by low field NMR experiments was mainly studied in this paper.NMR T2 spectrum of rock in tight oil reservoir was effectively measured using a series of optimized measurement parameters,and the features of T2 have been analysed.Based on the T2,the methods of calculating porosity,permeability and pore structure of tight oil reservoirs rocks were established with the results of porosity-permeability parameter measurement and high pressure mercury injection experiment.There are several parameters,including half echo time,echo number,repeated sampling frequency and inversion time,that have great influence on T2 measurement of tight oil reservoir rock.So the values of these parameters were selected from a lot of experimental data on tight core NMR,which were 80?s half echo time,1125echo numbers,5000 repeated sampling frequencies,0.01ms inversion starting time and 1000ms inversion stopping time.Many experiments of tight oil reservoir rocks have been conducted on porosity,permeability and high pressure mercury injection,and the results show that porosity between 0.04 and 0.08 is about 42.86%of the sample total;permeability of 0.010.05×10-3?m2 accounts for 38.10%of the sample total.Pore structure of tight oil reservoir rocks was divided into three classes including middle displacement pressureµn throat,high displacement pressureµn throat and high displacement pressure&nanometer throat.The tight oil reservoir rocks have characteristics such as small pore throat radius,poor sorting and weak flow capacity.Those parameters which can represent the pore throat size or filtration capacity have close pertinence with porosity and permeability.NMR T2 spectrum of tight oil reservoir rock mainly is made up of four peaks,which of short relaxation time and long relaxation time have largest and minimal peak area ratio respectively.Difference spectral method is proposed to analyze NMR porosity based on T2 measurement,which solves the problem that conventional method about NMR porosity measurement is not suitable for measuring tight oil reservoir rocks.An empirical model is present to calculate NMR permeability of tight oil reservoir rocks from T2 geometric mean,which can solve the problem that SDR model and Coates model are difficultly used for measuring permeability of tight oil reservoir rocks.Further,the relationship between T2 geometric mean and characteristic parameters of rock pore structure was analyzed,and there are four structure parameters,just like displacement pressure,average throat radius,max throat radius and the percentage of unsaturated pore volume,which are effectively expressed by T2 geometric mean.The research results in this thesis are significant to guide measurement of characteristic parameters on the tight oil reservoir rock with low field NMR. |
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