Study On Rock Physics And Log Evaluation Method Of Waterflooded Layer In Low Permeability Reservoir

Many developed oilfields in the Ordos Basin began water injection in the 1990s and have now entered a stage of high water cut and high recovery.The extremely low permeability,wettability and complicated water injection history make it more difficult to evaluate the waterflooded reservoir via logging curves.After the reservoir is flooded,the remaining oil distribution is seriously uneven,and the change law of resistivity in high resistivity reservoir is unclear.At the same time,there is a lack of logging evaluation method suitable for high resistivity waterflooded reservoir.The research for this paper is supported by the Major National Oil&Gas Specific Project of China（2016ZX05050008）.The main research area of this paper is the low-permeability waterflooded layer of the Upper Triassic Yanchang Formation in the Ordos Basin.The thesis conducts research on resistivity and NMR of complex wettable waterflooded layer and evaluation methods of waterflooded layer in low permeability reservoirs.Analyzed the high-resistivity and normal-resistivity reservoirs in the Ordos Basin,and clarified the changes in logging curves of the reservoirs after the injection of water with different salinities.In order to solve the problem of identification and evaluation of logging methods in high-resistance waterflooded formations,corresponding rock resistivity and nuclear magnetic resonance experiments were designed.Through a series of rock displacement experiments,the accumulation and flooding processes of different types of reservoirs were simulated.The changes of resistivity and NMR response of different reservoirs after waterflooding are clarified,and corresponding logging evaluation methods for waterflooded layers are established.The experimental results show that the resistivity of the waterflooded layer injected by the formation water decreases monotonously with the increase of saturation.After the reservoir is flooded with fresh water,the reservoir resistivity will first decrease and then increase due to the combined effect of the injected water and the decrease in oil saturation.Meanwhile,the resistivity changes of high-resistivity reservoirs and normal-resistivity reservoirs are different after waterflooding.NMR logging signals are less affected by formation water mineralization.However,when evaluating high-resistance waterflooded layers with low permeability,the difference between standard NMR T₂ spectra of different saturation states is small,which makes it hard to use standard NMR T₂ spectra to evaluate waterflooded reservoirs.In this study,by conducting NMR shift spectrum experiments under the condition of applying an external gradient magnetic field,the response characteristics of the T₂ spectrum of the NMR shift spectrum under different saturation states are clarified.Through the experiment of resistivity in the process of displacement of rock samples,the resistivity changes of different types of reservoirs after waterflooding are clear.Combining it with the phase permeability curve in low-permeability reservoirs,an evaluation method for different types of low-permeability waterflooded reservoirs is proposed.Through this evaluation method,the relative change of reservoir resistivity after water flooding is calibrated as water flooded level,which can be used in different types of waterflooded reservoir evaluation,including high-resistivity reservoir flooding by fresh water,formation water,and normal resistivity reservoir flooding by fresh water,formation water.When this method is applied to the evaluation of waterflooded layers in different blocks,the evaluation results are in good agreement with the production materials.The remaining oil index of NMR shift spectrum is proposed to evaluate waterflooded layers with different wettability.According to the difference in the diffusion properties of water and experimental oil in the pores of low-permeability rock samples,with the influence of a constant gradient magnetic field,the NMR T₂ spectra with the echo interval T_E=3.6 ms is measured.Based on it,the remaining oil index is established to evaluate of waterflooding level.By comparing with actual well data and production results,the application effect of the method is remarkable.This article proposes a method to characterize the pore structure of the reservoir based on the multifractal method.The multifractal parameters can be used to classify the Upper Triassic Chang 6 low-permeability reservoirs in the study area into three types according to the quality of the reservoirs,and the multifractal parameters of NMR logging can be used to evaluate the physical properties of the tight reservoir.A low-permeability waterflooded reservoir evaluation method based on logging flow units is proposed in low-permeability porous waterflooded reservoirs,which divides low-permeability porous reservoirs into three types of flow units.The plane distribution of the flow unit can be used to indicate the distribution of remaining oil after waterflooding,and it is in good agreement with the actual production water cut data.In the initial stage of water injection,the well areas with a high proportion of type1 flow units are preferentially flooded;in the high water cut stage,the remaining oil is mainly distributed in the well areas with a high proportion of type 3 flow units.