Experimental Study On The Saturation Model Of Volcanic Rock Based On Acoustic Parameters And Its Applications

With the exploration and development of oil and gas resources,more and more attention has been paid to volcanic gas reservoirs.The lithology of volcanic reservoir is complex and diverse,and it usually has the characteristics of low porosity,low permeability and strong heterogeneity.It is uncertain to use conventional logging data to evaluate the gas content of volcanic reservoir,which increases the difficulty of evaluation of volcanic reservoir.Based on the sensitivity of acoustic wave to the gas bearing state of rocks,it is necessary to establish a comprehensive evaluation method of fluid identification and saturation of volcanic reservoirs based on acoustic wave.The propagation speed of sound wave in the formation is affected by the formation lithology and porosity,and is closely related to the composition and distribution form of pore fluid.At present,there are many researches on the relationship between sonic velocity and saturation.It is the main method of evaluating saturation by seismic and acoustic logging to establish the model of formation gas saturation by using fluid replacement theory.At present,most of the experimental and theoretical researches are focused on sandstone reservoir.The acoustic velocity and elastic modulus of volcanic rock are not only affected by porosity and saturation,but also by complex lithology.More in-depth research is needed.Taking the volcanic rock of Nanpu 5 structure in Jidong area as the research object,the acoustic velocity experiment was carried out on 18 volcanic rock samples,and the saturation calculation formula was established,which was applied in Nanpu area,with good results compared with the gas test conclusion.In this paper,the relationship between P-wave velocity and saturation of volcanic rock samples during evaporation is measured.It is found that the relationship between P-wave velocity and saturation of samples of the same lithology shows similar characteristics.With the decrease of water saturation,the P-wave velocity of most rock samples decreases first and then remains unchanged.Some experimental samples are affected by the pore structure between grains,and the P-wave velocity saturation changes negatively in the low saturation stage.The results show that the effect of contact softening on P-wave velocity can not be ignored in the low saturation stage.With the decrease of porosity,the change of P-wave velocity with the water saturation is more obvious,which shows that the dense rock is more sensitive to the saturation,which is conducive to the use of acoustic data to identify gas bearing layers.In order to establish the petrophysical model and predict the reservoir fluid,the experimental results are compared with the calculation results of uniform fluid distribution model and patch model.The results show that the fluid distribution in the pores changes during the change of saturation: in the low and medium saturation section,the calculation results of Gassmann-Brie formula are in good agreement with the experimental data;in the high saturation section,the calculation results of patchy model are more close to the experimental data.Therefore,the uniform fluid distribution model and patchy model should be considered simultaneously in the calculation of saturation.Considering that the experimental sample is dense volcanic rock with small porosity,it is considered that in the early stage of evaporation,the decrease of saturation is mainly due to the evaporation of water in a small part of pores with good connectivity,while most of the pores in the rock are basically in saturated water state,at this time,the fluid distribution in the pores is consistent with the plaque saturation model;with the further decrease of saturation,all pores in the rock gradually change from saturated state After evaporation to dryness,the fluid in the pore meets the uniform fluid distribution model.According to the actual situation of volcanic rock reservoir,when the water saturation of dense volcanic rock reservoir is high and the gas content is less,the gas is mainly distributed in the large pores and the pores with good connectivity.In this case,the gas distribution in the rock conforms to the plaque theory;when the gas content in the reservoir is more,the gas is distributed in all pores,at this time,the gas distribution in the rock meets the uniform fluid distribution model.And the actual gas reservoir formation process is the process of gas drive water,so the distribution mode of this patchy-uniform combination is consistent with the actual reservoir fluid distribution.In this paper,the fluid distribution model is used to describe the rock modulus by combining the uniform model with the patch model.A hybrid formula is established based on the Brie formula to represent the bulk modulus of rock,namely the GassmannBrie-Patchy(G-B-P)model.Considering the effect of contact softening on acoustic velocity,the corrected G-B-P mixing formula in the middle and high saturation stage corresponds well with the measured data.The fluid identification factor based on acoustic parameters and the timefrequency analysis method based on array acoustic signals are used to identify the fluid in volcanic reservoir.Combined with a variety of qualitative and quantitative identification methods of acoustic gas bearing property,the comprehensive evaluation of volcanic reservoir saturation is carried out,and the evaluation effect is good.The volcanic reservoir in Nanpu area has obvious characteristics of low porosity and low permeability,so it is difficult to identify the reservoir fluid.Based on array acoustic logging data,this paper makes a logging evaluation of volcanic reservoir,which makes up for the difficulties encountered in conventional logging in volcanic reservoir evaluation,and provides a new idea for volcanic reservoir evaluation.