Experimental Study Of Acousitical Parameters And Its Applications In Tight Reservoir

With the exploration and development of oil and gas resources,more and more attention has been paid to tight and gas reservoirs.Tight sandstone and volcanic reservoir are two common tight reservoirs.However,tight sandstone is usually characterized by low porosity and permeability,low gas saturation and strong heterogeneity,which undoubtedly increases the difficulty of tight reservoir evaluation and development.And the applicability of Archie’s formula to evaluate conventional reservoir saturation based on electrical properties is greatly reduced.Therefore,it is particularly important to establish the relationship between non-electrical parameters and saturation when conventional reservoir production is declining.While an important discovery in petrophysics is the sensitivity of elastic parameters to fluids,and so the main research content of this paper is formation saturation evaluation and fluid identification based on elastic parameters.Therefore,the purpose of this paper is to evaluate saturation of the formation and identify the fluid type based on elastic parameters.In order to obtain the elastic parameters of the rock,the P-wave velocities of the tight sandstone were measured with changing saturation during imbibition and evaporation.The results show that the velocity curves during imbibition and that during evaporation cross with each other.During imbibition,with the increase of saturation,the P-wave velocity decreases slightly at beginning and then increases,and the growth rate increases with saturation.During evaporation,with the decrease of saturation the P-wave velocity first decreases slowly,then rapidly decreases,and finally increases.That is,the P-wave velocity difference between imbibition and evaporation is obvious,and the relationship of P-wave velocity versus saturation in the low saturation segment is negative,which is particularly obvious in the evaporation process.This indicates the following points:First,there is a close relationship between P-wave velocity and saturation,so it is feasible to use elastic parameters to evaluate saturation.Second,the difference in P-wave velocities during imbibition and evaporation shows that not only does saturation have an effect on P-wave velocities,but also the distribution of fluid in rock pores is crucial.Third,the existence of a small amount of water makes the P-wave velocity decrease,indicating that the contact softening effect of a small amount of water on rocks cannot be ignored.Therefore,the influence of fluid distribution and contact softening must be considered in establishing saturation model based on elastic parameters.In view of the experimental phenomenon that the p-wave velocity decreases with the increase of saturation,the original algorithm was developed to calculate the P-wave velocity with the changing saturation in this paper.At the same time,the influence factors of the declining quantity of the P-wave velocity and the saturation where the P-wave velocity turns are studied.It is found that the declining quantity is related closely with the specific surface and the clay content,indicating that connect softening is caused by both clay and surface action.There is a good positive correlation between the saturation at the inflection point and the NMR irreducible water saturation,which provides a new idea for the laboratory measurement of rock irreducible water saturation.CT scanning can be used to observe the distribution pattern of fluid in the pores of rocks with mcro-pores.However,the pore size of tight sandstone samples in this paper is very small,so the method is no longer applicable.In order to study the distribution pattern of rock pore fluid in the process of imbibition and evaporation,the NMR T₂spectra in the two processes were measured in this paper,and it is found that there was a significant difference in the NMR T₂ spectrum between the two processes.According to the existing fluid distribution model（uniform fluid distribution model and patchy fluid distribution model）,the nuclear magnetic T₂ spectrum was calculated with the changing saturation,by comparing the measured and calculated nuclear magnetic T₂spectrum,it is concluded that:in the process of imbibition,uniform-patchy-uniform distribution models are suitable in the imbibition process and patchy-uniform-uniform distribution models are suitable in the evaporation process.And this is the combined fluid distribution model established in this paper.In this paper,the P-wave velocity is obtained by calculating the elastic modulus varying with saturation according to the combined fluid distribution model with considering the influence of contact softening effect.In order to get the P-wave velocity,the relationship about contact softening are used to calculate the P-wave modulus in the low saturation segments of imbibition and evaporation,the White formula was used to calculate the bulk modulus in the middle saturation stage of imbibition and the high saturation stage of evaporation,and the Gassmann-Brie formula was used to calculate the bulk modulus in the high saturation stage of imbibition and the middle saturation stage of evaporation.The calculated results are in good agreement with the measured results,indicating that the combined fluid distribution model proposed in this paper is reliable.The formation process of the actual reservoir is a process of gas flooding,and the bound water in the formation is more inclined to exist in the form of film water.It indicates that the fluid distribution form in the actual formation is similar to that in the low and middle segments of evaporation process（uniform fluid distribution model）.Therefore,the Gassmann-Brie formula was used to invert the saturation according to the elastic modulus of the formation.Considering the impact of contact softening,the apparent dry sample bulk modulus was used to replace the real dry sample bulk modulus in the calculation of saturation.That is,the improved saturation model established in this paper is a saturation model based on elastic parameters,which takes into account the fluid distribution and contact softening effect.At the same time,two methods of calculating bulk modulus of apparent dry sample are given.The improved saturation model established in this paper is used to calculate the saturation of tight sandstone interval in the Ordos Basin.The improved saturation model is extended to the volcanic strata in the Wangfu fault depression,and the application effect is good,indicating that the method is applicable to tight reservoirs.,There is a big difference in the acoustic signal between the saturated water sample and the sample containing only a small amount of water,which indicates that it is feasible to use acoustic signal to identify reservoir fluid from the perspective of experiment.However,due to the great influence of environmental noise,the acoustic wave regularity measured in the experiment is poor.As a typical non-stationary signal,array acoustic logging signals contain abundant formation information,so the Empirical Mode Decomposition（EMD）and Smooth Pseudo Wigner Ville（SPWV）distribution were conducted based on array acoustic logging signals,and the quantitative indexes for gas-water identification on tight sandstone layers was established.By using time-frequency distribution characteristics and quantitative indexes,fluid identification is carried out on tight sandstone interval in Ordos Basin and volcanic rock in wangfu fault depression.The application effect is good,indicating that the method is applicable to tight reservoirs.,In addition,the P-wave velocity,the shear wave velocity and the density of samples saturated water and samples containing only a small amount of water.Then the bulk modulus,shear modulus and poisson’s ratio,lame coefficient such as elastic modulus were obtained.It is found that the elastic modulii of the saturated samples and samples containing only a small amount of water are obviously different,indicating that the elastic modulus can be used to identify the fluid type.The support vector machine（SVM）was introduced in this paper and the application effect is good.The innovations in this paper are as follows:（1）According to the NMR T₂spectrum,the distribution pattern of fluid in the pores of rocks is studied,and the comprehensive fluid distribution model is established during imbibition and evaporation.（2）The original algorithm was developed to establish the P-wave velocity-saturation relation in the low saturation stage considering the softening effect of rock.（3）The influencing factors of the saturation at the turning point of the P-wave velocity and the decrease of the P-wave velocity in the low-saturation segment in the evaporation process were determined.（4）According to the combined fluid model and considering the impact of contact softening,the P-wave wave velocity of rocks in the process of imbibition and evaporation is calculated.（5）An improved saturation model considering the fluid distribution and contact softening effect was proposed by substituting the apparent dry sample bulk modulus for the dry sample bulk modulus,and two formulas for calculating the apparent dry sample bulk modulus of reservoir were given.（6）Based on the time-frequency characteristics of array acoustic signals,a quantitative index for gas-water identification is established.The study in this paper is of great significance for the evaluation of tight reservoirs and provides a new idea for the application of seismic data.