An Unified Acoustic Velocity Model Of Shaly Sandstone

For the shaly sandstone,the minerals of the sandstone matrix,the elastic properties of the shale(clay),the shale content,and the distributions of the shale within the sand grains have great influences on its overall elastic properties and acoustic velocities.Up until now,few acoustic velocity models of shaly sandstone are found to take into account the influences of the shale distributions on the sandstone moduli and acoustic velocities systematically.Hence,it is essential to quantify their effects.In this research,we have combined the shale distribution types defined in geology,the cementation types,and the mechanical effects of shale,based on which the concept of critical shale content is proposed innovatively.Moreover,the shaly sandstone is classified into two types: Structure Shale Sandstone and Cementation Shale Sandstone.Based on the principles and applicability of Xu-White Model,CCT Model and Modified CCT Model combing with the definition of critical shale content,the acoustic velocity model for these two types of shaly sandstone are given,and an unified acoustic velocity model are established finally.Firstly,the analysis of the experimental core data and the well logging data of the changes of acoustic velocities with shale content provides the basis for the concept of critical shale content.It means that there exists a critical shale content,under which the acoustic velocities change with the shale content significantly.However,the acoustic velocities only slightly decrease or keep constant with the shale content above this value.When the shale content is smaller than the critical value,two situations will occur: on one hand,the acoustic velocities may increase with the shale content due to the cementation effect of the shale,which is distributed within the grains as the dispersive form,it corresponds to the cementation type with floating or point contact grains;on the other hand,the acoustic velocities may also decrease with the shale content.Under this condition,the shale is in the form of structural and dispersive shale.There is no cementation effect found for this case,it corresponds to the cementation type with sutured contact grains.Combined with the definition of critical shale content,the cementation types of the shaly sandstone are classified into two types: Structure shale sandstone(Type I)and Cementation shale sandstone(Type II),as stated above.These two types of shaly sandstones have different geological and physical properties,therefore,they should be studied separately(Chapter 2).Secondly,combined with the data from reservoir formation A,B,and C,the principles and applicability of Xu-White model are discussed.It is found that the shale is distributed as structural or dispersive shales for Type I shaly sandstone,whose acoustic velocities can be obtained through Xu-White model combined with the critical shale content: when the shale content is smaller than the critical value,the shale is distributed as the structural shale,the influences of shale porosity is negligible.Hence,we only need to consider the effects of shale on the moduli of the sandstone matrix when using Xu-White model.However,when its value is larger than the critical value,apart from the structural shale,there is also dispersive shale distributed in the pores.It has little effects on the matrix moduli.The structural shale plays the role of decreasing the matrix moduli,whereas the dispersive shale has effects on the pore structures when we apply Xu-White model(Chapter 3).Thirdly,the CCT model and modified CCT model are discussed for their principles and applicability based on reservoir formation D and E.It is found that modified CCT is applicable in the Type II shaly sandstone.In addition,apart from the cementation shale,the shale can also distribute in the pores,which has little effects on the moduli.Combing with the above characteristics,we proposed the following approach to obtain the acoustic velocities of this type: when the shale content is smaller than the critical value,the shale acts as the cement,its porosity is negligible.Under this condition,we can apply the modified CCT model to investigate the influences of cementation.However,when the shale content is larger than the critical value,apart from the structural shale,most shale may be distributed in the pores,the influences of these part shale need to be considered through Xu-White model in this circumstance(Chapter 3).Compared with the experimental and logging data,this approach can predict the acoustic velocities of Type II sandstone well(Chapter 4).Fourthly,an unified acoustic velocity model for these two types of shaly sandstone is given(Chapter 5).Finally,the conclusions for each research content are drawn,the influences of shale distributions and mechanical effects on the elastic properties of shaly sandstone are classified.It is suggested that more researches should be carried out based on the more comprehensive analysis of the data of the related experiments,well logging,and seismic exploration.At the same time,the differences between the predicted results and the measured data should be related to the compaction effects,which needs to be studied in the future.