ACOUSTIC WAVE VELOCITY IN SANDSTONE BASED ON PORE GEOMETRY AND PORE STRUCTURE

• Main Author: PRAKOSO (NIM : 32212005), SURYO
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/24426
• Institution: Institut Teknologi Bandung

Acoustic wave velocity is strongly influenced by the constituent of rock material composition. While the constituent of rocks material composition affects the level of pores system complexity that also impact the physical characteristic of rocks such as porosity, fluid saturation and permeability. The definition of this complexity is easier to reveal by using space characteristic i.e. dimension, shape and structure, or in fact, it is commonly expressed through geometry and structure. Therefore, the acoustic wave velocity is influenced by the pore geometry and the pore structure of the rock. In sandstone, the existence of clay minerals and other solids components can determine the complexity of the pores system. <br /> <br /> <br /> <br /> The previous studies merely emphasized on parameters regarding the type, distribution and volume of clay and the shape of pore and the pore size. However, the previous characterization of pore space complexity was too complex and was not associated with the characteristics resulted from diagenetic process. <br /> <br /> <br /> <br /> Considering that the pores internal structure characterization required petrography analysis data that are not always available, therefore, it is required the easier methods. The easier methods refer to the use of available physical properties of rock data, i.e. porosity and permeability, or the relationship between porosity and permeability and able to show such information. <br /> <br /> <br /> <br /> This research was intended to explore the relationship between the acoustic wave velocity and the complexity of pore systems through physical experiments on sandstones. The complexity of pore systems were approached by using quantification of pore geometry and pore structure based on dimensional analysis of Kozeny-Carman equation. The dimensional analysis has already been recognized for its ability to distinguish between one diagenetic result and other diagenetic results on carbonate rocks. It is expected that this analysis can also be applied on sandstones. The purpose of this research were to establish a more accurate emperically method to estimate the acoustic wave velocity based on petrophysical data, porosity and permeability and porosity and permeability estimation based on the acoustic wave velocity using an equation derived from the combination of the equations of Nur dkk. (1995), acoustic wave velocity and Kozeny (1972). <br /> <br /> <br /> <br /> The focus of this research was on sandstones because, when compared to carbonate rock, in general the materials composing sandstones more vary in terms of the type and the content of the clay, and the other minerals. This research employed 1094 core samples from 5 rock data sets of different sandstone reservoirs which comprised of the petrography and XRD data analysis, porosity, permeability, rocks density, capillary pressure as the function of water saturation, and acoustic wave velocity including both compressional wave velocity and shear wave velocity (Vp and Vs). The comprehensive analysis was conducted to identify the dominant factors which affected each group of rocks later associated with the acoustic wave velocity. <br /> <br /> <br /> <br /> In order to achieve the intended objective, grouping of the rocks was conducted, hereinafter referred as rock type, by applying the concept of pore geometry and pore structure similarity as the function of pore attribute. The similarity of rock textures includes the grain size, the sorting, the angularity and the grain roundness are the dominant factors which affect each rock type. A good quality of rock type tends to have good porosity and permeability which are composed of the large grain size, lower hardness, dominant quartz cement and kaolinite, and lower clay volume. This group has relatively larger pore geometry or larger hydraulic radius and simple pore structure. <br /> <br /> <br /> <br /> The results of plotting pore geometry variable againt pore structure variable show that P-wave velocity (Vpdry) data can be clearly grouped based on rock type. The rocks with similarity of pore geometry and pore structure were classified in the same group and have different variations of P-wave velocity from other groups. Each rock type has a similar pattern in that P-wave velocity (Vpdry) increases with pore geometry and pore structure variable. Since pore geometry is proportional to hydraulic radius, Vpdry increases with the increase in pore radius. Furthermore, based on the relationship of bulk modulus and P-wave velocity (Vpdry) with porosity by applying Nur approach (1995), it can be identified that each rock type has its own critical porosity. <br /> <br /> <br /> <br /> The results of this research indicates that rock type and critical porosity have an important role in estimating the porosity and permeability based on the P-wave velocity. The indication shows that there is a relationship between the porosity and permeability with P-wave velocity which can be used to estimate the porosity and permeability. Further, by applying the rock type equations (Wibowo, 2014) (k/&#61542;) 0.5 as function of (k/&#61542;3) to Vpdry, the empirical equations can be arranged to estimate Vpdry based on the porosity and permeability. The results of prediction for porosity, permeability and P-wave velocity are excellent by using this method, offering an alternative technique to be used in petroleum industry.