ROCK QUALITY GROUPING IN SANDSTONE FORMATION USING A CRITICAL POROSITY APPROACH AT FORMATION PRESSURE CONDITIONS

The methods of grouping reservoir rock types based on the physical properties of rocks that have been studied by previous researchers still have a relatively large value of uncertainty. This uncertainty arises in conditions where rock type grouping is carried out in wells that do not have core sample data. Where we know that in the oil and gas field, not all wells in the field are subjected to rock sampling, either routine core analysis or special core analysis. From these problems, the authors feel the need to carry out this study to create a method for classifying reservoir rock types based on the physical properties of rocks that can be used in wells that do not have core sample data. The rock types in the wells that do not have core samples will be grouped based on the critical porosity value of the rock obtained from the vp value in the acoustic log data owned by these wells. By making an approach model through the critical porosity of rocks from wells that have core sample data, wells that only have acoustic log data can be grouped by using the critical porosity approach which is generated from the vp value of the acoustic log.


INTRODUCTION
Each rock has different geometric characteristics and pore structures between one rock and another. The different characteristics of each rock can be grouped into rock groups that have similar rock quality. Previous researchers have conducted a lot of research in terms of grouping rock quality, such as Guo, G. et al. (2005) who proves that the geological process of rock deposition will affect the physical properties of rocks, (Guo, G. Et al, 2005) then Amafule (1993) made a hydraulic flow unit equation that classifies rock types against their hydrolic flow unit (HFU), each HFU has a proximity flow zone indicator (FZI). (Amafule, 1993).  made a rock grouping approach to the pore structure and pore geometry of rocks,  and there are several other researchers who grouped rock types with different methods, such as Gomes, J.S. et al (2008), Permadi& Wibowo (2009. The approach to grouping rock types that has been studied by previous researchers still has quite high uncertainty. The uncertainty in this approach occurs in wells that do not have core rock analysis data. To reduce uncertainty in wells that do not have core rock analysis data, researchers feel the need to conduct this research. This study focuses on obtaining an approach method for grouping rock types in wells that do not have core rock analysis data but have acoustic log data which has a value of P wave propagation velocity (Vp). With the Vp value, it will be possible to calculate the critical porosity value of the rocks in the well. The critical porosity value is used to make an approach so that the rocks can be grouped, each rock group having the same pore geometry character and pore structure will have different critical porosity values (Prakoso, S. Et al, 2017).
The author will do rock type through a critical porosity approach to pore geometry and pore structure so that rock types will be formed that have critical porosity values that are different from each rock type.
The data used in this research are routine core analysis data, special core analysis data, and log data from the sand stone formation KutaiBasin, the equipment used in conducting this research is a set of computers in a computer laboratory and software from the Department of Petroleum Engineering, Trisakti University, which has Petrel software installed. inside it.
The method used in conducting this research is Wibowo and Permadi method in analyzing pore geometry and pore structure of each core rock sample data with the following formula (Wibowo, A.S, 2014): With (k / ϕ) 0.5 = the pore geometry and k / ϕ3 = the pore structure. The value of critical porosity can be determined using the method of nur et al. by estimating using the parameters of the porosity and bulk modulus of each routine core and special core sample data. The formulas that can be used are as follows (Nur, A. Et al, 1995): With B = bulk modulus (Gpa), ϕ = porosity (frac), ϕc= critical porosity (frac), and Bm= bulk modulus mineral (Gpa) MetodeHashinShtrikman used for analysis mineral bulk modulus, In sandstone there are two dominant minerals, namely clay minerals and quartz, because there is more than one dominant mineral in the rock, calculations are carried out using this method, the following formula (Hashin, Z. and Shtrikman, S., 1963): With Bhs = bulk modulus of HashinShtrikman (Gpa) minerals, f1 = mineral fraction 1, and f2 = mineral fractions 2.
The bulk modulus value can be obtained from data P wave velocity (Vp) and S wave velocity (Vs) obtained from core sample measurement data using a sonic viewer. So that from the values of Vp and Vs can be obtained shear modulus and bulk modulus values that can be determined using the rock elasticity method with the following formula (Mavko, G., et al, 2009): With Vs as Velocity S (m / s), µ as shear modulus (Gpa), and ρ as density (gr / cc). The stages in conducting this research can be seen in the flow diagram of Figure 1 as follows:  The value of Vcl content in rocks will affect the size or bulk of the rock matrix modulus which is calculated using the HashinShtrikman model (Bhs.), Bhs value is needed to calculate the critical value of rock porosity.
So that by obtaining critical porosity values from each rock sample in dry conditions surface pressure can be calcified rock quality into several groups based on the pore geometry approach and the pore structure on critical porosity which can be seen in Figure 3. In figure 3, it can be seen that the relation of geometry and pore structure to critical porosity can produce a classification of rock quality into 10 rock types. Each rock group has pore geometry, pore structure and critical porosity that are different from other rock The classification of rock quality based on the critical porosity approach to geometry and pore structure can be verified against the results of rock quality grouping using the pore geometry approach and pore structure which can be seen in Figure 4. In Figure 4 it can be seen that the results of a verification the geometry approach and the pore structure on critical porosity with the pore geometry approach and the pore structure have groupings similar to 10 rock types, so it can be said that critical porosity can be used to determine rock quality groupings. Will dry and pressurized rock conditions must be converted into wet conditions and formation pressures so that any interior that does not have rock samples can be grouped into rock quality using acoustic log data.
Condition conversion is done by converting P wave velocity (Vp) to resemble the acoustic (A) (B) P-ISSN : 1907-0438 E-ISSN : 2614-7297 VOLUME IX No. 4, Desember 2020 Jurnal Petro 2020 http://trijurnal.lemlit.trisakti.ac.id/index.php/petro 171 log conditions at formation pressure. To be able to convert the P wave propagation state to rock samples that have log data measured at the state of surface pressure into a formation pressure state with the Pore Space Stiffness method which can be seen in Figure 5. In Figure 5 and 6 it can be seen that the more of pore space stiffness graph line results from the condition of the core data coinciding with the pore space stiffness line from the log data, will make the core Vp data converted to formation pressure close to the acoustic log Vp. The pore space stiffness constant value is generated from 1 / Pore Bulk modulus (Bϕ) taken from the slope constant of the log data pore space stiffness line. 0.046514 Vp value of core data that has been converted into formation pressure will affect the value of critical porosity which will determine rock quality grouping under formation pressure conditions using a geometry and pore structure on critical porosity approach. The classification of rock quality which based on the geometry approach and pore structure on critical porosity under the conditions of formation pressure can be seen in Figure 7. Limitation of critical porosity values on rock quality grouping of formation pressure conditions (@Plog) after a condition conversion has a difference to the boundary of critical porosity values still in the surface pressure condition (@Ps) which can be seen in table 3 The depth of log data that does not have core sample data can be grouped into rock quality that has been converted into formation pressure conditions using critical porosity values to be generated from acoustic logs and entered into groups of critical porosity values generated from core samples that have been converted into formation pressure conditions, grouping rock quality at uncored intervals can be seen in figure 8. P-ISSN : 1907-0438 E-ISSN : 2614-7297 VOLUME IX No. 4, Desember 2020 Jurnal Petro 2020 http://trijurnal.lemlit.trisakti.ac.id/index.php/petro