THE EFFECT OF CARBONATE RESERVOIR’S PORE TYPE ON COMPRESSIONAL ACOUSTIC WAVE’S VELOCITY (Vp) AND Vp MATRIX DETERMINATION IN DOWNHOLE CONDITION

• Main Author: ADELINA (NIM: 22215014), RIA
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/30401
• Institution: Institut Teknologi Bandung

Carbonate reservoir is a reservoir with high heterogeneity level. The heterogeneity is due to the effect of diagenesis processes since the deposition of carbonate. Diagenesis processes increase pore geometry and structure complexity of carbonate reservoir which cause little correlation between Vp and porosity. <br /> <br /> <br /> Heterogeneity due to diagenesis also occurs in minerals which compose carbonate’s matrix. Matrix mineral composition differences cause the variation of acoustic wave transit time of each carbonate reservoir. Information about transit time or Vp matrix is needed to be known for sonic porosity calculation. Porosity calculation is important step which is needed to be done correctly so hydrocarbon reserve can be calculated correctly too. Porosity calculated from sonic log is insensitive to secondary porosity of reservoir. Integrating porosity calculated from density log or neutron porosity log with sonic porosity is a good option to analyse the presence of secondary porosity in carbonate reservoir. <br /> <br /> <br /> Rock grouping based on pore geometry and structure similarity with PGS (Pore Geometry Structure) Plot Method was done in this research. Pore architecture complexity increase with the increasing of rock type number. The relationship between pore type and Vp in rock with simple pore system and rock with higher complexity pore system was been analysed by applying rock grouping PGS Plot Method. Ekstrapolation of PGS Plot of each rock type (RT) will meet at a point called convergence point that is when pore geometry value =0,045 and pore structure value = 0,002. Convergence point is defined as a value when rocks can not be differented based on pore architecture because the permeability value is too low. For practicle purpose, at convergence point, the rocks have small capillary tube with porosity = 1, so the rock is fully filled by fluid and have physical property value of saturated fluid. <br /> <br /> <br /> To predict Vp matrix, crossplot Vp – pore geometry and Vp – pore structure were done. Relationship between Vp – pore geometry and Vp – pore structure is defined by power law equation. Crossplot’s result between constant and exponent of the power law equation for each RT was extrapolated to convergence point of Vp – pore geometry exponent = 1 and Vp – pore structure exponent = 0,5. The value of Vp matrix is the extrapolation’s constant at exponent of convergence point that is 7.193,3 m/s and 6.601, 5 m/s. The different of calculated porosity value using Vp matrix = 7.193,3 m/s and Vp matrix = 6.601, 5 m/s is small so Vp matrix can be averaged to be 6.897,4 m/s. The difference is about 480,6 m/s when compared to Vp matrix of literature’s data which is commonly used for sonic porosity calculation. <br /> <br /> <br /> Pore type due to diagenesis processes is one of some matters that cause complexity in pore architecture of carbonate. Early stage of diagenesis processes will create simple pore architecture. The result of this research shows that the the pore type’s effect on Vp is clearly understood when the rocks are grouped by the similarity of pore geometry and pore structure using PGS Plot’s Method. In the same rock type and porosity, rocks with frame forming pore type (mouldic and vuggy) will have higher Vp than intercrystalline and interparticle pore type. Frame forming pore type generation will induce the generation of rock cement. The cement will increase rigidity of rock fabric and Vp. Fracture and dissolution which are happened in late stage of diagenesis processes, will increase the porosity and permeability but decrease Vp of rock.