PENGGUNAAN METODE VOLUMETRIC ANALYSIS (VOLAN) DALAM ANALISIS PETROFISIKA BATUPASIR GLAUKONITIK, RESERVOIR FA, KELOMPOK SIHAPAS, CEKUNGAN SUMATRA TENGAH

FRANSISKA ATIKA INDRIYANI, Sudarmaji, S.Si, M.Si

2015 | Skripsi | S1 GEOFISIKA

Analisis petrofisika reservoir batupasir FA, Kelompok Sihapas, Cekungan Sumatra Tengah dilaksanakan di PT. Chevron Pasific Indonesia. Laporan stratigrafi menyebutkan bahwa litologi reservoir FA didominasi oleh batupasir glaukonitik dan lapisan tipis batuan padat (tight rock). Kondisi litologi tersebut melatarbelakangi penggunaan metode Volumetric analysis (Volan) yang menggunakan gambar silang log neutron dan log densitas dalam evaluasi formasi. Metode Volan berbeda dengan metode konvensional. Metode konvensional menggunakan beberapa persamaan dalam evaluasi formasi dan menghitung volume serpih menggunakan persamaan linear log sinar gamma. Ketiadaan parameter untuk menghitung saturasi air dari data laboratorium juga menjadi permasalahan dalam penelitian, sehingga tujuan dari penelitian adalah: (1) melakukan analisis petrofisika yang lebih akurat dan optimis untuk reservoir FA dibandingkan dengan hasil dari metode konvensional; (2) melakukan evaluasi reservoir FA melalui perhitungan volume serpih, porositas, dan saturasi menggunakan metode dan parameter baru yang lebih tepat; dan (3) menghitung dan memetakan interval ekonomis pada reservoir FA secara lateral untuk interpretasi pola penyebaran hidrokarbon secara kualitatif. Evaluasi reservoir dilakukan dengan metode Volan, di mana saturasi air batupasir dihitung menggunakan persamaan Simandoux termodifikasi. Hasil rekontruksi metode konvensional digunakan untuk perbandingan dan kontrol kualitas hasil penelitian metode Volan. Zona net pay ditentukan melalui kalkulasi fractional flow. Keakuratan hasil dengan Metode Volan dibuktikan dari hubungan volume serpih, porositas total, porositas efektif, dan porositas batupasir. Hasil yang lebih optimis ditunjukan dengan penurunan total rata-rata volume serpih sebesar 14% dan kenaikan total rata-rata porositas efektif sebesar 5% dibandingkan hasil metode konvensional. Ketepatan perhitungan saturasi air divalidasi oleh data inti batuan dengan parameter baru a =1; m=1,6317; n=1,9187; Rw dari picket plot = 0,54 ohmm pada 77 derF; dan Rsh = 4,575 ohmm. Interval ekonomis ditentukan dengan nilai cut-off volume serpih 65%, cut-off porositas efektif 8%, dan cut-off saturasi air total pada 64%. Zona net pay terdeteksi pada 223 dari 302 sumur.

Petrophysical analysis of FA sandstone reservoir, Sihapas Group, Central Sumatra basin had conducted in PT. Chevron Pacific Indonesia. The stratigraphy report gives information about the existence of glauconitic sandstone and thin bed tight rock as the dominant lithology in FA reservoir. These lithology conditions become the background why volumetric analysis (Volan) method is applied. Volan method uses neutron log and density log crossplot in the formation evaluation. This method is different from the conventional method. Conventional method uses gamma ray linear equation for shale volume calculation. The absence of parameters from laboratory data to determine water saturation is also becomes a problem in this research. In order to solve those problems, this research goes to 3 goals which are: (1) doing the petrophysical analysis that is more precise and gives more optimism result for FA reservoir as compared to the conventional method result; (2) evaluating the FA reservoir through the determination of shale volume, porosity, and saturation which is evaluated using new appropriate method and new parameters; (3) doing calculation and lateral mapping of FA reservoir economic interval to get qualitative interpretation of hydrocarbon distribution. Reservoir evaluation is done by Volan method. In the other hand, sandstone water saturation is determined using modified Simandoux equation. The conventional method reconstruction results are used as comparison and quality control for the result from Volan method. Net pay zone is determined through fractional flow calculation. The Volan method results better in precision that are proven by the relation between shale volume, total porosity, effective porosity, and sandstone porosity. The more optimism results are showed by the decreasing of total average shale volume to 14% and increasing of the total average effective porosity to 5% if it is compared with conventional method result. The accuracy of water saturation determination using new parameter (a =1, m=1.6317, n=1.9187, Rw from picket plot = 0.54 ohmm at 77 degF, and Rsh = 4.575 ohmm) is validated by core data. Economic intervals are defined by the shale volume cut-off at 65%, effective porosity cut-off at 8%, and total water saturation cut-off at 64%. Net pay zone are detected at 223 of 302 wells.

Kata Kunci : Glaukonit, batuan padat, metode Volan, metode konvensional, zona net pay.