Overpressure dan Geomekanik Daerah Deepwater pada Lapangan "Verde", Selat Makassar

SARAH SAUSAN, Dr. Ir. Jarot Setyowiyoto, M.Sc.

2014 | Skripsi | S1 TEKNIK GEOLOGI

Penelitian ini bertujuan untuk menentukan karakteristik overpressure dan geomekanik daerah deepwater pada Lapangan "Verde", Selat Makassar beserta penyebab-penyebabnya. Daerah deepwater merupakan daerah beresiko tinggi sehingga kajian ini akan memberikan fondasi geologi yang sangat krusial bagi proses perencanaan pengeboran. Lapangan "Verde" merupakan lapangan deepwater (kedalaman air 6000 kaki) dan merupakan bagian dari Cekungan Kutai. Lapangan ini memiliki tiga sumur eksplorasi (Well-1, Well-2, and Well-3) yang menargetkan reservoar batupasir turbidit serta dua sumur rencana (Well-4 dan Well-5).Overpressure dikaji berdasarkan data log sumur dan tes tekanan RFT/MDT menggunakan metode Eaton (1975) dan kurva Bowers (1995). Geomekanik dikaji berdasarkan data log sumur, tes LOT dan FIT, serta hasil tes triaksial pada batuinti menggunakan kriteria Lade termodifikasi dan metode tekanan retakan Matthews dan Kelly (1967). Berdasarkan kajian overpressure, top overpressure dimulai pada ±7200 kaki TVD atau ±1200 kaki dari dasar laut. Profil overpressure meningkat secara gradual terhadap kedalaman. Berdasarkan kajian geomekanik, besar tensor-tensor tegangan in-situ mirip satu sama lain, begitu pula dengan kekuatan batuan. Nilai tekanan retakan semakin mendekati nilai tekanan litostatik sedangkan nilai tekanan shear failure secara konsisten berada di bawah tekanan pori. Penyebab utama overpressure ialah disequilibrium compaction dan ditambah kontribusi dari mekanisme non-pembebanan (diagenesis smektit-illit dan transfer lateral/efek sentroid) mulai kedalaman ±13500 kaki TVD. Penyebab kondisi geomekanik ialah pasifnya daerah penelitian secara tektonik, litologi deepwater yang cenderung cenderung seragam, nilai overpressure yang semakin membesar terhadap kedalaman, dan hubungan antara kekuatan batuan dan tegangan batuan yang sedemikian rupa sehingga batuan berada pada kondisi yang sangat stabil.

This research aims to discover overpressure and geomechanical characteristics of deepwater area in "Verde" field, Makassar Strait along with their origins. The research provides geological foundation for drilling operation planning, which is especially crucial for high-risk deepwater area. "Verde" field is a deepwater field (6000 ft water depth) that belongs to Kutai Basin. The field contains three exploration wells (Well-1, Well-2, and Well-3) which targeted turbiditic sandstone reservoirs and two proposed wells (Well-4 and Well-5). Overpresure was analyzed based on wireline logs and RFT/MDT pressure tests by using Eaton (1975) and Bower's curve (1995) method. Geomechanics was analyzed based on wireline logs, LOT and FIT tests, and core triaxial test results by using modified Lade criterion and Matthews and Kelly (1967) fracture pressure method. Based on overpressure analysis, overpressure top was found at ±7200 ft TVD which is ±1200 ft from mudline. Overpressure profile gradually increase with depth. Based on geomechanical analysis, in-situ stress tensors have similar magnitude with each other. Rock strength is also similar throughout the field for the same depth. Fracture pressure approaches lithostatic pressure whereas shear failure pressure is consistently below pore pressure. The origin of overpressure is disequilibrium compaction with secondary contribution of unloading mechanisms (smectite-illite diagenesis and lateral transfer/centroid effect) starting from ±13500 ft TVD. The cause of observed geomechanical conditions are the passiveness of tectonics, the relatively homogeneous deepwater lithology, increasing overpressure with depth, and the relationship between rock strength and rock stress which is such that the rock is in a very stable condition.

Kata Kunci : overpressure, pore pressure, geomekanik, modeling