Low Overpressure and Geomechanic Profile of Ainim to Daram Formations, PB Field, Onshore Area of Bintuni Basin, West Papua

RAHMAN FAJRIANTO, Jarot SETYOWIYOTO, Salahuddin HUSEIN

2016 | Tesis | S2 Teknik Geologi

Empat sumur eksplorasi telah di bor pada lapangan PB yang terletak di Cekungan Bintuni. Beberapa kondisi underbalanced selama pengeboran yang di klasifikasin sebagain low overpressure menyebabkan pengeboran sidetrack. Kebanyakan kondisi tersebut terjadi pada fase akhir pengeboran dimana akuisisi data wireline dilakukan pada Formasi Ainim hingga Daram. Studi ini didasari pada perhitungan data wireline log, sehingga fokus studi ini ada pada interval tersebut. Subsurface pressure berdasarkan perhitungan Bowers (1995) menggunakan data sonic memberikan hasil yang lebih baik dibdaningkan dengan Eaton (1975) menggunakan data resistivity dan sonic. Perhitungan fracture pressure atau minimum horizontal stress dilakukan berdasarkan Eaton (1969) menggunakan data sonic dan digabungkan dengan koreksi berdasarkan Marbun dkk (2015). Mekanisme pencetus overpressure dideterminasi menggunakan crossplot data neutron-density, korelasi log porositas, profil temperatur dan data kematangan. Profil geomekanik menunjukkan low overpressure dengan subsurface pressure diatas normal hydrostatic pressure, dibawah tekanan berat lumpur, dan dibawah vertical stress. Vertical stress terletak diatas minimum horizontal stress dan dibawah maximum horizontal stress. Arah minimum horizontal stress dari determinasi breakout mengindikasikan arah Barat daya hingga Timur laut. Mekanisme pencetus overpressure pada lapangan PB yaitu unloading mechanism yang secara dominan mencakup seluruh area penelitian, yang terdiri dari clay diagenesis, lateral transfer dan hydrocarbon generation. Mekanisme berikutnya yaitu loading mechanism (disequilibrium compaction) dengan kemungkinan transisi menjadi dominan unloading mechanism yang hanya muncul pada Formasi Daram, Jass dan Lower Vorwata dari sumur P-1 ST, juga pada Formasi Daram, Sebyar, Jass dan Lower Vorwata dari sumur P-4 ST.

Four exploration wells were drilled in PB field which located in Bintuni Basin�s. Some underbalanced condition during drilling which classified as low overpressure led the well to sidetrack. Most of them happened in the last section drilled which covered by wireline log data from Ainim to Daram Formations. The subsurface pressure calculated on this study based on Bowers (1995) which using sonic data. Meanwhile the fracture pressure or minimum horizontal stress calculated based on Eaton (1969) which considers the poisson ratio from sonic data and combined with correction based on Marbun et al., 2015. The overpressure generating mechanism was determined by using some neutron-density crossplots, porosity logs correlation, temperature profile and maturity data. Geomechanic profile shows the low ovepressure which located above the normal hydrostatic pressure and still below the mud weight pressure, and far below the vertical stress. The vertical stress is above the minimum horizontal stress and above the maximum horizontal stress. The minimum horizontal stress direction indicates the Northwest to Southeast direction. The main overpressure generating mechanism is unloading mechanism that dominantly covered the whole study area, consist of clay diagenesis, lateral transfer and hydrocarbon generation. The other is loading mechanism (disequilibrium compaction) with possible transition to mainly unloading mechanism which only occured on Daram, Jass and Lower Vorwata Formations of P-1 ST well, also on Daram, Sebyar, Jass and Lower Vorwata Formations of P-4 ST well.

Kata Kunci : low overpressure, vertical stress, horizontal stress, overpressure generating mechanism.