RESERVOIR UNCERTAINTY SIMULATION STUDY FOR GAMMA FIELD GAS INJECTION AND BLOWDOWN OPTIMIZATION BACHELOR THESIS Khalid Umar 12215013 Submitted as partial fulfillment of the requirements for the degree of BACHELOR OF ENGINEERING in Petroleum Engineering study program PETROLEUM ENGINEERING STUDY PROGRAM FACULTY OF MINING AND PETROLEUM ENGINEERING INSTITUT TEKNOLOGI BANDUNG 2019 1 Reservoir Uncertainty Simulation Study for Gamma Field Gas Injection and Blowdown Optimization Khalid Umar* and Hasian P. Septoratno Siregar** Copyright 2019, Institut Teknologi Bandung Abstract Gamma Field is located in Block B of South Natuna Sea, consisting of five separated reservoirs in three geological zones, and is being produced since 2008 with forty seven percent of the reservoir hydrocarbons located in the Upper Beta Massive West (UBMW) reservoir. UBMW is gas cap with an oil rim reservoir. In order to maximize the value, this reservoir is maintained with gas injection from Gamma-Teta field to effectively sweep the oil and to maintain the reservoir pressure with small portion of gas being exported. As the other gas fields became depleted faster than expected, it was decided to reduce gas injection rate in UBMW and produce more Gamma and Teta gas to increase gas sales volumes. Though UBMW reservoir is considered as a mature reservoir, there are still a lot of uncertainties. The purpose of this study is to recommend the optimum reservoir management scenario by capturing the range of uncertainties that could impact the Estimated Ultimate Recovery and by varying gas injection rate for the remaining years (partially blowing down the gas early). By using the existing base model, we adjust relative permeability and aquifer strength and size within history match tolerance then run forecast with different gas injection scenario to get the optimum scenario. The sensitivity study generate optimum hydrocarbon recovery and project NPV which are presented in the form of a range of values, which is very important for a better decision towards future reservoir management scenario. From this study, it is inferred that stopping gas injection becomes the optimum scenario with addition NPV of 0.35 MMUSD compared to the base case of 40 MMSCFD gas injection. Keywords: gas cap, oil rim, gas injection, reservoir uncertainties, relative permeability, aquifer, NPV Sari Lapangan Gamma terletak di Blok B Laut Natuna Selatan, teridiri dari lima reservoir terpisah dan tiga zona geologi, dan telah berproduksi sejak 2008 dengan empat puluh persen hidrokarbon reservoir berada di reservoir Upper Beta Massive West (UBMW). UBMW adalah reservoir minyak dengan tudung gas (gas cap) berupa “oil rim”. Untuk memaksimalkan potensinya, reservoir ini didukung dengan injeksi gas dari lapangan Gamma-Teta untuk secara efektif menyapu minyak dan menjaga tekanan reservoir dengan sebagian kecil gas yang terproduksi akan diekspor. Walaupun reservoir UBMW termasuk “mature reservoir”, masih terdapat ketidakpastian di sana. Tujuan studi ini adalah untuk merekomendasikan skenario manajemen reservoir optimum dengan mempertimbangkan rentang ketidakpastian yang dapat memengaruhi “Estimated Ultimate Recovery” dan dengan memvariasikan laju injeksi gas untuk periode waktu yang tersisa (“blowing down” sebagian gas di awal periode). Dengan menggunakan model dasar, kita mengatur nilai permeabilitas relatif serta kekuatan dan ukuran akuifer yang menghasilkan performa reservoir pada rentang toleransi “history match” lalu melakukan peramalan dengan skenario injeksi gas yang berbeda untuk mendapatkan skenario optimum. Studi kepekaan (“sensitivity”) memberikan rentang nilai hidrokarbon yang dapat terambil dan NPV proyek yang sangat penting untuk penentuan skenario manajemen reservoir yang lebih baik di masa yang akan datang. Berdasarkan studi ini, disimpulkan bahwa skenario optimum adalah dengan menghentikan gas injeksi yang akan menghasilkan tambahan NPV sebesar 0,35 MMUSD jika dibandingkan dengan skenario dasar yaitu gas injeksi sebesar 40 MMSCFD. Kata kunci: tudung gas, oil rim, injeksi gas, ketidakpastian reservoir, permeabilitas relatif, kekuatan akuifer, NPV *) Student of Petroleum Engineering Study Program, Institut Teknologi Bandung, 2015 batch **) Thesis Adviser in Petroleum Engineering Study Program, Institut Teknologi Bandung 2 3 1. INTRODUCTION 1.1 Background The Gamma Field 1 covers an area of approximately 18 km 2 in Eastern Hub – Block B Offshore PSC Area, South Natuna Sea, Indonesia.