The Challenges of Miscible Gas Injection Project: Towards Top Quartile Oil Recovery in an Extremely Tight Sour Reservoir
Authors: Cobanoglu M. , Jabri A. , Quintero Sarmiento O. , Tallentire L. , Hadhrami A. 
Published: March 2016

The field is located in the South Oman Salt sub-basin in Oman. The field is owned and operated by Petroleum Development Oman (PDO). The field is discovered in 1989 and it is 400m thick at a depth of 4000m. The permeability of the reservoir is extremely low (typically 1–100 μD). The oil in this field is very light, sour, low viscosity and volatile (48° API, bubble point pressure Pb=260 bar, H2S=1.5 mol%, CO2=2.5 mol%). The application of massive hydraulic fracturing (2–5 fracs/per well) combined with the favorable oil properties and under-saturated nature of the oil has made economic primary depletion development possible with initial oil rates (100–600 m3/day). Current oil Recovery Factor (RF) is 4.2% and expected to reach 10% through continued primary depletion.

In 2007, an assessment of potential EOR applications aimed at improving the UR of the field concluded that miscible gas injection (MGI) via hydraulically fractured wells in the crestal area of the field is feasible, once field pressure has sufficiently depleted. A phased development was defined, depletion (covered by Phases 1 and 2) followed by MGI (Phase 3). Pilot injection into 2 patterns in the crest of the reservoir (Phase 3A), will start Q1-2016 followed by a 2nd phase of crestal expansion MGI (Phase 3B) by 2026. A final stage could be extension of MGI to the full field. With the application of full field MGI, the recovery factor could reach 25%.

Feasibility study of Phase 3B has been completed in 2014 by: ▪Identification of the development concept and assessment of the urban planning options▪Assessment of the HSE risks and Urban planning requirements▪Update dynamic models and evaluate the Ultimate Recovery (UR) range▪Identify the key risk and uncertainties and mitigation plan for those risks▪Preparation of road map to move from pilot injection to extension of MGI.

Three main development concepts are identified and assessed against the key project drivers (HSE, NPV (Net Present Value), UR, UTC (Unit Technical Cost)). All three options are found to be technically feasible, however project remains very challenging in terms of cost and HSE exposure (i.e. the cheapest option carries the highest HSE risk and the option with the lowest HSE risk is economically not robust).Concept selection in this case depends mainly on balancing the business drivers and the success of pilot injection, which is designed to prove the MGI concept as well as to address key subsurface risks; e.g. gas breakthrough, sweep efficiency and injectivity. Learning from the Phase 3A injection phase (3A) and data acquisition are essential for a decision on the future expansion of MGI.

This paper will specifically cover the challenges of Miscible Gas Injection Project in order to get to the top quartile RF in such tight oil fields. As indicated above, this paper presents a methodology to complete a feasibility study for a tight sour oil field development with MGI as well as the design and evaluation of a pilot injection project with data gathering plans and a road map for extension of MGI.