EOR - Miscible Gas Injection in the South of Oman: A Bold Vision that Became Reality

Abstract

Abstract Since the mid 1990's a number of sour oil fields, comprising carbonate reservoirs and silicilyte reservoirs have been discovered in the South of Oman. The main common feature of all these fields, which stretch over an area of 150x50km, is the stratigraphic nature of the trap, whereby the reservoir and source rock is entirely encased by salt. The crude is light, sour and often heavily undersaturated. Despite significant depths of 3000-5000m and initial reservoir pressure gradients often far above the hydrostatic gradient, the primary depletion recovery is in general low, in the range of 10-15%. Wells generally quit from lift die-out at a fairly high production rate once the reservoir reach their bottom hole pressure limit, and no artificial lift method could successfully be implemented because of the high pressure, high sour and high GOR environment. This resulted in the need for secondary/tertiary development to access the significant oil volumes left in the fields. Plans towards the implementation of Enhanced Oil Recovery (EOR) by the mean of Miscible Gas Injection (MGI) were put in place covering most fields in the cluster. A structured implementation staircase was followed, gradually increasing the confidence in the recovery process through lab tests and field trials, and maturing field development projects through the opportunity realization steps. The commissioning of the Station-H and the implementation of the AN MGI 3A project mark the realization of the first tranche of MGI floods in Oman; it includes three projects:A full-field implementation in the Z-field, which started in April 2012 and is now injecting at 80% of the capacity into six injectors,A pilot flood in the S-field (S-Miniflood), which started in May 2015, injecting through 3 crestal wells into the pre-existing gas cap at a distance of 2000-2500m from the producer.A pilot project of two five-spot patterns in the AN field (3A), which started injecting in September 2015 At present injection has been going on for over 3 years at the full field level in Z-field, which is now showing a clear positive response. The reservoir pressure has risen above the Minimum Miscibility Pressure (MMP) of 380bar and excellent FTHP is observed (above 150bar) and producers that had previously reached lift die-out have been reopened. The asset's liquid production has increased by 3 fold and contributes to more than 15% of Oman's production. This project is the world's largest Enhanced Oil Recovery (EOR) with Sour Miscible Gas Injection (MGI) integrating oil and gas field developments. Sour gas is processed through a state-of-the-art processing facility for sweetening and re-injecting gas under MGI to increase oil recovery up to 50%, (i.e. 6 fold increase over primary RF). The two pilots (S-Miniflood and AN 3A) are also under heavy WRFM scrutiny for optimal flood management, and monitoring of KPI. They are expected to run for circa 4 yrs prior to unlocking the implementation at the full field scale. These projects provide the infrastructure for long term development of the area and a number of follow-on projects, which will make use of the existing facilities or lead to commissioning of larger facilities.