The Design of the First Miscible Sour Gasflood Project in Oman

Abstract

Abstract This paper summarizes parts of the strategies that were developed to demonstrate the feasibility of the first miscible sour gas injection project in one of the reservoirs of a cluster of fields in southern Oman. The hydrocarbons in the cluster are contained in carbonate "stringers", which are approximately 100m thick slabs of carbonate floating within salt at depths between 2.5 km to 5 km. Large quantities of sour gas with 3–4% H2S and 10–15% CO2 are available to be used as miscible agents. The cluster is developed in a phased manner. The key objective of phase one, producing via primary depletion, was to gather data from a number of different reservoirs to determine whether a miscible gas injection project is feasible. A balance between early delivery of new oil and the complex subsurface appraisal that takes resources & time is necessary. An example of a workflow that led to the construction of static & dynamic reservoir models with different realizations in one of the fields is described. This includes 3D seismic, well test and PVT data, well logs, correlations and interference testing. Advanced technologies have been utilized to monitor reservoir performance from Phase 1 and to forecast predicted oil recoveries for the miscible gas injection projects. The collection of production data and pressure performance along with appraisal drilling have provided valuable information to allow the reservoir models to be updated. It is illustrated that the emerging data can lead to subsurface concept refinements which have been included in the project design. The subsurface strategies are described as to how the information has been incorporated in the detailed facility design for this first miscible sour gas injection project in Oman. Introduction In the period between 1997 and 2005, Petroleum Development Oman (PDO) has discovered a major new oil province cluster, constituting 8 fields & 11 reservoirs1. The cluster is located in the south of Oman about 80 km from the nearest infrastructure (see Fig. 1a). The hydrocarbons in the cluster fields are contained in carbonate "stringer" reservoirs, which are approximately 100m thick slabs of carbonates floating within salt. They are referred to as stringers because of their stringtype appearance on seismic data; see Fig. 1b. All the fields within the cluster are deep and are characterised by high pressures with sour, light hydrocarbons. The cluster is developed in a staged/phased manner; see Fig. 2a. Each field goes through four stages of development, namely, appraisal & early primary production; full primary development to ideal Gasflood pressure, full miscible Gasflood development and finally gas blowdown via pressure depletion. The key objectives of phase 1 were firstly to start production from the cluster as early as practically possible and contribute to PDO production targets, and secondly to gather vital subsurface information to underpin an investment decision for a follow up development stage2,3. Phase 2 involves full field primary development in certain selected fields & includes the first miscible Gasflood development in Field A, as shown in Fig. 2b. Subsequent phases will involve full field primary development followed by a stepwise implementation of potential miscible gasflooding and ultimately reservoir pressure blowdown. In Phase 1, 4 fields (including field A) are being produced via primary depletion. They were selected based on different criteria (e.g. reservoir continuity, size, GOR, productivity). Although primary depletion provides a robust development, PDO decided to implement a development based on EOR miscible Gasflood at the earliest opportunity since it has the potential to increase the RF from about 10% (via depletion) to as high as 50%. An aggressive project schedule towards first oil was developed but at the same time subsurface learning was incorporated into the project plans. The process used to accomplish this goal is described in previous articles4,5.