Challenges and Learnings from Operating the Largest Off-Shore WAG in the Giant Al-Shaheen Field and Ways to Optimize Future WAG Developments

Abstract

Abstract The objective of this paper is to present the learning and the challenges from operating the largest offshore high pressure water alternating hydrocarbon gas injection (HC-WAG) pilot, with injection capacity of approximately 100 mmscf of gas injection per day in the giant Al-Shaheen field offshore Qatar. The paper also aims to share the methodology for analyzing the current HC-WAG pilot and optimizing the future development scenarios for extending the HC-WAG to the field scale. Al-Shaheen field, offshore Qatar, is a giant complex carbonate oil field characterized by thin oil column, low permeability, and large lateral variations in fluid properties, see Reference [1]. The field currently accounts for 45% of Qatar's total oil production, thanks to the implementation of an efficient water-flooding strategy along with implementation of the largest offshore water alternating gas injection projects. Although WAG has been around for some time, the industry focus has been mainly on miscible WAG, and the vast majority of these WAG fields have been onshore, see references [2, 3, and 4]. There is very limited information available on operating and optimizing immiscible HC-WAG operations, specially, in an offshore environment. Paper presents an analysis of operating and optimizing an offshore immiscible HC-WAG pilot from subsurface and operational point of view. Production data in conjunction with simulation results are used to assess the current HC-WAG development and to evaluate a range of future development scenarios testing the impact of different geological and petro-physical properties on HC-WAG recovery. The methodology presented is a unique way of evaluating operational strategy for operating and managing an offshore HC-WAG pilot. This paper summarizes the learning from evaluation of production and optimization strategy of the HC-WAG pilot based on various parameters related to operating such a pilot. The main evaluation parameters included are GOR constraints, WAG cycle lengths, composition of injected gas and the overall length of the project with regards to project economics. We will show that optimizing HC-WAG operations results in significant benefit for operating an offshore immiscible HC-WAG. The benefits are quantified in terms of incremental oil recovery & managed GOR from an optimized HC-WAG versus an un-optimized HC-WAG. The paper quantifies the impact of incremental oil based on WAG cycle length, the total WAG project duration, the WAG ratio, the total injectant slug size, tapering of the WAG cycle and injected gas composition. This paper is the first attempt to present the learning and challenges from one of the largest offshore high pressure HC-WAG pilot, currently being carried out in Al-Shaheen Field offshore Qatar, which has never been presented previously to the best of author's knowledge. The paper is divided into two parts, first part of the paper details the on-going WAG pilot and its background and the details of the lesson learnt from operating current WAG patterns. Second part details the strategy for optimizing future WAG developments in the field for full field WAG implementation.