Largest WAG Pilot in Giant Al-Shaheen Field Reveals Optimization Methods
Authors: Carpenter Chris 
Published: May 2019

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 190343, “Challenges and Learnings From Operating the Largest Offshore WAG in the Giant Al-Shaheen Field and Ways To Optimize Future WAG Developments,” by M. Pal, S. Furqan Gilani, and G. Tarsauliya, North Oil Company, prepared for the 2018 SPE EOR Conference at Oil and Gas West Asia, Muscat, Oman, 26–28 March. The paper has not been peer reviewed.

This paper discusses the operation of the largest offshore high-pressure water-alternating-gas (WAG) injection pilot using hydrocarbon (HC) gas, with a gas-injection capacity of approximately 100 MMscf/D, in the giant Al-Shaheen field offshore Qatar. The paper also analyzes the current HC-WAG pilot and optimizes future development scenarios for extending HC-WAG injection to the field scale.

Al-Shaheen WAG Background

As part of the field-development plan, a comprehensive enhanced oil recovery (EOR) screening study was performed that explored the range of available EOR techniques. On the basis of these studies, WAG injection was identified as one of the more-promising EOR options for the field.

Additional studies performed in 2006 and 2007 indicated a large full-field potential for WAG in Al-Shaheen, with an estimated recovery of hundreds of millions of barrels over and above water-flood recovery. Data analysis and intensive simulation work, including both a history match and several forecast sensitivities, were conducted to understand and to estimate WAG potential on Al-Shaheen reservoirs.

The Al-Shaheen WAG process is largely an immiscible process with some multi contact miscibility effects between injectors and producers. Overall, WAG has been a success in the field in terms of seeing incremental oil gains on several patterns. Although physically successful, the project had been challenging operationally. In spite of incremental gains, a number of operational challenges were encountered during the execution of the WAG pilot. These challenges mostly were related to WAG-cycle conversions and operational efficiency of gas-compression systems.

WAG Recovery Optimization

WAG will be a major part of the development of the mature-flank and heavy-oil areas of the field. The areas proposed for WAG are undersaturated with gas at reservoir conditions. Incremental recovery from WAG could be affected by thermodynamic effects caused by large variations in permeability, viscosity, and saturation pressure in the field along with several other variables described in the complete paper.

To analyze the effect of these optimization parameters on incremental oil recovery from WAG, a sector model with varying static and dynamic properties was built (Fig. 1). The properties of the sector model were based on the history-matched model of the neighboring areas. The well spacing was 700 ft, which lies in the middle of the well-spacing ranges of the ongoing WAG pilot wells (500 to 1,000 ft). The WAG sector model was initialized with a range of fixed oil API gravities. Different thermodynamic effects of gas injection have been examined on the sector model. The model is also simulated for different WAG cycle lengths (1/1, 3/3, 6/6, and 12/12), ratios (1:2, 2:1, and 1:3), and slug sizes (1–30 years of WAG). The effects of choking production wells and changes in injection gas composition were also tested on the sector model.