Successful Implementation of a Gas Injection Trial in a Low-Permeability Carbonate Reservoir, Offshore Qatar
Authors: Mogensen Kristian , Frank Soren , Noman Rashed 
Published: May 2016

This paper describes the planning and the successful implementation of a lean gas injection trial conducted in the low-permeability Kharaib B carbonate reservoir of the Al Shaheen field, offshore Qatar.

The main objectives of the injection trial were to determine whether premature gas breakthrough would occur in the neighbouring production wells and whether water injectivity after the gas flood would be reduced as a result of the presence of high gas saturation around the injection well. During a 6-month trial period, some 1.7 Bscf of produced gas was re-injected into a trial well drilled in a line-drive pattern and completed open-hole. Despite the substantial amount of gas injected, no premature gas breakthrough was observed in the neighbouring production wells located some 1,000 ft away. Furthermore, water injectivity normalised with length of reservoir section was comparable to or slightly higher than the nearby water injection wells, suggesting that gas injection did not have a negative impact on the subsequent water flooding efficiency.

Important aspects of the injection trial covered in this paper include the careful well selection, the prior data gathering, and the attentive daily monitoring of the process. Actual injection and production performance data were successfully history matched with a compositional reservoir simulation model, which required non-equilibrium initialisation to account for lateral reservoir fluid property variation, a tilted free water level, and a large transition zone. Details on the fluid property modelling can be found in Lindeloff et al. (2008).

Inspired by the positive results of the gas injection trial and the extensive experimental phase behaviour program, work is ongoing to implement gas injection as a water-alternating-gas (WAG) scheme to a larger number of wells belonging to a single platform location but encompassing two reservoirs. Results from preliminary reservoir simulation studies indicate significant scope for enhanced oil recovery from the Al Shaheen field due to improved sweep by the combined effect of water and gas.

Introduction

The Al Shaheen field is located in Block 5, off-shore Qatar, as seen in Figure 1. Production from the field commenced in 1992 from two thin separate Cretaceous carbonate formations and an overlying sandstone formation. The field development philosophy centres on long horizontal wells, some placed in radial, others in parallel line drive patterns of alternating water injectors and oil producers.

The carbonate reservoirs are characterized by relatively thin oil columns with a large areal extent (25 km by 45 km) with typical permeabilities in the 1–10 mD range. The reservoir fluid properties exhibit large lateral variations, with oil gravities ranging from 16–38 °API within the same reservoir. The field contains several gas caps and shows large variations in solution gas-oil ratio (GOR) and saturation pressures. A hypothesis for the origin of the complex fluid variations observed is that the reservoir has been charged by separate oil pulses followed by gas influx and biodegradation.