Abstract
In reservoirs that have been waterflooded or gas injected, it is still possible to recover a significant amount of the remaining oil by water-alternating-gas (WAG) injection. WAG injection has been successfully implemented in some waterflooded reservoirs. However, the physical processes underlying the complex three-phase flow in WAG have not been well understood.
A series of WAG experiments has been conducted, using high-pressure glass micromodels, with high quality images of the oil recovery processes being video recorded. The experiments were performed using water-wet, oil-wet and mixed-wet micromodels. The authors presented the results of the experiments of water-wet models in SPE ATC&E 2000 (SPE 63000)1 . This paper presents experimental results of oil-wet and mixed-wet models and demonstrates the difference in flow mechanisms of the WAG process under different wettability conditions.
Pore level fluid flow and distribution were studied and fluid saturations at different stages of the experiments were measured. The results showed that oil recovery was higher for WAG injection than for water or gas injection alone, under any of the wetting conditions. It was also observed that WAG recovery was higher for strongly oil-wet or mixed-wet models than for strongly water-wet one. Experimental results on water-wet models had highlighted the importance of corner filament flow of water in oil recovery process, with the initial water flood residual oil being trapped in the majority of pore space mainly surrounded by layers of water, and not in only large pores. The successive WAG cycles redistributed the fluids, creating fresh pathways for gas to enter the pores occupied by oil, hence, some of the oil which otherwise would not have been mobile under either gas or water injection alone was mobilised and produced. In a strongly oil-wet micromodel the flow resembled a piston type of displacement. The residual oil was only the amount staying in form of films on the pore surface, the filaments in the corners and some trapped in patches of pores surrounded by small throats. During gas injection following water injection, gas was observed to avoid entering most of the water-filled pores and preferentially invaded oil-filled pores. This is due to a much lower value of gas/oil interfacial tension than oil/water IFT. The mixed-wet model contained pores of varying degrees of wettability; from strongly water-wet and pores of intermediate wettability to strongly oil-wet pores. The displacement process in the mixed-wet model was a combination of those observed in the two extreme wettability conditions.
