Abstract

The use of WAG (water alternating gas) injection can potentially lead to improved oil recovery compared to injection of either gas or water alone, however the physical process is not well understood. Using high pressure glass micromodels, a series of WAG tests have been conducted using equilibrated fluids, with high quality images of the oil recovery processes operating during alternate WAG cycles being recorded. The tests were conducted using both water-wet and oil-wet micromodels. In this paper results of a typical water-wet test is presented (results of the oil-wet and mixed wet tests will be presented in a subsequent paper).

Water-wet micromodels were initially fully saturated with water and then displaced with oil to establish the connate water saturation. The micromodels were then flooded with water to observe the process of establishing the waterflood residual oil saturation (Sorw). Alternate cycles of gas and water injection were then conducted to observe three-phase flow and its associated oil recovery. The experiments were performed within the capillary dominated flow regime.

The results highlighted the importance of corner filament flow of water in the recovery process, with the initial waterflood residual oil saturation being trapped mainly in the centre of the majority of pore space surrounded by layers of water, and not in only large pores. The successive WAG cycles redistributed the oil in a way which resulted in improved oil recovery, hence, the oil which otherwise would not have been mobile under either gas or water injection alone was mobilised and produced.

It was identified that a limited number of WAG cycles were required to approach maximum oil recovery, after which additional recovery was minimal. All recovery processes were filmed and electronically stored using high resolution imaging, with oil recovery at the end of each flooding cycle being measured using image analysis techniques.

Introduction

Waterflooding, gas injection and water-alternating-gas injection (WAG) are well-established methods for improving oil recovery. In reservoirs that have been waterflooded, it is still possible to recover a significant part of the remaining oil by injecting gas alternately with water. Gas can occupy part of the pore space that otherwise would be occupied by oil, in the form of trapped gas saturation, thereby mobilising the remaining oil. Water, injected subsequently, will displace some of the remaining hydrocarbons (oil and gas), further reducing the residual oil saturation. Repetition of the WAG injection process can further improve the recovery of oil.

Christensen, Stenby and Skauge1 recently reported an excellent review of some sixty field-applications of WAG. Several field trials have been reported as being successful, e.g., in Kuparuk2, Snorre3 and Gulfaks fields4. Both immiscible4–6 and miscible gases7 have been used. A very large number of coreflood experiments8–12 and analytical and numerical simulations11,14 have been carried out. A recent study has considered the WAG process for improving the hydrocarbon recovery in gas/condensate reservoirs13. Most of the research work, conducted so far, has been on either core flooding8,9,10 or numerical simulation11,12, sometimes alongside field trials. The relationship between the injection gas/water ratio (GWR) and oil recovery has been empirically investigated using core displacement experiments, often at low pressure and generally with water wet cores8,10. Micromodels were used as early as 1960 for fluid displacement studies15. Some low-pressure micromodel studies of three-phase displacement have also been performed16,17. However, as far as we know, no micromodel visualisation of the WAG injection has been carried out to directly observe the physical processes taking place in the porous media, using live oil, live water in equilibrium with injection gas and models with different wettability. Larsen et al.18 reported some limited results of their WAG micromodel studies.

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