For heavy reservoirs, conventional oil recovery methods are polymer flooding and Steam-assisted Gravity Drainage (SAGD). Such methods are affected by various factors including reservoir thickness, formation heterogeneity, heat loss, and cost, especially in deep heavy oil reservoirs. In these deep reservoirs, their crude oil has a viscosity in the tens of thousands of centipoises at surface conditions, but only a few hundred centipoises at high temperature. There is sufficient mobility of crude oil making it possible for nitrogen (N2) huff-n-puff to be applied. Since foamy oil is a very significant production mechanism in huff-n-puff process, this study is to confirm the ability of nitrogen to form foamy oil in a deep heavy oil reservoir. This study includes two types of tests. The first category is the PVT tests to measure N2 solubility in heavy oil. It was found that the solubility of nitrogen in the oil sample at 50 °C and 7 MPa is 7.54 m3 /m3 . The second category is the pressure depletion tests conducted in a 1-D cylindrical model to observe the flow behavior. These tests are conducted consistently at four different pressure decline rates. It confirmed the possibility of foamy oil formed by nitrogen and summarized the variation of the flow pattern. This research is a preliminary basic study of N2 huff-n-puff in heavy oil reservoirs. Confirmation of the technology’s feasibility will significantly reduce the cost of exploiting heavy oil reservoir.

Introduction

Petroleum, although a nonrenewable energy source, will remain as the primary source of energy worldwide [23]. Unlike the increasing demand for oil, the scarcity of conventional oil reservoirs makes it a challenge to meet the demand. Fortunately, unconventional oil reservoirs account for 70% of total oil reserves worldwide [10]. Therefore, exploitation of unconventional oil reservoirs in the current oil industry has become the main focus to increase oil production [5,36,46]. Unconventional oil reservoirs are reservoirs with a wide variety of sources, which include oil sands, heavy oil, gas to liquids, tight oil, oil shale, and other liquids [36]. Among them, the recoverable reserves of tight oil and heavy oil are 47.0% and 29.7%, respectively. Tight oil and heavy oil are considered as the momentous resources for meeting the rapidly growing oil demand. Heavy oil generally consists of oil sand, extra heavy oil or bitumen, which are trapped in unconsolidated sandstones. These types of crude oil are extremely viscous, making extraction difficult. Due to the high viscosity, heavy oil and bitumen are unable to be pumped out by conventional production methods. Compared with conventional reservoirs, there is more residual oil in unconventional reservoirs after primary recovery. Nevertheless, due to the unique properties of heavy oil and the unconsolidated sandstones, the conventional displacement method of restoring reservoir pressure utilizing water, natural gas or carbon dioxide injection has no functional effect [19]. Thermal recovery is currently the preferred method to exploit heavy oil reservoirs [40]. Thermal methods mostly refer to steam injection, which is the most effective method to reduce heavy oil viscosity.