Experimental investigation of nitrogen-assisted SAGD in heavy-oil reservoirs

With the wide distribution of heavy oil reservoirs globally, effective heavy-oil thermal recovery methods are highly desirable for reduced overall investment cost and improved development of the oil industry. Pioneered by Butler, steam-assisted gravity drainage (SAGD) technology has been identified as a promising thermal recovery method for heavy oil development. Despite its advantages characterized by high oil recovery rate and high recovery factor, this technology has several disadvantages limiting its operation. For instance, it requires large steam consumption which is less economical and less productive due to significant heat losses. Also, it emits pollutant gases violating the newly enacted emission reduction and environmental protection measures. Recently, gas-assisted steam-assisted gravity drainage has been identified as a promising approach for improving the efficiency of steam-assisted gravity drainage related technologies. However, there is no enough experimental evidence to support their reliability as the studies are mainly based on theory and numerical simulations.

To this note, Dr. Songyan Li, Tingting Yu (Ph.D. student) and Professor Zhaomin Li from China University of Petroleum together with Dr. Kaiqiang Zhang at the University of Regina in Canada performed an experimental investigation of nitrogen-assisted SAGD in heavy oil reservoirs through a two-dimensional visualization simulation experiment. The visualization simulation was carried out by adjusting the length of the low-permeability interlayer. The main objective was to experimentally optimize the ratio of steam to nitrogen by analyzing the influence of nitrogen on steam bypass and breakthrough in the low permeability interlayer. The work is currently published in the research journal, Fuel.

The authors observed that for the case of nitrogen-assisted SAGD process, the optimal volume ratio of steam to nitrogen was 8:2. Compared to the experimental results of conventional steam-assisted gravity drainage, this approach recorded an increase in the maximum steam sweep efficiency from 46.39% to 71.34%, recovery factor from 27.78% to 49.12% and cumulative oil-steam ratio from 0.207 to 0.427- the largest values to be reported. This was attributed to the high-volume ratio of steam to nitrogen.  On the other hand, the steam velocity was observed to be affected by the nitrogen proportion. The steam velocity increased at low nitrogen proportion and decreased at high nitrogen proportion.

It was worth noting that for heterogeneous formation with a large number of low-permeability interlayers, nitrogen incorporation with steam can effectively facilitate the breakthrough of long interlayers while at the same time bypassing the short interlayers. This is because nitrogen has a relatively low percolating resistance, low interfacial tension with heavy oil and large diffusion coefficient which enable nitrogen to enter the low permeability interlayer thus reducing the resistance of subsequent steam seepage into the interlayer.

In summary, the research team presented a two-dimensional visualization of nitrogen-assisted steam-assisted gravity drainage in heavy oil reservoirs. The study insights highlight the significant role of nitrogen in expanding the steam-swept area in the process through an in-depth investigation and clarification of the synergetic action between the nitrogen and steam. Based on the research findings, Dr. Kaiqiang Zhang explained that their presented approach has a great potential for economic development of heavy-oil reservoirs with reduced emission and a negative impact on the environment.