Low-Permeable Reservoirs as High Potential Assets for EGR

Natural gas fields that are being developed in Ukraine, mainly relate to the high and medium permeability reservoirs, most of which are at the final stage of field life. In this situation one of the main sources of additional gas production is unconventional fields. This paper presents the analysis of challenges concerning development of low-permeable reservoirs and experimental results of conducted research, which provide the opportunity to establish technologies for enhance gas recovery factor. For this purpose, a series of laboratory experiments were carried out on the sand packed models of gas field with different permeability (from 9.7 to 93 mD) using natural gas. The pressure in the experiments varied from 1 to 10 MPa, temperature – 22-60 °C. According to the features of low-permeable gas fields development the research of displacement desorption with carbon dioxide and inert gas stripping by nitrogen was conducted. These studies also revealed the influence of pressure, temperature, reservoir permeability and non-hydrocarbon gases injection rate on the course of adsorption-desorption processes and their impact on the gas recovery factor. According to the experimental results of relative adsorption capacity determination it can be concluded that the carbon dioxide usage as the displacement agent can lead to producing adsorbed gas by more than 30% than by using nitrogen. To remove the adsorbed gas just reservoir pressure lowering is not enough due to the nature of adsorption isotherms. Particularly at pressure decreasing by 8-10 times compared to initial reservoir pressure only about 30-40% of the total amount of initially adsorbed gas is desorbed. And at considerable reservoir pressure reduction the further deposit development is not economically profitable. According to the results it was found that in the case of nitrogen usage the most effective method is full voidage replacement at injection pressure of 0.8 of the initial reservoir pressure, and in case of carbon dioxide usage - full voidage replacement method at pressure of 0.6 of the initial reservoir pressure. Taking into account availability of N2 and CO2, N2injection is recommended for further implementation. The influence of displacement agent injection pressure on gas recovery was experimentally proved. The physical sense of the processes taking place during natural gas desorption stimulation by non-hydrocarbon gases was justified. The effect of temperature, pressure and reservoir permeability on methane adsorption capacity were determined. The mathematical model for estimating adsorbed gas amount depending on the reservoir parameters was developed. Obtained results were summarized and recommendations for practical implementation of elaborated technological solutions were suggested.