Study On The Mechanism Of Increasing Oil Recovery By N ₂ In Huayingtai Metamorphic Rock Reservoir

Xinglongtai metamorphic rock reservoir is a fractured massive reservoir with bottom water, buried depth of the reservoir is2355m-4680m and the oil column height is2325m; The issues in productive process include a rapid decline of formation energy and oil production as well as water breakthrough at the bottom of the reservoir. To maintain the oil production rate of Xinglongtai metamorphic rock reservoir, it is necessary to explore the possibility of tridimensional development by using reservoir development features with gas injection at the top layers and water injection at the bottom layers of the reservoir, as a result, to investigate the mechanism of enhanced oil recovery in Xinglongtai metamorphic rock reservoir, a series of indoor experiments, such as N2injection flooding at the top of the core holder and water injection flooding at the bottom of the core holder, have been conducted in the lab.With the reservoir fluid and fractured cores of Xinglongtai metamorphic rock reservoir, physical simulation tests like phase state compatibility experiments by injecting gas to enhance solubility and expansion of oil, two phase flow in the process of gas/water injection flooding, and the long core test with fractured cores and high temperature/pressure are conducted, then combine with numerical simulation method, by conducting a serious of indoor tests and simulation analysis, the flooding mechanism of gas injection flooding at the top layers and water injection at the bottom layers in Xinglongtai metamorphic rock reservoir has been explored, the following conclusion can be reached:1) The percolation mechanism research on the metamorphic rock reservoir shows:the permeability of cores has been greatly improved after being fractured; the relative permeability test shows that the gas injection displacement procedure can only result in a moderate recovery; the highest pressure gradient calculated from the test is0.155MPa/m, the test on pressure gradient is the base for finding out the most appropriate pressure to improve the oil recovery of matrix or micro fracture.2) The reservoir fluid analysis shows the percentage of C1is45.47%, the total percentage of C2-C6is17.18%, and the percentage of C7+is36.92%, which is ranged as conventional crude oil. As the increasing amount of N2injected, the reservoir volume factor increase slightly, and the bubble point pressure increases with the increasing injected volume of N2, and the bubble point pressure is64MPa when the injected volume of the N2exceeds50%. It indicates that N2has a low solubility in the crude oil and can only make little contribution to the flooding process through oil expansion. The primary pressure of Xinglongtai metamorphic rock reservoir is38.6MPa, the MMP of N2injection displacement calculated from the P-X phase diagram/pseudo-ternary phase diagram/tubule simulation is higher, as a result, the long core test is an immiscible displacement process.3) There are two different types of displacement process, including gas injection from the top or water injection from the bottom of the core holder, in which the core holder is set with a30degree angle to the horizontal direction. The recovery of pulse injection displacement with N2is64.58%, the recovery of continuously injection displacement with N2is60.53%, the recovery of pulse injection displacement with water is69.51%and the recovery of continuously injection displacement with water is70.49%. The process of N2injection from the top of the core holder has resulted in a displacement efficiency over60%, because it is a gravity-stable displacement process, though it is also a immiscible flooding process, the flooding efficiency is still very high; simultaneously, the slow injection speed in the process of water flooding from the bottom of the core holder contribute to a high flooding efficiency for it has formed a piston-like displacement process. The outcome is the base to carry out the tridimensional development research of the reservoir.4) By calculating the enhanced oil recovery through imbibition while shutting in, we have investigated the contribution to the flooding efficiency of both matrix and microfracture, the recovery is higher in the water flooding process, and the recovery of matrix and microfracture mainly come from imbibition between the oil within matrix or microfracture and the injected gas or water, which indicates that the imbibition between oil and water is stronger than that of N2. Conclusion from long core test is the base to build up the theory of tridimensional development, including N2injection from the top layers of the reservoir, water injection from the bottom layers of the reservoir and producing oil from the middle layers of the reservoir at the same time.The permeability test shows the less the injecting pressure is, the lower the permeability will be. As a result, to maintain the reservoir pressure is of great significance to obtain a stable production from the oilfield.5) The numerical simulation investigation about N2injection from the top of the core holder and water injection from the bottom of the core holder has been carried out. The simulation of collaborative injection with water/N2shows that it’s of little importance whether the reservoir is totally perforated or not, and the recovery of reservoir with bottom water is lower than those without bottom water, the gas oil ratio of the reservoir without bottom water is much larger than those who have bottom water, indicating that if bottom water exists, the water will flow into the reservoir, prevent the injected gas front from breaking through the reservoir too soon and greatly lower the gas oil ratio, however, it will also affect the flooding efficiency.