Alternating Oil Production with Gas Injection as a Means of Enhancing Oil Recovery
Abstract Generated abstract
This paper argues that conventional simultaneous waterflooding and oil production can leave substantial oil unrecovered because injected water preferentially moves through larger pore channels and rapidly waters producing wells. Drawing on field examples from Arlan and Izbaskent, it contrasts low recovery under water injection with higher recovery during pressure depletion and proposes abandoning constant pressure maintenance. The author recommends cyclic operation in which reservoir pressure is first lowered, then restored by injecting hydrocarbon gases, preferably propane and butane, which may reduce oil viscosity and aid recovery from narrow pores. For Tomsk Oblast oils, the suggested operating range is depletion to about 20 ati followed by gas injection to 40 to 50 ati before resuming production.
Full Text
Alternating Oil Production with Gas Injection as a Means of Enhancing Oil Recovery from Reservoirs
L. A. Pukhlyakov
(Presented by Professor A. V. Aksarin)
Most of the oil fields in our country are currently being developed with the use of waterflooding. Moreover, oil production and water injection into the reservoirs are carried out simultaneously. This leads to certain undesirable phenomena. Namely, experiencing on the one hand a pressure deficit and on the other an excess of it, the reservoir fluid chooses the path of least resistance. This means that the water passes only through pores connected with one another by wide channels and does not penetrate into pores connected with one another by channels of comparatively small diameter. Such pores are the majority. As a result, significant quantities of oil do not take part in the resulting movement of the reservoir fluid and remain unrecovered from the reservoir, while the producing wells become watered out.
So says theory. And what does practice show us? It turns out that practice confirms this theoretical proposition, since by the time of water breakthrough into most producing wells, oil production in fields developed with the use of waterflooding proves to be negligibly small. Thus, in the development of the Arlan oil field in the northwestern part of the Bashkir ASSR, by the middle of 1968 water cut had reached 50% of the total number of wells, while oil production amounted to only 7% of the calculated reserves. Consequently, if watering continues at the same intensity (and we have every reason to believe that it will continue at precisely the same intensity), then total oil production from the field will amount to only 14%.
Where oil production is carried out without water injection, oil recovery proves to be much higher. Thus, in horizon IX of the Izbaskent field, oil production began in 1956 and at first (until 1965) was conducted without the use of water injection. During this time the reservoir pressure fell from 350 to 120 atm, but oil production amounted to 35% of the calculated reserves. From February 1962, water began to be injected into this horizon in order to maintain reservoir pressure. As a result, the reservoir pressure increased to 250 atm, but the water cut reached 90%. Without any radical measures, only an insignificant quantity of oil can be produced from this horizon, while the increase in production amounted to only 20%, and the total production from the horizon was 55%. Thus, here too, water injection into the reservoir carried out simultaneously with oil production led to the same effect as at the Arlan field; however, owing to the fact that beforehand, over the course of
during six years water had not been injected into the reservoirs, the total recovery proved to be more than three times higher.
To increase reservoir recovery, it is apparently necessary to abandon water flooding and, in general, the maintenance of reservoir pressure at a single level. The pressure within a deposit should alternately decrease and increase, and only in this way can we extract from the reservoir the oil that is located in pores with narrow channels. In doing so, to raise the pressure it is necessary to use not water but hydrocarbon gases, since they—especially gases such as propane and butane—not only do not hinder the release of oil from pores with narrow channels, but also facilitate it, because their presence reduces the viscosity of the oil.
A new question arises: to what extent should the pressures in oil deposits be lowered and raised?
The oils of Tomsk Oblast release approximately half of their associated gases when the pressure is reduced to 20 ati, and for different oils this amounts to from 20 to 50 m³/m³ under normal conditions, or 1–2 m³/m³ at a pressure of 20 ati. Thus, when the reservoir pressure is reduced to 20 ati, the volume of the gas–oil mixture will increase by a factor of 2–3; hence 50–70% of the oil can be extracted from the reservoir. A further reduction of reservoir pressure is undesirable, since it will be accompanied by a considerable increase in oil viscosity. Studies of these oils have shown that when degassed to 20 ati their viscosities increase by no more than 20–30%. With complete degassing, the viscosities of the oils of Tomsk Oblast increase by a factor of 5–10.
Consequently, after reservoir pressures have been reduced to 20 ati in the fields of Tomsk Oblast, it is necessary to stop oil withdrawal and inject gases into them, preferably propane and butane. When the pressure rises to 40–50 ati, oil withdrawal can be resumed, while the injection of gases into the reservoir is stopped.