The Department of Petroleum Resources (DPR) in their third quarter report on the petroleum industry announced that the country’s oil reserves may dry up in the next 45 to 50 years. Their calculations were based on the estimates of the nation’s proven plus probable oil reserves which stood at 32.39 billion barrels plus 5.19 billion barrels of condensate as at January 2008. With current production levels at 2.108 million barrels a day and an estimated 730 million barrels per year the DPR expects a 2.23% reserves depletion rate and 45.75 years reserve index. These announcements, alarming as they sound, are based on current reserves and do not take into account though undiscovered reserves in promising areas like the deep offshore Niger Delta, the Anambra and Chad basins plus extra oil that may be obtained in the future through enhanced recovery.
The question of how much reserves a country has is usually contentious as most claims by producing countries are typically unsubstantiated and sometimes over stated for political reasons. A clarification of the meaning of what comprises a countries petroleum reserves is therefore in order.
Proved reserves are those reserves claimed to have a reasonable certainty (normally at least 90% confidence) of being recoverable under existing economic and political conditions, and using existing technology. Industry specialists refer to this as P90 (i.e. having a 90% certainty of being produced). They are also known as 1P.
Probable reserves are based on median estimates, and claim a 50% confidence level of recovery. Industry specialists refer to this as P50 (i.e. having a 50% certainty of being produced) or 2P (proved plus probable)
Possible reserves have a less likely chance of being recovered than probable reserves. This term is often used for reserves which are claimed to have at least a 10% certainty of being produced (P10). Reasons for classifying reserves as possible include varying interpretations of geology, reserves not producible at commercial rates, uncertainty due to reserve infill (seepage from adjacent areas); projected reserves based on future recovery methods. They are referred to in the industry as 3P (proved plus probable plus possible).
All of the above add up to a country’s Oil Reserves the estimated quantities of crude oil that are claimed to be recoverable under existing economic and operating conditions. Oil reserves are furthermore, a fraction of the Oil in Place: the total estimated amount of oil in an oil reservoir, including both producible and non-producible oil. Most of the oil in place cannot be extracted because of the reservoir characteristics, limitations in the existing extraction technologies and economical considerations. With secondary and enhanced oil recovery techniques, which make use of different methods to get more of the oil in place, the recovery factor (the ratio of producible oil reserves to total oil in place for a given field) can increase significantly with time.
When a reservoir is tapped and begins to produce petroleum, its natural pressure, which pushes the oil towards the well, begins decline. This natural drive of the reservoir is usually as a result of associated gas found in the reservoir under pressure (gas drive) or water found in the reservoir (water drive). A time comes when the reservoirs internal pressure is no longer sufficient to force the oil to the well bore and to the surface. At this point only about 20 to 30 percent of the oil in place has been extracted. Secondary and Enhanced Oil recovery techniques are then deployed to get more oil out of the reservoir.
Secondary recovery involves the injection of fluids such as water or gas into to the well through injection wells in order to maintain the pressure within the reservoir and push hydrocarbons to the well bore. The most common secondary recovery techniques are gas injection and water flooding. Secondary recovery reaches its limits when considerable amounts of the injected gas or water are being produced by the production well. At this point about 40% of the oil in place would have been extracted.
Enhanced oil recovery is the third stage in the production process. It serves not only to restore formation pressure, but also to improve oil displacement or fluid flow in the reservoir. This is achieved by changing properties (usually the viscosity) of the oil inside the reservoir making it easier to be extracted. The three major types of enhanced oil recovery operations are chemical flooding (alkaline flooding or micellar-polymer flooding), miscible displacement (carbon dioxide [CO2] injection or hydrocarbon injection), and thermal recovery (steamflood or in-situ combustion).
In the chemical flooding process, special polymers or surfactants are injected into the reservoir. They work like detergents and ‘wash’ out the oil from the reservoir.
In miscible displacement process a gas such as CO2 is forced under pressure into the reservoir; the CO2 liquefies under pressure and dissolves the oil, helping it to flow more easily through the rock. The gas is usually followed by slug water which pushes the oil and gas mix towards the production well.
Thermal recovery is the most common and it is also used in the extraction of heavy or viscous oils. The process involves the introduction of heat into the reservoir to make the oil less viscous and more mobile. The 2 basic approaches to thermal recovery are: steam flooding where steam or hot water is injected into the reservoir under pressure. The other method, fireflood extraction, involves burning some of the oil in a controlled manner within the reservoir. This process requires special reservoir characteristics and is rarely done.
The major drawback of these techniques is the cost of carrying them out. Increasing oil prices (at least before the recent crash) have made many of these techniques more worth the while in squeezing out the extra oil form older reservoirs to increase reserves. As technology inevitably improves current methods of enhanced recovery will be improved upon and even made cheaper with better methods developed. The DPR’s estimates may end up being too conservative in the end.
This article (an abridged version) was originally written for Scitech360