IOR AND EOR – CHANCES FOR INCREASE OF OIL PRODUCTION AND RECOVERIES IN EXISTING , MATURE RESERVOIRS JOSIP SEČEN

After the application of conventional oil extraction techniques, about 2/3 of discovered reserves still remain to be produced. In view of high probability for oil presence and oil price increases, investments in research of possibilities for higher recoveries and higher production with application of appropriate IOR and EOR methods are justifi ed, naturally under economic conditions.


Introduction
Let us assume the position of owner or at least oil company director, whose main task is renewal of reserves, and whose business success is measured by reserve replacement ratio.
New recoverable reserves of oil and gas can be obtained in the following manner (3) : • Discovery of new reservoirs as a consequence of continuous pursuit of basic activity -exploration of domestic and foreign exploration areas; Figure 1 Proposed defi nitions of the EOR and IOR terms (8) Slika 1. Predložena defi nicija EOR metoda i IOR postupaka (8) • Improvement of reservoir models by application of new geophysical methods, state-of-the-art interpretation techniques and reinterpretation of all available data in combination with data obtained during reservoir exploitation (produced quantities of fl uid and change in reservoir pressure);

Exploration of domestic and foreign areas
Common problem of Central European oil companies is high degree of exploration of domestic exploration areas, without great probability for discovery of larger quantities of hydrocarbons.The situation in Croatia is a good example (1) .
The territory of Croatia is divided in three exploration areas: the Pannonian basin, the Dinarides and the Adriatic offshore.A total of 332,6 million m 3 of oil and condensate and 129,8 billion m 3 of natural gas were discovered so far from 34 oil fi elds and 20 gas and gas-condensate fi elds.All oil was discovered in the Pannonian basin, and most of gas comes from the Pannonian basin and partly from the Adriatic offshore.
Despite great efforts, there were no major discoveries in the Pannonian basin in the last 20 years.We have to answer two important questions: o Is the Pannonian basin suffi ciently explored, and o Is there a probability for discovery of new, larger quantities of hydrocarbons?
According to evaluations made within the framework of hydrocarbon potential investigation studies for the Croatian part of the Pannonian basin, two to four times more hydrocarbons were generated than they are discovered so far, which means that between 900 and 1 800 million m 3 of undiscovered quantities of oil equivalent could still be present in the subsurface.However, in spite of numerous seismic surveys and drilling of 850 exploratory and 2 780 production wells, these assumptions have not been confi rmed.
Evaluation of possible discoveries of hydrocarbons were made applying the Arps-Roberts method.based on areas and reserves of hydrocarbons in already discovered fi elds.The data from 52 oil, gas and gas-condensate fi elds were processed.
Results of evaluations are shown in Table 1.According to analysis results, exploration targets were small and probability for discovery was low, but they could still be attractive due to available infrastructure for treatment and transportation of hydrocarbons.These results point out the only possible solution -exploration of insuffi ciently explored domestic exploration areas, in this case primarily the Dinarides, and foreign exploration areas.Will it be independently or jointly with other companies depends on the fi nancial strength and company policy.

EOR methods
High depletion of oil reservoirs discovered several decades ago is attributable to high exploration of the larger part of the European continent.Conventional enhanced oil recovery methods, such as waterfl ooding and gas injection, were applied successfully in this area.We know about experiments with thermal processes in Romania and Hungary and displacement of oil with CO 2 was attempted in Austria, Hungary, etc.
According to natural depletion drives, recovery for all discovered oil reserves in Croatia was only about 25 % (2) .Recoveries were the smallest in the reservoirs with the largest reserves.Afterwards, following the world practice and effectiveness of natural water drive, reservoir pressure maintenance method by injection of water into 8 reservoirs/fi elds and gas injection into one fi eld has been applied since 1972.Waterfl ooding is applied on 58 % of all discovered reserves.By this processes recoverable oil reserves were increased by approx.30 million m 3 , and recovery from earlier mentioned 25,00 to 34,45 %.
By the end of 2002 total production of oil at recovery of 32,5 % reached 102,0 million m 3 and remaining recoverable oil quantities are estimated at about 7,0 million m 3 .The status of reserves, in percentages, is presented in Table 2.In spite of such good results achieved by waterfl ooding, oil production has been constantly decreasing since 1981.The question is how to stop such production decrease.The solution lies in emerging of an idea how to recover a part of remaining 205,2 million m 3 of currently unrecoverable oil reserves.
The idea how to increase oil recovery from waterfl ooded reservoirs in tertiary stage of production has been developing for a long time.The solution was found in CO 2 injection as the most effi cient method, since in addition to its positive characteristics and proved effi ciency, CO 2 is available in high quantities, at present discharged into the atmosphere.Possible application is expected in the near future, primarily on Ivanić and Žutica fi elds.Numerous laboratory analyses, studies and numeric simulations were carried out and they indicate possibility for recovery increase by 12 to 14 % under miscibility or near miscibility conditions.
Economic valuation for one of the projects (6) was done for different variants: • Variant A -extension of existing waterfl ooding system • Variant B -oil displacement with CO 2 .
with oil price at 20 $/bbl (125,8 $/m 3 ) and gas price at 0,12 $/m 3 and discounted rate of 10 %.The obtained results are as follows: • Positive project duration: variant A -10 years variant B -23 years or more After application in these fi elds, the method could be extended to other fi elds in the same region.

IOR processes
Good results should also be expected from application of IOR processes, regardless of the manner of realization.The study on possibilities to search for bypassed oil in highly waterfl ooded part of the reservoir on Šandrovac (7) fi eld was also prepared.
Analysis of sedimentation conditions, effective thicknesses, injected water fl ood front shifting as well as achieved recoveries from well drainage areas, helped locate the areas for 10 side-tracked holes and one horizontal well.They would double the quantity of oil in comparison with current production, with recovery rate increase by 5,8 %.Planned investments amount to approx.500 000 $ per well.With oil price at 16 $/bbl and discounted rate of 10 %, net present value of 15,1 million $ was obtained, while payout time was one year and 10 months.
All such projects are burdened with high water cut in produced fl uid.Hopefully, the problem will be solved in the near future.At a time when produced quantities of oil are accompanied by twice as large quantities of water, the problem is of global proportions and its solution would considerably reduce production costs.

Conclusion
These considerations point out the need for exploration in new promising areas and applying of IOR processes and EOR methods on existing fi elds.
Produced indigenous oil is more valuable for domestic economy than imported oil and oil companies, scientists and experts from this sphere of activity face a permanent task how to maintain production levels and increase recovery from existing old fi elds in a profi table manner.

Table 1
Calculation of ultimate number of fi elds in each size class Tablica 1. Izračunati konačni broj polja po klasama površina i zaliha ugljikovodika

Table 2
Oil recovery rates and oil reservoir depletion methods, %