In development and reactivation studies of old fields one is often faced with a lack of production and other data. Ways to reconstruct missing well production data and to reallocate comingled production for a multi-zone oilfield are presented. Individual well oil rates and water cuts were reconstructed based on field tests to fit reported cumulative oil and water values at a given time in the history of the field. To reallocate commingled production, typical correlations between oil rate, water cut and normalized cumulative oil production were derived using available data from single zone producers. All available production tests were used in the procedure to increase the reliability of the reallocation estimates.
To obtain proper results from conventional reservoir engineering procedures such as material balance analysis or performance prediction for an individual producing zone, reliable production information is needed. Any missing data should be filled in and allocation of commingled production should be checked and corrected if necessary. Such corrections were made to the available data for a reservoir simulation study of the Beda oil field.
The Beda field was discovered in Libya in 1959 and put on commercial production in 1964. A total of 65 wells were drilled of which 10 were D&A and 15 were completed as commingled producers. As of March 1996, 33 wells were suspended for various reasons and 22 wells were still producing. Field production peaked in late 1965 at 61,000 bbl/day and present production is about 2,400 bbl/day. Cumulative production from the field was just over 96 million barrels at the end of 1996. Production data was not available for the first two years of operation (i.e. 1964–1965) during which time about 20% of the total oil to date was produced.
There are three producing zones in the Beda field; the 4000 and 3900 foot zones and the shallow reservoir zones. Early production was mainly from the 3900 and 4000 foot zones and allocation of the commingled production did not appear, in some cases, to be reliable. This paper describes the procedures used to estimate the missing early production history from well test data and the procedure used to reallocate commingled production.
The Beda field, located in the Sirte basin in Libya (Figure 1), started producing in January, 1964. However, no historical production data was available until January, 1966. Available data was as follows; production tests with measured oil rate and water cut starting in 1964 and monthly oil and water production and initial cumulative production starting January, 1966 as well as completion and work over information. The challenge was to reconstruct missing production history based on this available information. At this stage no distinction was made between single zone and commingled producers.
Figure 2 shows the oil rate and water cut for a typical single zone producer from the 3900 foot reservoir (well B25). The reservoir is considered to be nearly volumetric, under a moderate to low edge water drive. Oil rates tested prior to January 1966 could be approximated with exponential time-dependence which is in good agreement with this drive. The problem was formulated in the following way:
