Abstract
At present time, a number of oil fields in Orenburg area are at a late stage of development. This leads to oil production decline, with some oil wells becoming so depleted that they have to be shut-in. This paper discusses a joint project to rehabilitate the Sorochinsko-Nikolskoe oil field in order to extend productive time of such wells by identifying candidate wells for horizontal sidetracking. The main goal of the project was the validation of horizontal sidetracking for long-term enhanced oil recovery. To achieve this objective, the joint group needed to select wells suitable for horizontal side tracking and predict both the initial daily oil production rate and cumulative oil production over five years. At the same time, the team had to minimize any risks associated with geological uncertainty. The main criteria for sidetrack selection included an initial oil production rate of more then 75 ton per day, followed by steady production for five years. An integrated approach was used to re-evaluate the development of the oil field. First step of this process was analysis of the oil field development and the identification of bypassed oil reserves to determine zones of interest. The next step was quality control of petrophysical and geological data sets and the analysis of match between initial models and actual production data. The result of this analysis confirmed the need to revise the existing models for a more accurate oil production forecast and to develop a water flooding strategy within the side track zones of interest. The next step was geological modeling based on petrophysical interpretation of porosity and fluid saturation.
Special feature of petrophisical interpretation is new algorithm for calculation of reservoir property. Base of this algorithm is petrophisical equations sort of "core-logging data" relationship for calculation of reservoir property as a porosity, permeability and fluid saturation. Petrophisical equation for open porosity in target formation was drawn from comparative analysis of core data porosity and neutron porosity. Since this equation was used for processing of all well data in this oil field, time for data processing has been significantly reduced and on the same time, required accuracy of petrophisical value of rock property was achieved. Interpretation results from previous researches on a data set from this oil field do not have a criterion for estimation of fluid saturation type and water-saturation cut-off for pay zone recognition. New algorithm for petrophisical interpretation gives clearly defined criteria for recognition of watersaturated, oil saturated and oil-water saturated pay zones for target carbonate formation. Comparison of log interpretation results based on new algorithm and previous one is shown in Fig. 1. Significantly that, the new interpretation results of oil saturation coefficient depend from porosity and resistivity while quantitative criteria for that calculation were obtained from analyses of acceptable well test results.
