Abstract

Oil production in Egypt is based on the development of mature fields with highly complex geological and reservoir characteristics; therefore, a great amount of creativity is required to operate these oil fields. One of the main elements for development of mature fields is to estimate the reserves and determine the amount and location of the remaining oil.

Material balance equations have been used in petroleum engineering for many years to estimate the original hydrocarbon in place. This paper documents the ability of using the analysis of the material balance results in the reservoir characterization and determination of the remaining oil location. The applicability of this work is confirmed by actual field case study (Shukheir Bay Field) in Offshore Shukheir Oil Company (an international joint venture company in Egypt). Such study is an original contribution to the knowledge of the material balance results analysis.

Introduction

The material balance equation in the reservoir engineering is based on the principle of the conservation of mass (Mass of fluids originally in place = Fluids produced + Remaining fluids in place). The general form of the material balance equation was first presented by Schilthuis in 1941.1 In this equation; the cumulative withdrawal of reservoir fluids is equated to the combined effects of fluid expansion in the reservoir resulting from a finite pressure drop, pore volume compaction, and water influx. In 1963, Havlena and Odeh 2,3 presented techniques for interpreting the material balance equation as a straight line, which makes it easy to apply graphical techniques. In particular, extrapolation of a straight line allows the prediction of future reservoir performance, while the parameters of the line often are simply related to in-place volumes or water influx performance.4

The results of the material balance calculations are affected strongly by the selection of the PVT data. The gas liberation in the reservoir changes with the reservoir pressure. In the case of reservoir fluids above/at the bubble point, as the pressure decline due to withdrawals, the gas librated from oil does not flow to the well but accumulates until the critical gas saturation is reached. When the critical gas saturation is reached near the well bore, the gas may be moving more rapidly than the oil (differential liberation) whereas the remainder of the area the liberated gas remains in contact with the oil (flash liberation). Therefore, flash liberation data more closely represent the reservoir liberation process.5

Shukheir Bay Field

Data used in this research was obtained from Shukheir Bay field (Offshore Shukheir Oil Company - OSOCO), which is located in the shallow water close to the western coast of the southern part of Gulf of Suez (about 20 km south of Gharib - Egypt- Fig. 1). The field has been developed by drilling four deviated wells from the shore line. Three wells (SHB-1, SHB-2 and SHB-4st) are completed in Lower Rudies Sands while the fourth well (SHB-3st) is completed in Karim formation.

In December 1980, Well SHB-1 was completed on Lower Rudies Upper Sand (Pay I) and started production with 2200 BOPD and 0.8 MMSCF/D gas. The initial reservoir pressure is 2470 psi; however, the bubble point pressure of the produced oil is 2241 psig. Since December 1980 till now, the main reservoir (Lower Rudies Sands) has produced a cumulative of about 5 MMSTB of 34 API gravity oil from two pay zones (Pay I and Pay II) through two wells (Wells SHB-1 and SHB-4st). Well SHB-2 was completed in an isolated dry zone and Well SHB-3 was completed in another formation (Karim Formation). The production performance curve of Lower Rudies Upper Sands (Pay I and Pay II) is shown in Fig. 2. Currently, the main producing well (Well SHB-1) is on jet pumping producing about 700 BOPD with 70% water cut and estimated GOR of 680 SCF/B. The reservoir pressure declined to its current value of about 1800 psig.

Recently, a complete reservoir study for the development of Shukheir Bay Field was performed. Material balance equation was used through the study to (1) estimate the original oil in place and the reservoir driving mechanisms, (2) identify the reservoir characteristics and provide more geological, engineering and structural understanding of the Lower Rudies reservoir, and (3) define the best location(s) of new producer(s) to be drilled in order to increase field production and enhance the recovery factor.7

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