ALIF OIL FIELD PERFORMANCE ANALYTICAL STUDY -YEMEN

Abstract

The present study highlights the results of reservoir analysis and performance prediction of Alif field, which located at near the centre of the Wadi Al-jawf- Marib basin- block (18), northeast of Yemen. The field was discovered in 1984. Production from the field began in March 1986. The original reservoir pressure is of 2,805 Pisa at the datum of 5,800 ft, approximate mid point of the oil column. The ِِfield produces from the Alif sand reservoir, the principle producing heroin of the field, it has excellent characteristics of good porosity (average close to 20%), low clay content (only some 4%) and consequent high permeability (average of 1,200 md). It is of Jurassic age. The ratio of gas cap volume to oil volume is 2.7 unusually high and very desirable. The reservoir rock is medium grained, clean and homogeneous sandstone. Until January, 2004 a total of 211 wells had been drilled in the field. Gas injection began in January, 1988 the average gas injection rate was 121MMSCFD through 10 wells. By January, 2004, the cumulative gas injection was 4 TSCF. The average Gas injection in the same date was 1MMMSCFD. Pressure gradients in the reservoir are small and correlate well with injection and withdrawal patterns. The purpose of this study is to assess the hydrocarbons in place and reserves, evaluate the prevailing drive mechanism, and predict the performance of Alif reservoir. Due to lack of detailed geological information about this reservoir to enhance the geologic model, the study reviews the available geological, reservoir engineering and production history using analytical techniques. The study describes and illustrates these methods, how they were used, and the results they achieved. Once of the primary purposes of this work was to develop a reservoir hydrocarbon characterization that could be used to predict the hydrocarbon reserves and recovery. The adjustment of reservoir fluid parameters produced differential liberation and flash PVT data as a function of pressure. The observed rapid equilibrium of pressure throughout the field justifies the application of the material balance equation (MBE) technique to calculate original oil in place (OOIP) and reserves. These calculations show that the reservoir contains approximately 871.83 MMSTB of initial oil in place (OIIP) and 3,562,136.5 MMSCF of the initial gas-cap gas in place (OGIP). The drive indexes indicate that, by, Sep,1998, 1.4 % of the oil recovery was obtained by depletion drive, 0.6 % by water drive, 97.93 % by gas-cap drive, of which, 11.54% attributed to natural gas cap expansion and the remainder, 86.38 to the gas injection. The results also demonstrate that the expansion-drive index (EDI) term can be neglected in the presence of a gas cap and inhibited natural water influx. Hence Alif field must be recognized as performed under strong gas drive mechanism. Prediction of the reservoir performance was carried out using two methods. The first one is Tarner’s method to generate a predictive model of cumulative hydrocarbon production as a function of declining reservoir pressure .The second method is the production decline curve analysis using the straight exponential and harmonic decline equations. Then the results were used with conjunction with the results from Tarner’s method. As of January 2007, the prediction show that, a total of 527.32 MMSTB of cumulative oil production (RF= 60.48 @ remaining reserves 23.9 MMSTB) is expected by 2010 (as the Yemen government is planning to turn the project for gas production by this date). However the field is technically capable to produce additional 74.4 MMSTB by 2030 (RF=69.5). Moreover, the field is estimated to contain 4,117 BSCF of gas in place. Based on prediction results a total of 3,474 BSCF (RF=84.73). can be produced at abandonment pressure of 500 psi. Assuming 20 years of field production, initial gas production rates of 900-1000 MMSCF/day, and average annual production rates of approximately of 173.7 BSCF /year are achievable