Abstract.

The Hassi-Messaoud field discovered in 1956 is producing with miscible gas injection for most zones and with waterflooding for other zones of the field.

The producing formation is Cambrian sandstones which are located at a variable depth from 3200 m to 3500 m, with an average thickness of 100 m and 4 reservoirs (Ri, Ra, R2, R3). The initial pressure was 482 kg/ cm2, this field has an oil with an API of 45" (0.806 g/cm3) and a variable bubble point from 140 to 200 kg/cm2.

Two zones have been flooded by gas from 1964, and later the gas was injected in the central zones in 1971-1979. In 1988-1992 the peripheral zones were also flooded by gas. The gas injection level was 5106 m3/d initially and increased as the needs increased, and new facilities (compressors, separators, etc.). The OIP in the gas injection zone is 55% of the total OIP meanwhile in the water flooded zone the OOIP is 15% of the total one.

Following the actual level of the static pressure, the miscible gas process will be extended to all the field except the zones which are flooded by water. This process is a Vaporising Gas Drive type (VGD), and the miscibility pressure is variable from 240 to 270 kg/cm2.

A good oil recovery of the main reservoir (Ra) was obtained and is due to the efficiency of the miscible gas flooding, a very low gas segregation, and high vertical anisotropy.

From the results of the breakthrough of the gas at the oil producers we found a low areal sweep efficiency with a channeling in the direction North-North-East/South-South-West.

Even when we got a good vertical sweep efficiency, there were still some sectors unflooded. We can recommend other development for this field to improve the recovery and the areal sweep efficiency.

INTRODUCTION

N Hassi Messaoud has produced since 1958. Drainage was of depletion type until the gas miscible injection started in 1964. In 1995 the areas under gas injection represent 55% of the OOIP (70% of the cumulative production), with 278 producers and 78 injectors.

Nine compression stations supply 36.5 lo6 Sm3/ day (1.3 lo9 Scf/day) at a pressure of 425 Kg/cm2 (6050 psi).

RESERVOIR DESCRIPTION

Geology Cambrian deposits (Figure 1) develop over several hundred meters constituting 4 reservoirs (R3, R2, Ra, Ri or D5); Ra is the best one (80 m thick and 10 mD) and is subdivided into 5 subreservoirs D1, ID, D2, D3, D4. Interbeddings of silts with small thickness and permeability, and variable lateral extension (decameter to kilometer) influence the fluids flow and the gas segregation.

The tectonics (Figure 2) generated networks of major (important throws) and minor faults mainly oriented NNE/SSW.

The diagenesis damaged strongly the petrophysical characteristics inducing an heterogeneity organized into strips of same direction as the major faults, with alternance of good or bad permeabilities.

Petrophysics

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