This paper presents a new reservoir description of an extremely heterogeneous, fissured sandstone reservoir. In order to develop a numerical reservoir model, detailed geological description consisted of considering fractures, fissures, shale breaks, calcite/dolomite cementation, and their vertical frequency. Also, the frequency of "clean sandstone" in a "dirty sandstone" matrix was correlated with reservoir macroscopic properties. This new description is first of is kind and has given an excellent history match for the primary recovery phase.
The giant oil reservoir of Hassi-Messaoud is located 625 km south south-west of Algiers, Algeria. This reservoir is a part of the triassic bassin of the Algerian Sahara. The reservoir was discovered in 1956 and has been known for highly complex heterogeneity with secondary fissures, cementation, and shale breaks. Consequently, even though more than 600 wells have been drilled in the reservoir, reservoir description remains incomplete and necessitates special techniques in order to prepare a proper reservoir model for numerical simulation. A detailed geological study ndicated that core analysis results were incapable of properly describing the reservoir - mainly because of secondary phenomena, such as fissures and local fractures. These fissures and fractures are also cemented in an irregular pattern to make the description more complex. This lithological problem is particularly important for the zone 24 - the subject of the present study. A particular reservoir description law developed for other zones of the reservoir failed to give proper history match and the problem had to be re-evaluated in order to develop a better description of the reservoir. This technique involved in dividing the reservoir in five major layers and using probability distribution of shale breaks and fissures. This enabled one to explain the difference between well test permeabilities and the core analysis results. The new description of the reservoir led to an excellent history match. Various prediction runs were then performed to investigate EOR prospects for this reservoir.
Oil was first discovered from the Hassi Messaoud oil field when well MD1 was drilled and completed in June, 1956. In May 1957, the drilling of OM1 at 7 kin north north west of MD1 confirmed the existence of this field. Ever since, more than 600 wells have been drilled in order to define and develop the field with an average spacing of 1.25 km. The development wells were drilled with smaller spacing for selected zones.1
The producing formation is a Cambrian sandstone encountered at 3100 m subsea. With oil in place over 2000 106 m3, Hassi Messaoud is among the giant oil fields of the world. The oil is light (45 ° API) with original reservoir pressure of 473 bars. After a few years of oil production, it appears that the whole field could be divided into relatively independent producing areas1 (Fig. 1). Even though geologically these areas are part of the same anticline, the fluid exchange in most cases are highly restricted by low permeabilities on the borders generally due to intense secondary geological phenomena.
