The Performance of WAG in a Stratified Reservoir, Hassi-Messaoud Field, Algeria

Abstract

Abstract Water Alternating Gas (WAG) process has been the subject of research, development, and field testing for a long time in Hassi Messaoud (HMD) field. There is still some disagreement in the interpretation of laboratory, field test data and in the selection of optimum parameters of injection such as (water/gas ratio, cycle period and miscibility conditions). The major reason for these problems is that the reservoir model does not mach the reference model. This work describes a method for designing an effective water/gas flooding in a stratified reservoir, and particularly in HMD. The proposed approach is based on a more realistic numerical model that takes into account the effect of three-phase flow, gravity and viscous forces in anisotropic media, which allows the determination of optimum parameters of water/gas injection. In addition, the performances of different WAG injection options were compared with the performance of waterflooding and gas injection. The simulation results illustrate the influence of WAG injection parameters, such as water-gas ratio, injection rates and cycle period on recovery process and select the optimum WAG ratio and WAG cycle. A more realistic reservoir model is the key to evaluate the performance of WAG injection in stratified reservoir, such as HMD. Based on the simulation results, Water-Alternating-Gas injection is the most successful process as compared to waterflooding and gas injection. The incremental oil recovery using WAG technique can reach 7% of OOIP and the GOR is more easily controlled than in conventional methods. Introduction Waterflooding is generally credited with producing 15 to 20 percent of the remaining petroleum. Miscible and immiscible gas injection can also account for 10 to 15 percent. In the search for increasing the recovery efficiency of these two mechanisms, it was found that the right combination of water and gas injections should yield an even higher percentage recovery. This combination, which is referred to as water-alternating-gas or WAG has been proven to be successful in the laboratory and field tests. Important factors affecting WAG injection are heterogeneity (stratification and anisotropy), wettability of the rock, fluid properties, miscibility conditions, injection technique, and WAG parameters. Stratification of a reservoir and ratio of gravity to viscous forces has a strong influence on the displacement process. Layered reservoirs may represent favorable geological conditions for gas injection. If a high-permeability layer, for instance, is situated below a low-permeability layer, it prevents quick gravity-segregation in the top zone towards the production intervals. Low vertical permeability of the layers also contributes to a better WAG sweep out. The main parameters that seem to influence the WAG process are injection rate, water-gas ratio, number of cycles and slug sizes. The water-gas ratio and number of cycles are major factors controlling the sweep of a reservoir by WAG displacement in stratified reservoirs, such as Hassi Messaoud. Reservoir Description Hassi Messaoud field is a flattened, broad, oval anticline trending north-northeast to south-southwest, parallel to a major fault zone. It covers almost 2,000 Km2 in the Oued Mya basin. The field has been subdivided into 26 zones (Figure 1) based on observed interwell pressure communication, the reservoir is in the Cambrian subdivided into four lithozones from the lower to the upper. R3 Lithozones Non-producing zones with a very low permeability and a high clay content averaging 30% (illite predominantly). The R3 section thickens from 275 m (902 ft) in well Md2 in the south central part of the field northward to 368 m (1207 ft) in well OMG 57 north of the field.