620 imbt. CARBONATE POROSITY PROBLEMS [Ch. 33 



during geologic time as a result of mountain building, erosion, and 

 submergence. Generally speaking, better carbonate porosity is found 

 in regions where there has been considerable tectonic activity. 



PRACTICAL APPLICATIONS 



As pointed out earlier in this treatment of carbonate porosity, rela- 

 tively few programs of secondary recovery in calcareous rocks have 

 been undertaken. The apparent lack of active interest in repressuring 

 carbonate reservoirs undoubtedly may be attributed to the lack of 

 success attained in those projects already undertaken. Figures are 

 not available from which the total number of failures can be counted. 

 Assumptions are dangerous, but in this instance it is safe to assume that 

 the majority of attempts have ended in failure or have achieved only 

 moderate success. Such a record certainly does not encourage con- 

 tinued attempts at carbonate reservoir repressuring. 



Most of the difficulty experienced is thought to be due to a lack of 

 fundamental data bearing on the nature of the porosity and the manner 

 in which it was formed. Seldom are data available, on the older 

 carbonate reservoirs, or for that matter on the more recently developed 

 reservoirs, that will permit the engineer to predict the character of 

 flow of fluids or gases through the voids. Of equal importance is the 

 determination of optimum rates of flow through these porous media. 

 Certain repressuring agents are better adapted to some types of poros- 

 ity than they are to others. All these considerations are important to 

 the engineer when he is considering the feasibility of secondary re- 

 covery from carbonate reservoirs. 



Reservoir mechanics is an involved study of reservoir behavior under 

 different producing conditions. In a sandstone reservoir the optimum 

 rate of fluid movement can be determined readily because of more 

 nearly uniform porosity and permeability. In carbonate rocks there 

 is wide variation in the reservoir index of porosity and permeability 

 which makes it difficult to handle the fluids in the most efficient man- 

 ner. At least 10 percent less oil is produced naturally from carbonate 

 reservoirs than from the average sandstone reservoir where conditions 

 of porosity, saturation, and character of the fluid are comparable. 



Often the average carbonate reservoir is produced at a rate too 

 high to permit the most efficient "sweeping" of the reservoir oil. 

 This is particularly true of those reservoirs under an active water drive. 

 Owing to great differences of porosity and permeability within the lime- 

 stone or dolomite, some of the fluid passes along fractures and crevices 



