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Fractured reservoir rocks may be considered as made up of at least two porosity-permeability systems: one which accounts for the largest void space fraction of the reservoir and which makes up the pore space between the grains (intergranular porosity); the other which accounts for the smallest void space fraction and which makes up the pore space of the fractures and fissures. The former generally exhibits a low permeability, whereas the latter provides channels of high permeability which are the main reservoir fluid carriers to the wells, provided sufficient lateral continuity exists. In most cases, vertical continuity of the highly permeable system is also present and gravity segregation of the reservoir fluids may exist to a large degree. Under certain conditions of re ervoir rock wettability, the fluids will segregate into two zones of saturation, a lower zone of high oil saturation and an upper zone of high gas saturation. This results in a rapidly increasing gas-oil ratio in the early stage of depletion and in a low ultimate oil recovery.
An alternative explanation of the behavior of fractured reservoir is also proposed whereby the low recovery by depletion drive is explained by capillary end effect which contributes to retaining a large fraction of the oil inside the "blocks" between fractures while permitting the evolution of solution gas. This end-effect capillary retention hypothesis would be particularly effective while low pressure gradients prevail within the reservoir.
The hypothesis submitted herein for the explanation of the low oil recoveries from fractured reservoirs are intended to stimulate thinking so that experiments may perhaps be designed for their verification and for finding some solution to increasing recovery from such fields. Various approaches to this problem are suggested, but they have not received as yet the sanction of field use.
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