A discussion on the interrelationships between five properties in reservoir evaluation

Reservoir pore spaces (incl. pores, fractures and vugs) are too complex to be predicted by use of the traditional interrelationships between the four properties of reservoirs, thus more and more contradictions occur in reservoir evaluation. A great number of case studies were made to reveal the causes of these contradictions and the corresponding solutions were also proposed. For the reservoirs with complex pore spaces, we found four common types of contradictions between porosity and permeability, porosity and water saturation, absolute permeability and effective permeability, and electrical property and hydrocarbon property. These contradictions are mainly caused by variation of pore types, pore-throat sizes and fracture occurrence. On this basis, the concept of geometrical property was presented and methods were discussed for qualitatively or quantitatively describing the geometrical properties of pores, fractures and vugs. The following findings were achieved. (1) For pores, two relationships were established between pores & throat sizes and rock textures, physical property & fluid property, and between pore types and fluid property & logging responses. (2) For fractures, five relationships were established between occurrence and pore texture index (m), radial extension and deep/shallow borehole resistivity, openness and fracture permeability, occurrence and matrix water saturation, and between development index and lithology. (3) For vugs, two relationships were established between size & connectivity and m value & three porosities derived from logging responses (neutron, density and sonic wave), and filling degree and logging responses. The interrelationships between geometrical property, lithology, physical property, fluid property and electrical property can significantly improve the evaluation of complex reservoirs such as carbonates.