Maharashtra Institute of Technology, India
An overview of porosity-permeability relationship in sand-shale and carbonate systems is discussed with available case studies. They are heterogeneous systems ranging from excellent reservoirs to tight-impermeable rocks. Large variation in composition, depositional textures and structures is common in occurrence. Carbonates system tend is complex and difficult to predict compared to the sand system. In general, based on statistical records these two systems together represent ninety-eight percent of reservoir rocks in the reported case histories of conventional petroleum system. In addition, ninety-six percent of source rocks are made of shale and mud component in carbonates while sixty-eight percent of them provide sealing mechanism in the petroleum geosystem from the available records.
The poroperm relationship with increasing depth is demonstrated using large dataset available on these systems., Other variables like density, pore size, shale volume and mineralogy have also been used wherever possible. Permeability dispersion in both systems is often anomalous particularly in the presence of mudstone in both systems as compared to porosity variation against depth suggesting heterogeneous behaviour on a core to log scale responsible in changing threshold pressure due to pore throat size. Calculation of shale volume and clay mineralogy plays a key role in sand shale system while grain size changes and diagenetic reactions control variation in the carbonate system while discussing the poroperm. Often features like bioturbation change the poroperm trend. Micro-scale anisotropy is one factor that is found to influence permeability which, in carbonates, can cover an extremely large range even in a single reservoir. Bedforms are influential in controlling permeability changes in the sand shale system. In general, depositional environment plays a key role in developing heterogeneity and also anisotropy. Fracturing may enhance permeability in sandstone reservoirs. Therefore, it is recommended to develop an analogy with the outcrop to understand the precise variation in the rocks.
Poroperm relationship with increasing depth particularly in the realization of gross to net thickness is crucial in improving understanding on quality of reservoir and thus certainty level is this high investment game of risk and uncertainty.