The Coates and SDR models for deriving permeability from NMR T2
NMR logging and formation testing are the most commonly used downhole methods for obtaining reservoir mobility or permeability data. Reliable formation tests provide good mobility estimates, but the cost and time required for the tests limit its use, often to a small number of discrete points. NMR logging, on the other hand, is acquired continuously. However, the commonly-used NMR permeability models, i.e., Coates model (1991) and geometric mean of relaxation time (T2GM Clearly, there is a fundamental deficiency in pore-body-size NMR permeability models for rock systems where the size of the pore body is not easily correlated to the size of the pore throat. To overcome this shortcoming, one needs to introduce a dynamic aspect of the pore system, such as pore connectivity into the model, which is the focus of this paper.
In our new approach, a pore-connectivity parameter is introduced to modify the Timur-Coates permeability model. Using NMR log data as input, the model was used to successfully estimate the permeability and movable fluid volume and in turn quantified the vuggy porosity in carbonate facies. The determination of the connectivity factor is a challenge in view of the large variation in pore geometries and structures inherent in carbonate lithofacies. To account for these variables, we studied various core samples from Middle East petroleum reservoir formations and also NMR data available from the NMR Carbonate Rock Catalogue.) model (also known as SDR model, Kenyon (1997)), are based on pore-body size which is encrypted in T2 decay. The models require that a certain correlation exists between the pore-body size and pore-throat size. Many carbonate reservoir formation rocks originated from lithification of carbonate sediments in various depositional environments. Carbonate reservoir formations differ from clastics, not only in mineralogy, grain morphology, and depositional environment, but also in post-depositional diagenetic processes. All of these aspects can affect the pore connectivity and permeability in a manner that is distinctively different from clastics. As a result, the pore-body-to-pore-throat size correlation frequently fails for carbonate lithologies where the connectivity among pores is not always associated with the pore size, especially for vuggy carbonates.
distribution data are based primarily on pore-body sizes and are thus more valid for rocks where a good correlation between pore-body-size to pore-throat-size exists, a characteristic not observed in many carbonate rocks. In the paper, we describe a modified Coates permeability model which takes into account the pore connectivity in carbonates. The model is applicable not only to vuggy carbonates but also is applicable to formation rocks that contain abundant mud and/or intragranular porosities. An attempt was made to relate the carbonate formation depositional environments and post-depositional diagenetic processes with the pore-connectivity parameter used in the modified permeability model, using core NMR, thin-section, and routine core analysis data. Examples were shown to use core, formation multi-test, or image data to estimate the pore-connectivity parameter and consequently, improve the accuracy of permeability estimation for complex carbonate formations where multiple facies present.
