Formation pore structure generally includes pore geometry, volume, connectivity and other properties of pore body and pore throat in the reservoir formation. Acquiring continuous pore structure information along the depth is very important to the identification of good quality reservoir and the evaluation of reservoir petrophysical or geological properties. It is well known that NMR T2 distribution reflects the distribution of different pore sizes very well under certain conditions. Other parameters such as permeability and total porosity can also be derived from T2 distribution. However, it is a relatively difficult problem in the exploration of the Ordos Basin of China as to how to establish a valid standard for identifying the quality and the type of pore structure as a function of depth using NMR logging data. We collect a series of core samples typical of the pore structures found in the Ordos Basin and use mercury injection capillary pressure curve as the standard to identify the quality of pore structure. Through an integrated analysis of their capillary pressure curves, T2 distribution, and the cumulative porosity curves of T2 distribution, we find that the key factor controlling the quality of core pore structure is not porosity but the percentage of pore components within a specific range of pore sizes in the whole pore system. Based on this finding we summarize the behavior of our experimental data and develop a method of utilizing T2 distribution data to evaluate the pore structure. We extract three parameters from the T2 distribution, each of which represents the percentage of one kind of pore components within a specific range of pore sizes in the whole pore system. The values of these three parameters and their relative magnitudes are very sensitive to the quality of pore structure and can be used to classify the type of the pore structure of the samples. The result is in good agreement with the method using mercury injection capillary pressure curves to evaluate the pore structure of the core samples. Field applications also demonstrated that our method is very sensitive in detecting the change of reservoir pore structure as a function of depth. In summary, based on our research from laboratory experiments to field trials, we have identified a key factor which defines the quality of formation pore structure and developed an evaluation method using NMR data.
The concept of formation pore structure discussed by geologists generally includes pore geometry, volume, connectivity and other properties of pore bodies and pore throats in the formation. But how do we define a good reservoir in terms of the pore structures? It is a relatively abstract idea that we should somehow quantify in order to make it useful. NMR and mercury injection are two frequently -used methods for petrophysicists to study the pore structures. Information of pore size distribution can be obtained from these two methods. Hence, it is only natural to use these two methods in our attempt to establish a criterion for good reservoirs based on pore structures.
