It is very important to understand reservoir pore structure information in finding interested zones and formulating advisable recovery scheme. Capillary pressure data is critical material in evaluating reservoir pore structure, from which, pore size distribution and pore structure parameters can be acquired, and by which, the quality of reservoir pore structure can also be identified.
With the analysis of hundreds of capillary pressure curves, it is found that they all have the same mercury-injection pressure, so the differences in shape and type of capillary pressure curves are controlled by the mercury-injection saturation. For capillary pressure curves with better quality, the mercury-injection saturation is relative higher in the same mercury-injection pressure, and the reservoir porosity and permeability are also higher, vice versa. Based on which, we develop a novel method to construct capillary pressure curves consecutively integrating reservoir porosity and permeability. Correlations among mercury-injection saturation and porosity and permeability in every mercury-injection pressure are established. With this correlation, the capillary pressure curves can be constructed.
Generally, capillary pressure curves are exhibited in semi-log coordinate, mercury-injection saturation is linear in X-axis and mercury-injection pressure is logarithm in Y-axis. If we display them in log-log axis, they will be hyperbolic curves. The ratio between mercury-injection saturation and mercury-injection pressure in the inflexions of hyperbolic curves are named Swanson parameters, which have a good relationship with reservoir synthesized index, from which, we can derive reservoir permeability.
The problem of evaluating reservoir pore structure consecutively is the limitation of quantity of capillary pressure data because of the expensive test cost. As is known to all that NMR T2 spectrum contains plenty of information of pore structure, porosity and permeability, we present a model to extract Swanson parameter from NMR T2 distribution. With this parameter, we can gain reservoir permeability, associating with NMR total porosity, the reservoir capillary pressure curves can be constructed consecutively. Comparison with the capillary pressure curves of mercury-injection experiment demonstrates that they are commensurate.
At last, an in-situ example is exhibited of evaluating reservoir pore structure using constructive capillary pressure curves consecutively by above method, which shows that it is available and accurate in detecting the change of reservoir pore structure as a function of depth.
It is very important to understand reservoir pore structure information for geologists and field development workers in calculating field recoverable reserves and formulating appropriate development programs. So acquiring reservoir pore structure information of interested intervals is the target for policy-maker of field, but no way is available at present. With the appearance of NMR logging technology, a new path is opened. From NMR T2 distribution reservoir pore structure information can be gained directly (Xiao L. Z., 1998, Coates G. R. et al., 2000 and Xiao L. et al., 2007, Zhou C. C et al., 2007). With the development of research, more and more difficulties appear, and the major problem is the influence on the shape of NMR T2 distribution when pore space is occupied by crude oil, especially light oil.
