Yana O. Karymova
||ITC “Gazprom Geologorazvedka”
Reservoirs of the Lower Berezovskaya subseries are fine-grained gaizes argillic to a variable degree. The reservoirs are commonly believed to be non-traditional due to an uncharacteristic combination of high porosity (at an average of 32%) and low permeability (below 1?10-3 um2). primarily. As this work shows, this is explained by the fact that the main part of pores (up to 75%) of the rocks in question is of subcapillary sizes making fluid movement practically impossible. At that, rock lithology affects the voids structure significantly. Thus, reservoirs with different content of the amorphous silica have different effective gas-filled porosity. Quantitate evaluation of the part of the differently sized pores among the voids in common is a lithological-capacitive model of the voids structure allowing describing these differences and forecasting possible effective gas-filled porosity of the reservoirs in question.
Materials and methods
Developing a lithological-capacitive model required materials gathered from four wells; the lithology of the collected cores was studied through the X-ray diffraction methods; the voids were studied through the mercury porosimetry methods, adsorptive structural analysis (ASA); general porosity of the samples were evaluated using naphtha saturation through the Preobrazhensky porosity method and natural water saturation of rocks was evaluated by the encapsulated core samples.
Mesopores represent over 50% of the voids of the non-traditional reservoirs of the Lower Berezovskaya subseries (2 nm to 50 nm) filled with cohesive waters; that is why gas movement is not possible along them. At that, the part of mesopores in reservoir LB1 is over 75%, which means that only fourth part of the pores can contain gas. This value is twice higher for reservoir LB2. Gas saturation of the rocks in question is expectedly related to the cumulative part of the macropores and capillary sized pores in them, which, when reduced, reduces the effective gas-filled porosity ratio. The gas-filled porosity ratio for reservoir LB1 characterized by the increased content of the OCT phase of the silica, is 7.5% at an average; for reservoir LB2 with almost absent OCT-phase, the ratio equals 13.5%. Lithological-capacitive models developed with the use of results of the mercury porosimetry and adsorptive structural analysis do not contradict each other in general, however they differ in certain details of the evaluation of the part of the capillary sized pores and macropores.
The lithological-capacitive model of the voids is a quantitative evaluation of the part of the pores of different sizes in the common voids allowing describing differences of the voids structure for different reservoirs lithology-wise, and forecasting possible efficiency of the gas- filled porosity of the reservoirs in question. Developing lithological-capacitive models of the voids structure of the reservoirs of the Lower Berezovskaya subseries of the north of the West Siberia requires using a complex of methods of core studying. The most adequate model is developed by combining the results of the adsorptive structural analysis with evaluations of the porosity through the Preobrazhensky porosity method for standard 30 x 30 mm samples.