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Determining the Accumulation Limits of the Physical Properties of Tight Sandstone Reservoirs in Eastern Ordos Basin

Abstract

Focusing on force analysis of natural gas in micropore-throat, this paper made a research on the upper and lower petrophysical limits of gas accumulation in tight sandstone reservoir of Eastern Ordos Basin. 1. Determination of lower limit of pore-throat radius Assuming thickness of irreducible water film as the minimum pore-throat radius for gas injecting into tight reservoir, based on the irreducible water saturation, it can be calculated thickness of irreducible water film for tight reservoir in study area mainly ranges from 5nm to 15nm, with a mean of 10.2nm, where values of He 8 Section and Shan 2 Section are 5nm-17nm and 4nm-9nm respectively. 2. Determination of Upper limit of pore-throat radius Calculation of upper limit can be divided into two cases: 2.1 Taking abnormal pressure of source rock into consideration Radius is in inversion proportion to abnormal pressure. At the end of Early Cretaceous, if abnormal pressure was larger than 5 MPa, gas driven by abnormal pressure and buoyancy, can inject into all reservoirs with 0.01μm pore-throat, i.e. all reservoirs with pores-throats which were not sealed by irreducible water. However, the corresponding value of abnormal pressure is 6.5 MPa in current geological conditions. 2.2 Regardless of abnormal pressure Without consideration of hydrodynamic force and abnormal high pressure, when capillary force is equal to buoyancy, the corresponding critical pore-throat radius value can be regarded as the upper limit of pore-throat, For the tight reservoir of He 8 Section, Taiyuan Formation and Shan 23 Section, at end of Early Cretaceous and present, which are 0.61μm, 1.55μm and 0.25μm and 0.46μm respectively. 3. Critical petrophysical limit The relation between median radius and permeability is significant. Therefore, based results of fitting correlation-ship between median pore-throat radius and petrophysical parameters, critical petrophysical properties can be predicted with critical pore-throat radius values. For He 8 Section, lower petrophysical limits of reservoir are 4% and 0.1×10-3μm2, upper petrophysical limits, i.e. porosity and permeability, of reservoir developed at the end of Early Cretaceous and present are 13%, 1.8×10-3μm2 and 14%, 2.0×10-3μm2 respectively. For Shan 23 Section, the corresponding values are, in order, 2% and 0.01×10-3μm2, 8% and 1.0×10-3μm2, 9% and 1.4×10-3μm2. For Taiyuan Formation, that are, in order, 3.5% and 0.02×10-3μm2, 11% and 1.1×10-3μm2, 12% and 2.0×10-3μm2.