Calculation method and application of cap rock's effective thickness

Key words:  cap rock  effective thickness  capillary sealing ability  evaluation criteria  logging interpretation  calculation and examination  equivalent breakthrough pressure

Abstract:

A method of computing cap rock's effective thickness is proposed based on that cap rock's heterogeneity may result in different capillary sealing ability within cap rock. Using data of thin section, SEM, breakthrough pressure, porosity, permeability and other test data of argillaceous cap rock and sandstone reservoir, the boundary between cap rock and reservoir can be defined, which includes breakthrough pressure limit, critical lithology(grain size) and petrophysics limit.Subsequentlythe evaluation criteria of cap rock's capillary sealing ability is established. With gamma ray curve, interval transit time curve and test data, a logging interpretation model is established to quantitatively recognize cap rock's lithology and to calculate porosity, permeability and breakthrough pressure.A comprehensive evaluation on capillary sealing ability of cap rock has been made to first remove the cap rock with no capillary sealing ability and then to obtain the effective thickness. Our results show that between cap rock and reservoir of X gas reservoir, the breakthrough pressure limit is 2 MPa;the critical lithology(grain size) is argillaceous coarse siltstone;the porosity limit is about 6%, and the permeability limit is about 0.02×10^-3 μm². Cap rock's capillary sealing ability has been divided into five grades from Ⅰ to Ⅴ. By removing grade Ⅳ and Ⅴwhich correspond to no capillary sealing ability, the effective thickness of single cap rock is obtained between 4 and 10 m. Correlation between the height of gas column and the effective thickness suggests the robustness of the method. Based on the effective thickness, a weighted average of breakthrough pressure has been taken to obtain equivalent breakthrough pressure, and the whole capillary sealing ability of cap rock has been evaluated as moderate effectively.