Reliability of velocity-deviation logs for shale content evaluation in clastic reservoirs: a case study, Egypt

Abstract

The shale content is important in reservoir quality evaluation. There are good relations between effective porosity, permeability, and shale content in clastic rocks. One of the main objectives of the velocity-deviation log is that it could indicate permeability qualitatively. The velocity-deviation logs could be calculated from density and neutron porosity logs in carbonate rocks. The velocity deviation calculated from the density log is more reliable than that from neutron log in various clastic rocks to assess its validity in shale volume estimation. This study is carried out in clastic reservoirs drilled by four wells from different hydrocarbon fields in various. Three wells act as train wells and the fourth one is used as a test well. The regression analysis method is used to show the effectiveness of the relationships between the different velocity deviations using density and neutron logs. The relations of the three train wells clarify that there are excellent reliable relations in case of the density log and poor in case of the neutron log. This may be attributed to the effects of hydrogen in shale lattice leading to erroneously overestimated neutron porosity. The correlation coefficients between the compiled three studied train wells and velocity deviation from density porosity log are high and good with very dependable relations. Shale content can be predicted from density velocity-deviation log by using these excellent relationships. The deduced equation is used to predict shale content in the fourth testing well with excellent reliability. The shale content is estimated by the random vector functional link (RVFL) network method to evaluate the predicted results. The matching between the three curves of Vsh, Vsh (VDd), and Vsh (RVFL) is very good with standard errors of Vsh (VDd) and Vsh (RVFL) are 0.13 and 0.17, respectively, which clarify that the two methods are close to each other.