Application of Microlog in Net to Gross Uncertainty Management

Abstract

Net to Gross (N/G) is a key input parameter into the estimation of Hydrocarbon In-Place volume. Errors in N/G evaluation could result in huge impact (up to 40%) on estimated in-place volumes which would in-turn affect the economics of reservoir/field development. Traditionally Gamma ray (GR) and/or SP logs are used for delineating reservoir sands from non-reservoirs. However, in some instances, these logs are not able to differentiate fully some reservoirs from non-reservoirs. In the case of GR, it is not able to resolve differences between shales and sands with high radioactive materials contents. Both GR and SP logs do not differentiate non reservoir cemented tight sand formations. This lack of contrast on the logs in such circumstances results in errors in the estimated net reservoir sands, leading to inaccuracies in the overall estimated reserves. To account for the deficiencies of these traditional techniques, a combination of neutron porosity and formation density logs may be deployed. These tools are able to provide contrasting petrophysical properties that differentiate between impervious formations and reservoir sands, thereby providing robust technique for improved net sand estimation. Incidentally, most old wells drilled in the 1950s up to mid 1980s do not have the neutron and the density logs. Yet for some early operators, a majority of the old wells were strategically positioned that the openhole data from them would be very crucial for estimation of reserves in the concerned fields. A cursory review of the log database reveals the abundance of Micrologs (Microlog Normal (MLN) and Micro Log Inverse (MLI)) in most of the early wells. Careful re-interpretation of these logs using appropriate techniques and principles have resulted in significant improvement in net-to-gross estimation in the absence of neutron and density logs. This paper aims at re-establishing the resourcefulness of microlog data in error reduction on estimated net to gross with a case study in ELJODA Field, Niger Delta Nigeria.