How Different Density-Neutron LWD Tools Response can Affect the Formation Evaluation Approach

Abstract

Log While Drilling (LWD) Density-Neutron allow to calculate, in real/near real time, a continuous porosity and shale volume curves. Sometimes log acquisitions from different tool types are available. In these cases, attention should be paid to the reading characteristics of the individual tools, to calibrate the petrophysical interpretation in a consistent way. Generally, the LWD interpretation can be considered more critical than Wire Line Log WLL because the operation timing strongly limits an accurate QC of the data. Different Nuclear tools involve different measurement system for density and different energy source as well as different neutron energy range for neutron. During the field development phase, the petrophysiscal analysis can deal with various type of Density and Neutron log, even in distinct phases of the same well, making the lithology recognition, the shale volume and the effective porosity evaluation more difficult and even misleading during the formation evaluation step. The aim of this paper is to show through different case histories, how the various tool types can affect the formation evaluation model and a possible approach to mitigate the problem. The case study includes logs from different WLL and LWD tools, different well diameters, deviations and lithologies. After a careful QC, the data sets have been processed in order to identify homogeneous intervals (from the lithology and the porosity point of view) in order to make the response comparison based only on the characteristic of the different tool types. Once the conditions for a correct comparison have been fixed, the statistical distribution within the homogeneous intervals was quantitatively described using histograms and cross-plots. The results of the analysis have proven the influence of lithology: silty-shale sequences show the most significant discrepancy between the tool responses, while the clean lithologies show less or negligible discrepancy. If not properly considered, the different Density-Neutron (DN) tool responses inside a homogeneous interval can strongly affect the output of the petrophysical interpretation, mostly on shale volume and effective porosity with a dangerous fallout also in the reservoir modelling. In the presented case study, the observed discrepancy from different LWD Density tool can vary between 0-0.02 gr/cc while for Neutron tools between 0-5 p.u. To quantify the impact of the mentioned discrepancies on the petrophysical interpretation, a deterministic interpretation model (calibrated against cores data) was used for shale volume and effective porosity calculation. The comparison between DN shale volume derived from different tools and Gamma Ray (GR) has been defined as simple but strategic approach to understand the meaning of the DN response and therefore to define the proper shale point in the DN cross-plot for the petrophysical interpretation. If DN logs are acquired with various tool types properly calibrated, the discrepancy between the log readings is only due to the tool characteristics, and may mislead the interpretation if the lithological parameters are not properly calibrated. Comparison with other lithological logs (e.g. GR) and calibration with core data is mandatory for a correct and consistent petrophysical interpretation when different acquisitions are available.