For over 40 years Pulsed Neutron Logging (PNL) has been used in the industry to determine hydrocarbon and water saturations for reservoir saturation monitoring. Earlier, this technique was mainly used for time-lapse contact movement analysis and for a qualitative assessment of the saturation. Present-day PNL tools can perform a quantitative assessment of petrophysical properties of the reservoir including mineralogy, porosity and saturation. The ability to record multiple independent measurements expands the applications of cased-hole evaluation to environments otherwise not accessible and to cases where a standalone evaluation is required.
PNL technology has been successfully used in the West Netherlands to estimate petrophysical properties, mineralogy, and for diagnosis of bypassed hydrocarbons and depleted zones. The cased-hole environment here tends to be complicated by the presence of multiple casing strings and completion hardware in the borehole, with variable annular and near-wellbore fluids. Every measurement is used and interpreted in relation to its statistical uncertainty and level of interpretability. In a complex scenario, near-wellbore conditions and well dynamics determine to what extent a standalone cased-hole evaluation can be performed.
The paper describes an example of log acquisition and data integration in a complex depositional environment. Cased-hole logging is performed to determine the oil potential in Lower Cretaceous reservoirs, following the discovery of an oil lead in a nearby field.
Important reservoir properties are measured through-casing and combined to compute porosity and mineralogy. Water saturation estimation done using porosity, mineralogy and measured sigma values is compared to oil saturation computed from total organic carbon (TOC) direct method. Carbon/Oxygen (C/O) ratio could not be applied in this wellbore environment because of the presence of multiple strings, large 12.25″ borehole, gas in the tubing and unknown mud filtrate in the invaded zone. As a result of the borehole conditions, the Near Carbon-Oxygen ratio versus Far Carbon-Oxygen ratio has a very narrow envelope (limited dynamic range) that cannot be interpreted. The presence of oil-based-mud invasion in combination with a large borehole limited the ability to confidently resolve the oil saturation in the uninvaded zone from TOC measurement. If considered independently, this could lead to an overestimation of the oil saturation of the uninvaded zone, as suggested by the comparison with sigma-based saturation computation.
A comprehensive evaluation approach that integrates cased-hole neutron porosity and sigma, elemental dry weights from capture and inelastic spectroscopy, including TOC, with resistivity from logging-while-drilling (LWD), is applied to reduce the reservoir characterization uncertainty and facilitate informed decision making. The combination of these log measurements into a quantitative multi-mineral solver analysis proved very useful in demonstrating that the oil lead identified in the formation has only residual oil and should not be pursued further as an appraisal opportunity.
A review and comparison of different interpretation methods with their associated uncertainties is required to constrain the petrophysical model. This further highlights the importance of data integration to maximize confidence in the evaluation results in such complex boreholes. The right amount of data exchange and communication between various disciplines led to a sound conclusion of the project that halted the execution of the appraisal opportunity; thus, avoiding an unnecessary expenditure of several million euros