Accurate petrophysical evaluations (formation lithology, porosity and water saturation) are essential to characterize potential reservoir zones and estimate in-place resources. Typically, these evaluations rely on acquisition of open hole logging measurements; however, for several reasons this is not always possible. This paper outlines two examples from tight gas reservoirs in two separate fields in offshore Abu Dhabi where open-hole data could not be acquired and petrophysical analysis was undertaken using cased hole log data. These evaluations successfully identified a number of gas saturated porous intervals in each well, one of which (based on the analysis results) was successfully flow tested.
Open-hole logging cannot always be performed because of well control concerns or bad borehole conditions requiring the well to be cased immediately. Historically, such scenarios resulted in acquisition of a cased hole dataset, which was typically not fit for petrophysical analysis (e.g., cased hole density and resistivity with their own limitations). This is especially the case in tight reservoirs where the pore volume contribution to a measurement value is correspondingly low. The new generation of Pulsed Neutron Capture (PNC) logging tools provides an opportunity to acquire a variety of independent measurements in challenging environments and enable standalone and accurate petrophysical evaluations.
The case studies presented in this paper summarize the workflows and petrophysical results from a suite of new generation cased hole logging tools. The petrophysical analysis outputs include lithology, porosity and hydrocarbon saturation. In the first case study from the Khuff Formation, open hole log data could not be acquired because of borehole stability. The cased hole petrophysical analysis workflow identified two gas saturated Dolomitic intervals with ~5 % porosity. This interpretation was confirmed by subsequent well testing operations with rates of 29 and 51 MMscf/d. In the second case study from the Araej Formation, open hole log data could not be acquired because of borehole conditions. The cased hole evaluation indicated elevated hydrocarbon saturations across an interval of the well, which coincides with high mud gas levels during drilling. This interval has been identified for development via a Smart Completion. The discussed cased hole log analysis workflow is not a complete replacement for an openhole log evaluation. However, the presented case studies highlight the sensitivity of the tools and the resultant potential to evaluate wells in technically challenging environments in the absence of open hole data.
These success stories of standalone cased hole evaluation have opened up myriad of possibilities in old and/or existing wells, especially where the open hole log data was previously inadequate or even absent. Accurate petrophysical evaluations will in turn provide better understanding of the reservoir and significantly help in planning future development of the reservoir.
