The presentation ”CONTEXT AND DISTRIBUTION MATRIXES – ONE PATH TO CONSISTENT AND EFFICIENT HANDLING OF UNCERTAINTY IN FORMATION EVALUATION” will be given by Kjetil Westeng, from Aker BP.
Understanding and quantifying the uncertainty in petrophysical interpretation inputs and results are increasingly important in subsurface settings as drilling targets and reservoirs are getting harder and more complex to evaluate. To make the right key decision in exploration or field development it is important to quantify the uncertainty of the interpreted property outputs. In our approach we assume the source of uncertainty originates from three sources, measured logs, the parameter settings used for our equation sets and finally also the imperfectness of the applied interpretation model. In-depth studies of the probability distribution of each parameter in every zone in every well are normally too time consuming to be part of standard well interpretation procedures. It would also open for too much subjectivity and inconsistency in how the uncertainty is deemed from case to case, and Petrophysicist to Petrophysicist. It is also known that the meta data describing the relevant conditions at the downhole sensor at every depth and in every zone is not always fully and perfectly measured or recorded and available to the analyst.
One can ask what the exact properties of the mud are (density, temperature, salinity, pad contact, pad tilt, etc.) depth increment for depth increment? Also, what is the sensor really measuring in heterogenous formation, is it at the lamina with m and n = 2 or was it also measuring some of the rock at the other side of the borehole with higher m and n? There will always be some error associated with both the measurements, and the parameters used as input to the petrophysical equation sets. There will even be some error in the models used, as the chosen interpretation model will fit better to some zones and depth intervals than others.
To help quantify the resulting uncertainty of the petrophysical interpretations we therefore have build a context-based approach where the interpreter specifies the context for the various key parameters, the input logs, and the applied model. Every context defined will call a predefined probability density functions fitting to the described scenario/situation as chosen by the interpreter. The predefined probability density functions are a product of consensus reached by a board of expert Petrophysicist.
Kjetil Westeng is Advanced Petrophysicist in Aker BP. He received his MSc degree in petroleum technology at NTNU in 2003. Since 2015 he has worked in Aker BP where part of his responsibility in addition to formation evaluation and to plan and follow up wells has been to develop the petrophysical interpretation methodology, algorithms, and multidisciplinary solutions.