The presentation “A METHOD TO IDENTIFY VERTICAL RESERVOIR PRESSURE COMMUNICATION BY COMBINING BOREHOLE SONIC AND HIGH-FREQUENCY ELECTRICAL IMAGING DATA” will be given by Knut Arne Birkeland, from Aker BP and Harish B. Datir, from SLB.
The presentation ”NMR Fluid Substitution – Pursuing the Fundamental Controlling Parameters of a Low-Mobility Reservoir” will be given by Søren Amdi Christensen, from Aker BP.
Downhole petrophysical measurements are typically governed by a combination of rock and fluid properties. The concept of fluid substitution (FS) is to create a measurement response at a different saturation than at which data are acquired downhole. In the context of nuclear magnetic resonance (NMR), FS is used to eliminate the hydrocarbon (HC) contribution from T1 and T2 distributions and thus to establish the pure response of a water-filled rock.
In 2015, Christensen et al. presented the first study conducting successful FS for a chalk reservoir. Based on theoretic considerations and laboratory measurements, the T2 geometric mean (T2gm) of the water-saturated rock is determined and converted into dominant pore throat radius (PTR) and Klinkenberg permeability (kk). The objective of this is to get a grip on the fundamental parameters controlling flow properties and the distribution of saturation in a North Sea low-mobility chalk reservoir, namely pore throat size and capillary entry pressure (Pce).
In the present study, the established FS methodology is applied to a chalk reservoir with 40 years of production history. In this case, however, the very basis of the FS model was seen to be challenged by the core data established for calibration. The core T2 and PTR distribution data acquired for calibrating the calculation of PTR and kk, was observed to deviate from the general and expected simple chalk behaviour. Despite these complications, a modified workflow was developed, enabling a successful calibration and application of the FS workflow.
The NMR FS methodology appears to be more versatile than originally concluded. The established model has shown the capability of identifying important flow units that otherwise would not have been appreciated as well as intervals with less favourable potential from a flow perspective that otherwise would have been modelled as “normal” rock based on the porosity-permeability transform established for the field.
In addition, the method has shown the ability to identifying rock with changed pore morphology due to compaction as well as revealing bad hole conditions that may not be readily appreciated from other downhole data. Under these circumstances the model fails to deliver results in line with the calibration, but the flawed output can be translated into information in terms of compaction and ongoing hole enlargement and thus the method has shown a new application.
Further work linking PTR and T2 distribution to sedimentary features observed on scanning electron microscopy energy dispersive spectroscopy (SEM-EDS) is required to fully understand the pore space system and thus determine the underlying principles of the deviation to the data processing introduced in this work. Hence the proposed workflow might show to be one of several steps towards arriving at the optimum data processing and calibration procedure for the Valhall-Hod field chalk reservoir rock.
Søren Amdi Christensen is petrophysical consultant working for AkerBP on the Valhall field. Having done a major part of his studies at Bergen University, Norway, he received his MSc degree in petrophysics and reservoir geology from Århus University, Denmark, in 1996. During his professional career he has worked as staff or consultant for various companies including DONG Energy, HESS, SAUDI ARAMCO. In recent years, his experience has branched out to include petrophysical consultancy for start-up companies in the geothermal- and CCS domain also. He is a data integration specialist with a special interest in the application of NMR. He has more than 25 years of experience with chalk evaluation and chalk field reservoir development. Formation evaluation and development of low-mobility reservoirs is a primary technical interest.