Presenter
Tom Bradley from Baker Hughes
Co-authors
Ali Eghballi, Elizabeth Von Wilamowitz-Moellendorff (Baker Hughes), Susanne Laumann, Phil Vardon, Auke Barnhoorn (Delft University of Technology)
Abstract
Introduction
Geothermal energy is growing in importance as a clean renewable energy source. Hence, the industry desires to quantify key formation properties. As financial returns in geothermal are considerably lower than in oil and gas, many commonly accepted oil and gas methodologies (including extensive downhole data acquisition) are not economically feasible.
Of key interest is if the formation will flow, from where, and at what rate. This is directly related to the amount of heat that can be produced and hence the economic return of a well. This is commonly quantified by well tests, however a well test will not give detailed information of what intervals are flowing, therefore a more detailed method of identifying flowing intervals is of significant interest to the industry.
Nuclear magnetic resonance (NMR) logging is an accepted method of estimating permeability from fractional porosity measurements, and hence to identify potential flowing intervals. With further benefits of not requiring a nuclear source, so has intrinsically lower HSE risks than other methods. However, NMR has historically been seen as a ‘slow’ log, and as a result is unattractive to geothermal operators because of increased rig time costs.
Procedure
As the slow logging speed is controlled by the slowest polarising fluid phase in the formation (commonly the hydrocarbons), long wait times are needed in NMR logging to polarise all fluid phases, resulting in slow logging speeds. It was theorised that in geothermal formations where no slow polarising hydrocarbons are present, wait times can be reduced significantly, and therefore logging speeds can be increased by an order of magnitude whilst still providing data equivalent to standard methodologies, hence making NMR logging considerably more attractive to geothermal operators.
Results/Observations
Two logging passes were conducted in a geothermal production well drilled with oil based mud. The formations of interest are high porosity and permeability water filled channel sandstones. The first log pass was a reference pass conducted using a standard NMR acquisition optimised for porosity, permeability and hydrocarbon typing, recorded at standard NMR logging speeds. The second pass was conducted using the experimental short wait time fast acquisition recorded at six times the logging speed of the standard pass. The porosities from the two passes were then compared. In many intervals the porosities were observed to match between the two passes. However in other intervals the fast pass was observed to measure porosity values approximately 25% lower. It was observed in intervals where porosity was underestimated, significant oil based mud filtrate invasion deeper than the NMR tool depth of investigation was present. This invaded OBMF will be insufficiently polarised by the short wait times resulting in porosity undercall. When this underpolarisation is accounted for, the standard and fast pass porosities were observed to match closely, hence supporting the theory of the fast logging approach.
Conclusions
We demonstrate that when no significant hydrocarbons are present, the new fast NMR logging methodology is a viable method to derive porosity, permeability, and reservoir quality in geothermal wells. This makes NMR logging considerably more attractive for geothermal formation evaluation as NMR can quantify several formation properties that cannot be quantified by other logs (for example partial porosities, permeability, reservoir quality), with considerably reduced HSE considerations compared to traditional nuclear porosity measurements
Where hydrocarbons or OBMF invasion are present, this should be accounted for: For example longer wait times can be used to sufficiently polarise any hydrocarbons/OBMF that is present; Optimisation of NMR acquisition sequence to identify hydrocarbons; Application of underpolarisation correction methodologies; Partial slow/partial fast to calibrate for OBM invasion; Consideration to use water based mud rather than oil based mud
Biography
Tom Bradley is an Associate Fellow with Baker Hughes, and a global subject matter expert for open hole formation evaluation, including nuclear magnetic resonance interpretation. He started his career in 1997 with Western Atlas International, and since 2005 has been based in Den Helder in the Netherlands. As part of his role, one of his key interests is the application oil and gas formation evaluation methodologies to geothermal formation evaluation