Fluid in place simulation analysis of net-to-gross, petrophysical cutoffs and upscaling
Adebola, Olanrewaju Oluropo
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The determination of net-to-gross through petrophysical cutoffs, usually outside the simulation engineer’s jurisdiction, is often the subject of confusion. Cutoffs are used to segregate rocks into “good” and “bad”, and the resultant net-to-gross is used to exclude the “bad rocks” from reservoir studies as it is assumed they do not contribute to flow. This work seeks to address and quantify how much error is involved taking this approach within the context of upscaling and to what extent the spatial distribution of these “bad rocks” portion of a reservoir affects the errors through a fluid-in-place analysis. A data-driven approach was taken in this work. The data was derived from a 40-acre, one injector – one producer two-dimensional model with two distinct fluid-in-place regions denoting good rock and bad rock regions. Different combinations of permeabilities and porosities for good and bad rocks were considered and a factorial design was used to examine the effect of these properties on the fluid production. The size and distribution of the bad rocks was varied to observe the effects of various net-to-gross ratios on the fluid-in-place and finally, a successive downscaling of the grid was done. The research shows that rocks that might be traditionally considered as non-reservoir can contribute to the production of a reservoir, sometimes, as much as 10% to the total recovery of the reservoir. Furthermore, in addition to the size of the “bad rock” the spatial distribution of the “bad rocks” are also important. The upscaling process also indicated that beyond the 5th degree of resolution, there is no need for higher resolution and eventually, there is little to no difference in the fluid production.