Ikon RokDoc - The Rock Physics System of Choice: Carbonates in RokDoc, Intro & Xu Payne

Carbonates

RokDoc for Carbonates

More than 50% of world oil is contained in carbonate reservoirs and RokDoc software is just as useful at modelling carbonate reservoirs as it is in siliciclastic reservoirs.

RokDoc starts with a simple shale/carbonate volume, each rock type with their own unique pore characteristics, and can build up to multipore, carbonate rock models, with bound and un-bound fluids inside. The Xu-Payne method divides the total pore volume into four components: (i) clay-related pores, (ii) inter-particle pores, (iii) microcracks and (iv) stiff (moldic) pores (fig 1). Each pore component is incrementally added to the rock matrix to calculate the elastic properties of the resultant effective medium. Individual pore components can be included into the model such that they are either isolated, or, in perfect fluid connectivity with the remaining pore space. This allows the user to decide exactly how the carbonate rock model interacts with Gassmann fluid substitution.

Figure 1. Building the model from Xu & Payne, 2009

Figure 2. P-wave vs. porosity cross-plot

Cross-plots of Vp V porosity (Figure 2) allow pore space analysis of the carbonate rock and provide input to pore space component inversion (Figure 3, track 3). Inversions yield pore shapes and volume proportions by fitting estimated Vp values to the measured well log data.

Figure 2: P-wave vs. porosity cross-plot for a 100% water saturated limestone reservoir interval. The solid lines show the reference trends for models containing only: 100% microcracks (dark red), 100% inter-granular pores (green) and 100% stiff pores (dark blue). The dashed lines show intermediate models containing contributions from 2 pore omponents.

Figure 3. RokDoc Well Viewer

Figure 3: Left-to-right: clay volume, total porosity, pore space component (pink = stiff pores, blue = microcracks) and Vp logs (black = measured, red = calculated) from the carbonate reservoir.

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