Tu D3 05 Fault Representation in Flow Simulation ...

Tu D3 05 Fault Representation in Flow Simulation Models: Sensitivity to Grid Structure V. Papanikolaou* (University College Dublin), T. Manzocchi (University College Dublin)

Summary Faults are represented into flow simulation models as 2D surfaces rather than 3D volumes. The conventional GRDECL format does not allow complicated structures to be incorporated in the simulation grid, while the StairStep and the GSG format are more flexible with regards to more complicated fault geometries. This geometrical flexibility of the grid formats is tested on a number of 3D models, representing fault geometries with different geometrical characteristics, with the scope of investigating the strength and weaknesses of these formats. An experiment focused on how differently the GRDECL and the Stair-Step formats behave with regards to accurate juxtapositions and flow behaviour. Initial results demonstrate that there is some deviation between the two formats, due to both incorrect juxtapositions of the Stair-Step format and non-orthogonality biases of the GRDECL format.

79th EAGE Conference & Exhibition 2017 Paris, France, 12-15 June 2017

Introduction It is widely known how important faults are for the entrapment of hydrocarbons and reservoir compartmentalization, since they may act variously as conduits, baffles or barriers to flow. Faults however never comprise one single surface; instead, they tend to form segmented arrays in both map view and cross-section and as displacement increases, they form 3D volumetric zones of variable complexity (e.g. Childs et al., 2009). Although the 3D nature of faults is well known, faults in flow simulation models are usually treated as 2D surfaces. Mainly due to grid limitations, the standard simulation grid format does not allow complicated fault geometries to be incorporated into production simulation models without simplifying the fault network; the 3D nature of faults is therefore ignored. Some authors have published new methods of representing 3D fault zones in production simulation models, either explicitly by using Local Grid Refinement (LGR) grids and the Fault Facies Modelling technique (Fredman et al., 2008), or implicitly by upscaling the geometrical sub-seismic fault zone characteristics (Manzocchi et al., 2008). The present work focuses on the geometrical flexibility generally available for fault representation in flow models, using the conventional GRDECL, the Stair-Step and the GSG format. Each format has a different impact on how faults are incorporated into the simulation grid and on subsequent flow behaviour. A number of 3D models have been built representing fault zone geometries with different geometrical characteristics. The formats were exported from Petrel and the models were then imported into the ECLIPSE 100 flow simulator. A focused experiment was performed to examine in more detail the differences between the two most commonly used flow simulation grid formats: the standard GRDECL and the Stair-Step format. Flow Simulation Grid Formats The GRDECL format is the most commonly used grid format and it is based on the Corner-Point Grid structure (Ponting, 1989). Four straight coordinate lines define the corners of each stack of cells and eight corner-points define the depth of each corner of individual cells. Faults are explicitly represented in the GRDECL format by defining different corner-point depth values between adjacent cells that the faults displace. Grid pillars have to be aligned along the faults and since the coordinate lines have to be straight, that limits possible, more complicated geometries. The use of stair-step faults in the GRDECL format (Stair-Step format) is the most commonly-accepted solution for modelling complicated structures (Gringarten et al., 2009). Most of the times however, stair-step faults result in a non-accurate representation of the across-fault juxtapositions. The recently-developed Generic Simulation Grid (GSG) is a generalization of the GRDECL format which allows also the use of truncated and/or curved pillars in the simulation grids (Schlumberger, 2014). Listric faults or fault geometries including branch-lines (e.g. synthetic faults) can now be modelled in this format. 3D Fault-Zone Representation Three different model cases (fig. 1) representing fault zones with different three-dimensional geometrical characteristics have been designed; a neutral relay zone with a breached and an unbreached relay ramp (fig. 1a), two faults forming a horizontal branch-line (fig. 1b) and a fully breached fault-bound lens (fig. 1c). These models consist of 14 alternating sand-shale units with constant bed thickness. Faulted versions of the models were built in the three formats and the behaviour was assessed by comparing flow simulation results.

79th EAGE Conference & Exhibition 2017 Paris, France, 12-15 June 2017

Figure 1: Model cases representing different fault zone geometries in 3D, used for testing the GRDECL, the Stair-Step and the GSG formats: A neutral relay (a), a horizontal branch-line model (b) and a fully breached fault-bound lens (c). Different grid formats have different impact on different fault geometries with regards to flow behavior and representation of across-fault juxtaposition. In the neutral relay model (fig. 1a) faults are vertical planes and since the GRDECL format requires pillars to be straight, this simple format is able to fully represent the modelled geometry. The model case in figure 1b contains two curved faults that form a horizontal branch-line. This geometry cannot be represented in the GRDECL format (due to pillar limitations) and the exported grid includes a faulted structure where the fault planes are straight and the branch-line is entirely missing. All cells below the branch-line are deactivated by default and no across-fault flow is present in the lowest permeable layer (fig. 2a). On the other hand, both the GSG and the Stair-Step format capture the initial fault geometry and both show similar flow behaviour (fig. 2b and 2c). The fully-breached faultbound lens (fig. 1c) is a very complicated structure and due to strong pillar limitations we have managed to build a model only with the Stair-Step format. Hence, although the GSG format is more flexible geometrically, it seems that the Stair-Step format is more easily applied. Questions remain, however, about the extent to which the geometrical simplifications associated with the Stair-Step format are able to properly honour across-fault juxtapositions. This is investigated by a detailed set of flow simulation models, described below.

Figure 2: Representation of the horizontal branch-line model (fig. 1b) in the GRDECL (a), the GSG (b) and the Stair-Step format (c). In the GRDECL format the branch-line cannot be preserved while the GSG and the Stair-Step formats are able to capture this geometry; however, only the GSG format is able to represent the parent model precisely. 79th EAGE Conference & Exhibition 2017 Paris, France, 12-15 June 2017

Juxtaposition Model Cases in a Single Fault A focused experiment of a model containing a single dipping fault with a lateral throw gradient was designed to understand how differently the GRDECL and the Stair-Step formats behave with regards to accurate juxtapositions and flow behaviour (fig. 3). The layers of the 14-layer model were designed as net- or non-net stochastically, with 10 realizations built at each of a five different Net:Gross cases. Production and injection wells were placed across the fault and the wells were completed in a single of randomly-chosen permeable layer in each realization, and a water-flood was simulated. The GRDECL format honours exactly the across-fault juxtapositions of the parent model, but the Stair-Stepped model, which is built prioritizing cell orthogonality over geometrical accuracy, has some juxtaposition windows absent and others of different sizes (fig. 3b and 3d).

Figure 3: Illustration of how the single fault model is represented in the GRDECL (a) and in the Stair-Step format (c) along with the relevant Allan diagrams (b) and (d), taken from a case of NTG = 0.6. The Allan diagrams show how differently the two formats behave with regards to across-fault juxtapositions. These juxtaposition-related issues have a significant impact on across-fault fluid flow (fig. 4). In the low NTG cases there is a good correlation between the two formats since flow is restricted due to either lack of permeable units (NTG = 0.2) or the high compartmentalization of the model (NTG = 0.4 and NTG = 0.6). In the high NTG cases where the permeable units are dominant (NTG = 0.8 and NTG = 0.95) deviation between the two formats increases. Results show that there is an earlier onset of water breakthrough in Stair-Step models with high NTG ratios (fig. 4a) while the total amount of oil produced is underestimated (fig. 4b). This initial study therefore suggests that the geometrical compromise offered by the Stair-stepped solution may be causing significant differences in flow behaviour due to incorrect juxtapositions, although it is unclear at present the extent to which the differences in behaviour are attributed to cell non-orthogonality biases in the GRDECL model, or to juxtaposition biases in the Stair-Step model. Work is ongoing to elucidate the result. 79th EAGE Conference & Exhibition 2017 Paris, France, 12-15 June 2017

Figure 4: Plots showing the Time to Water Breakthrough (a) and the Total Oil Production (b) in a 20-year simulation time. Both graphs show the deviation between the GRDECL and the Stair-Step models. Low NTG case models display a better correlation between the two formats than the high NTG ones. Conclusions Flow results between different fault geometries clarify the geometrical bottleneck behind the different simulation grid formats. A comparison between different fault geometries and different grid formats was made and the strengths and weaknesses of the latter were made clear. There are significant differences in flow response between initial models built with the GRDECL and the Stair-Step format. It is suggested that deviation between the two formats is a result of both the altered grid cell geometry and dimensions and the permeable/impermeable unit distribution. References Childs, C., Manzocchi, T., Walsh, J. J., Bonson, C., Nicol, A., and Schopfer, M. P. J., 2009, A geometric model of fault zone and fault rock thickness variations: Journal of Structural Geology, v. 31, p. 117-127. Fredman, N., Tveranger, J., Cardozo, N., Braathen, A., Soleng, H., Roe, P., Skorstad, A., and Syversveen, A. R., 2008, Fault facies modeling: Technique and approach for 3D conditioning and modeling of faulted grids: AAPG Bulletin, v. 92, p. 1457-1478. Gringarten, E., Haouesse, A., Arpat, B., and Nghiem, L., 2009, Advantages of Using Vertical Stair Step Faults in Reservoir Grids for Flow Simulation, SPE Reservoir Simulation Symposium 2009, Volume 2: The Woodlands, TX; United States, p. 1242-1249. Manzocchi, T., Heath, A. E., Palananthakumar, B., Childs, C., and Walsh, J. J., 2008, Faults in conventional simulation models: a consideration of representational assumptions and geological uncertainties: Petroleum Geoscience, v. 14, p. 91-110. Ponting, D. K., 1989, Corner Point Geometry in Reservoir Simulation: Proceedings of the First European Conference on the Mathematics of Oil Recovery, p. 46-65. Schlumberger, 2014, ECLIPSE 100 Technical Description.

79th EAGE Conference & Exhibition 2017 Paris, France, 12-15 June 2017