Corner-point grid

Abstract: A three-dimensional (3D) corner-point grid model gives a relatively accurate description of the structural properties and spatial distribution of oil and gas reservoirs than Cartesian grids. The finite element simulation of the stress field provides a relatively probable presentation of the in situ stress distribution. Both methods are of great importance to the exploration and development of oil and gas fields. Implementing the finite element simulation of in situ stress on a 3D corner-point grid model not only retains the structural attributes of a reservoir but also allows the accurate simulation of the 3D stress distribution. In this paper, we present a method for implementing the finite element simulation of in situ stress based on a 3D corner-point grid model. We first established a fine 3D reservoir model with corner-point grids and then converted the grids into corresponding 3D finite element grid models using a grid conversion algorithm. Next, we simulated the in situ stress distribution with the finite element method. The stress model is then resampled to corresponding corner-point grid geological models using the reverse algorithm. The grid conversion algorithm is to provide data support for the subsequent numerical simulation and other research efforts, thereby guaranteeing procedure continuity and data consistency. Finally, we simulated the stress distribution of a real oil field, the X region. Comparing the simulated result with the measured result, the high agreement validated the effectiveness and accuracy of the proposed method.

Abstract: Complex corner point grids are often encountered in reservoir simulation. Such grids can be severely faulted and the corner point cells near faults, and elsewhere, can be distorted. Also, extensive refinements may be used. Since such grids are usually presented without specific information regarding inter-cell contact, it is up to the reservoir simulator to decide when to connect blocks together to permit fluid flow, and also, in many cases, how to build the more extensive computational template required to model dispersive fluxes. The latter can be particularly complicated since directions both perpendicular to, and parallel with, the usual flow directions have to be resolved. Nevertheless, dispersive mixing is often a required feature, such as when modelling gas storage reservoirs where cushion gas and fuel gas commingle, or when examining convective mixing as a primary drive mechanism, such as for the VAPEX process. A corner point grid cell starts with eight points, and can be described as the volume contained within surfaces that are stretched across the corner points associated with each of its six faces. Face contact (or near-contact) can be specified as the requirement for flow to occur from cell to cell. The contact can be full (the four face corner points are shared by each cell), or partial. The first case is easy to detect, while the latter is more complicated, breaking down into sub-cases that will be described in this paper. Once contacts are established and overlap areas are determined, suitable transmissibilities can be defined. Refined grids need to be seamlessly incorporated into all of the above. These can be difficult to work with if several layers of refinement are present. Once connectivities have been determined, an extended computational molecule has to built around each cell to allow for the computation of dispersive fluxes to take place. Since directions have to be resolved both parallel with, and perpendicular to, the Darcy phase velocities, an enlarged template is required. An algorithm will be described for processing lists of cell contacts to devise a suitable template for resolution of parallel and perpendicular flows. Once the template has been built, a method for computing the required longitudinal and transverse dispersive fluxes will be given. Results of computations made using the above algorithms for building flow connections will be described. In particular, the effects of longitudinal and transverse dispersion will be shown.

Abstract: Static geological models representing complex geological systems are the prerequisite of dynamic model simulations applied for assessing subsurface processes. The corner point grid approach has been applied to represent the complexity in geometry, hydraulic connectivity, and heterogeneity found in these static geological models. Due to the occurrence of faults, pinch-outs, and eroded geological layers, corner point grids easily degenerate, which leads to model inconsistencies. This study introduces a workflow for converting heterogeneous geological models to consistent finite element models, accounting for regular and irregular hexahedral blocks of the corner point grid by converting to a set of hexahedra, prism, pyramid, and tetrahedral elements, based on the individual degeneration situation. Heterogeneous geological data such as permeability or porosity can be transferred layer-wise or on a block-wise basis. Additionally, well trajectories can be accurately mapped to the converted finite element mesh, to place the corresponding source terms. The developed workflow is tested on dedicated test cases and applied to convert a real complex field site from the North German Basin for use in a deep geothermal reservoir operation. The field application demonstrates the robustness and applicability of the newly developed conversion workflow and the suitability of the converted mesh for dynamic finite element reservoir model simulations.