| Keywords |
geothermal, well, shape, flow pathways, conceptual model, numerical, pressure transient analysis, fractional dimension, boundaries. |
| Abstract |
Pressure transient analysis (PTA) has come only relatively recently to the geothermal industry, where it is often performed using simpler analytical tools, which have been shown to be inapplicable to geothermal reservoirs, introducing significant error into the results. The results of PTA have many potential uses, informing on the reservoir proximate to the wellbore (skin), the wider reservoir permeability, reservoir boundaries and many more. Perhaps even more useful is the provision of information on the overall shape of the network of flow pathways in the reservoir. It has typically been assumed in standard models that this shape is a flat round disc, represented by a radial model grid. The flow pathways converge on the well equally from all horizontal directions, and hence the flow is two-dimensional (2D). For example, this would be the case where permeability is contained within a relatively horizontal formation, and the open-hole section of the well extends through the entire thickness of that formation (full penetration). Flow can also be one-dimensional (1D), with a planar shape. An example of this is where the flow to the well is dominated by flow along a permeable fault. Flow can also be three-dimensional (3D) with a spherical shape, converging on the well equally from all directions including vertically. For example, if the well is located in a formation in which permeability extends above and below the open-hole section (partial penetration). The network of flow pathways can also have some intermediate shape between these 1- 2- and 3- dimensional examples, and the dimension is no longer an integer (fractional dimension). Knowing the shape is clearly significant to the conceptual model of the geothermal field. Numerical PTA can assess the overall shape of the flow pathways to the well during a test, as different shapes will have characteristic features on the sensitive pressure derivative plot. The block volumes and connection areas of the standard radial grid can be modified to represent different shapes, either by implementing a calculation to alter the dimension of the grid (fractional dimension) or by a calculation to represent reservoir boundaries. The behaviour of the fractional dimension and boundary models are shown in the pressure derivative plot, and then demonstrated with field examples from New Zealand and Iceland. |