Defining the area of a geothermal field by drilling raises the question of how densely need the wells be drilled? Given a successful well, how large an area around it can be regarded as productive? This is similar to the proving of a mineral deposit, except that permeability in a geothermal field is typically very random, compared to relatively smooth variation of an ore deposit. Well performance depends primarily on two factors, temperature and permeability. Temperature varies relatively smoothly, but permeability can be highly heterogeneous. Standard geostatistical methods were applied to well success at Kawerau. All wells greater than 500m depth were used. Semivariograms were constructed for three statistics: steam flow at 7 bar gauge, Injectivity and well success. Well success was defined as 1 if the well was commercial, 0 if it was not.

At Kawerau, well success and injectivity are correlated over short distances, 100m or less, whereas well flow is correlated up to 500m. This latter reflects the effect of temperature - a high well flow requires high temperature, which in turn implies high temperatures within several hundred metres, improving the likely steam flow within that distance. Permeability is random over distances greater than 100m.

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