Fluid flow in the Earth's crust is controlled by pore-connectivity over scales from mm to km. Micro-scale pore-connectivity structures can appear as fractures on the meso-scale, and at macro-scales they can appear as displacement faults. Whatever their appearance, however, pore-connectivity structures are characterized by a single spatial correlation process attested by well-log spatial fluctuation systematics over the cm to km scale range. The crustal spatial correlation process is seen as 1/k power-law scaling of well-log Fourier power-spectra, S(k) ~ 1/k, over five decades of spatial frequency k from 1/km to 1/cm. Because pore-connectivity structures are due to random processes, they are inhomogeneously distributed in space. Power-law scaling spatial correlation means that one cannot meaningfully average pore-connectivity and associated permeability structures within or across geological layers. The effect of spatial correlation from 1/k scaling is apparent in different data related to fluid flow: well-logs, well-productivity, and well-core. Relevant accessible datasets have been analyzed from different geothermal fields in the world, e.g. Indonesia, New Zealand, Mexico, Germany and Lithuania. Measurements of well-core samples show that porosity spatially correlates with the logarithm of permeability. Where porosity is greater, permeability is very much greater due to strongly increased pore-connectivity leading to increased fluid flow. The combined effect of increased porosity and very much increased permeability results in lognormally distributed well productivities, as observed in crustal fluid flow systems across the world. Lognormality is particularly important in convective geothermal fields, where only very few wells give the necessary high flow output while the better part yielding low to moderate outputs are commercially useless. These natural fluid flow pathways can be altered by effects of field operations. Physical, chemical and biological processes can trigger blocking of natural flow structures and lead to exponentially declining injection curves. Small-scale changes in grain-scale pore-connectivity can lead to a huge negative influence on large-scale sustainability of geothermal systems. That is, strong negative effects caused by field operations flow can be based on the grain-scale connectivity nature of fluid flow in geological media. Such changes can be induced by chemical processes in the reservoir and the plant (precipitation, corrosion). Also, biological reactions can either directly affect the physical flow structure (biofilm) or interact with chemical reactions (triggered precipitation or corrosion). Therefore, it is important to consider the interaction of different processes that occur in geothermal reservoirs. These processes have been observed and analysed at field data from a geothermal plant in Lithuania. In particular, the observed injectivity decline for a low enthalpy heating plant provides an example of pore-connectivity reduction mechanisms during standard geothermal operations. Both understanding of location and structure of natural fluid pathways and how they can be altered during field operations are of greatest interest for sustainable reservoir management.

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