Fracture Intensity As A False Positive For Productivity: Indications of Geothermal Activity
Abstract
Crystalline basement fractured reservoirs can have highly productive fracture sets and still have low storage capacity, with the background matrix system containing most of the pore volume. The connection between the two systems is a function of fracture intensity (sigma) and the background permeability, to give the needed transmissibility between the two systems. If there is a weak connection then a successful DST at high rate from an open fracture might be insufficient for economic success as the fractures might not be able to sustain production.
A common approach is to target areas of high fracture intensity. The real target is the fractures that matter, as not all the fractures that appear open from a borehole image might be active. In some cases the response of the well to a higher sigma isn't a monotonic improvement. The effect of diagenesis caused by a geothermal system can chemically alter the background matrix and the permeability of the fractures that matter either through dissolution enhancement and/or fracture occlusion from mineral crystallization causing significant changes in permeability in the system. The reaction-infiltration flows have a feedback between the permeability and the rate of change of permeability, creating a lot of complexity. In the case of occlusion this diagenesis can damage zones with high sigma, creating false positives, whilst leaving lower sigma zones unaltered and potentially economic for an oil well. This study presents a case where a productive well has an inverse relationship between sigma and flow capacity. As the flow will occur in the highest permeability systems first, the diagenesis driven permeability enhancement will cause an unstable positive feedback, making good systems better. For the reverse system, it is the better originally permeability zones that are destroyed preferentially by the diagenesis. A complex fracture conceptual model and an uncertainty analysis with 40+ variables are discussed.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H51P1533S
- Keywords:
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- 1805 Computational hydrology;
- HYDROLOGYDE: 1829 Groundwater hydrology;
- HYDROLOGYDE: 1832 Groundwater transport;
- HYDROLOGYDE: 1869 Stochastic hydrology;
- HYDROLOGY