Abstract

Natural fractures are commonly encountered in several Oil Industry related activities: one has to think, for example, to mud losses or lost circulation problems when drilling fractured formations. In the same way the characterization of porosity and permeability is one of the most difficult task when dealing with naturally fractured reservoirs. In fact these properties depend on parameters which cannot be known with direct measurements, such as the geometric characteristics of the single fractures, their density, the extension of the networks they form, etc. Moreover, the conductivity of natural fractures depends on the physical mechanisms which govern the flow within them and, during the exploitation of the reservoirs, can diminish due to the increase of the effective stresses which tend to close the fractures.

Recently Eni/Agip has presented a methodology which characterizes the conductivity of the open fractures using real time monitoring of mud loss data while drilling. Starting from this previous work, a detailed characterization of single-phase flow within natural fractures has been performed via numerical simulations and analytical models. In particular, the effect of the geometric properties of the fractures (aperture, tortuosity and roughness) on the flow pattern and on the average conductivity have been investigated. The main results can be summarized as follows;

  • validation of the hypothesis of the laminar flow in the description of the flow dynamics in a small fracture;

  • development of flow patterns characterized by the presence of areas of high permeability that can be seen as network of channels;

  • correction of the so-called cubic law for hydraulic conductivity through the measure of the fractal dimension of the fracture surface.

Moreover, as it is shown with field examples, the interpretation of mud loss data on the basis of the acquired knowledge allowed to achieve a better evaluation of the near- well conductivity. These facts will provide more reliable and useful information when examining mud losses problems, and consequently the way to solve them, and are of primary importance for the first evaluation of productive intervals and for their selection for testing, completing, etc

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