ABSTRACT: Undiscovered Naturally Fractured Reservoirs--Why and How? An Engineer's Point of View

AGUILERA, ROBERTO, Servipetrol Ltd., Calgary, Alberta, Canada

Many hydrocarbon naturally fractured reservoirs around the world have not become profitable discoveries and have been abandoned because of (1) incorrect pressure extrapolations, (2) poor completions, and/or (3) failure to intersect the natural fractures.

This paper highlights problems associated with the three points mentioned above and presents recommendations on how to avoid such pitfalls.

Incorrect pressure extrapolations might occur when the infinite acting radial flow period has not been reached during the test. This can lead to the erroneous conclusion of quick depletion.

Conventional completions are performed in intervals that meet certain porosity, permeability, and water saturation cut-off criteria. This might be dangerous in some naturally fractured reservoirs where the largest degree of natural fracturing could be associated with the lowest porosities.

Vertical wells might not intercept vertical natural fractures. In these cases, a conventional test might yield negative results, even if the matrix blocks are hydrocarbon saturated. These poor results are due to the usually low matrix permeability of naturally fractured reservoirs. Hydrocarbons cannot flow from the tight matrix into the wellbore. However, hydrocarbons can flow very efficiently from the tight matrix into the natural fractures. The key to success is then intersecting the vertical fractures via directional or horizontal wells.

The above problems are illustrated with actual case histories from various places around the world.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)