NATURALLY FRACTURED RESERVOIRS

SPE DISTINGHISHED LECTURER

Dr. Roberto Aguilera was an SPE Distinguished Lecturer for the 2000-2001 season. Dr. Aguilera's presentation concentrated on case histories of Naturally Fractured Reservoirs from various places around the world. The following are the dates and locations where the presentations were held:

ABSTRACT OF LECTURE

This presentation will stay away from equations and theory and focus on practical aspects of naturally fractured reservoirs. Key points will be illustrated with case histories from reservoirs around the world.

The first example will highlight how a drastic gas production decline due to water encroachment in a field producing from a naturally fractured reservoir was stopped by producing more water from flank wells that had already watered-out completely. Although squeeze cementing is the first thing that comes to the engineer's mind when water cones suddenly through fractures, experience indicates that in many instances this solution is short-lived.

The second case will highlight the importance of long periods of swabbing. A key to proper evaluation of a fractured formation is to swab at least the amount of mud lost during drilling operations. Not doing so might lead to the abandonment of a reservoir that otherwise could be commercial. Many days of swabbing might be required specially if there have been significant mud losses. Underbalance drilling helps to solve this problem in many naturally fractured reservoirs.

The third case will highlight how poor assumptions regarding porosity, permeability, and water saturation cutoffs might lead to the improper completion  or abandonment of a well. There are reservoirs where the largest degree of fracturing is associated with the lowest porosities and matrix permeabilities. Introducing a cutoff might leave outside of the testing program the intervals with the largest degree of fracturing.

The fourth case will highlight problems associated with not intersecting 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. Commercial production of hydrocarbons is not possible from the tight matrix into the wellbore. However, hydrocarbons can flow very efficiently from the tight matrix into the natural fractures and from there to the wellbore. The key to success is to ensure the vertical fractures are intersected via directional or horizontal wells.

The fifth example will highlight poor well testing designs that are typically associated with very short flow and buildup periods. This can easily result in incorrect pressure extrapolations that can lead erroneously to the conclusion that the reservoir is depleting quickly.

The sixth example will highlight poor completions. They are many times the result of not drilling the wells thinking in terms of fractures. The most important thing next to finding fractures is not damaging the fractures. While completing a well, cement can travel very long distances through fractures and can induce deep-seated damage. To avoid this problem it is important to classify the fractured reservoir in terms of relative storage of the fractures to better decide on the most appropriate completion.

Some of the topics discussed during Dr. Aguilera's presentation can be found at Servipetrol Current Technical Notes and Previous Technical Notes.

Industry-Related Links

• APEGGA
• Society of Petroleum Engineers (SPE)
• Canadian Well Logging Society (CWLS)
• The Petroleum Society of CIM
• Canadian Society of Petroleum Geologists (CSPG)
• American Association of Petroleum Geologists (AAPG)

For comments, suggestions, or questions please email: aguilera@servipetrol.com

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