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:

October 10, 2000

Northern Michigan Section

Traverse City, MI

October 11, 2000

Michigan Section

Lansing, MI

October 12, 2000

Illinois Basin Section

Grayville, IL

November 28, 2000

London Section

London, England

November 29, 2000

Hungarian Section

Budapest, Hungary

December 1, 2000

Croatia Section

Zagreb, Croatia

December 4, 2000

Romanian Section

Bucharest, Romania

December 5, 2000

Tunisia Section

Tunis, Tunisia

January 16, 2001

Southwest Oklahoma Section

Duncan, OK

January 17, 2001

North Texas Section

Wichita Falls, Texas

January 18, 2001

Oklahoma City Section

Oklahoma City, OK

January 19, 2001

Panhandle Section

Borger, Texas

February 19, 2001

Billings Petroleum Section

Billings, MT

February 20, 2001

Wyoming Petroleum Section

Casper, WY

February 21, 2001

Western Wyoming Rock Springs Section

Rock Springs, WY

February 22, 2001

Big Horn Basin Petroleum Section

Cody, WY

February 23, 2001

Denver Section Study Group

Denver, CO

March 13, 2001

Pittsburgh Petroleum Section

Pittsburgh, PA

March 14, 2001

Northern West Virginia Section

Clarksburg, WV

March 15, 2001

East Kentucky Section

Paintsville, KY

March 20, 2001

Canadian Section

Calgary, Canada

April 23, 2001

Bolivian Section

Santa Cruz, Bolivia

April 24, 2001

Lima Section

Lima, Peru

April 25, 2001

Ecuador Section

Quito, Ecuador

April 26, 2001

Colombian Section

Bogota, Colombia

April 30, 2001

Caracas Petroleum Section

Caracas, Venezuela

May 2, 2001

Bahia-Sergipe Section

Salvador, Brazil

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.


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

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