ABSTRACT: Geological Reason for Horizontal Drilling

GRETENER, P. E., University of Calgary, Calgary, Alberta, Canada

The term reservoir heterogeneity has become a buzzword in the industry. It acknowledges the fact that most oil/gas reservoirs are extremely nonuniform, particularly in regard to the permeability distribution.

Any fluid produced by a well must penetrate the congested area next to the wellbore (skin). Near-well-permeability is critical to the productive potential of a well. This problem has been intensively investigated under the name of formation damage.

Putting those two concepts together we arrive at what can be called natural formation damage. The limited contact area of a well might just by chance be placed in a low permeability section of the reservoir, thus resulting in a marginal producer or even a D&A situation. Natural formation damage is an ailment common to most reservoirs.

How can this situation be improved? Two approaches are suggested:

1. By bulk reservoir surveying outlining the more favorable compartments. Logging does not qualify since it represents a process of digital sampling and does not provide the crucial continuity information. Geophysical methods such as 3-D seismic, tomography (X-hole) seismic, long step-out VSPs, and others come to mind.
2. By extending the contact area between the well and producing formation. Horizontal drilling offers this opportunity. Gaining access to some high-permeability compartments has an effect similar to the one produced by artificial fracing. The larger contact area also lowers the seepage flow rate for equal amounts of production, which in turn offers other advantages that will be discussed. (Successful horizontal drilling needs to be preceded by bulk reservoir surveying in most cases.)

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