Page 249 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

418 Reservoir Engineering

conditions such as boundaries or changes in fluids or fluid properties, can cause
the Horner plot to deviate from a straight line (see Figures 5-134 and 5-135).
Skin Factor. The skin effect refers to a zone of altered formation permeability
near a wellbore as a result of drilling, completion, or stimulation [231,232]. The
extent of altered permeability is expressed in terms of a skin factor, s, which is
positive for damage and negative for improvement. Skin factor can range from
about - 5 for a hydraulically fractured well to a theoretical limit of infinity for

5 5
n n

log [ (t t At)/&] log [( t + At) /At 3
BOUNDARY INTERFERENCE

'
log [ (t + At)/At] log [ (t+ At)/At] -1

PACKER FA1 LURES LATERAL INCREASE
PHASE SEPARATION IN MOBILITY
Figure 5-134. Downtrending Horner plots [13,180].