Page 528 - Petrophysics

496 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

Pores Natural Fractures Induced Fractures

Mud Pit Level
(a) (b) (c)
Figure 8.6. Mud loss indication andpit level behavior in pores, natural fractures, and
induced fractures:(a) gradual buildup in loss ratio with pressure; (b) sudden start and
exponential decline; and (cj loss can occur on increase in ECD as pumps are turned
off and on [141.

characteristics, such as porosity and permeability, will influence the
location and number of subsequent development wells and, therefore,
is of major economic significance. Stearns and Friedman [4], Aguilera
[13], Saidi [22] and Nelson [ 11 reviewed many of the approaches used to
detect and analyze naturally fractured reservoirs [ 1, 131. Some of these
methods are as follows (see Figure 8.6):

Loss of circulating fluids and an increase in penetration rate during
drilling are positive indications that a fractured, cavernous formation
has been penetrated (Figure 8.6).
Fractures and solution channels in cores provide direct information
on the nature of a reservoir. A detailed systematic study of the
cores must be made by the geologist in order to distinguish natural
fractures from those induced by the core handling process. Careful
examination of fracture faces and determination of density, length,
width, and orientation of fractures may lead to the ability to
distinguish fractures induced during coring from natural fractures.
Preferably, a naturally fractured formation should be analyzed
with full diameter cores. Plug data, which do not reflect the
permeability of fractures, often indicate a nonproductive formation,
whereas full diameter core data indicate hydrocarbon production. If
actual production rates are several-fold higher than those calculated
from permeability determined by core analysis, natural fractures
not observed in the core are suspected [15]. Low core recovery
efficiency-less than 50%-suggests a highly fractured carbonate
formation.
Logging tools are designed to respond differently to various wellbore
characteristics, such as lithology, porosity, and fluid saturations,
but not to natural fractures [16, 171. The presence of a large
number of open fractures, however, will affect the response of