Page 18 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

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fluids in it (rock-fluid system) are altered in the vicinity of (1.e. clay minerals from the drilling mud) are trapped on the
the borehole. A well’s borehole and the rock surrounding it side of the borehole and form mudcake (Rye: Fig. 1). Fluid
are contaminated by the drilling mud, which affects logging that filters into the formation during invasion ts called mud
measurements. Figure | is a schematic illustration of a filtrate (Rp Fig. 1). The resistivity values for drilling mud,
porous and permeable formation which is penetrated by a mudeake, and mud filtrate are recorded on a log’s header
borehole filled with drilling mud. (Fig. 2).
The definitions of each of the symbols used in Figure | invaded Zone—The zone which is invaded by mud filtrate
are listed as follows: is called the invaded zone. It consists of a flushed zone (Ryo)
and a transition or annulus (Rj) zone. The flushed zone
d, - hole diameter
(R,,,) occurs close to the borehole (Fig. 1) where the mud
d; - diameter of invaded zone (inner boundary; flushed
filtrate has almost completely flushed out a formation’s
zone)
hydrocarbons and/or water (Ry). The transition or annulus
d, - diameter of invaded zone (outer boundary; invaded
(R;) zone, where a formation’s fluids and mud filtrate are
zone)
mixed, occurs between the flushed (R,,) zone and the
:
A,, - radius of invaded zone (outer boundary)
uninvaded (R,) zone. The uninvaded zone is defined as the
h,. - thickness of mudcake
area bevond the invaded zone where a formation’ fluids are
R,, - resistivity’ of the drilling mud
uncontaminated by mud filtrate.
Ring - Tesistivity of the mudeake
The depth of mud filtrate invasion into the invaded zone
Rup - resistivity of mud filtrate
is referred to as the diameter of invasion (d; and dj; Fig. 1).
R, - resistivity of shale
The diameter of invasion 1s measured in inches or expressed
R, - resistivity of uninvaded zone (true resistivity)
as a ratio: dj/d, (where d, represents the borehole diameter).
- resistivity of formation water
The amount of invasion which takes place is dependent
- resistivity of flushed zone
upon the permeability of the mudcake and not upon the
Sy - water saturation of uninvaded zone
porosity of the rock. In general, an equal volume of mudd
S xo 7 Water saturation flushed zone
filtrate can invade low porosity and high porosity rocks if
Some of the more important symbols shown in Figure | are: the drilling muds have equal amounts of solid particles. The
Hole Diameter (dy)—Awell’s borehole size is described solid particles in the drilling muds coalesce and form an
by the outside diameter of the drill bit. But, the diameter of impermeable mudcake. The mudcake then acts as a barrier
the borehole may be larger or smaller than the bit diameter to further invasion. Because an equal volume of fluid can be
because of (1) wash out and/or collapse of shale and poorly invaded before an impermeable mudeake barrier forms. the
cemented porous rocks, or (2) build-up of mudcake on diameter of invasion will be greatest in low porosity rocks.
porous and permeable formations (Fig. 1). Borehole sizes This occurs because low porosity rocks have Jess storage
normally vary from 7 7/8 inches to 12 inches, and modern capacity or pore volume to fill with the invading fluid, and.
logging tools are designed to operate within these size as aresult, pores throughout a greater volunte of rock will
ranges. The size of the borehole is measured by acaliper be affected. General invasion diameters are:
log.
d/d, = 2 for high porosity rocks;
Drilling Mud (R,,)——Today, most wells are drilled with
dj/d, = 5 for intermediate porosity rocks;
rotary bits and use special mud as a circulating fluid. The
mud helps remove cuttings from the well bore, lubricate and and dj/d, = 10 for low porosity rocks.
cool the drill bit, and maintain an excess of borehole
L. - length (meter)
pressure over formation pressure. The excess of borehole Flushed Zone (Ry,)—The flushed zone extends only a
few inches from the well bore and is part of the invaded
pressure over formation pressure prevents blow-outs. zone. If invasion is deep or moderate, most often the
The density of the mud is kept high enough so that
hvdrostatic pressure in the mud column is always greater flushed zone is completely cleared of its formation water
than formation pressure. This pressure difference forces (R,,) by mud filtrate (R,,,). When oil is present in the
some of the drilling fluid to invade porous and permeable flushed zone, you can determine the degree of flushing by
formations. As invasion occurs, many of the solid particles mud filtrate from the difference between water saturations
in the flushed (S,,) zone and the uninvaded (S,,) zone (Fig.
oo
-y L). Usually, about 70 to 95% of the oil is flushed out; the
rx /
Resistivity (R) = tA remaining oil is called residual oil (S,, = [1.0 — S,.] where
R - resistivity in ohm-meters?/meters (ohm-meter)
Svo equals residual oil saturation [ROS]).
r -reststance (ohms)
Uninvaded Zone (R,)—The uninvaded zone ts located
A - cross sectional area (meters?)
beyond the invaded zone (Fig. 1). Pores in the uninvaded