BAKER HUGHES - Drilling Fluids Reference Manual

HORIZONTAL AND EXTENDED REACH DRILLING
to the selection of a suitable drilling fluid density being even more difficult. The extent and effect
of Equivalent Circulating Density (ECD) needs careful consideration.
No increase in mud weight with hole inclination is necessary across permeable formations such as
sands. Formations with reasonable matrix permeability can be drilled with nominal overbalance,
regardless of well trajectory or formation strength. Fracture gradient may decrease with increased
inclination.
In locally tectonically stressed areas, e.g. Colombia, the collapse gradient may exceed the fracture
gradient, even in vertical wells. In this case one or both of lost circulation and hole collapse must
be tolerated to some degree. Thus, there is a limitation that can be accomplished by fluid weight,
and it is therefore imperative that the virgin strength and integrity of the rock be maintained as
much as possible.
Shale Interaction
Interaction of the fluid with the formation which produces instability is primarily related to shale
bodies. Shales, which include other classifications such as mudstones, siltstones, and claystones, all
contain a relatively high percentage of hydratable and expandable clays. Typical mineral
compositions of a young marine shale and an older indurated shale are shown in Table 11-1.
It is the interaction of these clays with the water phase of the fluid that weakens the shale structure.
The absorption of water by the shales through capillarity or diffusion causes instability by creating
internal stress, weakening of cementing components of the shale, and lubricating micro-fractures
that may exist.
Classifications
Shales can be classified into three broad categories:
1. Soft, sticky shales
2. Dispersive shales
3. Hard, brittle shale
Component
Quartz
Feldspar
Calcite
Dolomite
Illite
Mixed Layer
Kaolinite
Chlorite
Typical Gumbo Shale, Gulf of
Mexico, % Composition
5 - 10
Trace
2 - 3
2 - 3
20 - 25
50 - 55*
5 - 10
5 – 10
* Mixed layer has 50% expandable layers
** Mixed layer has 25% - 35% expandable layers
Table 11 - 1 Shale Composition
Vicksburg Formation, Starr
County, Texas, % Composition
10 - 15
2 - 3
10 - 15
⎯
25 - 30
35 - 40**
3 - 5
5 - 10
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006. 11-9