Page 261 - Practical Well Planning and Drilling Manual

Section 2 revised 11/00/bc 1/17/01 12:04 PM Page 237

Drilling Fluids Program [ ]
2.5.8

A young “wet” shale (such as a gumbo shale) at a shallow depth,
will require a lower water phase salinity than a more diagenetically
mature shale (with higher shale salinity) at the same depth. The water
phase salinity requirements for any particular shale type (with the
same shale salinity) will increase with depth because matrix stress will
increase with depth.
In Figure 2-7, a shale that has a matrix stress of 5074 psi and a
shale salinity of approximately 100,000 ppm NaCl equivalents (inter-
cepts at A) is shown to require a 265,000 ppm CaCl water phase salin-
2
ity (B) to obtain balance of osmotic pressure.

Fig. 2-7 Oil-Mud-Water Phase Salinity Requirements to Balance Matrix Stress of
Shales (courtesy of Baroid)

How do you quantify the matrix stress at the rig site? How do you
come up with the shale salinity? As accurately as you practically can.
At the least, you may be able to quantify the minimum water phase
salinity that you require.

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