Page 198 - A Practical Companion to Reservoir Stimulation

PRACTICAL CONSIDERATIONS FOR FRACTURE TREATMENT DESIGN

P-4
pressure generated within these fractures. Realizing that the
Treatment Sizing net pressure is inversely proportional to fracture height helps
explain the difficulty of containing fractures to very small
P-4.1: Determination of Volumes of Fluids zones. Large fracture heights are easier to confine because
and Mass of Proppant the net pressure generated over a larger interval is much
The optimum volume of fluid and proppant is best deter- smaller. Small stress contrasts (< 250 psi) between the pay
mined by following the methodology outlined in Chapter 8 in zone and a barrier will almost never be sufficient for con-
Reservoir Stimulation. That chapter explains the optimization tainment unless relatively large fracture heights are obtained.
of fracture length and conductivity based on the net present Figure P-46 shows that a barrier with a stress contrast of 341 6
value (NPV) concept. The major parameters impacting the psi would be needed to contain a 1000-ft fracture with a gross
net present value of a fracture treatment are the reservoir height of 20 ft. Stress barriers of this magnitude do not
permeability, fracture height, fluid efficiency and residual generally exist. However, if the fracture height is increased to
damage to the proppant conductivity. 160 ft, a barrier with a stress contrast of 554 psi may contain
Height containment is a major constraint in determining the fracture. A stress contrast of this magnitude is often found
the volume of materials needed. Both fluid and proppant between sandstones and shales.
volumes will increase significantly as fracture height in- Fluid efficiency has a direct relationship on the volume of
creases. Figure P-45 is an example comparing total slurry fluid needed to obtain a given fracture length. The parameter
volumes needed to obtain a fracture half-length of 1000 ft that best quantifies efficiency is the leakoff coefficient. Fig-
when fracture height increases. As fracture height doubles ure P-47 shows that subtle changes in this fluid property can
from 20 ft to 40 ft, the volume of slurry increases from 22,000 have dramatic effects on the volume of fluid needed to obtain
gal to 66,000 gal. Before a treatment can be economically a given fracture length. By increasing the fluid loss coefficient
optimized, it is obvious that some knowledge about the from 0.001 ftl6 to 0.003 ft/6, the volume of fluid re-
anticipated gross fracture height is absolutely necessary. quired to obtain a 1000-ft fracture half-length increases from
A parametric study of this same example shows the net 65,000 gal to 103,000 gal of slurry. Although fluid loss

250,000

200.000

h
([I
v a 150,000
a,
5
-
P
-0
'5 100,000
ii

50,000

0 500 1000 1500
Fracture Length (ft)

Figure P-45-Effect of fracture height on the fluid volume required to generate a given fracture length.

P- 39