WATER JET CONFERENCE - Waterjet Technology Association

Bottom notch diameter was determined both indirectly by measuring the volume
of cuttings removed during notching and directly by means of a down hole caliper. The
volume of cuttings were measured by recording the change in height of settled material in
a 55 gallon drum used as a settling tank. The down hole caliper measured the insertion
distance of a spring steel tongue, Figure 5, which was forced into a horizontal orientation
over a series of rollers when engaged. The caliper was manually operated from the
surface by pushing a sliding rod downward relative to a fixed position caliper body which
could be lowered to the desired depth in the injection hole.
Initial analysis indicated horizontal separation should initiate from 2 foot diameter
bottom notches at an injection hole pressure of approximately 20 psi. Initial attempts at
injection required pressures in excess of 40 psi to induce slurry flow indicating that
renotching of each injection hole followed by immediate slurry injection was required.
The resistance to separation propagation at the tip of the notch was attributed to excessive
filter cake build-up resulting from slurry leak off occurring over a period of a few weeks.
By inducing propagation immediately after notching this build up would be avoided.
Higher injection pressures also lead to leakage up along the casings. Leakage into
the overburden in general often reached the surface in the vicinity of injection holes. In
the course of modifying the notching system to achieve the desired performance over 20
leaks to the surface were encountered requiring the development of a routine leak seal off
procedure. Initially portland cement grout was pumped under low pressure into the
leaking injection hole, allowed to set and through drilled in preparation for another
notching attempt at slightly greater depth. This operation proved time consuming and
made subsequent notching operation more difficult.
A second method of sealing off leaks was therefore adopted and refined. A thick
slurry containing swelling particles of varying diameter was pumped into the leaking
injection hole and injection pressure was gradually increased allowing these particles to
bridge in the leak channel and seal off with filter cake build-up. Upon attaining a
satisfactory pressure (35 to 40 psi) the hole was renotched followed by immediate
separation propagation.
Deformation analysis of an infinite half space indicated injection pressure
should be inversely proportional to notch diameter. The notching mechanism was
therefore modified to increase notch diameter to 4 feet. The jet system was now designed
to erode with a jet of 2 percent bentonite slurry in an air medium (figure 6). Slurry was
pumped down a 2 inch diameter line to a 1/2 inch diameter horizontal jet (figure 7).
Slurry and cuttings and excess air then returned up a 1 1/2 inch diameter return line from
a sump beneath the jet to a settling tank on surface. The air medium was maintained by a
slow inflow of compressed air at the casing collar. Back pressure was maintained by a
flow control valve throttling the return flow at the settling tank. Excess air bubbled into
the return line in the sump and was released at the settling tank. This system was used to
notch and propagate separations from intermediate injection holes. The initial 4 foot
notches were each expanded until they intersected the perimeter of the block. Each
separation was therefore propagated until it obtained a diameter of approximately 26 ft.
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