Page 25 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 19

Figure 1.10 LOT plot—Surface pressure vs volume pumped.
1. Leak-Off Pressure—The pressure at which the fracture begins to open and fluid
starts to leak off into the formation. This will be seen as a change in the slope of
the plot. At this point, the pump rate should be reduced.
2. Rupture Pressure—This is the maximum pressure the formation can sustain
before irreversible fracture occurs. This will be determined by a sharp drop in the
pressure being applied—pumping should be halted.
3. If no more pressure is applied at this point, most formations will recover to a cer-
tain degree, and the Propagation Pressure is determined when the pressure
becomes stable again.

• Bleed off and measure the volume of mud returned—record the
volume lost to the formation.
As pressure is increased during a LOT, three pressure stages are normally evi-
dent, and it is the operator’s decision as to which one will be taken as the pres-
sure on which to base subsequent formation integrity calculations (Fig. 1.10).
A major disadvantage of a LOT is that fracturing the formation can
weaken it, reducing fracture pressure to below the undisturbed value.
Propagation (recovery pressure) is normally lower than the original frac-
ture pressure, so the integrity of the formation during the drilling of the
next hole section is compromised to some degree.
During a LOT, there are two forces acting on the formation. Firstly,
there is the hydrostatic pressure from the column of mud, secondly,
applied pressure at the surface. Fracture pressure is:
• Mud hydrostatic pressure at the casing shoe 1 applied surface pressure.
• To calculate the fracture pressure as an equivalent mud weight:
Fracture pressure EMW 5 ðP surface =TVD=0:052Þ 1 Mud weight (1.2)