Fracture Pressure

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Fracture pressure
Fracture gradients Knowledge of the magnitude of formation fracture gradients is vital, especially when drilling into an abnormally pressured zone. Formation fracture gradient determines the maximum allowable mud weight that can be used (after incorporating an operational safety factor). Mechanism of Formation Fracturing A formation can be made tofracture by the application of fluid pressure to overcome the least line of resistance within the rock structure. Normally fractures will be propagated in a direction perpendicular to the least principal stress. Which of these three stresses is the least can be predicted by the fault activity in the area.

Figure 1: Distribution of Stress Planes in Unit of Rock Formation
Normal faultingindicates that the least principal stress is horizontal and probably about equal to the minimum horizontal stress required to avoid rock failure (figure 2).

Figure 2: Normal Fault
Thrust (reverse) faulting indicates the least principal stress to be of a vertical nature. It must be at least equal to the weight of the overburden, or about 1 psi/ft (figure 3).

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Figure 3:

Reverse Fault

Transcurrent faulting indicates that the least principal stress is horizontal but can be larger than the minimum required to avoid failure. It cannot be larger than the vertical stress (figure 4).

Figure 4:

Transcurrent Fault

To initiate the fracture fluid pressure must be transmitted to the formation, and to propagate thefracture this pressure must be maintained at a level greater than the least principal stress. If mud is used, then the fluid pressure has to overcome the filter cake deposited on the borehole wall, plus the pressure at which the formation fractures. It is therefore preferable to use a clear, low viscosity fluid when making a determination of the formation breakdown gradient. In practice, formationbreakdown gradients are determined by performing a Leak Off Test (LOT). This is normally done after drilling through a casing shoe in order to determine the maximum allowable mud gradient for the next section of open hole. The usual procedure is outlined below. a) Drill through the casing shoe and into new formation to a depth of 15 to 20 feet. Close the blowout preventers. Raise the surfacepressure in increments until bleed off is indicated. Alternatively, pumping could be maintained at a slow rate until bleed off.

b)

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Figure 7-5:

Typical Leak Off Test

During fracture testing it is generally not the coherent rock matrix that is sheared, rather the existing natural discontinuities (joints, parting, etc) whichare normally held together by the compressive stresses in the formation. When the hydraulic stress in the borehole equals the formation compressive stress, it effectively reduces the latter to zero across the crack. If the borehole pressure is then raised slightly, fluid will enter the crack.

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Leak off tests and fracturepressures Terzaghi and Peck (1948) states that S =σ + P

where

S = Overburden Stress σ = Matrix Stress P = Pore Pressure

As previously stated, subsurface stress is characterised by three unequal, mutually perpendicular principal stresses. The greatest stress is vertical (σv ) although the least principal stress (σh ) is of σ greatest interest as this indicates formation strength along theweakest plane. Hubbert and Willis (1957) Hubbert and Willis conducted laboratory experiments on core samples and derived the empirical relationships below. To extend a fracture σh = To induce a fracture σh = Hence 1 1 σv < σh < σv 3 2 or 1 1 (S - P) < σh < (S - P) 3 2 1 σv 2 1 σv 3

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Therefore, to extend FP = P + while to...