Page 204 - Applied Petroleum Geomechanics

In situ stress estimate 199

Figure 6.6 Pressure-dependent leak-off G-function plot to determine the closure
pressure.

However, these two measurements may give different results, and the
downhole measurement gives a lower value than the one from the
surface measurement. The downhole measurement gives a more realistic
result because it represents the downhole pressure of the tested formation. If
the injection pressure (pump pressure) is recorded at the surface, the
bottomhole pressure gradient can be calculated from the following equation:
p s
P l ¼ P mw þ (6.25)
0:051948D
where P l is the bottomhole pressure gradient (ppg); p s is the measured pump
pressure at the surface (psi); P mw is the downhole mud weight (or ESD) when
the LOT test is performed (ppg); D is the depth of the tested formation (ft).
It should be noted that if the LOT does not reach the formation
breakdown pressure, it is not a good test for in situ stress determination.

6.2.4 Minimum stress from diagnostic fracture injection test
The DFIT is another method to determine the closure pressure (e.g., Martin
et al., 2012). It can be used to discern both hydraulic fracturing character and
the reservoir pressure. It is often performed before the stimulation treatment
of a formation. It usually consists of pumping a certain amount
(500e2000 gal) of water (often containing a clay control additive) into a
reservoir rock at rates of 2e10 bbls/min. The pressure and injection rates are
measured throughout the fracture-injection test and recorded for subsequent
analysis. The principle in determination of closure pressure in the DFIT is