Interpretation of the Leak-off graph

Results should be plotted and interpreted on the large scale volume versus pressure plot during the test. "Leak-off” is a downhole event, indicated in the leak-off graph by the first deviation from the trend of either the initial static pressure or the final static pressure. In many cases it can only be identified positively if two points on the curve deviate from the trend. The "surface leak- off pressure" (pLO, surf.) is the (interpolated) value of the initial static pressure at the first indication of leak-off.

In a successful limit test no leak-off is observed when the initial static pressure reaches the

"surface limit pressure" (Plimsurf). It is confirmed that the borehole is strong enough to hold this additional pressure without formation breakdown (see Figure App. 4-1).

FIGURE APP. 4-1: LEAK-OFF GRAPH INDICATING ADEQUATE FORMATION STRENGTH

Various types of volume versus pressure plots can be encountered, depending on the kind of formation being tested as shown in the following three figures.

For a hard impermeable formation (eg. a shale) the pressure increases linearly with volume.

Leak-off is simple to identify, (Figure App. 4-2). For a permeable formation with an ineffective mudcake, the mud leaks away slowly, the graph is slightly curved, and the final static pressure curves away from the initial static pressure. Leak off should now be identified either by the change in curvature, or from the increasing difference between initial static and final static pressures (see Figure App. 4-3).

Especially in non-consolidated, plastic, loose or highly permeable formations where even low test pressures cause loss of mud the exact determination of the leak-off point is difficult. The initial static pressure will always be considerably higher than the final static pressure, and the graph will be curved considerably. Leak-off can only be established approximately from the leak-off graph, as shown in Figure App. 4-4. (In many cases the information

FIGURE APP. 4-2: LEAK-OFF GRAPH FOR IMPERMEABLE FORMATION

FIGURE APP. 4-3 LEAK-OFF GRAPH FOR PERMEABLE FORMATION

FIGURE APP. 4-4 LEAK-OFF GRAPH FOR UNCONSOLIDATED FORMATION

that no breakdown is observed will suffice, since it is obvious that the formation is weak and the main purpose of the test is to establish the absence of communication around the casing.) 4.8.2.4 Formation breakdown, fracture re-opening

Formation breakdown during a Limit or Leak-off test should be prevented, because a fracture may permanently impair the capability of the wellbore to withstand pressure . However, if breakdown occurs, it should be treated as an opportunity to derive real formation strength parameters.

"Formation breakdown" is indicated by a sharp pressure drop on surface (see Figure App. 4-5).

The highest pressure recorded immediately before the pressure drop, is the "surface breakdown pressure", (pFB,surf).

If formation breakdown occurs, pumping should be stopped, but the well should be kept closed-in, and the pressure decay curve should be recorded. "Fracture closure" is indicated by the stabilisation of the pressure decay curve to a constant pressure value. The FCP can be determined from the "surface fracture closure pressure", (pFC,surf). The results may be used to determine the in-situ stress, which may be very useful for future operations.

To confirm these observations, the test may be continued with a fracture re-opening cycle (see Figure App. 4-6). After the pressure is released, and the well is flowed back, the well is pressured up in steps. When the fracture re-opens, the pressure volume graph deviates from the trend (similar to leak-off), and the "surface fracture re-opening pressure", (pFR surf) can be determined. After re-opening, the well is shut-in and the PCP is again determined from the pressure decline. Theoretically the FRP and the FCP are equal, but differences may occur.

If the first and second FCP and the FRP are not consistent enough another cycle should be considered.

FIGURE APP. 4-5 LEAK-OFF GRAPH SHOWING FORMATION BREAKDOWN

FIGURE APP. 4-6 :LEAK-OFF GRAPH SHOWING FORMATION BREAKDOWN AND FRACTURE REOPENING

4.8.2.5 Reporting

Formation strength tests should be reported in a consistent manner. It is recommended to complete part 1 of the attached example report form before the test.

Actual measurements of pressures and volumes and an interpretation of the results should be reported as indicated in part 2 of the attached example report form. An accurate graph on a large scale of volume pumped versus surface pressure should be included in this report. It should be indicated whether leak-off or formation breakdown was observed.

Surface measurements of formation strength are converted to downhole pressures, assuming that the measurements relate to the formation just below the casing shoe:

pshoe = psurf + ρmud x (dshoe - dgauge) (App. 4-8) Downhole pressures can be converted to gradients or equivalent mud gradients using the conventions given in Chapter C section 3.3.

For Formation Breakdown the conversions are given below:

Formation strength gradients result from Eq. C-1 and Eq. C-2:

Land wells : FBG =

dfe d

FBP

shoe

−

Offshore : FBG =

− (relative to seabed)

Equivalent mud gradient results from Eq. C-3 : ρFB,shoe =

d

FBP

(relative to derrick floor) (Similar expressions can be given for Limit, Leak-off and Fracture Closure pressures).

If the measurement relates to a formation some distance below the casing shoe, the conversion may be slightly inaccurate (see the plots of FBP* versus FBP, FBG* versus FBG, ρ*FB versus ρ^∗FB in Figure App. 4-7). In practice this difference is not taken into account, except when a leak-off test is repeated at a different depth.

FIGURE APP. 4-7A : DEFINITION OF FORMATION-STRENGTH AND PORE-PRESSURE GRADIENTS (LAND LOCATION)

FIGURE APP. 4-7B : DEFINITION OF FORMATION-STRENGTH AND PORE-PRESSURE GRADIENTS (OFFSHORE LOCATION)