ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 16 OF 230 REVISION 0 ? Cuttings from normal pressured shales are small with
rounded edges and are generally flat, while cuttings from an abnormal pressure are often long and splintered with angular edges. As the differential between the pore
pressure and the drilling fluid hydrostatic head is reduced, the pressured shales will burst into the wellbore rather than having being drilled. This change in shape, along with an increase in the amount of cuttings at the surface, could be an indication that abnormal pressure has been encountered.
2.2.5.
Methods After Drilling
These are methods founded on the elaboration of the data from electrical logs such as: induction log (IES), sonic log (SL), formation density log (FDC), neutron log (NL). The most used methods for abnormal pressure detection are:
Induction Log (IES) Method:
Is used in sand and shale formations and consists in the plotting of the shale resistivity values at relative depths on a semilog graphic (depth in decimal scale and resistivity in logarithmical scale).
In formations, if they are normal compacted, the resistivity of the shales increases with depth but, in overpressure zones, it lowers with depth increase (Refer to figure .2.a).
Also it is possible to plot the values of the shale conductibility; in this case the plot will be symmetric to that described above. The method is acceptable only in shale salt water bearing formations which have sufficient and a constant level of salinity.
For the calculation of gradient, refer to the ?Overpressure Evaluation Manual?.
ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 17 OF 230 REVISION 0 Fig.1,2-1 INDUCTION LOG
1
Resistivity (OHMM)
10
100
1500
??????? ??????? ??????????????????????????????????????????????????????? To ?????? ?????????????????????????????????????????? 2000
2500
3000
3500
4000
4500
5000
Figure .2.A - Induction Log
Shale Formation Factor This is more sophisticated than the IES method described
above. It eliminates the inconveniences due to water salinity (Fsh) Method:
variation. It consists in the plotting of the shale factors on a semilog graph (depth in decimal scale and resistivity in
logarithmical scale)at relative depths. The ?Fsh? is calculated by the following formula:
Rsh Fsh???Rw
Where: Rsht Rw
=The shale resistivity read on the log in the points where they are most cleaned
= The formation water resistivity reported in
?Schlumberger?s tables on the ?log interpretation chart?.
The value of Fsh, increases with depth in normal compaction zones and lowers in overpressure zones (Refer to figure 2.b). For the gradients calculation, the ?Overpressure Evaluation Manual?.
ARPO ENI S.p.A. Agip Division IDENTIFICATION CODE STAP-P-1-M-6100
1
1500
F shale
10
0
100
PAGE 18 OF 230 REVISION 2000
2500
Depth (m) 3000
3500
4000
4500
5000
Figure 2.B - ‘F’ Shale
Sonic Log (SL) Method: Also termed ???t shale?, is the most widely used as, from
experience, it gives the most reliability. It consists in the plotting, on a semilog graph (depth in decimal scale and transit time in logarithmical scale) of the???t values (transit time) at relative depths.
The???t value (transit time) is read on sonic log in the shale points where they are cleanest;???t value lowers with the depth increase in normal compaction zones and increases with the depth in overpressure zones (Refer to figure 2.c) For the calculation of gradient, refer to the ?Overpressure Evaluation Manual?.
ARPO
ENI S.p.A. Agip Division
IDENTIFICATION CODE
STAP-P-1-M-6100
10 0 500 1000 1500
100
0
1000
PAGE
19 OF 230
REVISION
3500 Depth (m) 2000 2500 3000
?????????
2.3.
5000 4000 4500
????????? ?????????????????? ????????? ???????????????????????????????????????????? Top ???????? ???????????????? ???????? ???????????????????????????????? Figure 2.C Sonic log
TEMPERATURE PREDICTION
The temperature at various depths to which a well is drilled must be evaluated as it has a great influence on the properties of both the reservoir fluids and materials used in drilling operations.
The higher temperatures encountered at increasing depth usually have adverse effects upon materials used in drilling wells but may be beneficial in production as it lowers the viscosity of reservoir fluids allowing freer movement of the fluids through the reservoir rock. In drilling operations the treating chemicals materials and clays used in drilling mud become ineffective or unstable at higher temperatures and cement slurry thickening and setting times accelerate (also due to increasing pressure).
Another effect of temperature is the lowering of the strength and toughness of materials used in drilling and casing operations such as drillpipe and casing.
As technology improves and wells can be drilled even deeper, these problems become more prevalent.
ARPO ENI S.p.A. Agip Division 2.3.1.
Temperature Gradients
IDENTIFICATION CODE STAP-P-1-M-6100 PAGE 20 OF 230 REVISION 0 The temperature of the rocks at a given point, formation temperature, and relationship between temperature and depth is termed the thermal gradient. Temperature gradients around the world can vary from between 1oC in 110ft (35m) to 180ft (56m).
The heat source is radiated through the rock therefore it is obvious that temperature
gradients will differ throughout the various regions where there are different rocks. Seasonal variations in surface temperatures have little effect on gradients deeper than 100ft (30m) except in permafrost regions.
It is important therefore that the local temperature gradient is determined from previous drilling reports, offset well data or any other source. In most regions, the temperature gradient is well known and is only affected when in the vicinity of salt domes. If the temperature gradient is not known in a new area, it is recommended that a gradient of 3oC/100m be assumed.
The calculation of temperature at depth if the thermal gradient is known, is simply: T = Surface Ambient Temp + Depth/Gradient (Depth per Degree Temp)
2.3.2.
Temperature Logging
During the actual drilling of a well, temperature surveys will be taken at intervals which may help to confirm the accuracy of the temperature prediction.
Temperature measurement during drilling may be by simple thermometer or possibly by running thermal logs, however, the circulation of mud or other liquids tends to smooth out the temperature profile around the well bore and mask the distinction of the individual strata. Consequently the use of temperature logs during drilling is uncommon.