Overpressured zone

The -sonic log can also be used to detect overpressured zones. The sonic measurements until recently were available only on wireline. Now, MWD sonic tools have been developed adding one more parameter for overpressure detection while drilling. Two equations relating the formation porosity to the transit time are used ... 

Equation 4-258 gives better results in most formations, particularly in unconsolidated formations. Both of these equations show a variation of the transit time with the porosity. Since the porosity increases in overpressured zones, the transit time increases. Figure 4-333 shows a typical plot for the shale formations of a well in Jefferson County, Texas. 

Demonstration. A normal compaction trend can be seen down to 9,000 ft, then the overpressured zone is clearly outlined. At 12,000 ft the difference between the extrapolated normal trend transit time At and the measured transit time At is... 

The microsol scale technique is used to weigh a shale sample out of and in water, thus giving weight and volume. The results are plotted versus depth. Low shale densities (high porosity filled with water) indicate overpressured zone. A demonstration example is shown in Figure 4-336. 

Flowline temperature is also an overpressure indicator. In overpressured zones the formation temperature increases. The flow line temperature gradient (increase in temperature per 100 ft) can increase by 2° to 10° over the normal gradient. However, other effects (salt domes, lithology changes) may also cause gradient changes. 

Abnormal Formation Pressure Detection from Kicks. The kicks, or flow of formation fluids into the borehole, are the ultimate indication that the well has encountered an overpressured zone. Kick detection during drilling usually is achieved by use of a pit-volume indicator and/or a flow indicator. The usual pit-volume alert is 10 barrels drilling fluid volume increase. A differential mud flow indicator can also be used to detect kicks more quickly. 

Another important feature of mud logging and MWD/LWD, is the capability of overpressure zone detection. As was discussed previously in this section, overpressure estimates can be done during drilling with many techniques and accuracy increases with the number of parameters available. 

Pressure and stress reversion below the overpressured zone because of increased shale permeability... 

Facilities that are deemed critical or highly manned should be relocated out of the overpressure circles or provided with explosion protection measures. Other systems within these overpressure zones should be evaluated for the specific benefits of providing explosive protective design arrangements. An example is provided in Figure 16.1. 

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