Mr. Erik Milito - The House Committee on Natural Resources ...

54 API STANDARD 65-2
Cooke’s study [24,25] discovered fundamental mechanisms and explanations supporting some theories on loss of
hydrostatic pressure and invalidating others. <strong>The</strong>se discoveries were verified by others such as a similar separate
downhole sensor study (SPE 19552) by Morgan [40] while running and cementing casing in the North Sea. Morgan
also reported how these downhole measurements indicated the failure to set an ECP.
An analysis of downhole annular pressure measurements can explain other difficult to find or complex root causes for
other cause effects such as high ECD pressures when cementing liners. Brehme et al [41] et al found that downhole
pressure sensor measurements are a reliable method to help diagnose and evaluate liner running and liner
cementing operations. Cementing simulation computer model results were favorably compared to the actual
downhole pressures. Brehme et al [41] proposed this process to help predict and evaluate results in future cementing
operations by downhole gauge diagnosis of conditions not included in software models such as annular restrictions
by cuttings, hole cleaning performance, and liner hanger equipment functions. For example, this data can help
operators find answers for some annular flows linked to lost circulation events such as those caused by complete or
partial flow restrictions in liner overlaps or liner hanger bypass or cross-sectional areas plugged by cuttings not
removed during hole cleaning operations.
A.14 Some Key Results from Cooke's Study [24,25]
Some of the key results from Cooke’s study [24,25] are as follows.
1) Downhole sensor measurements proved that the loss of hydrostatic pressure in columns of unset cement may
be reduced to values below those found in cement mixwater (fresh, sea or saltwater) density gradients [see A.3,
no.4.a)]. See Figure A.2 (Figure 9 in SPE 11206) showing that the cement hydrostatic head from the sensor at
1900 ft. in well G decreased from 13.4 lb/gal to 2.5 lb/gal equivalent density in ca. 420 minutes. This
measurement is 6.0 lb/gal equivalent density below the average seawater gradient of 8.5 ppge. Cooke
concluded that SGS development caused the cement hydrostatic head to regress to 2.5 ppge based on hole
conditions such as formations with little or no permeability across and above the sensor at 1900 ft. This
invalidates the idea claimed in SPE 8255 [42] that the loss of hydrostatic head in the cement column never falls
below the cement’s mixwater density gradient. It also invalidates the associated practice [38] of adding salt in
cement slurries to increase the gradient and reduce the loss of hydrostatic head.
2) Surface pressure applied to the annulus may not reach the desired depth depending on drilling fluid and cement
properties such as gel strength development (A.3, no.4.b. and 5.7.8). See Figure A.3 (Figure 4 in SPE 11206)
that illustrates the lack of pressure response in 3 sensors at 4430 ft, 5454 ft, and 7412 ft in well B when surface
pressure is applied. Also notice the large amount of pressure applied after 2100 minutes was high enough to
break the cement SGS and allow hydrostatic pressure to be measured at 2 of the 3 sensors. Figure A.3 (Figure 2
in SPE 11206) also shows no pressure response from all sensors in well A by the applications of surface
pressure designed to test the validity of said practice described in SPE 8255 [42] . <strong>The</strong> surface pressure was
applied 24 minutes after the cement job ended. This uncertainty in transmitting hydrostatic pressure through
unset cement slurry at various times during a cement’s curing phase makes it unreliable to carry out the practice
of applying and maintaining an annular surface pressure to compensate for hydrostatic head pressure losses as
claimed in SPE 8255[42]. <strong>The</strong>refore, applying surface pressure by pumping into the top of the annulus should be
considered only for well control purposes such as controlling a kick in the annulus. However, in some recent
cases applying small amounts of surface pressure in the form of controlled pressure pulses has worked in some
wells to help prevent annular flows when the entire process, including the cement system, is properly engineered,
understood by all involved, and validated by relevant means such as the lab testing described in 5.7.8.
3) Prior to Cooke’s study [24,25] our industry did not have relevant field data to fully understand and confirm the
theoretical mechanisms accounting for the rapid losses of designed overbalances after cementing jobs. Cooke’s
measurement of this rapid decrease in cement column hydrostatic pressures to underbalanced conditions
across potential flow zones helped explain the cause of many LWC incidents. It also helped explain how other
annular flows such as cross-flows and underground blowouts were initiated. <strong>The</strong> understanding of how SCP
may initiate via different types of annular pathways was also improved. Figure A.4 (Figure 2 in SPE11206)
presents annular pressure and temperature measurements in well A that illustrate how the hydrostatic head