Page 30

Page 31

Photo by: Geir Ove Titlestad

Above: Handling the cement sacks during P&A activities 

Due to the depth it took some time to retrieve the cement cores. The cement was cored to 798 m. There were some 
small sections in between that contained soft cement. It was therefore difficult to get a good and solid 3 m long ce-
ment core. 

The well was subsequently tested to 73 bar and a good result was recorded. The cement plug is not considered as a 
permanent plug due to the open sandstone formation above.


DH6 was drilled in 2011 and is 435 m deep. The primary objective for this well was to test the shale formation above 
the sand formation. Before plugging no activities had taken place in the well since 2011. DH6 had no sign of gas pres-
ent or damage that might complicate P&A operations. 

In 2013 during the plugging operation a small injection test was performed before plugging. No change was recorded 
from the original test in 2011. 

The well was closed using bullheading. In this case the well was filled with water when the pump started to pump the 
cement; the water in front/ahead of the cement was pushed/injected into the formation so the cement placed itself 
across the formation. The well was then shut in for two days. DH6 was opened to the atmosphere and tests were un-
dertaken to verify the strength of the plug. The cement core sample had hardened as expected, and the upper cement 
plug was pressure tested to 100 bars. No pressure or flow from the well was recorded. 


DH7 was drilled in 2012 and is 703 m deep. The primary objective for this well was to test the sandstone formation 
at 670 - 703 m. Before the injection test in 2012 the well contained and produced natural gas with a surface wellhead 
pressure of 25 bars. After the injection test in 2012 the gas flow dried up and the well ceased to produce gas. 

In 2013 during the plugging operation the well was open up for a final injection test to verify that the well was open and 
accessible. The pressure decreased and stabilized as the well was shut down. 700 liter of G cement was pressurized 
and pumped down the work string used to administer cement to the bottom of the well.  

After 2 days the well was reopened. The cement plug was cored and pressure tested. No pressure was recorded and 
the cement plug was solid. There was no sign of gas. The work strings were removed.

DH7a required an additional “balanced cement plug”, to be installed inside the HQ string. The HQ string is permanently 
installed in the upper part of the well from surface and down to 300 – 500 m. The cement is pumped through a smaller 
work string inside the HQ string down to the plugging depth. The cement is pumped through the work string and the 
return is taken up the annulus the space between the HQ and the work string. 

See the figure for balanced cement plug. 

In September 2013 a final short step rate injection test was performed in DH4; the well was filled with 800 liter of wa-
ter before pressure was registered. When the pump started up a small amount of gas mixed with water was released 
from the BQ- NQ annulus. This was monitored and found to be an insignificant amount.

The well was refilled with 640 liters of water to register pressure and the injection test began. Over 6 hours the pump 
rate and pressure were monitored. The surface data pressure fluctuated, but over 6 hours a clear trend was estab-
lished, the pressure increased slowly.  

The down-hole sensor was retrieved in mid-October 40 days after the test. When the well was open up for retrieval 
of the sensor there was no gas or pressure in the well. For details, see summary of injection and falloff data by Larsen 
(2013) (UNIS CO2 Lab report 2013-X). 

Based on the low injection pressure and the registered under-pressure of the formation there was a concern that the 
heavy cement could disappear into the formation and therefore not seal of the well bore. 100 liters of cement was 
pumped into the well before adding celloflake to the cement. Celloflake is a lost circulation material used to close off 
pores and cracks in the sand formation.  An additional 380 liter of cement was pumped into the well. In total 550 liter 
of G cement and a 100 liter of saltwater with 11% salt content was used to displace the cement. The planned top of 
cement was at 725 m. During tagging of the cement 4 days later the cement was found to be 100 m above the planned 
top.  This 100 m section was removed by coring.