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Case study Fluid Loss - Onshore Buzau

Case study Fluid Loss - Onshore Buzau

Onshore, Buzau

Fluid Loss Fluid Loss

This case study presents key experiences from an LCM call-out for total loss of circulation during an onshore drilling operation in Buzau, Romania.

Onshore, Buzau, Romania

Density & type of drilling fluid
1.05 SG WBM System

Drilling Challenges

  • Total losses encountered drilling vertical fault zones in limestone formations on the 6″ interval
  • Continuous minor to partial losses drilling fractured limestone formations on the 6’’ interval


The Operator had been experiencing severe to total losses in excess of 5000 m3 on a vertical exploration well in the Buzau region of Romania. In total, in excess of 5000 m3 of polymer based WBM had been lost while drilling fractured limestone and vertical fault zones on the 6’’ interval of the well.

Prior to receiving the call-out, the Operator had exhausted all conventional LCM options with the fluids company with no success. This had resulted in a cement plug being set on the previous interval, before drilling to TD and running the casing.

The Operator encountered the same type of total loss scenario when drilling ahead on the subsequent 6’’ interval and then requested a call-out from Dosco PetroServices to provide an alternative LCM solution.

A two-pronged LCM strategy was developed to deal with both the total loss scenario anticipated to be encountered from the karstified vertical fault zones with Stoploss, and a Fracseal LCM program to deal with continuous losses while drilling ahead. The second part of strategy included the use of sweep pills pumped on the fly while drilling and also larger volumes of the same Fracseal LCM pill formulation spotted on bottom when the loss rate exceeded an acceptable rate of losses > 10 m3/hr.

Develop a strategy for the LCM application

Cure total losses and enable drilling to continue



The job was a call-out during the operation to provide a different LCM strategy with different products to help cure the losses encountered on the well. On the recommendation of the Dosco personnel at rig site a rig specific Standard Operating Procedure was developed for the pumping of Stoploss using the installed PBL sub in the BHA.

A separate LCM program with Fracseal was also designed using a dedicated LCM rig pit for the expected losses when drilling continued after curing the total losses. This would keep the losses within a manageable range and reduce the NPT related to more time consuming and costly reactive measures to deal with the losses.


The loss zone was located at a depth of 3,436 mMD in a suspected vertical fault zone. At this depth, it was not possible to pass and the Operator had been stuck for a prolonged period while troubleshooting alternatives to solve the problem and allow drilling to continue.

After arriving at the rig site and discussing with the team there, work was started on developing a specific LCM strategy to manage the risks from the heavy losses in a way that would enable drilling to continue.

Following consultation with the Operator and the fluids company, it was agreed to start preparations for the application of a Stoploss LCM pill at a concentration of 95 kg/m3. Prior to taking this decision, an attempt was made to RIH and continue drilling on two occasions. In both instances, conventional LCM with CaCO3 had been pumped without success. On the second attempt, the high concentration of coarse CaCO3 had caused a number of the drill collars and the drill bit to plug and the BHA was POOH.

A concentration of 95 kg/m3 was selected for the application based on the scale of losses and the suspected type of thief zone. The plan was to mix a volume equivalent to 2 pumpable pills. This provided for a second pill to help plug the thief zone which was believed to require a large volume of LCM material to finally stop the losses.

A total volume of 30 m3 of the LCM was pumped to cure the losses that were encountered.

A separate 35 m3 tank was used for the mixing and storage of the Stoploss LCM pill slurry. For the pumping operation, it was advised to use the cement unit in order to isolate the LCM application from the rig pumps.

The first 2 LCM pills were pumped in quick succession. After pumping the first pill and observing decreasing loss rate, the BHA was pulled up to the last casing shoe and the well monitored while using the ECD to squeeze the pill into the formation. The loss rate was observed to reduce by 50%. Prior to pumping the second pill, a 5 m3 Fracseal LCM pill was pumped ahead and the loss rate evaluated. It was then lined up to the cement unit for spotting the second Stoploss pill 50 m off bottom and the BHA pulled out to the casing shoe for a hesitation squeeze to force as much of the pill as possible into the loss zone.

Maximum pressure achieved during the hesitation squeeze was 10 bar.): The loss rate after finishing the LCM application was recorded as static. a third LCM pill was prepared and pumped 16 hours later to further strengthen the bridge inside the loss zone. The same procedure as for the second pill was followed by pulling up to the previous casing shoe to perform the hesitation squeeze. The pill was spotted with 700 lpm, and final pressure recorded as holding at 10 bar. After re-establishing circulation at 700 lpm, the well was monitored and found to be stable. The total volume pumped to cure the losses was 30 m3 LCM material indicating a large loss zone taking a large volume before effectively plugging back deep into the loss fracture network.

TD for the interval was based on passing the loss zone with a target for casing setting point in the deeper and more compact limestone. The losses continued to cause problems for the operation. A combination of Fracseal based sweep pills pumped on the fly and larger volumes of the same LCM pill spotted on bottom were used to successfully manage the losses and allow drilling to section TD without the requirement for long delays or trips out of hole.


The use of Stoploss cut down significantly on the NPT related to curing the losses and enabled drilling to continue. A total of 130 m3 LCM material was pumped downhole on the operation in attempts to cure the total losses at depth 3,436 m. The Stoploss volume of this total was 30 m3, pumped as 3 separate LCM pills at a concentration of 95 kg/m3 each. In comparison, the other LCM formulations which were tried unsuccessfully had concentrations between 376 kg/m3 to 440 kg/m3.

A detailed summary of LCM pills pumped on the operation can be viewed in the table below.

The Fracseal strategy for managing losses while drilling was able to get the Operator to target TD without unnecessary disruptions to the operation caused by long delays to deal with curing losses. No further total loss scenarios were intercepted and the subsequent casing run operation for the 6’’ interval was run with no major issues reported.