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Case study Fluid Loss - Onshore Buzau

Fluid Loss products Oil & Gas

Onshore, Northern Vienna Basin

Fluid Loss Fluid Loss

This Case Study presents the key experiences of the application of Drilchem LCM products on a recent onshore HPHT well in the northern Vienna Basin.

Location
Onshore, Northern Vienna Basin

Density & type of drilling fluid
1.15 SG – 1.85 SG WBM (KCI/Glydrill)
1.85 SG – 2.17 SG OBM (Versatherm)

Drilling Challenges

Pannonian channels consisting of soft and unconsolidated Mittelpannon formations penetrated on the top hole vertical phase of the well.

Hole instability from microfractures in the silt and shale layers, and in the loosely consolidated sands in the transition zone from Sarmatian to Upper Badenian formations. Pressure ramp while drilling the Upper Badenian formation.

Loss of circulation risk in the Top Conglomerates consisting of coarse sandstone, shales and carbonates on the 12 ¼’’ interval drilling to top of reservoir.

Background

The exploration well ALTLICHTENWARTH TIEF 1/1a targeted a large 4-way dip closure in the northern Vienna Basin. This was a HPHT well with a maximum downhole pressure at final well TD of 838 bar. On the basis of offset well data specific strategies were developed to manage the level of risk anticipated from unconsolidated formations on the top hole 24” interval, a steep pressure ramp in transition zone to HP on the 18 1/2” interval, and for the risk of losses drilling the 12 1/4” interval through coarse conglomerates to top of the reservoir at 3,633 m.

A Fracseal pretreatment program was designed for the operation to minimize seepage losses and thereby prevent/reduce filtrate invasion and control swelling and enhance wellbore stability. Stoploss LCM was mobilized as a contingency to mitigate severe to total losses predicted to be a challenge on the 12 1/4” HPHT interval while drilling coarse sandstone, shales and carbonates in the Top Conglomerate formation.

Contribute to smooth drilling operations by reducing NPT and enabling drilling to achieve targets without the need for contingencies

Execute the mud treatment according to program. Maintain target concentrations in the Active mud system based on benchmark testing. Manage any challenges with contingency plans

Objectives

Planning

A Fracseal pretreatment program was designed for the operation to minimize seepage losses and thereby prevent/reduce filtrate invasion and control swelling and enhance wellbore stability. Stoploss LCM was mobilized as a contingency to mitigate severe to total losses predicted to be a challenge on the 12 1/4” HPHT interval while drilling coarse sandstone, shales and carbonates in the Top Conglomerate formation.

The top hole and intermediate sections of the well were drilled with a WBM system. Ahead of drilling the lower intermediate 12 1/4” HPHT phase of the well, the well was displaced to Versatherm OBM system.

During planning for the operation, Fracseal had been selected for WBS on the 18 1/2” section where a pressure ramp from 1.15 SG to 2.2 SG was expected. The product was also planned to be used as background LCM treatment on the 12 1/4” HPHT phase of the well with a high risk of losses to conglomerate formations and an uncertain and variable PP regime.

Prior to spudding the well, it was also decided to run a sweep pill program on the 24” top hole section to assist in stabilizing the unconsolidated Pontian shales and high permeable sands in the middle Pannonian formation while improving hole cleaning in large diameter with high ROP.

Action

Top hole – 24″ section
The top hole 24” section was drilled to 800 m. A Fracseal sweep pill program was run on the section, pumping 5 m3 pills with 35 kg/m3 at a frequency of 2 times per stand drilled. The section saw improved hole cleaning and the unconsolidated shales and high permeable sands on the section were successfully stabilized.

18.5″ section
The 18.5” section was drilled to 2,600 m, including a pressure ramp from 1,600 m to 2,500 m where the MW was increased stepwise according to a prearranged schedule from 1.15 SG to 1.50 SG. While drilling the mud was continuously treated with Fracseal F in the range of 5 – 10 kg/m3 concentration. This was done by mixing 30 m3 batches of premix with Fracseal concentration in the range of 15 – 30 kg/m3. A wiper trip performed at 1,611 m back to the 20” casing shoe, prior to entering the transition to HP drilling environment, confirmed the hole conditions to be good. After calling TD at 2,600 m and pulling out of hole, the liner became differentially stuck while connecting the liner hanger. After cutting the MW and freeing the string, a sidetrack was initiated at depth 2,423 m. The mud treatment with Fracseal was adjusted to 5 kg/m3 on the conditioning run ahead of the sidetrack and confirmed at rig site by testing.

Sidetrack
The sidetrack was drilled in slide- and rotary mode to 2,595 m for section TD. While drilling the mud was continuously treated with Fracseal by using a dedicated premix pit and a diaphragm pump to maintain the bridging treatment independently of any other mud treatment being made. The change was made to avoid any unnecessary and avoidable disruptions to the continuous treatment.

12.25″ interval
The 12.25” interval objective was to use Fracseal as background LCM to reduce the risk of losses while drilling through conglomerate formations and case this formation off for TD at the top of the reservoir. The contingency for the section if not able to achieve planned depth was to set 11 3/8” liner and drilled additional interval to top of reservoir. The hole was displaced from 1.85 SG WBM to 1.85 SG Versatherm OBM prior to starting operations on the 12.25 interval. Drilling the last quarter of the shoe track, the Fracseal concentration in the active was adjusted to 8 kg/m3 by bleeding in a 20 m3 pill with 113 kg/m3 Fracseal. After the shoe track was drilled and the hole circulated clean, a 5 m3 WBS pill with 42 kg/m3 Fracseal was spot on bottom for the planned LOT. The LOT was performed with a MW of 1.85 SG, applying surface back pressure at 104 bar and achieved a final LOT value of EMW 2.20 SG. A second LOT was performed and a new WBS pill spotted this time with 72 kg/m3 Fracseal concentration. The final LOT value achieved was 2.17 SG.

After performing the LOT, the DO BHA was POOH and the RSS drilling BHA was RIH for starting drilling operations on the 12.25’’ hole interval. The 12.25’’ interval was drilled with MPD system installed for holding constant BHP at connections. Starting MW for drilling the section was 1.85 SG. In advance of drilling operations starting, as contingency for losses, a 14 m3 LCM pill with 57 kg/m3 Fracseal + 28 kg/m3 CaCO3M, and a 14 m3 LCM pill with 100 kg/m3 Stoploss were mixed and kept on standby. While drilling, the background LCM was built into the active at a concentration of 10 kg/m3 Fracseal and maintained according to hole conditions and progress by continuously bleeding-in high concentration premix pills. Entering the conglomerates, the premix concentration was adjusted from 20 kg/m3 to 35 kg/m3 Fracseal to provide a more aggressive treatment strategy. The rate of transfer was increased from 3 m3/hr to 5 m3/ hr.

Drilling penetrated multiple high permeability sandstone layers on the section. The lower than expected LOT value at the 13 3/8’’ casing shoe at 2.20 SG resulted in a lower FG and a smaller drilling window. The MW was kept at just above the PP while drilling and dynamic pore pressure tests performed at regular intervals to evaluate gas levels and adjust MW. While drilling, in response to DPPT’s, the MW was increased from initial 1.85 SG to 2.11 SG at final section TD (3,630 m) with a PP at this point of 2.14 SG and an ECD of 2.17 SG (equivalent to the fracture reopening pressure at 13 3/8’’ casing shoe as confirmed by second LOT).

The wiper trip performed at section TD confirmed the hole conditions to be good with a tight spots recorded between 3,273 m to 3,600 m and reamed. At the 13 3/8’’ casing shoe, depth 3,018 m, a flow check was performed and confirmed static hole conditions with no recorded losses. Prior to running the 9 5/8’’ casing, Fracseal was screened out. The casing was run to depth 2,598 m where losses were observed at an initial rate of 200 ltr/hr. Just out from the 13 3/8’’ casing shoe at depth 3,038 m the gels were broken and a loss rate of 2,500 lpm was observed. A 15 m3 LCM pill with 57 kg/m3 Fracseal and 28 kg/m3 CaCO3 was pumped to fight the increasing loss rate. At section TD, circulation B/U was started at FR 400 lpm and increased to 800 lpm with no losses observed.

Ahead of the spacer for the cement job a 10 m3 LCM pill with 35 kg/m3 Fracseal was pumped as a cushion to minimize losses during the operation. The cement job was performed with no recorded losses.

Results

Feedback from the operator was that the products and services at rig site contributed to the well being a technical success. They saved the operator considerable time on several hole sections including potential lost circulation and resultant well control issues, and proved synergies exist between primary mud supplier and specialist service companies.

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