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Case study Fluid Loss - Onshore Buzau

Drilchem LCM products to reduce cost

Onshore, Prahova

LCM Products LCM Products

This Case Study presents the key experiences of the application of Drilchem LCM products on a recent onshore well, Prahova in Romania.

Location
Onshore, Prahova, Romania

Density & type of drilling fluid
1.15 SG – 1.76 SG KCl WBM
1.76 SG – 1.90 SG NADF

Drilling Challenges

Hole instability from poorly consolidated loose gravel, sand banks and pebble matrix of the Rom Dacian formation on the 22’’ interval.

Highly reactive, water sensitive shales of the Pontian formation on the 17 ½’’ interval causing a range of hole instability related drilling challenges.

Hole instability from drilling micro-fractures in the interbedded sands and marls of the Lower Burdigalian formation on the 12 ¼’’ x 14 ¾’’ interval.

Losses in the range minor seepage to total losses on multiple intervals of the well.

Background

This exploration well targeted the marine turbiditic sands of the Aquitinanian and the Oligocene expected at a depth of 4,700 m on the Northern sector of the Tintea-Baicoi-Floresti field, part of the Baicoi exploration block V. This was an HPHT well with a maximum downhole pressure at final well TD of 1,080 bar.

On the basis of offset well data, WBS strategies were developed to manage the level of risks drilling unconsolidated sandbanks and gravels, and also for the risk of events like pack-offs from drilling coal stringers on the deeper part of the 22’’ interval.

The highly water sensitive and reactive shales of the Pontian formation were planned mitigated by a bridging strategy to minimize the pore pressure transmission, maintain hole stability and prevent formation breakouts. The main criteria for the 16’’ casing setting point was to isolate the Pontian formation and for this objective an LCM contingency plan was also put in place to mitigate the risk of losses while drilling the fine sands intercalated with shale of the Meotian formation and into the limestone of the Sarmatian for the 16’’ casing setting point.

For the 12 ¼’’ interval, a bridging strategy was also planned to assist in maintaining WBS while drilling down to the top of the Salt-Breccia formation expected to represent the reservoir cap. LCM contingency plans were also put in place to mitigate the risk of potential losses drilling interbedded sandstone with shales and marls on the Lower Burdigalian formation.

The top hole and intermediate sections of the well were drilled with a KCl WBM system. Ahead of drilling the lower intermediate 12 ¼’’ interval, the well was displaced to a NADF system.

Implement the WBS strategy according to program and contribute to drilling operations going as planned with minimal NPT

Put into action the mud treatment as per program. Monitor the treatment by use of the fluid invasion test program based on benchmark testing to establish target ranges of concentrations in the active system

Manage all challenges and deviations to the planned schedule of drilling with contingency plans

Objectives

Planning

During the planning phase for the well, the identified risks of hole instability were addressed by designing a WBS strategy based on the use of Fracseal as the main drilling fluid additive in the bridging package. A pretreatment program was designed for stabilizing unconsolidated formations, sealing microfractures and controlling swelling shales thereby enhancing wellbore stability. Stoploss LCM was mobilized as a contingency product for the risk of losses on multiple sections while drilling the well.

Action

22″ interval
The interval presented hole stability problems out from the 24’’ casing shoe at 301 m due to a poor consolidation in the rock matrix composed of gravel, pebbles and sand banks. The first phase of the interval was drilled without treating the active mud system with a Fracseal bridging package for the first 700 m. After reaching TD at 2,198 mMD and pulling out of hole to surface, the subsequent 18 5/8’’ casing run encountered obstructions from depth 301 m, observing tight spots and attempting to wash down. At depth 344 m, no further progress was possible after several attempts with different parameters to pass without success.

The casing running equipment was rigged down and a clean-out BHA made ready for the wiper trip. Prior to performing the wiper trip, the MW was increased from 1.17 SG to 1.21 SG to give a higher hydrostatic and to help to enhance the integrity of the poor consolidation in the rock matrix.

Included in the clean-out program, the Fracseal bridging package was used on the first phase of the interval where the obstruction to running the casing was encountered. This would increase the integrity of the poor consolidation in the rock matrix and contribute to maintaining stability in the formation at a higher hydrostatic.

The Fracseal bridging package was introduced for this phase of the operation by running a sweep pill program pumping 10 m3 sweep pills with 20 kg/m3 Fracseal and 10 kg/m3 CaCO3 for every stand drilled.

The second casing run was successfully landed without obstruction or issues, and cemented in place with no recorded losses. During the wiper trip the hole was reamed down from the 24’’ casing shoe at 301 m to section TD at 2,198 mMD. The Fracseal bridging package was added on the upper untreated parts of the interval to build a stabilizing and consolidating filtercake for the obstruction zone starting from 301 mMD, and continued for the rest of the wiper trip to repair the filtercake removed by the reaming operations. The extra clean-out trip could have been eliminated if a Fracseal bridging strategy had been used for drilling the interval initially.

17 1/2″ Interval
Completing the 17 ½’’ interval on the well was seen as critical for the project. Borehole stability was a major issue in offset wells. The previous well had been abandoned on the 17 ½’’ interval due to stuck pipes and BHAs left in the hole after several failed attempts to sidetrack and successfully pass the problem zone.

A pretreatment program with Fracseal had been designed to manage wellbore stability, cavings accumulation and formation breakouts while drilling the Pontian shales, fine sands intercalated with shales of the Meotian, and into the Sarmatian limestone for the 16’’ casing setting point at 3,335 mMD.

The drilling program had set a safe MW of 1.40 SG for drilling the Pontian shales. During operations this was re-evaluated and the use of Fracseal to strengthen the formation enabled increases of the MW up to 1.55 SG for drilling the lower part of the Pontian consisting of Marly facies with very thin interbedded sandlayers.

The bridging package was added to the active system before starting to drill the interval with an initial formulation of 10 kg/m3 Fracseal. Adjustments to the bridging package were applied during drilling to further enhance the wellbore shielding effect of the treatment. A sulfonated asphalt drilling fluid additive was introduced at a concentration of 10 kg/m3 to improve the performance of the bridging package on the lower depths of the interval. The treatment was maintained according to hole conditions such as morphology and quantity of cavings observed at the shakers, and the rate of product depletion downhole monitored on a continuous basis using the fluid invasion test program.

The 16’’ casing was run to setting point with no reported issues. There was a small clearance of 1.5’’ for the casing in the 17 ½’’ hole. The use of Fracseal for extending the safe time window for swelling shales in the Pontian and assisting in strengthening the formation for the higher MW’s delivered a slick hole for the casing run.

The interval was successfully drilled avoiding the potential risks from the Pontian shales notorious for causing hole collapses and lost BHA’s in the area around the Baicoi field. The WBS strategy proved successful for managing the risks on the interval and the wellbore was strong enough to take additional MW increases up to 1.55 SG without resulting in shear-rock failure. This was 15 points above the recommended maximum MW set in planning for the operation and based on all prior experience by the operator from drilling the same formation in the area. There was an absence of bulky cavings as a result of pore pressure transmission – 95% of cavings in the cavings register were pressure related and due to insufficient MW.

12 1/4″ Interval
The original 12 ¼’’ x 14 ¾’’ wellbore experienced a hole collapse at depth 4,288 m caused by severe hole instability in the sandstones and shales of the Upper Burdigalian. The first attempted sidetrack was abandoned due to a stuck event caused by the hole caving in. The subsequent sidetrack was planned initiated above the 13 5/8’’ casing shoe and planned to drill through the Upper Burdigalian consisting mainly of calcareous claystone with a defined hard stop at 4,390 m unless in the case of encountering the Salt-Breccia. In this event, drilling would continue until safely through the salt in order to isolate this for the next 10 ½’’ x 12 ¼’’ interval. A separate run was planned for enlarging the hole to 14 ¾’’ with underreamer.

Fracseal was selected as the main drilling fluid additive in the bridging strategy. The objective underpinning use of the product was to assist in strengthening the formation ahead of the planned FIT. In the Amended Drilling program, it was stated that an FIT value under 1.90 SG would require mitigating measures since the strength at the casing shoe then was not able to carry the expected ECD while drilling. The product would also be used to close microfractures and provide the required hole stability with increased MW for drilling the pressure ramp expected on the deeper parts of the Lower Burdigalian.

The bridging strategy used on the interval included the use of a dedicated pit for bleeding-in high concentration premix pills with Fracseal, the option for direct additions of sxs/hr as a fast way to respond to signs of hole instability, and the use of sweep pills. A more frequent monitoring program was also applied with a higher number of tests carried out at sample points in the active return pit and headerbox before the shakers to keep tighter control over the rate of product depletion.

The mud was treated with a high concentration Fracseal premix pill (57 kg/m3 Fracseal, 8 kg/m3 CaCO3 F, and 6 kg/m3 Sulfonated Asphalt) over the course of 2 full circulations as per program for the planned FIT. The final achieved result for the FIT was 1.95 SG giving a safe FG at the csg shoe for drilling the section.

Drilling the section, treatment was continued by bleeding-in Fracseal based premix pills in order to maintain the bridging package in the active system. TG gas levels were observed between 4% to 16% and understood to be coming from gas bearing sandlayers. The well was shut-in several times. Pack-off tendencies were encountered and wash-up/ream down procedures were used at each connection. The cavings/cuttings percentage ranged from 60/40 to 20/80, mainly mechanical cavings (claystone / sandstone) from reaming. Drilling ahead the MW was increased in stages from 1.85 SG to 1.90 SG and there was observed a close correspondence between the increased MW and control/reduction of the gas levels.

The use of Fracseal in the bridging strategy helped to strengthen the wellbore which was critical for closing microfractures in the formation while operating close to the FG at the casing shoe and enabling drilling with the higher than planned MW’s. The hole instability challenges encountered on the 2 previous attempts to drill the interval were experienced on the second sidetrack but managed in a sustainable way by the combined use of mud treatment with Fracseal and adjustments to the MW for the successful application of the WBS strategy.

Enlarging the hole to 14 ¾’’, the treatment with the bridging package was continued to compensate for the filtercake removed by reaming the hole to a larger diameter. The 11 ¾’’ liner was run slowly due to high lifting forces caused by the viscous mud in the hole and tight clearance between the liner and the 13 5/8’’ casing. At 3,805 mMD, the liner became differentially stuck. Attempt was made to free the liner by reducing the MW to 1.85 SG. After this showed no improvement, the liner was set and cemented in place with no losses experienced.

Results

The successful completion of the second sidetrack for the interval was critical for continued operations on the project. Drilling of the unsuccessful original wellbore and the failed first sidetrack cost 60 days to the operation. The second sidetrack took 24 days to drill and cement the casing on. If they had failed to reach an appropriate casing setting point, the well could have been plugged and abandoned. The more aggressive bridging strategy put into play on the second sidetrack ensured a continuous and very flexible flow of premix into the active system and this is seen as a major contributing factor in providing the necessary wellbore shielding effect needed to stabilize the Upper and Lower Burdigalian. This also proved important in making possible the higher MW’s used to control the gas levels without causing formation breakdown.

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