< Formate Matters

Let it flow...

Three gas wells completed and change was needed. Faced with poor productivity and operational challenges, Total needed to alter reservoir fluid strategy on its fourth well in the Martin Linge field, offshore Norway.

The challenges with Martin Linge gas wells were unique. I’ve never known such high permeability sandstone – up to several darcies – and porosity of over 20% at depths of 3,800 meters. Reservoir pressure was up to 745 bar with temperatures from 135°C to 145°C,” says Eirik Jøntvedt, senior specialist engineer at Total E&P Norge. “We drilled and completed the earlier reservoir sections with non-aqueous-based mud (NABM), but suffered stuck pipe, barite sag, exhausting cleanup operations and low productivity. It was time to re-evaluate,” explains Eirik.

Cesium formate-based drill-in and screen-running fluid significantly outperformed non-aqueous-based mud to deliver a problem-free gas well in the Martin Linge field

“We needed a heavy reservoir drill-in fluid that flows easily through sandscreens, facilitates effective cleanup and exhibits good fluid-loss properties. Working with Cabot and M-I SWACO, we found these qualities in low-solids 2.07 g/cm3 cesium/potassium brine-based fluid,” says Eirik.

First for Total
Total has used cesium formate many times for completions and perforations, as well as drill-stem tests and plug and abandonment operations, but never as reservoir drill-in and screen-running fluid. Indeed, it has been used successfully for Martin Linge completions. “We chose cesium formate to install the upper completion in the first three gas wells, so we knew the fluid, but hadn’t drilled with it,” explains Eirik. “We were confident after running numerous assessments, including several satisfactory return-permeability tests, but it’s still exciting to put theory into practice.”

A drilling comparison
As earlier reservoir sections were drilled through the same formation in the same conditions with the same bit, results are easily comparable. For the three wells drilled with NABM, ROP was low at 2 to 3 m/hr, especially in shale sections. In the fourth gas well, there were no physical limits for ROP with formate fluid and the rate was set at a much higher 5 to 15 m/hr with weight on bit more than halved. Equivalent circulating density and standpipe pressure were lower and more stable, while the low-solids cesium formate fluid also reduced stick-slip responses for downhole tools (SPE 189550).

“Perhaps the most surprising aspect of the operation was the ease with which we drilled through difficult coal sections. We got stuck here twice earlier and needed to sidetrack one well, but the only indication that we were drilling through coal with formate fluid was in the cuttings. During trips, logging and running screens, we did not experience any borehole instability,” says Eirik.

Well controlled
Total found that cesium formate drill-in fluid provided superior well control. Elimination of weighting solids removed sag risk, while low gas-diffusion rates allowed instant kick detection. These benefits and the fluid’s favorable thermophysical properties meant less circulation and quicker verification of static well conditions. Eleven flow checks and real-time downhole equivalent static density measurements confirmed very good stability.

Low losses
“A fear of subsurface fluid losses and inconclusive formation damage results from initial laboratory tests were key reasons why formates were not originally specified as the primary reservoir drill-in fluid,” says Eirik. He continues: “Therefore it was very satisfying to see fluid filtration properties and rheology in the field surpassing laboratory tests and comfortably within our strict specifications. In fact, there were no subsurface losses at all.”

A clean slate
Cleanup was a major challenge on the first three wells. In well four, cesium formate provided smoother startup, stabilized flow faster and kept screens free during the rapid cleanup operation. Despite using shorter cleanup time, the productivity index was double that of wells two and three, with the first not producing at all. “All indications are that the well is good to go,” says Eirik.

“Cesium formate-based drilling fluid has outperformed the previous NABM. We have been very happy with Cabot’s and M-I SWACO’s professional service and, of course, the reservoir fluid. It has quickly delivered a well with high production potential without operational problems. I don’t know of any other fluid that could have done this given the specific challenges faced,” concludes Eirik.