New rheology fluid simplifies engineering, enhances performance

Proven in deepwater and extended-reach wells, the next-generation flat rheology invert drilling fluid simplifies system engineering and maintenance without sacrificing performance. The upgrade is especially beneficial in deep plays where thermal stability and fluid control are essential to avoid complications that substantially raise risks for operators.

M-I SWACO’s flat rheology invert drilling fluid (FRIDF) has performed consistently due to the correct combination of emulsifier, wetting agent, rheology modifiers, and supplementary viscosifiers. But multiple products complicated engineering and management in the field. Two years ago M-I SWACO set out to revamp the fluid system for better field handling and better rheology control without compromising performance. Now a new flat rheology system (NFRS) contains only a single emulsifier for both emulsification and surface wetting.

This typical fluid composition of NFRS resulted in reducing the breaking circulating pressure, standpipe pressure, and ECD. (Images courtesy of M-I SWACO)

A decade ago, the company developed its original FRIDF system to combat the problem of thinning fluids at deeper depths with higher temperatures. Thinner fluid contributed to poor hole cleaning, barite sagging, and lost circulation.

But the FRIDF relied on two organoclays, one emulsifier, one wetting agent, and two rheology modifiers to generate “flat rheology” with consistent readings of 6 rpm, yield point, and 10-minute gel strength from 4.5°C to 65.5°C (40°F to 150°F).

The product’s 10-minute gel strength was considered excessive, especially with the fluid system contaminated with low-gravity solids (LGS). One rheology modifier tended to become less effective at temperatures above 121°C (250°F), and another had to be added to maintain the flat rheology profile.

The NFRS has a simpler fluid formulation, lower gel strengths, and flatter high-temperature profiles than the original FRIDF. This is enabled through the use of a new emulsifier and rheology modifier package. The NFRS delivers lower equivalent circulating density (ECD), standpipe pressure, and breaking circulation pressure than the original FRIDF while providing similar drilling performance.

Field trials conducted in extended-reach drilling (ERD) and deepwater wells confirmed that many operational benefits can be achieved using the NFRS. No major lost circulation was encountered due to good hole cleaning and low ECD impact in both field trials. This was especially critical for the deepwater field trial where low margin windows were encountered. The new system was proven to be contamination-tolerant, including both drilled solids and brine contamination.

New flat rheology system

After hundreds of tests, one combination of emulsifier and rheology modifier demonstrated desirable flat rheology profiles and low gel strengths without sacrificing other fluid properties.

The new emulsifier includes chemistry that imparts both emulsification and surface wetting. The result is stable emulsion to improve solids tolerance, enhance the flatness of the rheology profile, and provide less increase in gel strengths in the presence of LGS.

The emulsifier can be used to formulate invert drilling fluids with synthetic-to-water (SWR) ratios varying from 60:40 to 90:10.

The new rheology modifier, also based on novel chemistry with additional LGS tolerance, is more efficient and temperature-stable. It provides enhanced barite suspension while minimizing barite sag tendency. Table 1 shows the typical fluid composition of the NFRS. Figure 1 shows the plots of 6-rpm reading, yield point, and 10-minute gel strengths of the original FRIDF and the NFRS of similar mud weight and solids content.

The new rheology modifier with additional low-gravity solids tolerance is more efficient and temperature-stable and provides enhanced barite suspension while minimizing barite sag tendency. This is a comparison of the rheology profiles of the original FRIDF and NFRS in the temperature range from 4.5°C to 65.5°C (40°F to 150°F)

The NFRS displayed comparable 6-rpm readings and higher yield point but lower 10-minute gel strengths. With similar amounts of LGS, it demonstrated better solids tolerance to allow for faster drilling with good hole cleaning. It also can minimize problems induced by high ECD.

The new fluid showed reasonably good flat profiles even after aging at 149°C and 177°C (300°F and 350°F). This is a significant improvement over the original system in which rheology profiles could deteriorate in higher temperatures as the rheology modifier underwent thermal degradation.

The new emulsifier and rheology modifier were used in separate systems with mineral oil and paraffin oil as base fluids. The fluids again showed reasonably flat rheology profiles after heat-aging at temperatures up to 121°C (250°F). Lab tests of the NFRS and original FRIDF systems revealed similar barite sag control performance in a simulated flow loop with 60° hole angle and no pipe rotation.

But hydraulic simulations revealed that the NFRS performed better in reducing the breaking circulation pressure, standpipe pressure, and ECD without sacrificing the hole cleaning index (HCI).

Extended-reach well

Field trials were conducted on one extended-reach well and one deepwater well. For the ERD, the NFRS, with mineral oil as the base fluid, was used to drill the 17½-in. and 12?-in. sections with a relatively low mud weight.

With comparable low-end rheology designed for hole cleaning, the NFRS had much lower 10-second and 10-minute gel strengths. It showed a flat profile with 6-rpm and 3-rpm readings, almost identical over a wide temperature range. The fluid also exhibited relatively low but flat and nonprogressive gel strengths with sufficiently high low-end rheology and yield point. The drilling fluid was prepared with a final oil-to-water ratio of 70:30.

Throughout drilling, stable rheological properties were maintained to deliver good hole cleaning under optimized ROP.

Since this was an ERD well, barite sag tendency was determined using a specially designed sag flowloop tester to allow mud weight changes to be measured at different angles with different eccentricities of the drillstring under various fluid circulation and pipe rotation conditions.

No mud weight changes, thus no barite sag, were observed even at low annular velocity and zero-rpm pipe rotation. Subsequent testing of the field mud with a higher mud weight revealed similar results. The NFRS experienced flat rheology with very little thinning at high temperatures versus conventional nonaqueous drilling fluids (NAF) used in offset wells.

The NFRS fluid reduced back-reaming time through improved hole cleaning, a lower standpipe pressure, and increased flow rates. It also reduced plastic viscosity of the fluid, improved low-gravity solids tolerance, reduced breaking-circulation pressure, and lowered drilling fluid cost.

Deepwater exploration well

In the deepwater Gulf of Mexico, the NFRS was used from top-hole to total depth over a conventional NAF and the original FRIDF system because of the likelihood of encountering narrow operational windows in several intervals that can cause severe losses.

The mud weight varied from 10 lb/gal up to 17.2 lb/gal. The synthetic-to-water ratio was run between 68:32 and 79:21 but mostly between 70:30 and 75:25, even with high mud weights.

During the drilling operations, a severe brine influx occurred in the top intervals. The NFRS took the brine kick without suffering severe changes in fluid properties. In the lower intervals with a high mud weight but slower ROP, the content of LGS increased sharply. But the fluid rheology remained stable without significant increases in gel strengths.

For the last interval, the maximum bottomhole temperature was expected to reach 177°C. To ensure the flat rheology profile and low gel strengths, a specially designed polymer was used as a supplementary rheology modifier. The additive was used only in the last two intervals, and the concentration used was relatively low at about 0.5 lb/bbl.

Despite the relatively high mud weight and high LGS content toward the last two intervals, the NFRS consistently exhibited relatively low and nonprogressive 10-minute gels at all temperatures.

This property allowed the well to be drilled to a greater depth for a higher well pressure evaluation. Table 2 shows rheological properties for the deepwater NFRS.

With the rheological properties of the NFRS used in a deepwater well, the new fluid consistently delivered lower breaking circulation pressure than the original FRIDF for similar ECD and HCI.

With both fluid systems having similar low shear rate viscosity, simulation results showed that the NFRS consistently delivers lower breaking circulation pressure than the original FRIDF for a similar ECD and HCI.

The NFRS also shows a potential in reducing tripping times compared to the original FRIDF.

Acknowledgement

This article was adapted from SPE 154682, which was presented at the Society of Petroleum Engineers Oil and Gas India Conference and Exhibition in Mumbai, India, March 28-30, 2012.