New, water-based mud system is successful as an environmentally compliant alternative to nonaqueous fluids on Gulf of Mexico shelf wells.

The United States Environmental Protection Agency restricts drilling waste discharges. SBM whole mud discharge is not allowed. Water-based mud (WBM) whole mud and cuttings can be discharged provided the fluid meets aquatic toxicity standards. Oil-based mud (OBM) is "zero discharge" and must be transported to shore for disposal or injected at the well site.
An environmental initiative by Chevron has led to the use of a new, high-performance, WBM (HPWBM) in place of nonaqueous fluids (NAF) on shelf wells in the Gulf of Mexico. The new HPWBM is permitted for discharge of whole mud and cuttings.
First-generation HPWBM
This HPWBM design focused primarily on pore pressure reduction in shale. It used a novel approach to increase the membrane efficiency of shale by decreasing shale permeability. The first-generation HPWBM was field tested on two shelf wells in the South Timbalier field of the Gulf of Mexico.
Well #1 South Timbalier
This reentry well used a 4¾-in. by 6¾-in. pollycrystalline diamond compact (PDC) and reaming-while-drilling (RWD) assembly. Rheological properties of the fluid were stable, mud weight was controlled with minimal dilution, torque and drag were minimized and the well bore was extremely stable throughout the interval. Average rate of penetration (ROP) on an offset well drilled from the same platform over a similar interval using SBM and a PDC bit was 34 ft/hr (10 m/hr), compared to 25 ft/hr (7.6 m/hr) with the early HPWBM. Inspection showed that several jets in the PDC bit were completely blocked. The RWD assembly and PDC bit were not severely balled.
A mill-tooth bit drilled the remainder of the well at an average ROP of 25 ft/hr (7.6 m/hr) while sliding and rotating. Despite foaming problems, the shaker screens were able to process flow without mud losses.
The first-generation sealing polymer was identified as the root cause of foaming and air entrainment. The foaming was managed with defoamers but was still significant. The ROP enhancer was added after repeated incidents of slow ROP. Some bit balling may have occurred before it was added.
Hydraulic horsepower at the PDC bit was based on historical use of SBM and not on values recommended for WBM.
Well #2 South Timbalier
Wellbore stability characteristics of the first-generation HPWBM were very good. Foaming was reduced, but still persistent. Hole quality was excellent, with no tight spots, unusual torque and drag or fill on bottom. The ROP enhancer was used as a contingency. Improvements in ROP and sliding were observed after its addition.
Re-engineering work focused on eliminating foaming, increasing ROP with PDC bits, improving clay and gumbo inhibition and optimizing bit hydraulics.
Next generation HPWBM
The source of the foaming problem was the surfactant, added to help maintain particle size distribution of the original sealing polymer. A second-generation micronized sealing polymer was extremely stable in the presence of high salts and did not require the use of surfactants to maintain particle size. Laboratory tests confirmed that stability and particle size of the new sealing polymer in 20% NaCl were improved and that it performed equally well in pore pressure transmission (PPT) tests. The new sealing polymer also exhibited superior compatibility with other products in the system, with significant improvements in foam reduction, filtration control and rheological properties. The pressure transmission and membrane efficiency characteristics of the HPWBM closely mirror that of SBM (Figure 1).
Bit balling and accretion were minimized and ROP increased by a patented, anti-balling and accretion additive. A proprietary method was developed to inject the additive so that a continual, non-emulsified stream is available at the bit while drilling. This minimizes emulsification, reduces concentrations and allows the material to coat metal and rock surfaces. Clay and gumbo inhibition was further improved by an environmentally acceptable, water-soluble clay-hydration suppressant (CHS). The CHS effectively inhibits reactive clays and gumbo from hydrating and becoming plastic, which also reduces bit balling.
Field trials
The new HPWBM provides: 1) shale stability, 2) gumbo and clay stability, 3) cuttings stability and solids removal efficiency, 4) high ROP, 5) minimized bit balling and accretion, 6) torque and drag reduction and 7) minimized differential sticking.
Pre-well planning focused on optimizing bit hydraulics. A target bit horsepower/sq in. of >2.5 HSI with a minimum of 2.0 HSI was recommended for optimized bit hydraulics.
West Cameron Well
A total of 5,668 ft was drilled in the 12¼-in. interval, at an average ROP of 93 ft/hr (28 m/hr), comparable to the NAF offset wells. Wellbore and cuttings stability were excellent. Three wiper and two round trips were made without tight-hole, fill-on-bottom or gumbo-attack problems. The interval was drilled and casing run without problems. The foaming problem was solved. Balling and accretion on tool joints and stabilizer of the 12¼-in. drilling assembly were prevented.
Inability of the shakers to handle high flow rates and mud losses in the massive sand sections caused a large reduction in ROP. Without these problems, the interval could have been drilled as fast as desired (+/-300 ft/hr, or 92 m/hr, instantaneous).
Eugene Island Well #1
Key concerns included hole cleaning, lost circulation and differential sticking in depleted sands. Also, gumbo attacks had been a recurring problem on offset wells drilled with conventional WBM. Average ROP in this well was 105 ft/hr (32 m/hr), with no gumbo attacks, bit balling or accretion. The interval was drilled and casing run without problems.
Eugene Island Well #2
HPWBM was used on this well after repeated diesel-based NAF mud losses as high as 60 bbl/hr and failure of remedial treatments with lost-circulation materials. Because of possible losses at an anticipated mud weight of 16.9 lb/gal in the production sand, an openhole displacement from NAF to was made. The system handled NAF contamination well. Drilling continued to total depth, with a maximum angle of 63°, mud weight of 16.1 lb/gal and bottomhole temperature of 150°F (65.5°C). The interval was safely drilled, the HPWBM had stable properties despite NAF contamination, mud losses were significantly reduced compared to NAF, and the HPWBM eliminated problems related to ballooning in the fractured shale. The interval was drilled and casing run without problems.
Ship Shoal Well
A rotary steerable assembly was used to drill the 97/8-in. and 6½-in. intervals. A PDC bit was used to drill the upper portion of the 97/8-in. interval, at an average ROP of 115 ft/hr (35 m/hr). A mill-tooth bit was used to drill the lower portion at a controlled ROP for reservoir navigation. The interval was drilled at an average (PDC and mill-tooth bit) ROP of 102 ft/hr (31 m/hr). The 75/8-in. casing string was run to bottom without problems. The 6½-in. interval was then drilled at a controlled ROP of 55 ft/hr (17 m/hr).
Comparison of field results
First-generation HPWBM problems were resolved with the new system. There were no further incidents of foaming. The system(s) exhibited stable rheological and filtration control properties on all wells. The key area of improvement was in ROP. Performance of the HPWBM compares favorably with NAF.
Environmental benefits
In most cases, use of HPWBM allows cuttings and whole mud to be discharged overboard. Using HPWBM in place of NAF (OBM and SBM) provides the performance and economic benefits of NAF, while setting a new benchmark of environmental compliance.

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