In the world':s first full-scale field trial of a low viscosity, ester-based mud (EBM), Kerr-McGee successfully drilled and completed a deep East Breaks well in the Gulf of Mexico in January 2001. The high-angle well was displaced to low viscosity EBM while drilling out the 20-in. casing. The same fluid was used throughout the well to a measured depth of over 15,000 ft. The low viscosity EBM was selected for the well to provide a valid test of the performance characteristics of the fluid known to have the most environmentally friendly attributes of any available.
Ester-based mud has long been identified by the US Environmental Protection Agency (EPA) and US Minerals Management Service (MMS) as the "greenest" drilling fluid due to its rapid biodegradability in seabed cuttings piles. In a 1999 report, the EPA encouraged the development of "new esters with lower viscosity." However, until the development of two new thinners designed specifically for use with EBMs, the rheological properties of the ester fluids did not compare well with the more commonly used internal olefins (IOs). Cold temperatures associated with deepwater drilling caused the original EBMs to thicken, making it difficult to control gel strengths and surge pressures. Petrofree LV, the new low viscosity EBM by Baroid Drilling Fluids, was tested on Kerr-McGee's East Breaks well. This fluid showed an excellent response to the specially designed thinners and exhibited good rheological properties over a variety of temperatures.
Properties of the low viscosity ester-based muds used on the well.
This new, low viscosity EBM, which is part of a product service line of Halliburton Energy Services, is in line with the ongoing commitment to providing a drilling fluid that is both EPA-compliant and operationally efficient in deepwater locations.
The biodegradation of different synthetic-based muds (SBMs) varies widely under aerobic conditions, and even more so in the anaerobic conditions found in cuttings deposits. Unlike other synthetic-based fluids, EBMs biodegrade quickly under anaerobic conditions, so that cuttings piles generated from ester fluid drilling operations do not persist.
Consequently, there are two retention on cuttings (ROC) limits, depending on the SBM in use. The maximum ROC allowed for IO fluids is 6.9%. By contrast, the MMS allows up to 9.4% oil on cuttings (OOC) with ester-based fluid cuttings, a 36% increase over the IO limit. Reducing the ROC to 6.9% often requires additional processing and drying of cuttings after they pass through conventional solids control equipment. The 9.4% limit required for EBMs can be achieved with many rigs' existing solids control equipment.
Before the low viscosity EBM could be considered as a viable alternative to other SBMs, its drilling performance had to match or exceed the performance of the internal olefins. The young, unconsolidated formations encountered in deepwater locations are extremely water-sensitive and must be drilled with an inhibitive fluid. The high cost of deep-water drilling operations also demands a drilling fluid that can provide an efficient rate of penetration, adequate hole cleaning and good lubricity. High viscosities and rheological pro- perties are definitely undesirable in these wells, where the margin between pore pressure and fracture gradient (PP/FG) is often very narrow. Controlling the equivalent circulating density (ECD) and minimizing surge pressures while running casing are critical factors in reaching total depth with a usable wellbore. The risk of loss of circulation and related hole problems is exacerbated when the drilling fluid thickens excessively under static or cold temperature conditions.
Testing the EBM
Kerr-McGee chose to test the low viscosity EBM on this well to determine if the ECD and surge pressures could be controlled sufficiently to avoid the aforementioned hole problems and to evaluate how well conventional solids control equipment would clean up cuttings generated with this drilling fluid. If the low viscosity EBM could be run without extra processing steps to reduce ROC, and could be formulated at a lower ester/water ratio, and could still match the performance of IO fluids, it would certainly be an attractive alternative to other SBMs. These advantages could offset the higher cost per barrel for the ester base oil and help provide the operator with better insurance against environmental issues.
The fluid used on the East Breaks well was prepared onshore and shipped to the rig with slightly higher rheological properties than would be used while drilling. Base oil was added to the rig':s active system to dilute the fluid as needed. It is anticipated that larger fifth generation semisubmersibles and drillships would have adequate base oil storage and mud plant facilities to build and maintain a low viscosity EBM without total reliance on onshore plants.
The subject well was located in a water depth of 3,675 ft. The water temperature at the mudline was 38-40°F, while the bottom-hole temperature was 165!F. At an average flowline temperature of 68!F, the funnel viscosity measurements ranged from 55 sec/qt with a 9.1 ppg fluid to 71 sec/qt with a 13.1 ppg fluid at total depth. The yield point was generally 12-20 lbs/100 sq ft. The gel strengths remained flat throughout the well: typically 8/9 in the 17 1/2-in. hole, and 11/13 in the 12 1/4-in. hole. The pressure-while-drilling (PWD) log indicated that actual pressure losses while drilling did not exceed the values required to protect hole stability. Data from an offset well, where the low viscosity EBM was also tested, indicated the hydraulic parameters typical of the system.
The thinners used with the low viscosity EBM system prevented the fluid from thickening beyond an acceptable range. Each thinner is designed to meet a specific temperature-related need. One addresses the effects of cold temperatures, while the other helps the fluid adapt to both hot and cold temperatures. To make cuttings removal more efficient in this high-angle hole, Kerr-McGee implemented a sweep program, alternating downhole sweeps every two stands drilled and also sweeping the riser every five stands. The sweeps resulted in approximately 30% cuttings increase over normal returns. Prior to running casing, a 200-bbl low viscosity sweep was pumped, followed by a 200-bbl high viscosity sweep.
As the well approached production zone depth, a wiper trip was made to help ensure the wellbore was in good condition. The mud weight was raised to 12.8 ppg, then 13.1 ppg to minimize drag and reduce background gas readings. A final sweep at this point showed that the hole was clear of cuttings. Three logging runs were made with no problems. Only 16 bbl of mud were lost while running the final casing string. The cement bond log showed cement placement was satisfactory.
Lower maintenance costs
The low viscosity ester-based mud was run at lower ester/water ratios than most SBMs, which are usually run at around an 80/20 oil/water ratio, generating less oil-on-cuttings, better environmental protection, lower initial cost per barrel of fluid, and lower maintenance costs for the active system. Even at the 70/30 ratio, the low viscosity EBM proved to be extremely lubricious and helpful in reducing the risk of stuck pipe.
The drilling fluids specialists on the well reported that mud dilution and maintenance costs could be reduced by upgrading the solids control system on the rig. The shakers and mud cleaners were undersized for the pump rates and volumes of mud required to drill the well; the shaker screen mesh size could not be reduced sufficiently to do an efficient cleaning job, which led to underperformance with the mud cleaners. However, there was no apparent need to add extra processing equipment not normally found on offshore rigs.
The East Breaks drilling program allowed for 15.5 drilling and logging days (excluding flat time related to running and cementing casing). The actual number of days to accomplish this was 14, thus reducing cost estimate. The wellbore in the 12 1/4-in. hole – the production interval – was noted to be in particularly good condition.
Viable in deepwater
As a result of this extensive field trial, the low-viscosity, ester-based fluid has proved it can be viable in deepwater drilling conditions. At present it can be formulated at densities up to 17.5 ppg, and, as mentioned above, can be run at lower ester/water ratios than are normally required in other synthetic-based fluids. It can remain stable at temperatures up to 320°F. Cold temperature rheological properties can be controlled within clearly acceptable ranges with the use of the recently developed fit-for-purpose thinners. These attributes, along with its environmental advantages, make low viscosity ester-based fluid a strong contender for use in the Gulf of Mexico and elsewhere.
Doug Hyatt is the Global Product Champion for Non Aqueous Fluids at Baroid Drilling Fluids in Houston. His technical expertise is in the area of mud technology with emphasis on Non Aqueous Fluids.
Vic Estes is a Drilling Engineering Advisor in the Worldwide Deepwater Drilling group of Kerr-McGee Oil & Gas Corp. Estes has most recently been the project-drilling engineer for Kerr-McGee
Philip J Vice is a technical professional in the Gulf of Mexico for Baroid, a Halliburton Company. Vice has recently been working in-house at Kerr-McGee Oil & Gas Corp. coordinating the drilling fluids, on Kerr-McGee