High biodegradability and relatively low toxicity have long made esters universally recognized as the best base fluids for synthetic-based muds in regards to environmental performance. A major limiting factor in the use of ester-based fluids, particularly in deep water, is the inherently high kinematic viscosity, a condition that is magnified in the cold temperatures encountered in deepwater risers. These higher viscosities are believed to be especially critical in deepwater wells where lack of overburden causes a severely narrowed window between pore pressures and fracture gradients. Other implications of these higher viscosities include limitations on oil/water ratios, mud weights, and drill solids tolerance. A new low viscosity (LV) ester has been developed which overcomes these limitations while maintaining the significant environmental advantages of the original esters. The new LV ester has a kinematic viscosity nearly equal to that of currently used internal olefins. It allows formulations of drilling fluids that can be used effectively in virtually all drilling applications in the Gulf of Mexico (GOM), including deep water. The GOM test protocol was developed to prove the viability of the LV ester as a base fluid. This protocol was designed to consider all conditions drilling fluids are expected to encounter during the course of a well. It evaluates the temperature stability, low temperature rheological properties, and the contamination tolerance of drilling fluids formulated with an LV ester base fluid. Three LV ester fluids were subjected to this protocol: an 11.0 ppg mud; a 14.0 ppg mud; and a 16.0 ppg mud. Results from this extensive laboratory testing indicate new LV ester-based fluids have overcome previous limitations, exhibiting exceptional cold temperature rheological properties, the ability to use a wide range of mud weights and oil/water ratios, and a high tolerance to contamination. In July 2000, an LV ester-based drilling fluid was used to drill a 3651 ft. interval of a GOM well in 3,669 ft. of water. The LV ester fluid performed well in comparison to the same interval drilled on an offset well with an internal olefin fluid. The initial results from that field trial are included in this paper. Development of Esters In 1985, development began on a fully biodegradable base fluid at the request of operators facing restrictions on the use of and discharges from conventional oil base fluids. Esters were found to be the most suitable naturally derived base fluids in terms of potential for use in drilling fluids, being exceptional lubricants, and showing low toxicity and a high degree of both aerobic and anaerobic biodegradability. Ester fluid provides similar shale stabilization and superior lubricity to mineral oil-based mud, and yet also satisfies environmental parameters. The first trial of the conventional ester-based fluid in February 1990, took place in Norwegian waters and was a technical and economic success. Since then, nearly 400 wells have been drilled worldwide using this C12-C14 ester-based system (Table 1). The release of ester-based fluids into the global drilling fluids market initiated the era of synthetic-based invert drilling fluids. Following the success of esters, other drilling fluids classed as synthetics were formulated, but these synthetics have not matched the environmental performance of ester-based drilling fluids. One of the most important criteria that they have failed to meet is that of biodegradability; additionally, some have been rejected as a result of poor eco-toxicological or technical performance. Drivers for Low Viscosity Ester A low viscosity base fluid was required to give operators the choice of using a system that fulfils more technically demanding requirements than existing ester-based systems, with a particular value in the deeper, colder environments increasingly explored today. The conventional ester is a C12-C14 ester manufactured from palm oil. Research demonstrates that C8 esters have lower viscosities than their conventional equivalents over a wide temperature range. This led to the development of a lower viscosity C8 ester base fluid.
Summary Diesel-based spotting fluids have historically been used to free differentially stuck pipe. Increasing environmental regulatory pressures have prompted the development of environmentally acceptable water-based spotting fluids. In spite of their widespread application, these new fluids have had only sporadic success in freeing differentially stuck pipe in the field. An extensive laboratory research project was initiated to identify potential chemistries that could offer increased performance and maintain environmental acceptability. Literature reviews revealed that previous spotting-fluid research had focused mainly on the spotting fluid providing lubrication between the drillpipe and mud filter cakes. Laboratory data also revealed that the best preforming conventional spotting fluids were highly toxic to mysid shrimp. In accordance with the new findings, a two-phase spot was developed. This paper details a field application that validated the concept of the two-phase spot, and discusses its method of application. Introduction Diesel oil has historically been considered capable of promoting fast filtration through a clay-based filter cake and lubricating the drillpipe. The environmentally safe lubricant-type spotting fluids1–6 were observed in laboratory tests to be good lubricants, but were found to filter slowly through water-based filter cakes. This new research showed that when salt solutions with a low activity coefficient were combined with environmentally-safe lubricants, low torque levels were produced in differentially sticking tests. These findings prompted the re-evaluation of differential-sticking theory, and led to the development of a two-phase spot for freeing differentially stuck pipe. Previous Research A brief review of previous research is helpful in better understanding the filtration and lubrication requirements of a successful water-based spotting fluid. Helmick and Longley7 have shown that both the differential pressure and adhesion between the filter cake and drillpipe affect the magnitude of the pull-out force needed to free a stuck pipe. Some scientists believed that the filter cake should crack when exposed to the spotting fluids. No conclusive evidence, however, has been reported indicating a strong correlation between filter-cake cracking and the final force needed to free the stuck pipe. Outmans8 developed a mathematical model describing differential sticking. In developing the model, he assumed that "oil spotting fluids" are virtually impermeable to clay-based filter cakes. The spotting fluid replaces the void volume between the filter cake and drillpipe created by the dewatering of the filter cake under a differential pressure. Ayers and O'Reily9 saw differences in performance of the different oil-based spots. They reported that 51% of their wells were freed with diesel-oil-based spotting fluids, while only 33% were freed with mineral-oil-based spots. Clark10 observed that low-density diesel-oil-based spotting fluids had the shortest release times for freeing stuck pipe. He also observed that five out of eight of the low-density (less than 12 lbm/gal) commercial water-based spots will free stuck pipe, but only after being exposed to the spotting fluid for a minimum of 20 hours. Bland11 emulsified a brine into an environmentally safe glycol to aid the penetration of the glycol lubricant through the filter cake. Test Equipment and Procedures A number of tests were run in the laboratory to evaluate the effectiveness of various chemical compounds. Environmentally safe candidates were first screened through a series of filtration tests, and then through a series of differential-sticking tests. The data collected for the various chemical compounds were compared to similar data collected for diesel-based spotting fluids. The two fluids with filtration and lubricating characteristics most similar to diesel were then field tested. Bioassay Range Finder Test. Bioassay range finder tests were performed to catalog the environmental acceptability of the various candidate materials. Generic Mud 7 was treated with various additives at a concentration of 5 vol%. Mysid shrimp were then placed in a several seawater solutions mixed with the treated Generic Mud 7 at concentrations ranging from 1.8 to 32 vol%. API Low-Pressure Filtration Test. American Petroleum Inst. (API) low-pressure filtration tests were used to differentiate the penetration capabilities of the spotting fluid candidate materials through clay-based filter cakes. To build a filter cake for spotting-fluid screening purposes, a 16-lbm/gal seawater/lignosulfonate mud was filtered at 100 psi until 10 mL of filtrate was collected. This procedure ensured that each fluid was filtered through a filter cake with a thickness of 0.125 in. The seawater/lignosulfonate mud was decanted and 70 mL of the spotting-fluid chemicals were placed on the top of the previously deposited filter cakes. API filtration tests were then run for a 24-hour period at 100-psi differential pressure. The filtrate volumes of the various spotting-fluid mixtures were then recorded. Differential-Sticking-Test Device. Fig. 1 shows a schematic of the Fann® differential-sticking tester used to collect the data. The test device allows a filter cake to be deposited at room temperature and under differential pressures of up to 1,000 psi. Filter paper and a metal screen are placed in the device to hold the filter cake in place while measuring the torque. A torque wrench is attached to a metal disk with a 2-in. diameter and 0.625 in. thickness. The torque wrench used in the experiments can measure torque up to 300 (±5) lbf-in. Bioassay Test Data Bioassay data were analyzed to determine the effects of the various spotting fluids on mysid shrimp. Fig. 2 shows the relative toxicity that several organic liquids had on the mysid shrimp. Diesel oil, octanol, decanol, and methyl soyate killed all 20 shrimp when the shrimp were exposed to 3.2 vol% of Generic Mud 7 treated with 5 vol% of these additives. The relative toxicities of the other additives listed in Fig. 2 were much less acute. The less-toxic additives have higher molecular weight and decreased water solubility. Fig. 3 shows that calcium salts at all concentrations have LC50 values above the minimum 30,000-ppm suspended particulate phase (SPP) value required under current regulations for discharge in the Gulf of Mexico. When mixed with field muds, the calcium-brine/drilling-mud mixtures will have higher LC50 values than those shown in Fig. 3. Bioassay Range Finder Test. Bioassay range finder tests were performed to catalog the environmental acceptability of the various candidate materials. Generic Mud 7 was treated with various additives at a concentration of 5 vol%. Mysid shrimp were then placed in a several seawater solutions mixed with the treated Generic Mud 7 at concentrations ranging from 1.8 to 32 vol%. API Low-Pressure Filtration Test. American Petroleum Inst. (API) low-pressure filtration tests were used to differentiate the penetration capabilities of the spotting fluid candidate materials through clay-based filter cakes. To build a filter cake for spotting-fluid screening purposes, a 16-lbm/gal seawater/lignosulfonate mud was filtered at 100 psi until 10 mL of filtrate was collected. This procedure ensured that each fluid was filtered through a filter cake with a thickness of 0.125 in. The seawater/lignosulfonate mud was decanted and 70 mL of the spotting-fluid chemicals were placed on the top of the previously deposited filter cakes. API filtration tests were then run for a 24-hour period at 100-psi differential pressure. The filtrate volumes of the various spotting-fluid mixtures were then recorded. Differential-Sticking-Test Device. Fig. 1 shows a schematic of the Fann® differential-sticking tester used to collect the data. The test device allows a filter cake to be deposited at room temperature and under differential pressures of up to 1,000 psi. Filter paper and a metal screen are placed in the device to hold the filter cake in place while measuring the torque. A torque wrench is attached to a metal disk with a 2-in. diameter and 0.625 in. thickness. The torque wrench used in the experiments can measure torque up to 300 (±5) lbf-in.
Proposal Since the introduction of invert emulsion fluids in the 1960s, oil-based fluids (OBF) and synthetic-based fluids (SBF) have been formulated with a similar group of components: base oil, organophilic clay and lignite, lime, CaCl2 brine, and emulsifier. The family of invert emulsion fluids has remained closely related in terms of mud properties and performance expectations. In 2001, an SBF formulated entirely without commercial clays or lignites was introduced in the Gulf of Mexico (GOM). Rheological properties are controlled through the emulsion characteristics, a radical departure from accepted solids suspension mechanisms. The behavior of this unique fluid has changed perceptions about what constitutes "good mud." The clay-free, emulsion-based fluid (System) has consistently prevented detectable barite sag on 80+ wells drilled with it to date. Based on observed fluid densities after long static periods (an 8-day logging run in one case) and verified by modular dynamic test (MDT) log data on numerous high-angle wells, the fluid's unique emulsion structure and wetting characteristics prevent settling of barite and other solids. Attempts to control barite sag with conventional clay-based SBFs have produced mixed results. A 14.0-lb/gal SBF treated with sag-prevention organophilic clay showed a 0.3-lb/gal density reduction in the initial return flow and a 0.3-lb/gal density increase in the tail flow after a 52-hr logging run. This 0.6-lb/gal variation in density was deemed "manageable." However, the treatment resulted in a 20% increase in the funnel viscosity at 75°F, demonstrating an adverse effect on the rheology after increasing the clay concentration by only 0.2 lb/bbl. At lower temperatures encountered at the sea floor, this effect would be amplified. In addition to preventing barite sag, the System has provided other important field-documented performance advantages:Whole mud losses reduced by an average of 60% while drilling, running casing, and cementing (with 80% reductions reported on several deepwater wells).Significantly lower ECDs, validated by pressure-while-drilling (PWD) data.High, flat gel strengths that break with minimal initiation pressure, validated by PWD data.Highest standard of compliance with environmental regulations governing GOM operations. The System is currently under consideration for use offshore West Africa, offshore Brazil, and the Asia-Pacific region. Introduction Since the introduction of invert emulsion fluids in the 1960s, OBFs and SBFs have been formulated with a similar group of components: base oil, organophilic clay and lignite, lime, CaCl2 brine, and emulsifier.1 The family of invert emulsion fluids has remained closely related in terms of mud properties and performance expectations. The invert emulsion fluids were developed to help maximize rates of penetration (ROP), increase lubricity in directional and horizontal wells, and minimize wellbore stability problems such as those caused by reactive shales.2 Until operators began drilling in deepwater locations, where the pore pressure / fracture gradient (PP/FG) margin is often very narrow, the standard formulations provided satisfactory performance. However, the issues raised by deepwater drilling and changing environmental regulations led to a closer examination of several seemingly essential additives. Organophilic Clay The most widely used primary viscosifier for OBFs and SBFs is organophilic clay. Organophilic clay is bentonite that has been treated with an amine to make it yield in oil. However, organophilic clay requires significant shear and circulating time to yield fully. Overtreatment resulting from this delayed response often causes excessive viscosity, and the problem is compounded when the fluid is at ambient temperature or worse, is exposed to cold temperatures at the seabed in deepwater locations.
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