Ammonia-oxidizing microorganisms compete with phytoplankton for reduced nitrogen in the euphotic zone and provide oxidized nitrogen to other microbes present in the sea. We report 15 NH z 4 oxidation rate measurements made at 5-20-m resolution using an in situ array and quantification of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in corresponding samples from the upper water column and oxygen minimum zone (OMZ) of the Gulf of California (GOC) and eastern tropical North Pacific Ocean (ETNP). 15 NH z 4 oxidation rates varied substantially with depth and between stations: they were greatest at the base of the euphotic zone, and maximum rates were up to 28-fold greater than rates measured within 5-10 m. Pyrosequencing and quantitative polymerase chain reactions (QPCR) indicated that AOA were present throughout the water column at all latitudes and always outnumbered AOB. AOB constituted only 39 of 432,240 16S ribosomal ribonucleic acid gene sequences produced via pyrosequencing but were more abundant at greater depths and higher latitudes. 15 NH z 4 oxidation rates were correlated with AOA abundance at some stations and were detectable in 96% of samples, including depths where oxygen concentrations were , 5 mmol kg 21 and depths within the euphotic zone, where up to 42% of ammonia oxidation occurred. Ammonia is rapidly oxidized within discrete depth intervals in the GOC and ETNP; while pyrosequencing and QPCR demonstrate that AOB are confined to deeper portions of the water column, AOA appear to be active within the euphotic zone, where they may quickly respond to nitrogen inputs.

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Silicate drilling fluids have been used for more than 60 years. While the early versions of silicate-base systems proved very inhibitive, controlling mud properties was difficult because proper polymer additives were not available for maintaining rheology and fluid loss control. Furthermore, the solids control equipment available at the time was ineffective in removing solids. During the last decade, silicate-base systems have been re-engineered to provide a state-of-the-art and environmentally acceptable, water-base system that provides the inhibition approaching that of an invert emulsion field. The proposed mechanism of inhibition is the surface adsorption and chemical reaction of the silicate polymer with the formation surface. This "coating" effectively provides a thin pressure and chemical barrier on the surface of the wellbore. This formation/silicate interaction has raised a number of concerns regarding both the lubricity and the formation damage potential of the fluid. This paper investigates systematically both the lubrication characteristics of the fluid and the effect of silicate-base systems on formation production and evaluation. The results show promise in expanding the application of silicates as reservoir drill-in fluids and in the drilling of critical high-angle wells where lubricity is a primary design criterion. The data obtained in this study utilizes purpose-built lubricity and formation damage equipment to critically look at the problems. From this investigation the authors will reveal that the coefficient of friction of silicate fluids, as observed in the laboratory and measured in the field, does not differ significantly from that of other water-base fluids. Furthermore, the thin physical and chemical barrier formed during the reaction of the silicate polymer with the formation surface prevents fluids and fine particles from migrating into the formation, thereby minimizing damage to the producing zone. Introduction Silicate-based drilling fluids are probably the most misunderstood systems in the industry. Since the introduction of sodium silicate fluids in the the 1930's, silicate chemistry has demonstrated outstanding shale and formation inhibition characteristics. However, rheology and filtration properties had been difficult to control. Over the past few years, the development of modern polymer technology provided fluid design tools that proved effective in controlling both the rheological and filtration properties of silicate systems. Complementing advanced polymer technology was the design of new solids control and process technology that made silicate-base products effective well-bore stabilizing agents. Improvement in inhibition not only contributes to wellbore stability but also to minimizing dilution volumes, minimizing costs. The low dilution rates and toxicity of the system also provide an excellent fluid with minimal environment impact. In the 1980's, Wingrave's research on shale stability found that silicates, used in conjunction with the potassium ion and specific polymers, combined for an effective shale-stabilizing package.1 During the last decade the industry has effectively used sodium silicate in conventional polymer fluid formulations to provide an effective water-base shale stabilizing system.2,3,4,5 High-performance polymers and efficient solids removal equipment can now provide the required fluid performance and maintenance necessary for efficient and cost-effective silicate chemistry. Furthermore, its capacity to provide inhibition competitive with invert emulsion fluids makes silicate systems an ideal alternative for optimum wellbore stability, without the environmental limitations of an invert emulsion fluid.

Historically, the surface holes of the wells in the West Azeri field of the Caspian Sea were drilled conventionally with seawater and gel sweeps. However, seawater destabilized the highly reactive soil formations in the surface interval, resulting in unacceptable movement of the 20-in. casing. Further examination identified mechanical and chemical destabilization of the reactive shallow soils as the root causes of this instability. This paper describes the application of a silicate-based drilling fluid system1,2,3,4,5,7,8 in tandem with a unique Mud Recovery System (MRS) that combined to stabilize the problematic formation and allowed for the successful setting of the 20-in. casing without the lateral movement experienced in offset wells. The operator conducted laboratory tests to identify possible fluid alternatives that would provide the necessary chemical stability to provide support for the template and casing. The laboratory investigation considered conventional SBM, glycol/polymer/KCl and silicate/polymer/KCl inhibitive water-based fluids, and a KCl/NaCl/high-performance waterbased mud (HPWBM) as possible alternatives. The oil-based, HPWBM and silicate systems demonstrated excellent chemical stabilization. Offshore discharge restrictions negated the use of oil-based drilling fluids if the MRS was used. The HPWBM provided an alternative but logistics eliminated it from the initial consideration, thus opening the door for the first-time use of the silicate-based system in the Caspian Sea. The authors also review the application of the MRS system that utilized the rig pumps to deliver the fluid down the drill pipe and up the annulus to specially designed equipment on the sea floor. The latter used a sub-sea disc pump to transfer the fluid to the surface for conventional solids processing and maintenance. This technique provided a dual gradient to control the equivalent circulating density. Three field tests were conducted to determine the best fluid and mud weight for use in drilling the surface holes for the operator's template in the West Azeri field. The first test with the MRS unit used a conventional silicate/polymer/KCl system; the second test employed a synthetic-based drilling fluid with a pin-connector and a riser. A third trial utilized the silicate/polymer/KCl system with increased concentration levels of silicate and KCl and a higher mud weight. The authors will review the fluid planning, the performance of the three field trials, and the results which have been implemented into an ongoing field development program. Introduction The shallow soils in the West Azeri field are highly reactive and deformable; the pore fluid is nearly salt-saturated whereas the Caspian seawater is merely brackish. Initial drilling experience using conventional practice of seawater and high viscous sweeps resulted in severe washouts and pack-offs with eventual lost circulation. The initial template was abandoned due the unacceptable movement of the surface casings. In order to solve the instability problem, laboratory inhibition tests using Azeri soil samples were run to identify possible drilling fluid solutions. Inhibition tests in the laboratory showed the soil dispersed rapidly in seawater and various brines, thereby, identifying that salinity alone was not the solution.10 Geomechanical studies suggested that an increased mud weight was required in addition to chemical stabilization.9 The application of the mud recovery system (MRS) provided a solution to allow using a higher mud weight, minimizing the effect on the equivalent circulating density (ECD) at the shoe. The unit provides transfer of the fluid and solids from the seafloor to the surface via a sub sea disc pump. This technique provides a dual gradient condition resulting in ECD pressures lower than those observed with a conventional riser. Laboratory tests evaluated several inhibitive water-based alternatives including glycol, silicate and high-performance water-based muds (HPWBM). Caspian seawater and kerosene (simulating OBM performance) were used as reference benchmarks. Based on this data, as well as logistics contingency, a silicate system was selected for a trial evaluation in the Azeri field. The objective of the field trial was to set 30-in. conductor and drill a 24-in. hole without disturbing the surrounding formation. Additional criteria for success were satisfactory drilling performance, wellbore strength, and an in-gauge wellbore.

Two exploratory wells were recently drilled in the North Sea which used a specially designed polymer drilling fluid to provide optimum shale and borehole stabilization. The first well was a deep Jurassic well in the Frigg and Balder area of the Norwegian North Sea; the second well was drilled in the Ekofisk area. (Figure 1.) FIGURE 1SHALE STABILITY MECHANISM The mud for the reactive formation intervals was formulated to provide chemical stability for the reactive tertiary clays and claystones. Conventional fluid loss and rheology control polymers (polyanionic cellulose and xanthan gum, respectively) were used to achieve and maintain the required performance properties. Shale stability was obtained using a novel.additive composed of a polyvinyl alcohol polymer, potassium silicate, and potassium carbonate. Shale stabilization is based on a unique new theoretical mechanism which was initially postulated in SPE 16687, "New Chemical Package Produces an Improved Shale Inhibitive Water-Based Drilling Fluid System," and presented at the 62th SPE Conference, Dallas, Texas, 1987 (Wingrave et al). Further details are included in this paper. In the deeper intervals, high temperature polymer additives were used to maintain favorable rheological and filtration properties. An injection system was used to inject prehydrated liquid polymer concentrate directly into the suction. This procedure was used to obtain optimum polymer efficiency and performance in the management of the drilling fluid system. Drilling and drilling fluid performance data is presented for the 17.5", 12.25", and 8.5" hole sections. Mud weights from 10.0 to 16.0 ppg were required in the drilling program. Drilling and fluid performance data from the first well (Frigg and Balder area), designated Well A, is compared with an earlier offset well in the same block, Well B, which was drilled with a conventional KCl/PHPA/Biopolymer/PAC polymer system. Caliper data, borehole stability relative to exposure period, solids removal efficiency performance, wireline log quality, trouble-free casing and cementing operations, and minimal fluid-related hole problems while drilling - all provide supportive data indicative of superior chemical stabilization of the wellbore. U.S. and Norway toxicity results support its use as an environmentally acceptable water base additive for offshore and onshore application.