Improving the Working Environment and Drilling Economics Through Better Understanding of Oil-Based Drilling Fluid Chemistry

Abstract

Summary The relationships between oil-based drilling fluid composition and the associated vapors have been quantified. The study documents the long- and short-term changes in base oil selection and fluid compositions. The study is based on the generation and assessment of data collected from the vapor emissions of oil-based drilling fluids with varying compositions and at a range of temperatures. The purpose of the study was to enable a better understanding of the potential working environment hazards when using mineral oil-based drilling fluids. One result of this understanding is that new rig construction designs and older rig upgrades can be performed more informatively. This will reduce the repetition of continual modification costs. Introduction Controlling the amount of vapor in the workplace is important for occupational health and safety reasons. It is commonly believed that most vapors in the working areas come from the mineral base oil of the drilling fluid. This is due to the base oil being the greater volume of the drilling fluid, generally in excess of 50%. Studies have shown this belief to be erroneous. The total hydrocarbon vapor from oil-based drilling fluids also includes vapor contributions from drilling fluid additives and possibly from hydrocarbon-bearing formations which have been drilled with the fluid. This means that when formulating a suitable drilling fluid it is important to minimize those components in the formulations that could be hazardous. Generally it is the lighter hydrocarbon components that are most hazardous to personnel. The traditional approach to minimizing the concentration of organic vapor in the atmosphere has involved improving the characteristics of the base oils. However, field studies have shown that reducing the vapor concentration in the headspace above the pure base oil, from 100 parts per million (ppm) to 10 ppm, for example, does not necessarily lead to the same percentage reduction of vapor in the work place atmosphere issued from the whole drilling fluid (see the Appendix). The term headspace refers to the vapor phase associated with and in equilibrium with a respective substance or blend of substances, liquid and/or solid. A brief field history is given to support the reasons for the continued use of oil-based drilling fluids by the operator. In addition, the driving criteria for ongoing working environment studies are discussed. Phillips Petroleum Company Norway (PPCoN) had used water based drilling fluids for drilling operations until the early 1990's. Drilling down to the chalk reservoir has been through tertiary clay formations primarily of the Miocene, Oligocene and Eocene ages. The clay formations have pore pressures of up to 1.68 specific gravity fluid weight equivalent. The clays are very reactive and have a smectite bentonitic content of up to 35%. These characteristics provide very unstable drilling conditions in the presence of aqueous drilling fluids. Subsidence of the depleted reservoir sections, most notably in the Ekofisk field, has added to formation instabilities especially when using water-based drilling fluids. The drilling of wells with increasingly greater deviations and more complex well paths necessitated the use of nonaqueous synthetic oil-based drilling fluids to stabilize the exposed formations being drilled. Since 1990 there has been wider and more successful use of these fluids. However the Norwegian state pollution control authorities and the European pollution authorities were not entirely comfortable with the use and discharge of synthetic oil-based drilling fluids. Consequently PPCoN moved towards an increased use of mineral oil-based drilling fluids. This practice was coupled with the reinjection of cuttings and of contaminated oil-based drilling fluid. This form of drilling operation has eliminated any negative disturbance of the marine environment since there is a no discharge to the sea. PPCoN has also been challenged on two strategy fronts for its intention of using mineral oil-based drilling fluids. These challenges have been from the following.The Norwegian Petroleum Directorate (NPD) regulatory controlling document, "Quality assurance when documenting chemical hazards to health and environment."An item of concern resulting from an NPD audit of the 2/4X platform, a new platform purposefully designed in 1994 to use oil-based drilling fluids. The item reads, "… The NPD would like to point out that choosing oil-based drilling fluids is controversial, from a working environment point of view, and would like to emphasize that the selection of equipment and ventilation systems must be thoroughly evaluated." The hazards for personnel of uncontrolled exposure to the vapors and mists from oil-based drilling fluids includes impacts on the central nervous system such as dizziness, tiredness, headaches and nausea, and conditions such as pneumonia, bronchitis, bronchial asthma and possibly cancer.1 The seriousness is emphasized knowing that some mineral base oils can evaporate at a rate of 1 vol%/10 h at 70°C.1 This temperature is not uncommon for a drilling fluid during drilling operations. Headspace Measurement Observations From Laboratory Studies In this section some of the possible sources of vapor emissions from oil-based drilling fluids are described and alternative ways of reducing them are discussed. The concentration of vapor in the headspace of a diesel base oil is typically 100 ppm but may be as high as 1,000 ppm at 80°C. By comparison the best of the more recent generation base oils with low headspace characteristics have maximum headspace concentrations of less than 30 ppm at 80°C. It is also important to note that the vapors given off by a diesel base oil, for example, contain more toxic compounds such as polycyclic aromatics than the more highly refined base oils described herein.

Occupational exposure to airborne contaminants during offshore oil drilling

“…Previously reported oil mist and vapour concentrations are, in contrast, oen based on samples collected to assess compliance with limit values or to document effect of technical control measures. 5,7,11 Exposure to oil mist and oil vapour in this study was low when compared to occupational exposure limits (OEL) in Norway of 0.6 and 30 mg m À3 (12 h TWA), respectively, as well as the OEL of 5 mg m À3 for inhalable oil mist from rened mineral oil (8 h TWA) recommended by the Scientic Committee on OEL, European Commission of Employment, Social Affairs and Inclusion. [21][22][23] Studies addressing possible health effects due to inhalation of oil mist and oil vapour exposure during oil drilling are lacking.…”

“…2 Although oil companies have their own surveillance programs, scientic publications presenting data on occupational airborne exposure during oil drilling are sparse. [3][4][5][6] These publications have shown that the total hydrocarbon air concentrations based on personal samples collected from the North Sea oil elds, are ranging from 10-200 mg m À3 at the drill oor and from 20-450 mg m À3 in the shaker area in the late 1980s. Later studies indicated oil mist air concentrations below 1 mg m À3 in personal samples.…”

Synthesis and application of sodium 2‐acrylamido‐2‐methylpropane sulphonate/N‐vinylcaprolactam/divinyl benzene as a high‐performance viscosifier in water‐based drilling fluid

“…The structure of SDKP was characterized by FT-IR, 1 H-NMR, elemental analysis, and GPC. The optimal SDKP was obtained under the optimum reaction conditions: initiator concentration (by weight of monomer) 5 0.20%, reaction temperature 5 65 8C, mole ratio of NVCL/AMPS/DVB 5 60.0:38.0:2.0, SDS concentration (by weight of monomer) 5 2.0%, and pH value 5 7.…”

“…1,2 However, as the move was made into high-temperature, high-pressure (HTHP) wells, a main challenge with water-based drilling fluid was related to the thermal degradation of drilling fluid additives, which could lead to rheological instability, filtration-control degradation, etc. 1,2 However, as the move was made into high-temperature, high-pressure (HTHP) wells, a main challenge with water-based drilling fluid was related to the thermal degradation of drilling fluid additives, which could lead to rheological instability, filtration-control degradation, etc.…”

Application of a new family of organosilicon quadripolymer as a fluid loss additive for drilling fluid at high temperature

“…At extreme temperatures, special formulations and systems must be used 12–14. Since the 1980s, environmental legislation has increased restrictions on the use of oil‐based fluids, which possess high thermal stability, and progressively new water based formulations presenting improved properties for specific applications at high temperature are required 15–18. To avoid such problems, effective additives (especially fluid loss additives) are used to stabilize water‐bentonite suspensions at high temperature 19–24…”