Low-Toxicity Oil Muds: A Knowledge of Downhole Rheological Behavior Assists Successful Field Application
- T.J. Bailey (British Petroleum Co. Plc.) | P.A. Bern (British Petroleum Co. Plc.) | F. McEwan (BP Petroleum Development Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- April 1986
- Document Type
- Journal Paper
- 107 - 114
- 1986. Society of Petroleum Engineers
- 1.6.3 Drilling Optimisation, 6.5.3 Waste Management, 4.1.2 Separation and Treating, 1.11 Drilling Fluids and Materials, 1.6 Drilling Operations, 5.3.4 Integration of geomechanics in models, 2.2.3 Fluid Loss Control, 4.1.5 Processing Equipment, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.5 Drill Bits
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Summary. The introduction of oil-based drilling fluids has led to significant improvements in drilling efficiency for many North Sea operations. The original formulations that were based on diesel are generally being replaced by the lower-toxicity mineral oils. One factor crucial to the successful application of mineral oils has been a knowledge of their downhole rheological characteristics. This paper describes a purpose-built viscometer capable of simulating realistic downhole purpose-built viscometer capable of simulating realistic downhole temperature (300C [572F]) and pressure (100 MPa [1,000 bar]) conditions. Results from this study are used to indicate how surface-fluid rheologies can be tailored to provide adequate downhole viscosities, thus reducing total system pressure losses.
Until Spring 1982, most of our North Sea wells were drilled with water-based muds. KCl/polymer muds proved to be particularly successful for drilling the Tertiary "gumbo-like" mudstones overlying the Forties reservoir. The successful use of these muds proved to be time-dependent, however, especially when drilling highly deviated wells. Any delays in drilling operations often resulted in significant wellbore instability problems. The development of low-toxicity oil muds provided the opportunity to use invert-oil-emulsion drilling muds on development, exploration, and appraisal wells. We currently use low-toxicity, invert-oil-emulsion drilling muds that are based on a low-toxicity mineral oil. Oil muds based on this low-toxicity oil have been used recently for developmental drilling on Forties, Magnus, and several exploration wells in the central and northern sectors of the North Sea. Drilling fluids engineers recognized the potential of using the British Petroleum (BP) Co.'s low-toxicity mineral oil as a base oil for invert-oil-emulsion drilling muds in 1981. Vastly improved drilling rates could be achieved with the combination of oil muds and the new polycrystalline diamond compact (PDC) bits. Also, the U.K. government's acceptance of using low-toxicity oil muds without cuttings cleaning equipments justified the decision to use oil-based muds. This development was of particular significance to our North Sea operations because of the presence of thick, reactive shale sequences in the central and northern sectors. A full screening program was undertaken to determine whether the low-toxicity mineral oil was an acceptable alternative to diesel oil. Compared with fluids based on diesel oil and other low-toxicity oils available in 1981, the muds formulated with our low-toxicity oils were more rheologically stable in the presence of contaminants and were more versatile.
The rheological behavior of oil muds under downhole conditions must be known if their full potential is to be reached. Problems with higher-than-anticipated circulating pressures have been experienced and have resulted in loss of drilling efficiency. Previous work in the area of high-temperature/high-pressure (HT/HP) rheology of oil muds has illustrated their behavior under downhole conditions. These studies, however, show that the nature of the base oil affects the degree of viscosity change caused by temperature and pressure. Furthermore, most previous work has been conducted on muds formulated previous work has been conducted on muds formulated with diesel as their base oil. De Wolfe et al. have published data on certain low-toxicity muds and have published data on certain low-toxicity muds and have concluded that each specific system needs to be assessed individually. To investigate the reasons for the higher circulating pressures experienced with oil muds in the field, the pressures experienced with oil muds in the field, the downhole rheological behavior of our mineral-oil-based muds has been examined with a purpose-built viscometer.
The main advantage of using invert-oil-emulsion drilling fluids in the North Sea is the improved drilling of the long, reactive, gumbo-like claystone shale sequences. Significant improvements in hole stability and drilling rates have been seen compared with inhibited water-based muds, especially when drilling deviated wells. Our experience with oil muds in the North Sea was limited. Diesel-based muds were used to drill one or two wells before the introduction of low-toxicity oils. The cuttings wash systems available at that time, however, were found to be ineffective.
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