Prediction of Rheological Behavior of Ester-Based Drilling Fluids Under Downhole Conditions
- Terry Hemphill (Baroid Drilling Fluids)
- Document ID
- Society of Petroleum Engineers
- International Petroleum Conference and Exhibition of Mexico, 5-7 March, Villahermosa, Mexico
- Publication Date
- Document Type
- Conference Paper
- 1996. Society of Petroleum Engineers
- 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.6 Drilling Operations, 1.11 Drilling Fluids and Materials, 4.1.2 Separation and Treating, 4.1.5 Processing Equipment
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The accurate prediction of drilling fluid rheological behavior under downhole conditions has become increasingly important, especially on highly-deviated and extended-reach wells drilled with invert emulsions. On these wells, hole cleaning and cuttings suspension parameters should be maintained within optimum ranges for best results, and firm control over unnecessary increases in annular pressure losses and Equivalent Circulating Densities (ECDs) must be exercised. Rather than calculate downhole hydraulics using surface data, more accurate simulation of fluid properties downhole should be used.
Rheological tests that simulated field conditions were run in the laboratory on an ester-based drilling fluid from the field. The rheological behavior of the fluid was tested under varying ranges of temperature, pressure, and ester/water ratios. A predictive downhole rheological model of the ester-based drilling fluid was constructed using over eight hundred (800) fluid viscosity measurements. Using the surface rheological measurements and downhole temperature and pressure, the computer algorithm rapidly calculates fluid properties under the specific downhole conditions.
As a general rule, invert emulsion fluids exhibit much greater fluctuations in rheological behavior with temperature and pressure than do water-based drilling fluids. The thermal expansion and compression of water is much less than that of commonly-used base oils, esters, etc. Hence, drilling fluids formulated with base oils and esters will have properties that can vary widely under downhole conditions, something that has major implications for the accurate calculation of mud hydraulics and equivalent circulating densities.
Before discussing the work that forms the basis of this paper, it is helpful to review the literature with regard to predictions of downhole rheological behavior of invert emulsion drilling fluids. In general, researchers have found that fluid viscosities of invert emulsions decrease with increasing temperature and increase somewhat with increasing pressure. In an early work by McMordie et al, different viscosity profiles were identified for diesel-based inverts containing oil-wet inorganic solids as viscosifiers and inverts containing large amounts of asphaltic materials. Two sets of coefficients for temperature and pressure were required for a specific mud, one set valid for shear rates above 191 sec-1 and the other set for shear rates below 191 sec-1.
In a later work on the subject, muds formulated in the laboratory and field muds were tested under elevated temperature (100-350 F) and pressure (0-10,000 psi) conditions. Fluid rheological measurements were made on fluids having different mud densities and oil/water ratios. The authors noted that fluid viscosities changed the greatest with temperatures between 100 F and 200 F.
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