Summary
Oil-based muds (OBMs) have been developed to combat drilling problems caused
by shale hydration. This paper presents experimental data that show the factors
that control the movement of oil filtrate into a shale, as described by its
"entrance pressure." Although the oil filtrate of the OBM does not hydrate the
shale, it can penetrate the shale, increase the pore pressure, and cause
wellbore failure. It is of primary interest to understand this when troublesome
shales are drilled. It is also important to understand how factors such as the
emulsifier concentration in the OBM and the porosity of the shale affect this
entrance pressure.
The objective of this study is to determine and quantify the factors that
control the oil entry pressures of shales. For this purpose, five OBMs,
containing different concentrations of emulsifiers, water, and oil, were
studied.
The study was also intended to determine the effect of shale porosity on
entrance pressure. Data are reported for the Arco China shale samples having
porosities of 1.8 (native), 3.9, and 4.9%.
It was observed that for a given OBM water content, as the emulsifier
concentration increased, the required entrance pressure and the electrical
stability (ES) of the mud increased. Also, for shales that had higher
porosities and, therefore, larger pore throats, the oil entrance pressure
decreased.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
14 July 2008
- Meeting paper published:
21 September 2008
- Revised manuscript received:
30 April 2009
- Manuscript approved:
11 June 2009
- Published online:
28 January 2010
- Version of record:
11 March 2010
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