Summary
Oil-based drilling fluids are used extensively in drilling activities
worldwide. During the drilling process, because of overbalance pressure in the
mud column, the filtrate of oil-based mud invades the formation. This
hydrocarbon-based filtrate mixes with the formation hydrocarbon, which can
cause major difficulties in obtaining a representative reservoir-fluid sample.
Despite the recent improvements in sampling, obtaining a contamination-free
formation fluid is a major challenge, particularly in openhole wells.
Depending on the type and conditions of the reservoir, the oil-based-mud
filtrate is totally or partially miscible with the formation fluid.
Oil-based-mud filtrate dissolves completely in reservoir oil; therefore, the
captured sample contains the true reservoir oil with added filtrate. Gas
condensate (lean gas condensate in particular) is often not fully miscible with
mud filtrate. In this case, the mass exchange between gas condensate and mud
filtrate makes the sample unrepresentative of the reservoir fluid.
In this study, the impact of sample contamination with oil-based-mud
filtrate on different types of reservoir fluids, including gas condensate and
volatile-oil samples, is investigated. Two simple methods are suggested to
retrieve the uncontaminated composition from a contaminated sample in which mud
filtrate is totally dissolved in the formation fluid (i.e., reservoir-oil
samples). A tracer-based technique is also developed to determine the
composition of an uncontaminated reservoir-fluid sample from a sample
contaminated with oil-based-mud filtrate, particularly for those cases in which
the two fluids are partially miscible. The tracers are added to the drilling
fluid, with the additional cost to the drilling-mud preparation being
negligible. The capability of the developed techniques has been examined
against deliberately contaminated reservoir-fluid samples under controlled
conditions in the laboratory. The results indicate the reliability of the
proposed methods.
Introduction
Historically, most drilling in the North Sea has used water‑based muds;
however, drilling certain formations with water‑based muds can be difficult,
primarily because of the hole instability caused by the swelling of
water‑absorbing rock. Problems of this type can be greatly alleviated by using
mud suspended in an oil (rather than water) base. These oil-based muds also
provide better lubrication and achieve significant increases in drilling
progress (Davies et al. 1984). In recent years, oil-based drilling fluid has
been used extensively in drilling activities in the North Sea. During the
drilling process, because of overbalance pressure in the mud column, the mud
filtrate invades the reservoir formation. Using an oil-based mud in the
drilling, the mud filtrate can mix with the formation fluid. This can cause
major difficulties in obtaining high-quality formation-fluid samples. Depending
on the type and conditions of the reservoir, the mud filtrate can be totally or
partially miscible with the formation fluid. This can alter the composition and
phase behavior of the reservoir fluid significantly. Hence, the measured data
using the collected formation-fluid samples need to be corrected for the
contamination.
In this study, contamination of different types of reservoir fluids with
oil-based-mud filtrate, where the two fluids are partially or totally miscible,
is discussed. Practical decontamination techniques are proposed to retrieve the
original fluid composition from contaminated samples.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
11 March 2005
- Meeting paper published:
13 June 2005
- Revised manuscript received:
4 August 2006
- Manuscript approved:
10 October 2006
- Version of record:
20 February 2007
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