Summary
A uniquely designed water-based mud (WBM) was used in two shelf areas of the
Gulf of Mexico to drill highly reactive gumbo sections. The drilling
performance and inhibition in these shale intervals resembled that of
synthetic-based muds (SBMs) previously used in offset wells. Owing to new
environmental regulations, obtaining the drilling-performance characteristics
of a synthetic-based fluid and the stability of a water-based fluid is a high
priority for Gulf of Mexico operators.
A special characteristic of the WBM fluid is its ability to deliver SBM-like
shale inhibition without use of any salts, through an ideal combination of a
polyamine shale inhibitor, a polymeric encapsulator, and an antiaccretion
additive.
This paper presents field data comparing the drilling performance and the
inhibition in gumbo-shale sections of the new WBM and the previously used SBM.
The combination of this inhibitive WBM and the rotary-steerable tool produced
an 85% higher rate of penetration (ROP) than in similar wells in the area
drilled with SBM and mud motors. No accretion or bit balling was noted on
bottomhole assemblies (BHAs) during bit runs, and the cuttings drilled were
well encapsulated and protected from hydration. Economic savings were realized
by reducing the drilling time and by not requiring the essential
waste-management equipment and engineering associated with SBM-cuttings
discharge. Furthermore, the new high-performance WBM was easily prepared at the
wellsite with seawater, which also translated into cost savings by reducing the
volumes handled by the workboats.
These field applications demonstrate that this uniquely designed WBM can
effectively replace invert-emulsion fluids in many highly reactive shale
sections without sacrificing drilling performance or increasing environmental
risk.
Introduction
Drilling the gumbo-shale sections in the Gulf of Mexico has always been
challenging. When a WBM is used to drill this section, problems such as
severe bit balling, stuck pipe, and plugging of the flowline with large pieces
of soft shale are not uncommon. The severe washout associated with the lack of
shale inhibition may also lead to ineffective cementing of the casing
strings.
In the past 5 years, the drilling problems encountered prompted more and
more operators to drill these sections with synthetic-based drilling fluids.
However, recent changes in environmental regulations impose a limit of 6.9%
retention-of-oil-on-cuttings (ROC) for olefin SBM and 9.4% for ester SBM. This
translates into the use of cuttings dryers before discharging the cuttings into
the ocean and also running daily compliance testing and reporting. The hole
size of the intermediate gumbo sections may vary from 14 to 17 in., generating
a significant amount of cuttings to be processed.
A highly inhibitive water-based mud (HIWBM) was designed to provide near-SBM
drilling performance without the limiting aspects of environmental
compliance.1–5 This fluid has a very simple formulation and can be mixed
in fresh water, seawater, or NaCl brine, up to saturation. A seawater-based
HIWBM was used in two shelf applications in the Gulf of Mexico with very good
drilling performance and wellbore stability, comparable to offset wells drilled
with SBM.
© 2005. Society of Petroleum Engineers
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History
- Original manuscript received:
27 January 2004
- Revised manuscript received:
19 October 2004
- Manuscript approved:
22 November 2004
- Version of record:
15 March 2005
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