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Abstract
Ultra high temperature, high pressure (uHTHP) conditions have a different
definition depending on the region and the operator and Service Company. In
this paper the definition used for uHTHP fluid performance is that of a fluid
able to perform above 500°F and 30,000 psi. This paper describes the
development of innovative drilling fluids specific to these well
conditions.
When bottomhole temperatures exceed 400°F, the design and engineering of
drilling fluids can be challenging. Drilling fluids that destabilize can cause
a variety of fluid control problems that could lead to drilling and completion
issues. With Invert emulsion fluids, the major challenges encountered under
these conditions are related to the thermal degradation of the emulsifier and
wetting package that can lead to gelation and syneresis. Another challenge is
fluid loss which is related to the emulsion stability and to the degradation of
the fluid loss control additives. Finally, efficient control over the
rheological properties – critical to the success of any well - can also be
challenging, where effects from emulsion instability, filtration control
degradation and rheology control additive degradation are coupled with
increases in drilled solids, rapidly leading to rheological instability. This
can manifest itself as high fluctuating rheologies and gelation, or loss of
rheological properties that can give rise to sag of weight material, both
potentially leading to associated well control problems.
The paper describes the development of the new fluid system designed for such
uHTHP environments, highlighting the chemical differences and compares the test
data of the system with more conventional HTHP invert emulsion fluids. Data is
presented showing the stability and performance of the new fluid over extended
exposure to temperature >500oF, demonstrating tolerance to various
contaminations and showing the rheological behavior and stability to 600oF and
40,000 psi.
Introduction
Over the last decade, many service companies have addressed the need of new
drilling fluid systems that could match the advances in drilling technologies,
namely high temperature and high pressure (HTHP) or extreme environments that
require fluids with special performance from superior suspension properties to
minimize or eliminate barite sag, additives to minimize subsurface losses,
control over the potential for differential sticking and generally fluids
designed to improve wellbore stability. The narrow operational window between
pore pressure and fracture pressure that is often a feature of operating in
these extreme environments requires drilling fluids designed specifically to
minimize ECD (Equivalent Circulating Density). Selecting the proper additives
for the extreme environment becomes critical for appropriate rheological
properties and fluid loss control. A multitude of synthetic polymers come to
mind for their applications, however the utilization of those in the conditions
under study showed limitations, the same reasoning goes for the commonly used
polyamide and amido-amine based emulsifiers which have limitations on
performance as temperatures exceed 450°F. The main characteristics of oil-based
muds (OBM), i.e. high lubricity, low fluid loss, stability in adverse
conditions and thin filter-cake, make them particularly suitable for HPHT
wells.
Generally oil or synthetic based invert emulsion drilling fluids show superior
temperature stability when compared to water based drilling fluids. Oil based
drilling fluids also typically exhibit a lower coefficient of friction and
provide a thinner, more lubricious, filter cake and are therefore the right
choice for these extreme applications, where longer open hole sections, and
areas of pipe contact prevail.
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