Summary
The paper summarizes a 10-year research program at the University of
Stavanger in borehole fracturing and mud design. Novel fracturing cells and mud
cells were built to better understand the mechanisms that lead to circulation
losses. Numerous experiments were conducted using both oil- and water-based
drilling fluids.
The paper presents a new mechanistic model for fracturing called "the
elastoplastic-barrier model." It is different from other recent models, and
it is verified with laboratory experiments. In simple terms, it defines optimal
barrier filtrate loss to place particles in the loss zone, and the mechanical
strength of the particles required to resist losses. Selected laboratory
experiments are presented demonstrating that borehole fracturing resistance can
be improved significantly by changing the mud composition.
While testing commercial lost-circulation-material (LCM) products, it was
found that some worked well, some were poor, and some worked only in synergy
with others. On the basis of these findings, the composition of an optimal LCM
pill will be presented. Nonpetroleum products also have been tested to search
for improvements in mud design. One result is that calcium carbonate can be
replaced with more-efficient materials. We also have shown that adding small
amounts of carbon fiber has a positive effect.
This research has been conducted in close cooperation with major mud
companies and operators. A field case is presented from a shallow field. The
mud was designed and tested during operation at the laboratory of the
University of Stavanger. The result was a clear increase in fracture pressure,
resulting in a successful operation.
Experimental work
A large industrial project, DEA-13 (Morita et al. 1990; Onyia 1994), was
undertaken in the early 1990s to investigate lost-circulation problems with
oil-based drilling fluids. Good understanding came out of this project.
Publications by Morita et al. (1990) and Onyia (1994) give a good overview of
these results. Many of the observations reported in DEA-13 have been seen
during the work reported in this paper. There is one significant difference:
Whereas DEA-13 focused on oil-based drilling fluids, the present work has been
concerned mainly with water-based drilling fluids.
At the University of Stavanger, experimental fracturing research has been
carried out during the past 10 years. This work has resulted in several PhD
theses and a number of master’s theses. Recognizing that borehole-stability
mechanisms are not understood fully, the research has had to focus on the
fundamental physics and chemistry.
Fig. 1 shows a fracturing cell where specially prepared, hollow concrete
cores are fractured. The setup also allows for mud circulation to ensure that
mud particles are well distributed inside the hole. The cell is rated to 69
MPa, and the axial load, the confining pressure, and the borehole pressure can
be varied independently. Many oil- and water-based drilling fluids have been
tested, along with other novel ideas such as changing rock wettability or
creating other chemical barriers. Cores with circular, oval, and triangular
holes have been tested to study the effects of hole geometry.
Fig. 2 shows typical results from the fracturing experiments. The commonly
used Kirsch equation (Kirsch 1898) is used as a reference. The Kirsch equation
defines the theoretical fracture pressure with a nonpenetrating situation, such
as when using drilling muds. From Fig. 2 it can be seen that only one of the
measured fracture pressures agrees with the theoretical model; the two others
are much larger. Several conclusions have come out of this research,
including:
1. The theoretical Kirsch model underestimates
the fracture pressure in general.
2. There is significant variation in fracture
pressure, depending on the quality of the mud.
This shows that the fracture pressure can be increased by designing a better
mud.
To study the mud and the filter cake, several devices have been constructed.
Fig. 3 shows a mud cell provided with six outlets containing artificial
fractures of various dimensions. The mud is circulated with a low-pressure pump
to develop a filter cake across the slots. At this stage, a high-pressure pump
increases the pressure until the mudcake breaks down. In this way, we can study
the stability and the strength of the mudcake. We have used many common muds
and additives and have observed that reducing the number of additives often
gives a better mud. We also have studied nonpetroleum products to look for
improvements. Some of this will be discussed later.
© 2008. Society of Petroleum Engineers
View full textPDF
(
671 KB
)
History
- Original manuscript received:
21 November 2006
- Meeting paper published:
20 February 2007
- Revised manuscript received:
18 January 2008
- Manuscript approved:
8 February 2008
- Version of record:
15 September 2008
View Cart
