Summary
The complexity of oil and gas wells has increased significantly during the
past decades. In addition to improved equipment, a better understanding of the
subsurface environment is required to efficiently drill these wells.
Fracturing, as related to circulation losses, is a continuous challenge when
drilling these wells. For that reason two modeling activities have been pursued
in Norway during the past decades: 1) establishing a fracturing model for
shallow sediments offshore, and 2) developing a fracture model for deepwater
drilling.
Data from seabed investigations as well as from conductor and surface
casings were collected and normalized for varying water depths. This model
works well and has been used for many years. When deepwater drilling started in
1997, the same concept was applied, and the results show that the same type of
model also applies for deepwater drilling. Wells from Norway, UK, the Gulf of
Mexico, Angola, and Brazil have been analyzed and show remarkably similar
behavior when the water depth is considered.
This paper brings this work further by presenting a generalized fracture
model for shallow sediments in relaxed depositional environments. Data
normalization is a key method for making the fracture model applicable for all
water depths. Normalization methods are presented so fracture data can be
converted to other water depths and serve as a prognosis for new wells (based
on rig floor elevation, water depth, differences in overburden stresses, and on
mud properties).
Several field cases are presented with water depths ranging from 380 m to
1350 m, demonstrating the wide applicability of the new model.
It is shown that the generalized fracture model for shallow sediments
provides a good correlation with an error within a few percent.
Introduction
The shallow formations have not received much attention as compared to
deeper formations. However, experience shows that both shallow gas and
circulation losses are often challenges. In Norway, there has been an increased
focus on both the design of the upper-casing strings and rock mechanics
behavior of the less-consolidated sediments. Shallow sediments may exhibit
properties similar to surface soils, resulting in (with deepwater wells in
particular) an increased number of drilling problems. Some of these are weak
top holes with low-fracture strength, borehole collapse, water flow into the
borehole, cementing problems, riser problems, and shallow gas to mention a few.
This paper addresses issues related to circulation losses and fracturing.
© 2008. Society of Petroleum Engineers
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History
- Original manuscript received:
2 June 2006
- Meeting paper published:
11 September 2006
- Revised manuscript received:
5 December 2007
- Manuscript approved:
10 January 2008
- Version of record:
20 June 2008
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