Fracture Model for General Offshore Applications
- Eirik Kaarstad (U. of Stavanger) | Bernt Sigve Aadnoy (U. of Stavanger)
- Document ID
- Society of Petroleum Engineers
- SPE Asia Pacific Oil & Gas Conference and Exhibition, 11-13 September, Adelaide, Australia
- Publication Date
- Document Type
- Conference Paper
- 2006. Society of Petroleum Engineers
- 3.3.2 Borehole Imaging and Wellbore Seismic, 2 Well Completion, 1.2.2 Geomechanics, 1.6 Drilling Operations, 2.1.7 Deepwater Completions Design, 1.7 Pressure Management, 1.10 Drilling Equipment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation, 1.11 Drilling Fluids and Materials
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The complexity of oil and gas wells has increased significantly the past decades. In addition to improved equipment, a better understanding of the subsurface environment is required to drill these wells efficiently.
Two modelling activities have been pursued in Norway the past decades: 1) Establishing a fracturing model for shallow penetration offshore, and 2) develop a fracture model for deepwater drilling.
Data from seabed investigations and 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. The results show that the same type of model applies also for deepwater drilling.
Wells from Norway, UK, the Gulf, Angola and Brazil have been analyzed. They show remarkably similar behaviour when the water depth is considered.
This paper brings this work further by presenting a generalized fracture model for relaxed depositional environments that works for all water depths. Normalization methods will be presented so fracture data can be converted to other water depths (serve as prognosis for new wells), based on rig floor elevation, water depth, differences in overburden stresses, and on mud properties.
Several field cases will be presented with water depths ranging from 380 m to 1350 m, demonstrating the wide applicability of the new model.
It will be shown that the generalized fracture model provides a good correlation with an error within a few percent.
The fracture prognosis
In the following, a brief summary over the most used modelling techniques will be given. The most common methods to predict fracture pressure are:
Fracture pressure from elastic theory
This is the most common model, which is used in hard rock. For vertical wells it defines the critical fracture pressure from the equation1:
When leak-off data are available, this equation can be used to construct stress profiles. Although very simple, this is the most important equation in applied rock mechanics, as it is used to normalize data and to establish correlations.
For shallow penetration below seabed, which we are concerned with here, we usually assume that no tectonic effects exist, and that an average horizontal stress is present. This is verified in the North Sea in the paper by Aadnoy et. al.2 The equation above has been presented in many forms, e.g. including poroelasticity effects. We usually do not have rock data for more complex versions, and these effects are of second order.
Elastic theory is based in in-situ stress loading. The geotechnical models are often based on material properties. Basically, the geotechnical science is concerned with the upper parts of the ground. One may argue that the shallow penetration from the sea-bottom is similar to shallow penetration on land.
Geotechnical engineering also uses the fracture model discussed earlier. However, many other models are also used. The cavity-expansion theory assumes that the soil around the borehole experience large displacements. For the cylindrical cavity to propagate, the borehole pressure must exceed a critical pressure given by:
where: su = shear strength and G = shear modulus.
This model usually over predicts the fracture pressure. It is therefore not much used. Andersen et. al.3 explains this as the fracturing pressure usually is lower than the cavity expansion pressure. The borehole may therefore fracture before a cavity may form.
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