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Research Papers: Petroleum Engineering

Simulating Multizone Fracturing in Vertical Wells

[+] Author and Article Information
Wei Wang, Arash Dahi Taleghani

Louisiana State University,
Crafts and Hawkins,
Department of Petroleum Engineering,
Baton Rouge, LA 70803

Contributed by the Petroleum Division of ASME for publication in the JOURNAL OF ENERGY RESOURCES TECHNOLOGY. Manuscript received October 24, 2013; final manuscript received May 7, 2014; published online June 5, 2014. Assoc. Editor: Andrew K. Wojtanowicz.

J. Energy Resour. Technol 136(4), 042902 (Jun 05, 2014) (8 pages) Paper No: JERT-13-1304; doi: 10.1115/1.4027691 History: Received October 24, 2013; Revised May 07, 2014

Numerous multizone multistage hydraulic fracturing treatments are now being executed in low permeability oil and gas fields around the world. Due to the limited access to the subsurface, post-treatment assessments are mainly limited to few techniques such as tiltmeter, microseismic, and tracer-logs. The first two techniques are mainly used to determine fracture extension; however, fracture height and fracture initiation at all perforation clusters could only be confirmed through radioactive tracer logs or detailed pressure analysis. In this paper, we consider real examples from a field from Central America and investigate potential problems that led to the limited generation of fractures in multizone treatments. For instance, some of the postfrac radioactive logs show very low concentration of tracers at some perforated zones in comparison with other zones. On the other hand in some cases, tracer logs indicate the presence of tracers in deeper or shallower zones. Different reasons could cause fracture growth in nonperforated zones, including but not limited to: perforation design problems, casing/cement integrity problems, lack of containment, instability of fracture growth in one or some of the zones, and finally making a mistake in selecting lithology for fracturing. In this paper, some of these issues have been examined for a few sample wells using treatment pressure data, petrophysical logs, and postfrac tracer logs. Some recommendations in designing the length and arrangement of perforations to avoid these problems in future fracturing jobs are provided at the end of this paper.

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Figures

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Fig. 1

Tracer log of a tight oil vertical well in North America, fractures are initiated along the whole perforated zone and stayed contained in this zone after propagation. Horizontal tracer mark at 1207 m looks to be the bedding plane artifact.

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Fig. 2

A two-stage fracturing job has been performed as shown above. The first stage shows limited fluid entry. In the second stage, tracers were only tracked in the shallower part of the zone.

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Fig. 3

An example of fracture growth beyond the perforate intervals, in this case, pumping more fluid only adds to the fracture height not the fracture length

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Fig. 4

Linear softening traction–separation law for the cohesive element under pure shear loading (left) and pure normal loading (right)

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Fig. 5

A schematic picture of the numerical model with one potential hydraulic fracture zone and the interface between two planes are shown in (a). It also shows the model's dimensions, boundary, and loading conditions, and meshes for the three-dimensional model (shown in (b)).

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Fig. 6

Injection rate variation at the perforation clusters

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Fig. 7

A schematic picture of fracture (shown in red) propagation in a model with two perforation clusters, where larger flow is diverted into the lower large perforation zone. The dimensionless time is as shown in Fig. 6.

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Fig. 8

A schematic picture of fracture propagation through two perforation clusters, where larger volume of flow is diverted into the lower large perforation zone. In this example, the rock is considered to be heterogeneous.

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