Abstract
A review of important issues in steam injection in naturally fractured
reservoirs (NFRs) is presented. The effect of temperature on physical
properties of crude oils and rocks and the thermo-chemical alteration of crude
oil are discussed.
The recovery of oil from NFRs can be modelled as a two step process: first
the oil is expelled from the matrix blocks through mechanisms that impose a
pressure gradient within each matrix block and then it is swept through the
fracture network to a production well by mechanisms that impose a pressure
gradient within the fracture network. The recovery mechanisms associated with
steam injection in NFRs and their characteristic times are presented. The most
important recovery mechanism in matrix blocks is differential thermal expansion
between oil and the matrix pore volume and the strongest mechanism in fracture
network is the reduction of viscosity ratio (μo/μw). The matrix
oil recovery mechanisms are relatively independent of oil gravity, making steam
an equally attractive recovery process in fractured light and heavy oil
reservoirs.
The mechanism and impact of CO2 generation during steam injection
in carbonate reservoirs are discussed. The rate of CO2 generation is
controlled by the rate of heat conduction from fracture into the matrix. For a
specific reservoir the rate of heat conduction is a function of temperature and
injection rate of steam and these can be optimized to make use of the in
situ generated CO2.
Introduction
Heavy oil in naturally fractured carbonate reservoirs is an important
resource, which accounts for one-third of total heavy oil worldwide. Many
fractured reservoirs in the Middle East, former Soviet Union and Canada are
candidates for thermal heavy oil recovery. Steam injection processes, which
have been used extensively to recover heavy oil from non-fractured reservoirs,
were not applied to fractured reservoirs until recently. This was primarily
based on the belief that the injected steam would bypass the oil through the
fractures and would be ineffective in recovering the oil. However, the results
of experimental, theoretical and pilot tests which have appeared in the
literature since early 1980s, show the feasibility of heavy oil recovery from
fractured reservoirs using steam injection.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
2 April 2007
- Meeting paper published:
12 June 2007
- Revised manuscript received:
13 November 2009
- Manuscript approved:
5 December 2009
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