Summary
A considerable portion of the world’s hydrocarbon endowment is in carbonate
reservoirs. Carbonate reservoirs usually exhibit low porosity and may be
fractured. These two characteristics along with oil-to-mixed wet rock
properties usually result in lowered hydrocarbon recovery rates. When enhanced
oil recovery (EOR) strategies are pursued, the injected fluids will likely flow
through the fracture network and bypass the oil in the rock matrix. The high
permeability in the fracture network and the low equivalent porous volume
result in early breakthrough of the injected fluids. Infill drilling programs
and well conformance strategies—mostly gas and water shutoff—have been
effectively used to mitigate the early breakthrough and increase oil recovery.
In most cases, however, 40 to 50% of the original oil in place (OOIP) is not
produced.
A large number of EOR field projects in carbonate reservoirs have been
referenced in the literature since the early 1970s. These field projects
demonstrate the technical feasibility of various EOR methods in carbonate
reservoirs. However, because of the collapse in oil prices, most of the
aforementioned project plans have been abandoned. This paper presents a
comprehensive compilation of EOR (Gas, Chemical, and Thermal methods) field
experiences in carbonate reservoirs within the US, as an attempt to identify
key variables and project design parameters for future evaluation and
revitalization of mature carbonate reservoirs.
Carbon dioxide flooding [continuous or water-alternating gas (WAG)] is the
dominant EOR process used in the US This is because of the high availability of
low-cost CO2. CO2 EOR in particular represents the logical first step towards
viable geologic carbon storage and sequestration. EOR chemical methods in
carbonate reservoirs, especially polymer flooding, have been widely tested in
US carbonate reservoirs. However, EOR chemical methods have made a marginal
contribution, relatively, in terms of total oil recovered.
Our study includes a brief overview of current laboratory (e.g. wettability
changes and novel chemical additives) and field (e.g. injectivity enhancement)
experiences in EOR chemical methods in carbonate formations. A brief discussion
surrounding the screening methods used to identify viable EOR opportunities in
carbonate fields based on past and present experiences is also included.
Introduction
Carbonate reservoirs are naturally-fractured geologic formations
characterized by heterogeneous porosity and permeability distributions. In the
case of low porosity and low permeability carbonate rocks (more specifically
rock matrices), the fluid flow in the reservoir can be completely dependent on
the fracture network while the matrix only plays a source role (analogous to
tight sand formations and natural gas flow). In the case of porous carbonate
rocks, fracture networks can cause uneven sweeping of the reservoir, leading to
early breakthrough of injected fluids in the producing wells and resulting in
low recovery factors. The abundance of carbonate reservoirs has been the
subject of numerous studies attempting to characterize their heterogeneities,
classify different types of fractured reservoirs, and determine how rock and
fluid properties have an impact on ultimate recovery (Roehl et al. 1985; Allan
and Qing Sun 2003; Carr et al. 2001; Grave et al. 2000; Benson et al. 1998;
Wardlaw 1996).
The TORIS database (maintained by the US Department of Energy) indicates 22%
of the OOIP in the US is contained in shallow-shelf carbonate reservoirs.
Currently in the US, these types of reservoirs exist in more than 14 states
with over 70% of the OOIP located in reservoirs in Texas and New Mexico, mostly
concentrated in the Permian Basin (Nuckols 1992, Xie et al. 2005). Over the
last three decades, primary production, waterflooding, and CO2 floods, combined
with infill drilling programs, have been the most widely used recovery methods.
However, other EOR strategies have been tested in the past and there are
currently several active research programs seeking alternatives to increase the
recovery factor of these mostly light crude-oil reservoirs economically.
Although the Permian Basin (west Texas and southeast New Mexico) can be
considered mature, its potential for improved oil recovery is still very high.
A recent study reports that there is an estimated 30 billion barrels of mobile
oil in the Permian Basin, reiterating the strategic importance of EOR
technologies for carbonate reservoirs and their impact on US oil production
(Nuckols 1992; Xie et al. 2005; Seethepalli et al. 2004; Moritis 2004; Cole
2003; Moritis 2003; Dutton et al. 2004).
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
16 February 2006
- Meeting paper published:
22 April 2006
- Revised manuscript received:
2 February 2007
- Manuscript approved:
4 March 2007
- Version of record:
20 December 2007
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