Abstract
Cross-linked polymer flooding can increase oil recovery 10 to 20% over
conventional waterflooding, while using a lower concentration of polymer than
conventional polymer flooding. Based on production data from a developed oil
field, the average incremental oil recovery by polymer flooding is only 10%,
indicating that much of the oil is bypassed. We need to know how this bypassed
oil is distributed to design an optimal development strategy.
Numerical simulations have been performed to study the mechanisms of polymer
flooding. For the coarsening downward sedimentary cycle, the simulation model
was divided into five layers with average geometric permeabilities of 100 mD,
300 mD, 500 mD, 980 mD and 2,190 mD, respectively. Five spot well patterns with
180 m distance between each injector and producer has been used. The injecting
well was controlled by an injection rate of 100 m3/d and shut down
as the water ratio reached 98%. Cross-linked polymer injection was introduced
after the water cut was as high as 90% and stopped as the water cut reached
98%.
It was found that most of the residual oil after polymer flooding existed in
small areas far away from the diagonal line for heterogeneous reservoirs, and
in layers with low or intermediate permeabilities for cycle reservoirs.
Introduction
The Xiaermen Oil Field lies in Biyang County, Henan Province. It is located in
the middle of a large northeast fault edge, to the east of Biyang Sag, Nanxiang
Basin (Figure 1). The structure of this field is a brachy-anticline in an
east-west direction complicated by four major faults and many minor faults
adjacent to the main oil-bearing area. Among these faults, a series of small
faults to the east were originated from the big fault (Figure 1). The maximum
height of the trap is 275 m.
The Xiaermen sand was deposited by deltaic fans(1). Considerable
variation in sand thickness occurring over short lateral distances is
characteristic of this particular deposit and thick layers with high
permeability are patchy and isolated. The sediment underlying the project area
is finely grained and relatively clean quartz sand. The average porosity is
about 24% and the mean geometric permeability is 2 µm2. The porosity
and permeability is generally high, but fairly heterogeneous.
The petroleum industry has recognized the problem of inefficient oil recovery
by conventional waterflooding. Therefore, extensive research has been conducted
using new techniques of enhanced oil recovery (EOR) of the
reservoirs(2). Polymer flooding is a significant method used for
this purpose, in which the mobility of the displacing phase can be reduced
effectively by the addition of small amounts of water-soluble polymers. Polymer
flooding will enhance oil recovery by increasing the water phase viscosity,
improving the water/oil mobility ratio and improving sweep efficiency. Polymer
flooding may have beneficial effects in relatively homogeneous oil reservoirs.
However, the improvements might be more modest in a heterogeneous reservoir. By
adding a cross-linking agent into the polymer and then forming a 3D reticulate
texture through the intermolecular cross-linking, a cross-linked polymer can
increase the viscosity of the injected fluid.
© 2009. Petroleum Society of Canada (now Society of Petroleum
Engineers)
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History
- Original manuscript received:
22 March 2007
- Meeting paper published:
12 June 2007
- Revised manuscript received:
17 February 2009
- Manuscript approved:
2 March 2009
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