Numerical Simulation of Oil Recovery After Cross-Linked Polymer Flooding
Authors: Xie C.J. , Guan Z.L. , Blunt M. , Zhou H. 
Published: March 2010

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.