Abstract

Wenmingzai (WMZ) is a mature water-flooded Oilfield, which is characterized by high temperature (80°F), high salinity (8×104mg/L) and high heterogeneity. After long time of waterflooding, the heterogeneity is getting worse and worse. Many injection wells were treated by injecting water-plugging agents, but the treatment results are fading with treatment times. To improve treatment results, a combination technology of profile control and mini-type surfactant flooding has been applied since 1998. This paper has introduced the laboratory experiment evaluation and field application results of the combination technology. Based on the salinity, hardness and temperature of this oilfield, a new non-ion surfactant has been developed. Effects of temperature, surfactant concentration, salinity and formation water hardness on surface tension between the surfactant and reservoir oil have been studied. Results show the tension has a magnitude of 10−3 mN/N at 65° and 10−2 mN/N at 80° under reservoir brine conditions, and it can resist 5×104mg/L of brine and 1500mg/L of Ca2+ and Mg2+. Adsorption of the surfactant onto reservoir cores has been studied by static and dynamic experiments. Results show that the dynamic adsorption density is less than 0.5g/g. Core flooding experiments were carried out to prove that the surfactant could improve recovery by more than 10%. Two application examples were analyzed to show how to choose candidate wells and how to operate the injection procedures. This combination technology can be widely used to improve oil recovery for mature oilfields.

Introduction

In view of the worse and worse effects of profile control on more matured water-flooding oilfield, a combination technology of profile control and surfactant flooding?CTPCSF? has been presented. The combination technology, an organic combination of profile control and water shutoff in secondary recovery with surfactant flooding in tertiary recovery, is a surfactant flooding based on a full profile control. Totally 16 well times of tests have been conducted in Wenmingzhai oilfield since 1998, from which some successful experience has been obtained and some lessons in failure have been generalized.

Presentation of the Problems

The application of profile control, water shutoff and tertiary recovery in heterogeneous oilfields is limited, thus affecting EOR effects[1].

1. Profile Control Limit
(1) Effective times of profile control

After initial profile control of an injection well, the single well production increase effect is obvious, but with increase in repeated profile control times, it becomes bad gradually. Field tests indicate that unlimited profile control cannot be conducted in an injection well and there exists a limit of profile control times due to being affected by economic benefits.

(2) Profile control mechanism

Waterflood recovery rate=sweep efficiency×displacement efficiency, i.e., ?=Es×ED. The mechanism of profile control is to enhance the recovery rate only through increasing the sweep efficiency, i.e., Es. Therefore, plugging agents take effect only within the range of enhancing the sweep efficiency. This is a limit to the action mechanism of profile control.

2. General Limit to Applications of Oil Displacement Agents in Tertiary Recovery

For the oilfields of relatively serious heterogeneity and especially under the condition that there are large channels, the sweep efficiency of injection water is relatively small, and if purely oil displacement agents for tertiary recovery are adopted, they take effect only within the range of relatively small sweep efficiency, whereas the remaining oil part of high oil saturation cannot be in contact with the oil displacement agents and thus the oil displacement effects cannot be fully developed.

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