document preview

Abstract

Gathering of Oil-water-gas mixtures in oil fields is usually performed on condition that the gathering temperature is above the pour point of the crude oil or above the paraffin precipitation temperature. Therefore, all the mixtures that are gathered are in a fluid state (liquid and gas). However, when gathering crude oil that has a high pour point and high paraffin precipitation temperature in cold regions, the mixture must be heated to maintain the high temperature and a large amount of energy is consumed.

Daqing Oil Field experimented on a large scale (2,300 wells) in the field the gathering of oil-gas-water at temperatures lower than the crude oil pour point, which means that the mixture is gathered simultaneously as a sticky solid (crude oil), gas and liquid (water). This paper introduces the results in the laboratory of adding a Fluid Phase Conversion Agent into the mixture and the characteristics of the mixture at different temperatures. The paper also introduces the field results of adding a Phase Conversion Agent to the gathered fluid, the problems that arose and how they were solved.

The field results of gathering produced fluids from more than 2,300 wells are very good. The gathering of produced fluids proceeded normally, energy consumption and gathering costs were lowered. At the same time, some actual data on the rheology of simultaneous gathering of a sticky solid, liquid and gas was obtained.

Introduction

At present, most oil fields gather oil-gas-water at temperatures above the crude oil pour point or even above the paraffin precipitation point, otherwise the crude oil would stick to the gathering pipe walls and block it up.

In cold regions (for instance, the temperature of Daqing in winter is -30∼-40°C), when gathering high pour point crude oil (the pour point of Daqing crude is 25∼35°C), the oil-gas-water mixture must be heated above its pour point, sometimes above the paraffin precipitation point. When heating the fluid mixture, a large amount of energy (electricity, oil or gas) is consumed. For example, on the average 10∼12 m³ of natural gas is burnt to heat up a ton of crude oil in the gathered fluid in Daqing. At the same time, the cost of gathering the crude oil mixture increases.

In the lab and field, Daqing experimented to add Fluid Phase Conversion Agents into the gathered fluid mixture to see if the crude oil mixture could be gathered and transferred at temperatures below the crude oil pour point. This paper introduces the results of the experiments in the lab and field.

Results of Experiments in the Lab

Emulsification characteristics.

When the water cut is below 60%, the crude oil and water produced from wells is a mixture with oil as the outer phase. When the water cut is above 60%, the fluid is a mixture with water as the outer phase. The emulsions in the mixture are quite stable.

After a certain amount (about 50∼150mg/L) of Fluid Phase Conversion Agent is added to the produced fluid mixture and the water cut is below 40%, it is a mixture with oil as the outer phase. When the water cut is above 40%, water is the outer phase. The oil-water mixture is not stable, in less than 30 minutes, by gravity, the oil and water are basically totally separated.

The above shows that to gather the oil-water mixture and not have the oil stick to the wall of the steel pipe, the water cut of the mixture, when using Conversion Agent, should be at least higher than 40%.

Changes in the Wall Sticking Temperature.

This is a very important parameter to determine if the fluid mixture can be gathered at a lower temperature. When the temperature is below the pour point, the crude oil turns into a sticky solid. If it is dispersed in the water and gas with water as the outer phase and it does not stick to the wall of the steel pipe, then low temperature fluid gathering might be realized; if the crude oil sticks to the pipe wall, than the gathering pipeline will be plugged up.