Abstract

Micro and macro evaluations were conducted to understand modified enzyme EOR mechanisms using various fluids. Huff n puff tests and flooding tests were carried out with variable modified enzymes concentrations in several reservoirs. Resulted production performances were considerably improved. At reservoir temperature 50–80°C, lab experiment indicated that the modified enzyme solution was adaptable to both light and heavy oil and was not sensitive to minerals water with bivalent cation (1000mg/l) and high salinity (10%). The enzyme working performances were further enhanced with microorganism occurrence in the solution. Micro modeling experiment revealed that spontaneous emulsification and solubilization could take place between the modified enzyme and crude with emulsion particle 2–6µm in diameter, which is produced through striping as a result of solubilization. Core desorption and flooding experiments presented that desorbed crude volume and displacement efficiency were related to modified enzyme concentrations which usually ranged 5 to 10%with the optimum being 8%. In optimal conditions, recovery can be averagely increased by 16.9%. Experiments also proved that the modified enzyme and crude oil could form emulsion but the emulsion was not stable. One specific pilot test with modified enzyme had achieved additional oil production 22,869 bbl.

Introduction

With time in oil development, some oil components precipitate and composites of organic and inorganic scales deposit. Formation fine grains migrate and clay swells. These are barriers to fluid flow and productivity, causing much oil unrecoverable underground. Modified enzymes were applied to improve biological activity of normal enzymes and protection of pay through opening pore paths and thus to introduce, stimulate and merge remaining oil into flows. Huff and puff tests and flooding tests have been implemented in Dagang, shengli, Daqing and others both in China and abroad all show improved performance [1–6, 10–11]. Yet it is not quite clear why and to what extent it works. So a collection of experiments were performed on modified enzyme EOR mechanism evaluation. A field test was made and is presented here.

Enzyme Working Mechanism

The environment-friendly enzyme agent is a water soluble product which can strongly release oil from reservoir grain surface. It can alter pay rock from oil-wet to water-wet, reducing interfacial tension of grains and oil flow resistance through pores.

Enzyme works in three stages. First it makes attachment, subsequently takes biochemical reactions and creates enzyme/oil complex, Fig.1. Then, the complex decomposes themselves into induced-fits and particles. The induce fits would continuously decompose and release enzyme until oil and enzyme are separate and consequently enzyme is restored.

Different from its chemical and bacterial counterparts, the above process is biological. It doesn't change any oil property or produce any derivatives. Instead, the process restores the environment-friendly enzyme to its original state after working. Theoretically, the enzyme will never be consumed out. In fact, its effectiveness and activity will degrade after processes. But that is solved by added sacrificial agents.

Enzyme Modification

Enzyme SL is a product of gene, cell and enzyme engineering techniques. To produce it, the bacteria that can separate oil from sands are selected and DNA is extracted. The selected bacteria were put into a nutrition solution and cultivated with high protein. Then the bacteria are removed of all of its activity to be a non-active catalyst. The enzyme resolves in water, not in oil, and can split hydrocarbons from rock grains.

To reduce cost, it is necessary to make modifications for less cost. The enzyme is combined with chemical surfactants to achieve synergy that is almost equivalent to the American enzyme efficacy. In a pay zone, the surfactants will make absorption on and effect on rocks, fluids and oil. Some surfactants act as sacrificial agents to ensure minimum even zero decrease of the original enzyme concentration and its activity.

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