Abstract
Background: A hydraulic fracture is a key technology to increase production of the low permeability oil fields. Fracture additives such as gels, friction reducers, pH adjusters and clay stabilizers were injected into the underground. While more than 50% of the fracture fluid remains underground. The residue of fracture fluid comes out with the produced liquid (a mixture of crude oil and water) in the subsequent oil recovery process, which results in a highly stable crude oil-water emulsion. Objective: The stability and stable mechanism of the emulsion with fracture fluid have been experimentally investigated. Materials and Methods: The influences of fracture additives and components of crude oil on the stability of emulsion were investigated by bottle test and microscopic examination. The interfacial tension and modulus of dilation were explored by a spinning drop interfacial tension meter and an interface expansion rheometer, respectively. Results: The fracture additives played the key role on the emulsion stability. On one hand, the interface energy of oil-water was reduced by friction reducer (IFT was decreased from 24.0 mN/m to 1.9 mN/m), which was a favor for the formation of an emulsion. On the other hand, the dilational modulus of crude oil-water film was increased by hydroxypropyl guar and pH adjuster (Na2CO3) to form a viscoelastic film, which resulted in a highly stable emulsion. Conclusion: The residual fracture fluid accompanied by produced liquid resulted in a highly stable emulsion. The emulsion with fracture additives was difficult to be broken, which may affect the normal production of the oil field. A positive strategy such as developing demulsifier with high efficient should be put onto the schedule.
Highlights
Low-permeability oil reservoirs have become one of the most important sources of energy in China and in North America [1 - 3]
The interface energy of oil-water was reduced by friction reducer (IFT was decreased from 24.0 mN/m to 1.9 mN/m), which was a favor for the formation of an emulsion
The dilational modulus of crude oil-water film was increased by hydroxypropyl guar and pH adjuster (Na2CO3) to form a viscoelastic film, which resulted in a highly stable emulsion
Summary
Low-permeability oil reservoirs have become one of the most important sources of energy in China and in North America [1 - 3]. Typical hydraulic fracture additives include gels, friction reducers, crosslinkers, pH adjusters, clay stabilizers and breakers. At temperatures below 66 ◦C, enzyme breakers can be used to break guar gels [10, 14], but under higher temperature condition and at pH above 10.5, the effectiveness of most enzymes is reduced Inorganic breakers, such as ammonium, potassium, and sodium salts of peroxydisulfate, are used at concentrations ranging from 1 to 400 mg L−1 [15]. A hydraulic fracture is a key technology to increase production of the low permeability oil fields. Fracture additives such as gels, friction reducers, pH adjusters and clay stabilizers were injected into the underground. The residue of fracture fluid comes out with the produced liquid (a mixture of crude oil and water) in the subsequent oil recovery process, which results in a highly stable crude oil-water emulsion
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