Study on a Novel Viscosity Reducer for High Viscosity and Low Permeability Reservoirs
Authors: Zhang Fan , Ma Desheng , Tian Maozhang , Luo Wenli , Zhu Youyi , Luo Yousong , Liu Wanlu 
Published: February 2017

Abstract
Reservoirs of high viscosity and low permeability are abundant and widely distributed in various countries, which can contribute an important percentage of oil output in the world. Conventional chemical flooding are suitable for low viscosity oil (less than 30 mPa·s) and medium/high permeability reservoirs (more than 50mD). However, it is a great challenge to applied chemical flooding technology for higher viscosity (30-500mPa·s) oil and lower permeability (1-50mD) reservoirs. On the one hand, the high contents of resin and asphaltene, or the formation of wax crystals in low reservoir temperature, lead to high oil viscosity and low oil recovery; On the other hand, low permeability can cause injection difficulty, and conventional chemical agents (polymers and formulations of chemical combination flooding) are difficult to inject. At present, the main exploitation mode is water flooding. However, because of high oil viscosity and low oil fluidity, the water flooding recovery is only about 15%. So it is very necessary to develop effective development technology.
It is a good choice to develop water flooding with the intelligent viscosity reducer to decrease oil viscosity and improve oil fluidity, which has small molecular weight and can inject low permeability reservoirs easily. The performances of the viscosity reducer were studied in detail, including viscosity reduction efficiency, tripping oil film capacity, interfacial property, oil-displacement efficiency etc. The novel viscosity reducer with intellectual property show excellent properties. The viscosity reduction efficiency of the viscosity reducer was more than 80% (from 64.4 mPa·s to 10.3 mPa·s), and it could strip oil film within 40 seconds quickly, and could achieve ultra-low Interfacial Tension (IFT, less than 1.0×10-2mN/m).Alkali (NaOH or Na2CO3) could help to increase the viscosity reducing effects, and improve the stripping oil film capacity and interfacial property.The viscosity reducer had high oil-displacement efficiency; after water flooding, Oil recoveries increased 23% (OOIP, Original Oil In Place) with thehelp of this viscosity reducer.The novel viscosity reducer has viscoelasticity and shear-thinning ability. The monomer molecules can form the three-dimensional network structure in aqueous solution with high viscosity, which plays a role in enlarging the swept volume. With the increase of the shear rate, the three-dimensional network structure is broken down into the monomer molecules, and the viscosity decreases rapidly, so the viscosity reducer solution can inject low permeability reservoir easily.Compared with conventional polymer and polymer-surfactant, this viscosity reducer solution had better shear resistance and injectivity, and could filter membrane (0.2μm pore) effectively.
The novel viscosity reducer can substitute for conventional polymer and polymer-surfactant in chemical flooding. This paper provides insights of a new effective EOR way for high viscosity and low permeability reservoirs.