A Novel Separate Layer Injection Technique for Polymer Flooding in Daqing Oil Field

Abstract

Daqing Oil Field is the largest continental oil field in China. It is a large heterogeneous sandstone oil field with various permeability layers in longitudinal. Polymer flooding plays an important role in Daqing Oil Field which has recovered 10 million tons oil per year for 11 years. In the process of commingled polymer injection, most of the solution enters high permeability layers. Consequently, low permeability layers cannot be swept effectively which limits the recovery rate of polymer flooding. Based on the successful experiences of separate layer water flooding technology applied in Daqing Oil Field, several separate layer polymer flooding techniques were developed. The purpose of this technology is to inject polymer solution into pay zones separately with different pressure and flow rate according to reservoir properties, getting a stable injection profile of polymer solution in longitudinal direction so as to improve the overall recovery rate. The key points of this technology lie in three respects: First, the injected polymer solution should have a stable flow rate and pressure difference in the whole duration of polymer flooding. Second, the injected polymer solution should keep a higher viscosity to ensure EOR result of polymer flooding. Thirdly, the economic factor must be considered. This paper presents a unique separate polymer injection technology which solves the above issues effectively. The injected nozzle is designed into a special streamline style with different stages. When polymer solution flows through the nozzle, the molecular chains of polymer will expand and contract, creating an additional pressure difference. The more stages it has, the higher pressure difference can be reached. Furthermore, the shape of nozzle is optimized by simulation tools to ensure the minimum viscosity loss of polymer solution during the injection. After the optimal design, the maximum additional pressure difference of this nozzle can reach 2.5 MPa at the injection flowrate of 50 m3/d as well as viscosity loss ratio is less than 6%. This technology has been applied in 6464 wells in Daqing Oil Field. The producing degree of reserve was improved from 62.8 to 74.7% and oil recovery rate was improved by 2%. It is a remarkable core technology for sandstone oil fields in polymer flooding period. This technology has a brilliant future in the development of mature oil field.