Study of Remaining-Oil Distribution After Polymer Injection

Abstract

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 133808, ’Study on Remaining-Oil Distribution After Polymer Injection,’ by Jiecheng Cheng, SPE, Daqing Oil Field Company; Jianguang Wei, Daqing Oil Field Company and China University of Petroleum; Kaoping Song, SPE, Northeast Petroleum University; and Peihui Han, SPE, Daqing Oil Field Company, prepared for the 2010 SPE Annual Technical Conference and Exhibition, Florence, Italy, 19-22 September. The paper has not been peer reviewed. The recovery efficiency by polymer flooding is 10% higher than that of water-flooding for the main reservoirs in the Daqing oil field. To enhance oil recovery further, it is important to know how much oil remains in the reservoir after polymer injection along with its distribution. This study used logging data from 200 new wells for a macroanalysis on remaining-oil distribution after polymer injection. Samples from cored wells, before and after polymer flooding, were studied by use of core-membrane-fluorescence technology to analyze microscopic remaining-oil distribution. Introduction Large-scale polymer flooding has been ongoing in the Daqing oil field since 1996. Yearly oil production from the polymer process has exceeded 107 tonnes for 8 years. Polymer injection has become the leading technology for controling oil-production decline in the field. Field experience indicates that the viscosity and elasticity (viscoelasticity) of the polymer solution can increase the swept volume greatly and enhance the oil-recovery efficiency, resulting in oil recovery by polymer injection in major reservoirs that is 10% higher than that of waterflooding, but approximately 50% of the crude-oil volume remains after polymer injection. The remaining oil can be developed by high-viscoelasticity-polymer injection, alkaline/surfactant/polymer solutions, or foam flooding. But first, characteristics of the remaining-oil distribution must be described accurately. Therefore, it is important to study the remaining-oil distribution to improve oil recovery further and to adjust subsequent polymer injection. This study evaluated the remaining-oil distribution, both macro and micro, after polymer injection in the PI reservoirs of the SZ zones of the Daqing oil field. Macrodistribution of Remaining Oil Up to June 2010, 14 test blocks had completed polymer injection and had begun water injection. Target intervals of polymer flooding are the PI reservoirs, which are fluvial deposits with widespread and thick oil layers. The sand body consists mainly of braided-channel sand, meandering sand, and distributary-channel sand. The macrodistribution of the remaining oil in the PI reservoirs was studied on the basis of log-interpretation data from infill wells and cored-well data (before and after polymer flooding) from three typical test blocks. The study showed that after polymer injection, the distribution of remaining oil was more scattered horizontally, and the average oil saturation for these three cored wells was reduced by 11.9%, from 52.8% before polymer flooding to 40.9% after polymer flooding. The flushing degree of the oil layer in the vertical direction increased significantly, and average swept thickness improved from 68.6% before polymer injection to 89.4% after polymer flooding.