Abstract

Xing 4–6 is a multilayer sandstone reservoir which has already experienced a secondary water flood in the entire reservoir and a tertiary polymer injection in certain formations. The polymer flooding began in 2001 as a typical polymer flooding field in Daqing. Currently, the Xing 4–6 is reaching the end of the designed polymer injection. In this paper, a cyclic water flood after the polymer injection is investigated in this field. The research is principally on the basis of reservoir simulations. After the history matching of the water flooding, a polymer flooding model describing target polymer flooding layer was extracted from the water flooding model to simulate the polymer injection process. By combining the development information and simulation results, the efficiency of the polymer flood was analyzed as the fundamental of cyclic injection investigation. Based on the plenty of geological study and multidisciplinary investigation, a cyclic flooding model describing Putaohua I formation was established.

Two cyclic injection cases were designed. Two cyclic units were defined in the model: the priority payzone P I33was considered as an independent cyclic unit due to its considerable proportion of the original oil in place (OOIP); other six layers were set as the second unit. In one case, the cyclic water was only conducted in the first unit; the other unit still employed continuous water injection. In the other case, two units were alternatingly involved in the cyclic injection. The predictions showed that the vertical sweep efficiency had been improved in upper zones which have vertical connectivity with lower layers due to the pressure pulsation and capillary pumping of the wetting phase in the low-permeability zones, especially the zones with high heterogeneities. Areally, the dead oils which were formed by the un-improvement of injection and production system were also mobilized. The cyclic water injection after polymer flooding improved the ultimate recovery. One more significant advantage was the extended use of the current well pattern. Comparing two cases, the case with only one cyclic unit involved had a better development efficiency in the priority payzone P I33 than that of the case with alternating cyclic injection, the incremental recovery in P I33 was 1.21%, which was 0.37% higher than that of the alternating case. Because the case with alternating cyclic injection took all zones into the consideration, the entire flooding efficiency was better than the other. The incremental recovery of alternating cyclic injection was 0.84% which was 0.32% higher than that of one cyclic unit injection on the reservoir scale. The detailed analysis in each case was also presented.

Introduction

Polymer flooding is playing an important role in the stable output of Daqing, the largest field in China, which started its polymer floods since the last 90's. Currently, most of polymer flooding fields in Daqing are getting close to the ends of their injections. Figuring out a superseded way is important to the control of production decline and the utilization of existing well networks. In this paper, an investigation of cyclic water injection after polymer flooding in the Xing 4–6 is presented. Over several decades of research, cyclic water injection has been developed as a mature technique to improve water flooding efficiency. In this research, the pre-investigation is focused on the cyclic injection after the polymer flood instead of the chase water of continuous injection. Cyclic water injection has been developed to a successful technique in improving water flooding efficiency in heterogeneous reservoirs.

Geological overview

Daqing oil field was discovered in a continental fluvial-delta sedimentary environment and features complex geologic conditions and high permeability contrasts. The North Xingshugang is one of the most typical fields in the Daqing (Liu and Han, 2001). Most pilot experiments are carried out in this region. If any test or experiment is successful in the north Xingshugang, it will be a great reference for most subfields in the Daqing oilfield. For this reason, the Xing 4–6 field of the North Xingshugang Unit was chosen as the commercial pilot for the high preformed-polymer flooding.

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