EOR Technology Practice of Natural Gas Flooding: A Case Study on Donghe I Carboniferous Reservoir with the Characteristics of Ultra-Deep, Very Thick and 3 Highs
Authors: Zhou Wei , Zhou Daiyu , Fan Kun , Lian Liming , Bian Wanjiang , Shao Guangqiang , Yu Hongwei , Wu Zangyuan , Zhang Min , Li Jun , Han Haishui , Bai Yin , Fan Jiawei , Kuang Xiyu , Zhang Liang 
Published: January 2020

This paper provides a case study for the application of gas assisted gravity drainage (GAGD) EOR in the Donghe I carboniferous reservoir, which is ultra-deep (&gt;16,000 ft), thick (150-400 ft) and has high temperature, pressure, and salinity values of 410 K, 6500–8500 psi and &gt; 200,000 ppm, respectively. The study aims to mitigate the problems faced by this reservoir such as high degree of reserve recovery (40%), high water cut (&gt; 82%), low reservoir pressure (pressure maintenance level &lt; 0.65), and low production rate.

The techniques described in this paper are based on natural gas miscible flooding in which the GAGD technique occupies the dominant position, with the plane displacement technique being assisted. A reservoir description focusing on fine characterization of the interlayer has been applied combined with a compositional ten-million-grid-scale reservoir simulation model to make the filtration mechanism be straightforwardness. Laboratory experiments were conducted to reveal the phase behavior of reservoir fluids after gas flooding. Several key methods have been established including gas breakthrough indication by composition, well pattern reconstruction, injection/production matching, typical treatment for typical wells, and injection/production ratio control.

The aforementioned techniques and methods mentioned have been carried out in Donghe I carboniferous reservoir as part of a geology-reservoir-engineering-economics-HSE integral project. The daily gas injection rate at the pilot site is around 1.4 MMCF/D using 4 horizontal wells as gas injectors and the injection/production ratio is over 1.5, which is considered a satisfactory result. As of 2018, there are 13 response producers of 25 wells in this reservoir, of which 6 wells showed doubling or tripling of daily production and 5 wells transferred from artificial lifting to natural flow, with production of two of the producers exceeding 650 BPOD. The cumulated oil production and gas injection have respectively reached 1.6 MMbbl and 10.2 BCF. The injected gas serves as an effective supplement to reservoir energy, causing the pressure maintenance level to climb up to 0.8. The gas injection has also had a positive effect on the composite production decrement and composite water cut of this reservoir. The composite production decrement has declined from 15.5%/A before gas flooding to about zero after gas flooding till date, and the production still has a trend to change from constant rate to rising rate. Simultaneously, the water cut which was previously increasing has now started dropping down.

The promoted GAGD EOR technique showcased in this paper effectively combines a series of technical and theoretical methods including natural gas miscible flooding, GAGD with plane gas displacement, gas breakthrough indication by composition, injection/production matching, typical treatment for typical well, injection/production ratio controlling, ten-million-grid-scale compositional-model simulation, and laboratory experiments on phase behavior. All these aspects combined have led to a favorable field test performance in the Donghe I reservoir.