Feasibility study of gas injection in low permeability reservoirs of Changqing oilfield

Abstract

Changqing is the largest petroleum-producing field in China and one-third of its production is attributed to the formations with permeability lower than 1 mD. Based on the recent successes of gas injection pilots in North America, we investigated the feasibility of gas injection in the low permeability Chang 63 reservoir of Changqing. An eight-component fluid characterization, which fitted the measured PVT data, was used in a dual-porosity compositional model. A typical well pattern was selected for the simulation study. The key input parameters were adjusted to match the historical data. Huff-n-Puff using different gases shows that the richer the injected gas, the higher the oil production. C3H8 huff-n-puff achieves the best performance, increasing the cumulative oil production by a factor of 2.28 after 5 cycles, then followed by C2H6 and the produced gas. CH4 demonstrates a lower recovery factor (RF) than waterflood, because its minimum miscibility pressure is close to the maximum allowable injection pressure, i.e., the minimum horizontal stress. With the current well placement design where the producer is at the reservoir top, the miscible bank, which forms at the front of lean gas injection, will be displaced towards the reservoir bottom even out of the stimulated reservoir volume (SRV), undermining its performance. Rich gas is more compatible, as the miscible bank forms at the injection tail. Based on the fracture spacingsfrom the published work, we could, for the first time, verify the technical feasibility of rich gas injection in Chang 63 following the presented compositional modeling framework.