Naturally fractured-caved Ordovician carbonate reservoirs in China have some distinctive characteristics: deep buried depth, developed multi-scale fractures, vugs and caves, no movable oil in pores, poor reservoir connectivity, isolated reservoirs with small developed volume and high production decline rate. How to effectively develop this kind of reservoirs is a major challenge. This paper presents the detailed development strategies of gas injection huff and puff for different representative fracture-cave type patterns after primary depletion and waterflooding.

Based on the geological study, representative reservoir patterns of fractured-caved carbonate reservoirs can be determined. And combined seismic interpretation, geological study and dynamic characterization, different development patterns can be identified and characterized. After that, simulation models of different development patterns with different fracture-cave architectures are built, and development strategy for representative isolated development patterns with gas injection huff-and-puff are detailed studied and optimized.

The technology is applied to K fractured-caved Ordovician carbonate reservoir of Tarim Basin in China. The mid-depth of K reservoir is 6500m with well annual production decline rate higher than 25%, and current oil recovery is only around 7% of OOIP. Through the geological and dynamic characterization, 3 representative reservoir type patterns are identified and characterized. And simulation models of the 3 patterns with different fracture-cave architectures for the three patterns are built to investigate the optimized development strategies of gas injection huff-and-puff. The optimized parameters include gas injection rate, cyclic soak time, cyclic injection volume, liquid off-take rate, etc., which are applied to the gas injection pilots in K reservoir, and the results show gas injection huff-and-puff can highly improve oil recovery.

This paper offers a case study on optimizing the development strategy of different reservoir patterns for fractured-caved carbonate reservoirs. It also provides a methodology and an improved oil recovery reference case for engineers and geologists to develop other similar reservoirs.

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