Abstract

Currently many lean gas EOR pilot projects are implemented in Eagle Ford shale. The major component of lean gas is methane. From the field feedback, there is always large discrepancy between production forecast (or reservoir simulation) and the field results. The natural fracture system is complex and the communication between natural fracture, matrix and hydraulic fractures is even more complicated. Comparing connecting natural fracture between wells with short and low fracture conductivity, the well interference and the resulted optimal well spacing significantly change. In this study, according to field feedback, some connecting natural fractures with high fracture conductivity are mapped between wells to better represent the field geology and production status.

A composition reservoir simulation model is built for Eagle Ford shale. Typical production curve of Eagle Ford shale is matched for the first three years of primary production, lean gas Huff and Puff (HNP) process is simulated for the next three years for the parent wells and child wells. As the main composition of lean gas is methane, different methane adsorption effects are quantified between wells to investigate its influence on production. For lean gas Huff and Puff process, normally the Minimum Miscibility Pressure (MMP) is above 4000 psi. During lean gas cycling process, methane is adsorbed and desorbed, and the effective methane amount used to enhance miscibility between gas and oil phases is reduced, thus the reservoir pressure is not elevated to as high as no gas adsorption case. The methane adsorption effect significantly affects the oil and gas production. Relative permeability hysteresis and capillary pressure hysteresis (gas trapping effect) are the first time systematically studied to quantify the gas EOR performance in pad level production of unconventional reservoirs. Considering gas trapping effects, the cumulative gas injection amount and production amount is much better matched to the field pilot results. The oil incremental benefit of gas Huff and puff process considering gas trapping effect is quantified.

To our best knowledge, this is the first time that both methane adsorption and gas trapping effects are studied for pad level Huff and Puff process with the realistic connecting natural fractures between wells. Better matching of the production data with pilot results confirms the successful application of these two mechanisms. Considering the realistic complex natural fracture effects also greatly contributes to the correct production forecast and efficient design of gas EOR project for unconventional reservoirs such as Eagle Ford shale and other major basins.

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