Spontaneous imbibition is important in oil recovery from fractured and low permeability tight gas reservoirs. Gas-water spontaneous imbibition experiments were conducted using low permeability heterogeneous limestone core plugs. The interfacial tension was changed by adding differing amounts of alcohol to water. It was observed that the true residual saturation of gas is very small for all cases. A much larger pseudo-residual saturation was achieved early in the imbibition stage but gas continued to be produced at extremely low rates until the true saturation was reached. The gas-water spontaneous imbibition performance was modeled using a mathematical model where the porous medium is represented as a bundle of equal but tortuous capillary tubes. Input data for this model was obtained from serial thin sections using a state of the art image processing system and a computerized microscope. The gaps between the thin sections were constructed using geostatistical techniques. It was observed that the model successfully explained the imbibition process in samples where pores with varying circularity were present. Average number of pore throats meeting at one pore in the pore skeleton ( coordination number) was less than six for all cases.