Stress sensitive experiments for abnormal overpressure carbonate reservoirs: A case from the Kenkiyak fractured-porous oil field in the littoral Caspian Basin

According to the characteristics of overpressure carbonate reservoirs of the Kenkiyak pre-salt oil field in the littoral Caspian Basin, cores were made artificially and the stress sensibility of matrix cores and cores with non-packed, semi-packed and fully packed fractures were analyzed. The method of gas measurement was used in the experiment. The confining pressure of the sample cores was first increased and then decreased. When the confining pressure became stable, porosity and permeability data from each pressure point were obtained to analyze stress sensibility of cores. The results showed that the stress sensibility of these four types of cores with a descending order is cores with non-packed fractures, cores with semi-packed fractures, cores with packed fractures, and matrix cores. With the decrease in packing degree of core fractures, the stress sensitivity of core permeability increased and the recovery degree of permeability decreased; the recovery degree of porosity and permeability of the matrix cores and cores with fully packed fractures was high with the recovery of pressure and these two types of cores showed elastic-plastic features. By contrast, the recovery degree of porosity and permeability of cores with partly packed and unpacked fractures was lower and these two types of cores showed plastic features. With the increase of confining pressure, the variation of porosity and permeability showed relatively regular exponential variations.