Hydrocarbon production decline behaviors in low permeability reservoirs and conventional reservoirs are significantly different; in such a way that high initial rate is usually observed for a short period in tight formation wells, then the well productivity drops dramatically. The rapid production decline in shale wells has a strong relationship with the loss of rock matrix permeability and reservoir conductivity, resulting from the dissipation of pore pressure during reservoir depletion. This paper presents experimental studies of stress-dependent compaction in tight reservoirs and its impact on long-term recovery.
The magnitude of rock permeability change depends on the in-situ effective stress, which is a combined effect of reservoir pore pressure and confining stress. In this research, permeabilities of a series of shale and sandstone core plugs are measured in the laboratory using pressure transimission test technique. The samples are tested under multiple confining stress and pore pressure combinations. Several confining stress are pre-set to represent different reservoir stress conditions. Different pore pressures are then applied to mimic reservoir depletion process under specific confining stress conditions. The permeability is calculated for all scenarios and pressure dependent permeability behaviors are analyzed for each type of sample.
The interpretations of effective stress dependent permeability show that different types of rocks have different permeability decline signatures responding to the depletion effect. In addition, tight rocks with different dominant porous media (i.e., matrix and fractures) are found to have different permeability behaviors. Characteristics of permeability decline at different pore pressure intervals are analyzed and the critical reservoir pressure and depletion index (i.e., ratio of pore pressure to overburden stress) are identified for different types of rocks where significant permeability change occurs. Based on the stress dependent permeability measurements, the Biot Coefficient is also determined for different tight samples.
Recognizing permeability decline signatures of tight reservoirs provides significant insights into long-term dynamic reservoir conductivity monitoring and contributes to the field management practices. It is indicated from the study that the permeability decline in tight reservoir wells can be significant with continuous depletion while maintaining reservoir pressure above the critical level has the potential to mitigate the sharp production decline in tight formations and therefore enhance ultimate hydrocarbon recovery.