An in-situ capillary pressure measurement method to characterize pore structure of tight formation

Tight sandstones usually contain pore throats ranging from nano-scale to micro-scale with poor connectivity. Few existing capillary pressure obtainment methods are capable of providing accurate in-situ capillary pressures for tight formations. This work has developed and verified a method (SMD) to measure the in-situ capillary pressure specially for tight formations under high pressure, high temperature and confining pressure with native formation fluids. The capillary pressures converted from the porous plate method (PP), the mercury intrusion method (MICP) and the self-developed method under ambient temperature (SDM-A) were compared with the in-situ capillary pressures to examine errors of the converted capillary pressures. Furthermore, the in-situ capillary pressures and the pore structure of a target tight formation were investigated through the SDM. Results show that the relative errors of the SDM are lower than 20% with an average value of 7.8%. Average relative errors are around 29.5%, 12% and 6% for the converted capillary pressures from the MICP, PP and SDM-A, respectively. This indicates that it is better to measure the in-situ capillary pressure directly through the easy and reliable SDM than to convert from other capillary pressures. The median in-situ capillary pressures of the target tight rock samples range from 0.1 MPa to 0.8 MPa. The pore throat radiuses are mainly from 0.1 μm to 1.0 μm, and the movable oil saturation is between 11.72% and 66.21% with an average value of 34.92%. This paper provides a new method to accurately characterize the in-situ pore structure and can provide guidance for oil-gas field development.