MAGNETIC RESONANCE IMAGING OF OIL RECOVERY DURING SPONTANEOUS IMBIBITION FROM CORES WITH TWO-ENDS OPEN BOUNDARY CONDITION

摘要

Spontaneous counter-current imbibition into cores with different boundary conditions progresses at significantly different rates.In order to compare experiments conducted using different boundary conditions,one needs to be able to scale results for boundary conditions.One approach is based on the characteristic length.The definition by Ma et al. in 1997 provided satisfactory scaling for strongly water-wet high permeability Berea sandstone.The boundary conditions used in previous experiments include All-FacesOpen (linear and radial,counter-current imbibition),One-End-Open(linear,countercurrent imbibition),Two-Ends-Open(linear,counter- and/or co-current imbibition) and Two-Ends-Closed(radial,counter-current imbibition).The characteristic length for TwoEnds-Open (TEO) boundary condition was expected to scale as if the process consists of linear imbibition into two separate One-End-Open(OEO) cores with length equal to half that of the TEO core.However,as further data were accumulated,TEO data often exhibited inconsistencies in scaling,usually with dimensionless times being longer than expected.These observations triggered more detailed investigation.The TEO geometry is a special case because it allows the possibility of recording recovery by imbibition separately from each of the producing end faces,and can therefore be used to test the theory behind the characteristic length.Separate production from each end face revealed that oil production was often asymmetrical,with more than 85% of the oil recovery from one side in some cases.The pressure generated within the core during imbibition was also recorded.Also,spontaneous imbibition was imaged using Magnetic Resonance Imaging(MRI) to track the movement of water and oil within the core plug.Brine imbibition was usually symmetrical and oil production asymmetrical.In two tests,an imbibition brine front was developed almost entirely from one end face of the core and moved in a piston-like manner.In each test,the oil produced from each open end face was highly asymmetrical.In repeated tests with the core plug orientation inverted,the primary imbibition front was still established from the same end face,no matter whether gravity aids or opposes imbibition.This demonstrated that both co-current and countercurrent flows occur during imbibition.The TEO imbibition results present a special case that is amenable to detailed investigation.