Gas/Oil Relative Permeability Calibration With Observed Time Drainage Performance

Abstract

Abstract An empirical technique to calibrate gas-oil relative permeability data to observed time drainage performance from repeat Compensated Neutron Log (CNL) data analysis has been developed. The technique will yield a unique estimate of endpoint residual oil saturations by observing the rate of time drainage after gas has moved through a logged interval. The technique will also establish the correct relationship between the kv/kh ratio and the exponent in the oil relative permeability expression (the Corey exponent) by noting the absolute oil saturation value existing at the onset of time drainage, as measured in a CNL logging program. By estimating the kv/kh ratio of a horizon by core work, statistical log interpretations, vertical pulse test, etc., the corresponding required oil exponent is determined. Thus, the endpoint residual oil saturation as well as the shape of the relative permeability curve can be calibrated based on the observed time drainage performance. performance. Some important observations made in the study are:After the gas front has moved through an interval, the rate of oil drainage is independent of voidage rate and PV gas throughput. This is also true for a bypassed oil lens on top of a shale.The rate of drainage is insensitive to vertical locale, kv/kh ratio, and the relative permeability exponent. These parameters affect the absolute oil saturation values over time, but do not significantly impact the rate of oil saturation change behind the gas front.After gas front passage, the rate of oil saturation change behind a gas front is most affected by the residual oil saturation to gas.A relationship between the kv/kh ratio and the oil relative permeability exponent exists that will correctly yield the oil saturation found at the onset of time drainage. Introduction The purpose of this project is to, develop a methodology to calibrate gas-oil relative permeability (kg-ko) data used in field performance reservoir modelling to the observed time drainage performance that has been measured in a field CNL logging program. Time drainage is defined as the changes in oil saturation that occur after gas has swept through an interval. The actual conversion of the repeat CNL porosity tool response (gas-effect) to changes in saturation values has been documented in previous papers. The observed time drainage performance shown in Figure 1 is a normalized plot of oil saturation versus days of time drainage for a subject field. At zero days of time drainage, gas front passage has just occurred. A best-fit line passage has just occurred. A best-fit line regressed through the composite data indicates an average reduction of 2.0 oil saturation units in the first year of time drainage. During years two and three, the data indicates an average reduction of 0.8 and 0.5 oil saturation units, respectively. During year 6, time drainage reduces oil saturations by less than 0.1 saturation units. P. 155