Thermal Decay Time Logging At Ekofisk, Case Study Of A Multiple Use Tool

Abstract

Abstract Thermal decay time (TDT) logging has proven to be a multiple use tool in the thick carbonate reservoirs at Ekofisk and nearby fields. Phillips Petroleum Company Norway has initiated a program to take a baseline log in each development well shortly after it has been put on production, to be followed by periodic future logs. The original reason for obtaining a baseline log was to determine the TDT response at initial conditions in each well for comparison against future TDT logs, i.e., the usual 'time-lapse' technique, to monitor changes in water, gas and oil saturations at various stages of depletion. The Danian and Maestrichtian limestone reservoirs are commonly acid-fractured during completion in wells in the Ekofisk Area fields, so the baseline TDT log was also intended to determine how much correction for the 'acid effect' would have to be applied in each well. The baseline TDT logs by themselves, however, are proving to be useful tools in applications other than those originally planned. The effect of acid on TDT log response has allowed the logs to be used to evaluate the effectiveness of acidizing and diverting techniques. Restimulations have been recommended and evaluated using TDT logs in conjunction with cased hole production logs. The usefulness of the TDT log in tracking the horizontal and vertical distribution of injection gas is also being evaluated. Phillips believes that thermal decay time logging will be a basic, multiple use reservoir monitoring tool throughout the life of the Ekofisk Area fields. INTRODUCTION Soon after Phillips Petroleum Company Norway began development drilling in the Ekofisk Area fields (Ekofisk plus 5 other major carbonate reservoir fields and one small sandstone reservoir field) the need was recognized to gather data about initial conditions in each well to provide the basis for future reservoir monitoring efforts. A program was established, therefore, to obtain, in addition to normal openhole logs and completion tests, cased hole production logs and a baseline thermal neutron decay time (TDT*) log soon after the start of production in each well. Initially it was believed that the baseline TDT logs would be of limited use until comparisons could be made with future TDT logs, in the usual time lapse technique, to observe fluid saturation changes in the reservoir. In practice, however, the baseline TDT logs have proven useful in several unexpected ways. THE TDT LOG Since the development and introduction in the late 1960's of pulsed neutron logs to measure thermal decay times, TDT logs and, even more importantly, the techniques for their use and interpretation have matured greatly. TDT logging is now one of the most useful reservoir monitoring tools available to the industry. When first introduced by Dresser Atlas as the 'Neutron Lifetime Log', and later by Schlumberger as the 'Thermal Neutron Decay Time Log', TDT logs were basically used for locating undepleted oil and gas zones behind casing in older fields or determining the source of high water cut production.