Pulsed Neutron Capture Log Evaluation Using Synthetic Base Logs (includes associated papers 10920 and 10939 )

Abstract

The evaluation of recompletion opportunities using pulsed neutron logs ismade easier by preparing predictive synthetic logs using openhole log data.Wellsite personnel can assess drainage quickly by comparing the actual logwith the synthetic expected response, thus avoiding time-consuming trialcalculations. Introduction The pulsed neutron capture (PNC) log has proved tobe a valuable production surveillance tool for theU.S. gulf coast environment since its introduction in1962. In this geographic area, more than 900 PNClogging runs have been made in producing Shell wellsalone. We have come to rely on this tool as a reliableaid for basing many of our remedial andrecompletion decisions.In conventional PNC log evaluation, watersaturation (Sw) calculated from openhole logs iscompared with Sw computed from the cased-holePNC log. Drainage is indicated when a statisticallysignificant difference in Sw is observed. Althoughthis technique is a practical procedure, the calculations can be time-consuming and subject tohidden errors.A nearly foolproof method of assessing drainagefrom PNC log data is the time-lapse technique. Inthis method, a base PNC log is run early in the life of the well, preferably before water production begins.When subsequent drainage requires assessment, asecond log is run and compared with the first. If thewellbore environment (production equipment, annular fluids) has not changed greatly, differencesbetween the logs can be caused only by reservoir fluidchange. The problem with this approach, of course, is the expense and practicality of building a completebase-log inventory. It usually is not justifiable.This paper presents a simple computer techniquefor creating synthetic base PNC logs using theoriginal openhole log information. Offering many ofthe advantages of actual base-log data, the computer technique is easy to use and surprisingly accurate. Inaddition, synthetics can indicate quickly whether aPNC log can be interpreted reliably under adverseconditions. This can help the engineer choose morewisely among evaluation alternatives before a loggingcommitment is made. Example Fig. 1 shows an example of a synthetic pulsedneutron log made using this technique. An openholegamma-ray induction log is displayed on the left;cased-hole PNC curves (synthetic and actual) areshown on the right. Note the capture cross sectioncurve of a Schlumberger TDT-K (TM) log ("Sigma log"-the dashed curve) run 1 year after the well wasdrilled and cased. This curve is flanked by two logtraces, Sigma i and Sigma d, generated by the computer process usingopenhole logs. The Sigma i curve represents a "synthetic"PNC sigma curve at initial conditions. It shows whatan actural PNC base log would have looked likebefore the zone was drained. The d curve representsthe fully drained condition, in which the zone is totally water saturated. This curve represents the endpoint of drainage.(Actually, this condition will neverquite be reached since some residual hydrocarbonsaturation always remains after waterflood inwater-wet reservoirs.) JPT P. 667＾