USING OPENHOLE AND CASED HOLE RESISTIVITY LOGS TOMONITOR GAS HYDRATE DISSOCIATION DURING A THERMALTEST IN THE MALLIK 5L-38 RESEARCH WELL, MACKENZIE DELTA,CANADA

摘要

Gas hydrates (naturally occurring ice-like combinationsof gas and water) have the potential to provide vastamounts of natural gas from the world's oceans andpolar regions. However, producing gas economicallyfrom hydrates entails major technical challenges.Proposed recovery methods such as dissociating ormelting gas hydrates by heating or depressurization arecurrently being tested. One such test was conductedrecently in northern Canada by the partners in theMallik 2002 Gas Hydrate Production Research WellProgram.This paper describes how resistivity logs were used todetermine the size of the annular region of gas hydratedissociation that occurred around the wellbore duringthe thermal test in the Mallik 5L-38 well. An openholelogging suite, run prior to the thermal test, includedarray induction, array laterolog, nuclear magneticresonance and 1.1-GHz electromagnetic propagationlogs. The reservoir saturation tool was run both beforeand after the thermal test to monitor formation changes.A cased hole formation resistivity log was run after thetest.Baseline resistivity values in each formation layer (Rt)were established from the deep laterolog data. Theresistivity in the region of gas hydrate dissociation nearthe wellbore (Rxo) was determined from electromagneticpropagation and reservoir saturation toolmeasurements. The radius of hydrate dissociation as afunction of depth was then determined by means ofiterative forward modeling of cased hole formationresistivity tool response.Pretest computer simulations had predicted thatdissociation would take place at a constant radius overthe 13-ft test interval. However, the post-test resistivitymodeling showed that this was not the case. Theresistivity-derived dissociation radius was greatest nearthe outlet of the pipe that circulated hot water in thewellbore, where the highest temperatures wererecorded. The radius was smallest near the center ofthe test interval, where a conglomerate section with lowvalues of porosity and permeability inhibiteddissociation. The free gas volume calculated from theresistivity-derived dissociation radii yielded a valuewithin 20% of surface gauge measurements. Theseresults show that the inversion of resistivitymeasurements holds promise for use in future gashydrate monitoring.