RETROGRADE CONDENSATION OR WATER IMBIBITION ? A CASE STUDY OF GAS WELL PRODUCTIVITY DECLINE BEFORE AND AFTER HYDRAULIC FRACTURING

Abstract

Studies have been carried out to diagnose the cause of productivity decline for the Kaimiro-1 well in the Kaimiro gas field, Taranaki Basin, New Zealand. The gas flow rate for Kaimiro-1, declined from 5 MMSCFD (0.14 Mm3 per day) in 1983 to about 0.6 MMSCFD (0.017 Mm3 per day) in 1993, immediately prior to hydraulic fracturing. While hydraulic fracturing initially increased production rates, long term post-fracture results have been disappointing. The volumetric gas-in-place for the field was estimated to be at least 100 BCF (2.83 Gm3), whereas the total cumulative gas recovery to date is 5.1 BCF (0.14 Gm3). During the production period prior to hydraulic fracturing, reservoir pressure declined from an initial 6,109 psi to 5,625 psi (42.1 MPa to 38.8 MPa). The well has produced water at low rates over its entire production history. Analysis of pressure build-up data showed a continued decline trend in effective reservoir permea­bility with time. Thus, it appeal s that the productivity decline is due to a decrease in effective permeability to gas and not to natural depletion. However, the exact origin of this decrease in effective permeability has been the subject of much controversy. Two competing theories regarding the decrease in permeabiiity have been proposed: retrograde condensation and water imbibition. Based on black oil and compositional simulation studies of pre-and post-fracture production, together with hindsight analysis of hydraulic fracturing, it is concluded that retrograde condensation is more likely to be the primary cause of productivity decline.