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Characterization of Deep Tight Carbonate Gas Plays With an Integrated Interpretation - Case Study From North Kuwait

Abstract

In North Kuwait, the classic deep, tight gas play has been tested successfully in two wells with a matrix porosity of 5 pu and permeability of 0.01 mD but still continues to remain a conundrum in terms of commercial productivity. Though, these tight dolomites have proven commercial production from two other wells, where they are naturally fractured. However, natural fractures are limited in extent and observed in only a few wells. Hence the matrix driven production is of prime importance to develop these tight reservoirs, as these dolomites are extensive in space and time in area. The dolomites are of Pliensbachian to Hettangian age of Mesozoic Era. These are cyclic deposits with anhydrite and tight limestone in repetitive cycles of intertidal to subtidal environments of deposition. The first horizontal well drilled (1000′), intended to delineate the matrix driven tight gas play in the targeted dolomite. However, the twin dolomite bodies above it within the argillaceous section were found to be promising based on SCAL data in the equivalent zone in the nearby well. On testing it produced oil and gas, confirming the validity of the tight gas play. The significance of recent SCAL (XRD, MICP, SEM and Thin sections) and petrophysical SCAL (formation resistivity factor and resistivity index) data from the equivalent cored interval of two adjacent wells enabled a petrophysical re-evaluation of the target zone and argillaceous section. Recomputed fluid saturations showed presence of hydrocarbons. MICP data indicated three possible contributing zones within the argillaceous section. These intervals are predominantly dolomites, based on XRD analysis. On comparison with global tight gas reservoirs, the closest analogue to North Kuwait tight dolomites appears to be the Atoka Dolomite in Berlin Field, Western Oklahoma, where the average porosity and permeability have comparable values of 8% & 0.1 mD at the same depth range with high clay content. In the analogue, acid stimulation in the early stages of testing resulted in formation damage because of the released clays in dolomites. The successful completion adopted in this commercially producing field is hydro-fracturing instead of acid stimulation. The current study has facilitated in understanding the complex lithological assemblages and petrophysical re-evaluation for a possible reserve accretion and need to adopt successful completion mechanism in line with the analogue discussed.