--> Quantitative Formation Evaluation of Hith-Gotnia Evaporites by Integrating Logging-While-Drilling and Cased Hole Wireline Measurements

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Quantitative Formation Evaluation of Hith-Gotnia Evaporites by Integrating Logging-While-Drilling and Cased Hole Wireline Measurements

Abstract

The Gotnia and overlying Hith formations are evaporitic deposits with inter beds of limestone. Discovery of hydrocarbon in these carbonates identified them as new exploration targets. During Kimmeridgian and Tithonian times, a hyper-saline setting prevailed which attributed to the deposition of thick anhydrites, salts, with occasional shales and limestones. These sediments are named as Hith and Gotnia Formations in Kuwait. Sharp contrast in the lithology of salt-anhydrite and high pressure in interbedded limestone layers makes this formation extremely challenging to drill and evaluate. Well data acquisition, namely well logs and cores, has been a major challenge owing to the gas kicks conditions had been encountered while drilling. Hith and Gotnia formations are drilled using mud weights in the range of 18 ppg to 21 ppg to counter the high formation pressure. For obvious safety reasons, casing is set immediately after drilling, limiting open hole data acquisition. Due to complex lithology and low porosity, a comprehensive acquisition and interpretation methodology is required for carrying out a quantitative formation evaluation to assess the prospectivity. A workflow using open hole LWD quad combo measurements acquired in “wash down” mode, cased hole wireline measurements and mudlogs has been devised to optimize formation evaluation in the Gotnia-Hith sequences. Neutron-induced spectroscopy and natural gamma ray spectroscopy are acquired behind casing. A thick, homogeneous anhydrite marker is identified on the mudlogs and offsets on the cased hole spectroscopy measurements are guided to match the lithology across this marker. This is done to eliminate the effect of casing and cement signal on the spectroscopy measurements. Natural gamma ray spectroscopy results showed that the limestone layers were radioactive, resulting in erroneous shale volume computation using open hole gamma ray. The lithology and matrix properties derived using spectroscopy measurements are combined with other open hole measurements in a multi mineral model for quantitative formation evaluation. Formation evaluation utilizing the workflow described above was carried out on multiple exploration wells. The results were first used to optimize the selection of intervals to be tested. The first well tested using the workflow resulted in oil flow validating the hydrocarbon potential of these reservoirs and opened doors for development of the Hith and Gotnia formations.