WATER SATURATION GRADIENT (SWGRAD) AS A NEW MATHEMATICAL SOLUTION FOR HYDROCARBON IDENTIFICATION IN THE OIL AND GAS FIELD WITH HIGH UNCERTAINTY OF WATER SALINITY: CASE STUDY IN TUNU FIELD, DELTA MAHAKAM

摘要

Water saturation (Sw) is the ratio of water volume to pore volume. Sw is a classical method to differentiate hydrocarbon zone and water zone, if water salinity is well known. In Mahakam area, fluid identification is challenging due to varying formation water salinity both vertically as well as laterally. Therefore, conventional water saturation computation from open hole logs is not used directly to differentiate hydrocarbon and water or as cutoff to define netpay due to high uncertainty on formation water salinity. An innovative mathematical approach, referred to as SWGRAD is developed by using several of water saturations computed with series of formation water salinity values. SWGRAD is the gradient of water saturation to salinity changes. The gradient in hydrocarbon bearing zone is significantly less than in water bearing zone, therefore it can be applied to differentiate hydrocarbon zone and water zone to help fluid interpretation in the oil and gas fields in Mahakam. The result of this technique has been evaluated by using real time down hole fluid analysis (FA) and perforation results. The success ratio of this technique compared with FA and perforation results is very convincing. This paper details the method and its application in most of Mahakam Fields, East Kalimantan. This technique, referred to as SWGRAD, provides a new alternative technique that might be applied in oil and gas fields outside Mahakam, by better identifying hydrocarbon content & optimizing the data acquisition.