Reservoir compartmentalization and fluid property determination using a modular dynamic tester (MDT): case study of an Algerian oil field

Abstract

Compartmentalizing the reservoir and determining its initial properties are key steps in the development of an oil field. These steps are mainly performed to determine the number and types of fluids in the reservoir. They are also carried out to establish the initial reservoir pressure and reservoir potential. For each fluid zone, it is essential to identify the fluid present and its range. The oil reservoir examined in the present case study consisted of different zones with ambiguous fluid limits. Thus, the pressure gradients and variations in this reservoir were measured using a modular formation dynamics tester (MDT). This device can be employed to discriminate between formation fluids in real time. An analytical analysis of the measured pressure gradients allowed the reservoir compartments to be defined, taking into account the types of reservoir fluids present. The investigation also aimed to identify the initial reservoir conditions and establish the free water level (FWL) in each fluid zone. Thus, with similar setting, reservoir characterization and heterogeneity may constitute a key factor approach to be examined. The oilfield analyzed in this work, which is part of the Hassi Berkine basin in the Saharan platform (southeastern Algeria), has an anticlinal structure. From a sedimentological point of view, it consists of fluvial-continental deposits. Some erosional effects of the Hercynian unconformity influence the top of the Frasnian clays present. The main hydrocarbon reservoir is located in the Lower Triassic Clayey Sandstone (TAGI), the current name for the Triassic reservoir in the Sahara platform in the investigated area. The TAGI is divided into three levels: the upper TAGI (TAGI-U), middle TAGI (TAGI-M), and lower TAGI (TAGI-L). Data revealed several pressure gradients along the depth profile of each borehole section, implying significant changes in fluid properties. Substantial differences in pressure between oilfield boreholes (e.g., between the boreholes through TAGI-U+M and TAGI-L) were observed. The equivalent densities for the pressure gradients indicate that the fluid succession in the deposits is oil (TAGI-U+M), water (TAGI-M), and then oil again (TAGI-L). A stratigraphic barrier between TAGI U+M and TAGI-L was identified. By superimposing the pressure gradients observed in wells 1 and 2, two reservoir compartments were deduced. Furthermore, based on the pressure evolution with depth and the density record, two free water levels (FWL) in addition to the initial pressure were identified for the reservoir.