Characterization of flow units in shaly sand reservoirs—Hassi R'mel Oil Rim, Algeria

Abstract

The concept of hydraulic flow units as viewed by Amaefule et al. [Amaefule, J.O., Altumbay, M., Tiab, D., Kersey, D.G., Keelan, D.K., 1993. Enhanced reservoir description: using core and log data to identify hydraulic (Flow) units and predict permeability in uncored intervals/wells. Paper SPE 26436. SPE Annual Technical Conference and Exhibition Houston, TX, USA. October 3–5] was first introduced for the purpose of predicting permeability in uncored intervals. This concept is strongly related to the flow zone indicator which is assumed to be the unique parameter that can distinguish between flow units. However, almost all published works that are based on this concept use a semi-log plot of the reservoir quality index versus porosity to identify flow units regardless of depth. This approach leads, in some instances, to locate the same flow units in non adjacent horizons. Hence, some questions can be addressed, such as: how one can determine the location of the Flow Units? Is the Flow Zone Indicator (FZI) sufficient to characterize them? Is it possible to determine Flow Unit pattern? If yes, then how? And what kind of information can bring such a characterization? This paper is devoted to answering these questions and to characterize flow units in shaly sand reservoirs using conditional simulation based on geological layering and the flow zone index concept in a thin section of the Oil Rim of Hassi R'mel field, Algeria. This concept requires that shale be first characterized by determining its amount, type and form of deposition. A new correlation was then developed to determine horizontal permeability in uncored intervals. Using available data several correlations were obtained for estimating vertical permeability. Combining geological units with FZI layering, it was possible to identify and locate the flow units with respect to depth. Using the estimated vertical permeability a new Flow Zone Indicator concept (FZIv) has been introduced. Geostatistical techniques of sequential Gaussian simulation was used to obtain 3 equi-probable realizations of the reservoir attributes such as porosity, horizontal and vertical permeability, shale volume, FZI and FZIv. The resulting 3D models for FZI and FZIv yielded well defined patterns of the flow units, with reference to horizontal and vertical flow. The models revealed excellent images of the fluid flow trend, which relatively agreed with the direction of channels obtained from sedimentological studies. In order to select the best model, production history of the Oil Rim of Hassi R'mel field, Algeria, was critically analyzed. This analysis revealed that the conditionally simulated case with a seed of 1000 is the most realistic model to be used for future simulation purposes. Four 3-D models of reservoir attributes using Kriging and Gaussian conditional simulation were constructed. Six flow units within the three main geological units were identified and located. The orientation of the preferential flow is North–South.