A Facies-Based Approach to Distribution of Petrophysical Properties in an Aeolian Sandstone Reservoir, Unayzah Formation, Saudi Arabia

Abstract

Defining and distributing petrophysical reservoir property within a simulation ready geocellular model is an industry-wide dilemma; the result of which has a large business impact on reservoir management decisions. The objective of this study is to develop porosity-to-permeability transforms, multiphase relative permeability and capillary pressure curves for each distinct depositional facies. The key to this process is the ability to recognize each of the four eolian facies and demonstrate that even at deep formation; the original depositional facies controls the petrophysical properties. Through core and image-log studies, the geologist divided the reservoir into the four distinct depositional facies that comprise a "wet" eolian depositional system: dune, sand sheet, paleosol, and playa. Each facies was distributed proportionately by zone (vertically) and by region (area) in a 3D geocellular model using an object based modeling technique and a wet eolian depositional model. The dune facies is the best reservoir unit followed by sand sheet; together the dune and sand sheet comprise a high proportion of the reservoir gross thickness. Conventional and special core analysis measurements were conducted on hundreds of core samples from three gas wells. The core data were thoroughly evaluated for pertophysical properties, and then grouped by depositional facies. Porosity-to-permeability transforms, multiphase dynamic relative permeability and capillary pressure were derived for each facies. The reservoir properties were then geostatistically distributed within each facies object using the facies and zone specific transforms. Results showed a quantifiable porosity-to-permeability transform, and corresponding capillary pressure and condensate-gas relative permeability for each depositional facies in the reservoir. This paper shows that by grouping the reservoir into recognizable depositional faceis has significantly enhanced the distribution of reservoir and fluid properties, which were incorporated into a compositional model. This will result in a dependable fluid-flow model that will support investment, reservoir development and reservoir management decisions. Introduction Saudi Aramco has conducted a successful integrated study to fast track develop a large, rich gas-condensate field located south of Ghawar in Saudi Arabia. The main goal of the study was to establish reliable rock and fluid properties to characterize a deep eolian sandstone reservoir composed of four distinct depositional facies. The Unayzah-A reservoir is Permian in age and consists of well-developed eolian sandstones and associated inter-dune and lacustrine deposits. Based on thorough and extensive geological assessment, geologists have divided the reservoir into four discrete geologic facies: dune, sand sheet, paleosol, and playa. The best reservoir units are observed in the dunes followed by sand sheets throughout the field. (MDT) Pressures, core, and log data1 indicated that dunes and sand sheet in the Upper Unayzah A are more permeable than the thick Main Unayzah A unit due to the clay choking effect in the main Unayzah A. The paleosl and playa contains low reservoir quality rocks, low porosity, and very low permeability (<1 md), thus they are considered as non-reservoir and act as baffles and barriers that separate flow units. The study has utilized conventional and special core analysis conducted on three development wells drilled throughout the south and north margins of the field. To reduce the uncertainties, the core data were thoroughly evaluated and then were grouped by the depositional facies. Consequently, an engineering process was conducted to generate permeability-porosity transforms and multiphase dynamic relative permeability and capillary pressure for each facies. These rock and multi-phase fluid properties data proved to build a robust reservoir compositional simulation model. The resulting data were analyzed and integrated with geological and log interpretations to allow a more reliable characterization of the pay interval into flow units for completion design and reservoir modeling.