Carbonate Reservoir Rock Typing - The Link between Geology and SCAL

Abstract

Abstract Reservoir rock typing is a process by which geological facies are characterized by their dynamic behavior. The dynamic behavior of the facies is assessed by studying the rock texture, the diagenetic processes which overprinted the initial fabric, and the interaction between the rock itself and the fluids. Porosity, permeability and pore size distributions characterize the rock texture while capillary pressure, relative permeability and wettability describe the rock-fluid interaction. Reservoir rock typing is a synergetic process between geology and petrophysics/SCAL. It is therefore a process by which various petrophysical parameters and dynamic measurements obtained from SCAL are integrated in a consistent manner with geological facies (lithofacies) to estimate their flow (dynamic) behavior. The relationships between lithofacies and reservoir rock types (RRTs) is complex because of the inter-play between facies, diagenetic processes and the rock-fluid interaction (wettability changes) in the reservoir. Similar lithofacies, deposited under the same depositional environments, may exhibit different petrophysical properties due to diagenesis. Therefore, lithofacies deposited under similar geological conditions may experience different diagenetic processes resulting in different petrophysical groups with distinct porosity-permeability relationship, capillary pressure profile and water saturation (Sw) for a given height above the Free Water Level (FWL). On the contrary, lithofacies deposited in different depositional environments, might exhibit similar petrophysical properties and dynamic behavior. The authors emphasize on the need to have a good understanding of the original facies, depositional environments, subsequent diagenetic processes and rock-fluid interaction (via SCAL) to be able to unravel the relationships between lithofacies, petrophysical groups and rock types. A workflow for carbonate rock typing addressing some of the industry pitfalls and the differences between lithofacies, petrophysical groups and rock types are presented in this paper. Introduction - Nomenclature Before proceeding into the rock type description and its link with geology and SCAL, it is important to provide a few basic definitions of the common technical terminologies found in the literature such as lithofacies, facies associations, petrophysical groups, rock types and flow units. In this paper we define lithofacies or lithofacies types as a depositional facies, or lithotype, based on sedimentary texture (Dunham 1962; Embry and Klovan 1971), grain types (skeletal grains, peloids, ooids, etc.), and, optionally, sedimentary structures (cross-bedding, bioturbation, lamination, etc.). Typical lithofacies types are skeletal wackestone, skeletal-peloid packstone or cross-bedded ooid grainstone. Facies associations are groups or bins of lithofacies from the same depositional environment/facies tracks with common f-k relationships/trends. Petrophysical groups are units of rocks (can consist of multiple lithofacies) with similar petrophysical correlations and common porosity and permeability bins in the f, k domain. Petrophysical groups can be characterized by conventional core analysis and Mercury Injection Capillar Pressure (MICP) data. Flow units are petrophysical groups with spatial continuity at the Field scale. They define the hydraulic units or flow zones (layering) of the reservoir, which should be preserved during upscaling. So, flow units, hydraulic units and speed zones are all expressions with similar meaning. Rock types are units of rock deposited under similar geological conditions (not a must), which experienced similar diagenetic processes resulting in a unique pore network and wettability, which is translated into a unique f-k relationship, capillary pressure profile and relative permeability dataset for a given height above the FWL. Rock types are therefore characterized using SCAL data. MICP and conventional core analysis are not enough to characterize rock types.