Gas shales are economically viable hydrocarbon prospects that have proven to be successful in North America. Unlike conventional hydrocarbon prospects, gas shales serve as the source, seal, and the reservoir rock. Generating commercial production from these unique lithofacies requires stimulation through extensive hydraulic fracturing. The absence of an accurate petrophysical model for these unconventional plays makes the prediction of economic productivity and fracturing success risky.This paper presents an integrated approach to petrophysical evaluation of shale gas reservoirs, specifically, the Barnett Shale from the Fort Worth basin is used as an example. The approach makes use of different formation evaluation data, including density, neutron, acoustic, nuclear magnetic resonance, and geochemical logging data. This combination of logging measurements is used to provide lithology, stratigraphy and mineralogy. It also differentiates source rock intervals, classifies depositional facies by their petrophysical and geomechanical properties, and quantifies total organic carbon. The analysis is also employed to locate optimal completion intervals, zones preferable for horizontal sections, and intervals of possible fracture propagation attenuation. Resistivity image analysis complements the approach with the identification of natural and drilling induced fractures. We compare results from three different wells to show the effectiveness of the method for shale gas characterization.The methodology presented provides a means to understand the geomechanical and petrophysical properties of the Barnett Shale. This knowledge can be used to design a selective completion strategy that has the potential to reduce fracturing expenses and optimize well productivity. Though developed specifically for the Barnett Shale, the underlying ideas are applicable to other thermogenic shale gas plays in North America.
[1] An accurate description of water-or oil-bearing reservoirs strongly depends on a robust determination of their petrophysical parameters, e.g., porosity, permeability and fluid distribution. Downhole logging measurements are the primary means to formation evaluation; however, they do not directly provide the petrophysical properties of interest. To interpret well logging data, a range of empirical models are usually employed. These empirical relationships, however, lack scientific basis and usually represent generalizations of the observed trends. Since macroscopic rock properties vary depending on their microstructure, we suggest using a pore-scale approach to establish links between various petrophysical properties of sedimentary rocks. We outline a method for computing formation permeability using the proposed rock models. The method utilizes NMR (Nuclear Magnetic Resonance) logging data for the information about porosity and grain size. We also present an approach for prediction of acoustic velocities of model rocks. The proposed methodology is applied to the field data, and the corresponding interpretation results are included in this paper.Citation: Gladkikh, M., D. Jacobi, and F. Mendez (2007), Pore geometric modeling for petrophysical interpretation of downhole formation evaluation data, Water Resour. Res., 43, W12S08,
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractUsing an interfacial tracer technique, our experiments show qualitatively different trends of total interfacial area between the wetting and non-wetting phases as a function of saturation, depending on whether the system is strongly or weakly wetted. A strongly wetted system is defined as one in which the wetting phase can spread as a thin film on the solid surface. We assess the relative contributions of fluid/fluid and fluid/solid interfaces to the total area using thermodynamic arguments. The fluid/solid contribution to area plays a crucial role in explaining the measurements. The influence of interfacial area on relative permeability is not straightforward. Simple analysis based upon pore-level distribution of phases in a model porous medium allows quantifying the differences in the relative permeabilities for both weakly and strongly wetted systems, measured simultaneously with the interfacial area. Relative permeability correlates with fluid/solid area but not with fluid/fluid interfacial area.
Summary Many factors affect flow performance of perforated completions, including perforation-tunnel geometry, drilling and perforation damage, and formation-permeability anisotropy. The combined effect of these factors is usually accounted for by means of a single parameter: total-skin factor, which is an important input parameter for inflow-performance-relationship prediction and reservoir simulation. The Karakas and Tariq (1991) semianalytical skin-factor model is the most commonly used in the industry (Bell et al. 1995; Kabir and Salmachi 2009; Zhan et al. 2012). It assumes that the total-skin factor can be expressed as a linear combination of horizontal skin, vertical skin considering permeability anisotropy, perforation-damaged-zone skin, and other skins. The purpose of this study is to investigate the validity of the Karakas-Tariq semianalytical model in a realistic operational range of perforating-parameter values. For this purpose, we use computational-fluid-dynamics (CFD) software to simulate the production flow of a vertical cased-and-perforated well in a representative 3D geometric formation. We consider three effects: drilling damage, perforation damage (crushed zone around the perforation tunnel), and permeability anisotropy, assuming no pressure drop along the interior of perforation tunnels. All combinations of the three effects are considered. Computed skins are compared with the semianalytical skin model of Karakas and Tariq (1991). Computed results show good comparisons between skin factors calculated by use of CFD software and the Karakas and Tariq model (1991) for most cases. However, significant deviations in skin-factor comparisons are observed when both perforation damage and formation anisotropy exist if considering permeability anisotropy in the crushed zone. We also conclude that an additional skin-factor term, related to the ratio of the modified wellbore radius to the original wellbore radius, should be explicitly listed in the Karakas and Tariq model (1991) for perforation tunnels extending beyond the drilling-damage zone. Calculated CFD skin factors can be used as a database for predictive prejob analysis. Deviation between skin factors calculated by use of CFD and the Karakas and Tariq (1991) model highlights the need for improving industry methods to estimate skin factor in vertical perforated completions if considering crushed-zone anisotropy.
There are many factors affecting flow performance of perforated completions. These factors include perforation tunnel geometry, drilling and perforation damage, formation anisotropy, etc. The combined effect of these factors is usually accounted for via a single parameter – total skin factor, which is an important input parameter for Inflow Performance Relationship (IPR) prediction and reservoir simulation. The commonly used semi-analytical skin factor model assumes total skin factor can be expressed as a linear combination of horizontal skin, vertical skin considering anisotropy, perforation-damaged-zone skin, and other skins. The purpose of this study is to investigate the validity of the semi-analytical model in the realistic operational range of perforating parameter values. For this purpose, we use computational fluid dynamics (CFD) software to simulate the production flow of a cased-and-perforated well in a representative 3D geometric formation. We consider three effects: drilling damage, perforation damage (crushed zone around the perforation tunnel), and anisotropy, assuming no pressure drop along the interior of perforation tunnel. All combinations of the three effects are considered. Computed skins are compared with the semi-analytical skin model used in the industry. Computed results show good comparisons between skin factors calculated using CFD software and the model for most cases. However, significant deviations in skin factor comparisons are observed when both perforation damage and formation anisotropy exist. We also conclude that an additional skin factor term, related to the ratio of modified wellbore radius to the original wellbore radius, should be explicitly listed in the model for the case of perforations tunnel extending beyond the drilling damage zone. Calculated CFD skin factors can be used as a database for predictive pre-job analysis. Deviation between skin factors calculated using CFD and the model highlights the need for improving industry methods to estimate skin factor in perforated completions.
Productivity of a cased-and-perforated well depends on completion parameters (charge, shot density and phasing), formation properties (lithology, anisotropy, permeability, fluid properties and saturations), and environmental conditions (pressure and temperature). A shaped charge produces a jet of dense, typically solid material traveling at very high velocity which penetrates casing, cement, and formation. This penetration process creates a tunnel in the rock connecting the reservoir and the wellbore. Stress waves generated during penetration damage the rock around the tunnel, creating a complex zone of mechanically deformed rock material. Explosive and metallic debris may mix with damaged rock material. Depending on the specific conditions (mechanical rock properties, permeability, fluid viscosities, interfacial tensions, pressures and fluid storage volumes), reverse surge flow following the jet penetration partially or completely clears the tunnel of the charge and rock debris. The resulting tunnel is a rugose, tapered cylinder roughly characterized by its diameter and total depth of penetration. The degree of cleanup and total depth of penetration are two of the most critical parameters influencing flow performance of a perforated well. This work focuses on predicting penetration depth and providing a summary of recent progress and advances in this area. A review of industry penetration prediction methods is presented, including relationships based on surface perforation tests in concrete targets, as well as correlations of penetration depth with properties that could be measured by downhole logging tools. Despite a variety of available methods and published experimental data, penetration depth results are often inconsistent with each other and of questionable use in predicting actual downhole penetration. Moreover, most studies have concentrated on the influence of a single penetration variable. There have been no systematic studies of the interaction of parameters on perforation penetration. This paper emphasizes the critical need for such a study, given the difficulty of downhole measurements of penetration depth to finally achieve reliable penetration predictions, and suggests future directions and conditions necessary for such a study. Introduction Prior to the adoption of perforation technology in the oilfield industry, wells were completed "open hole" or "shot hole" (barefoot), sometimes employing liners. The perforated-casing completion was an important and necessary development as wells got deeper, and reservoir conditions became more complex. Early gun perforators were "bullet" devices, using actual projectiles (usually steel bullets) to penetrate the well casing. Prior to the introduction of lined shaped-charge perforators, almost all research emphasis was placed on increasing the depth of bullet penetration. Shaped-charge perforators were adapted for oilfield industry from the military in the 1950s (Pugh et al. 1952; Eichelberger and Pugh 1952; Turechek and Lindsay 1953; Eichelberger 1956) and have displaced the old bullet perforators (nearly to extinction) since then. This paper is focused on depth of penetration (DOP) of downhole shaped-charge perforators and summarizes recent progress and advances in understanding and predicting this important parameter. Productivity of cased-and-perforated wells is influenced by many different factors, including completion (charge characteristics, shot density and phasing), formation properties (lithology, anisotropy, permeability, fluid properties and saturations), and environmental conditions (pressure and temperature). The main goal of natural-perforation completion is to optimize production flow by re-establishing connectivity between wellbore and reservoir, and maximizing the effective wellbore diameter. Drilling and completion operations result in various types of formation damage, including drilling damage (mechanical damage, mud filtrate invasion, and fines migration), perforation damage (crushed zone of reduced permeability around perforating tunnels), partial penetration, well deviation, etc.
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