Heterogeneous Porosity Distribution Research Articles

Practical Application of NMR Logging in Carbonate Reservoirs

Summary Nuclear magnetic resonance (NMR) data acquisition and interpretation in carbonate reservoirs is much more challenging than in sandstones, where it is a well-established technology. Heterogeneous porosity distribution, a broad range of pore sizes, a wide variety of complex textures, and low surface relaxivity combine to complicate the picture considerably. The successful practical application of NMR in these reservoirs requires the development of acquisition and interpretation techniques specifically suited to the task. In carbonate reservoirs dominated by intercrystalline or inter-granular porosity, NMR can deliver accurate estimates of porosity, permeability, bound fluid volume, and residual oil saturation (ROS). In heterogeneous carbonate reservoirs more complex interpretation models are required, normally based on the integration of whole core and log data. NMR answer products, based on these new techniques, are presented and validated with core data and by comparison with other logging measurements.

The Evaluation of the Quality of Cork Planks by Image Analysis

The quality evaluation of cork planks in relation to porosity may use image analysis techniques. Results are presented on the number, dimensions, form and orientation of the lenticular channels in tangential, transverse and radial sections of cork planks of different thickness and qualities. The porosity coefficient (% pore area in relation to total area) varied between 1.1%-18.9% and 2,1%-16.4%, respectively in tangential and transverse/radial sections and is determined mostly by the number of pores larger than 0.81 mm 2 and by their average area. The porosity may be used to define cork quality classes. Due to the heterogeneous porosity distribution in cork planks, an assessment of quality made on small cork samples will give a result with a large error. For an error below 15%, the observation in the tangential section should be made on a sample with a minimum area of 225 cm 2 (e.g. 15 cm x 15 cm) and in the transverse and radial sections on a sample with a minimum length of 15 cm.

The origin and distribution of porosity in the Zechsteinkalk (Upper Permian) of Hewett Field, southern North Sea

The Zechsteinkalk of the Hewett Field is characterized by extremely heterogeneous porosity distribution. The two most closely spaced wells (only 200 m apart) show one of the largest differences in average porosity (1.8 to 7.6%). In order to locate future wells and achieve 'reasonable' OGIP estimates, it is therefore necessary to model porosity distribution. Facies type has some influence on porosity development. Tidal flat muds developed at the shoreward margins of shoals have only very low porosity, whereas inter-shoal and main shoal facies have moderate porosity, on average. Extreme porosity variation within facies is caused by differential diagenesis. Porosity has been created by leaching and dolomitization and destroyed by early carbonate cementation and by anhydrite cementation. Anhydrite is volumetrically the most significant, occluding 20% (bulk volume) porosity in many intervals and completely cementing many fractures. The scale of the variability that can be derived from modelling of porosity-affecting diagenetic processes is mostly much greater than that found between closely spaced wells, particularly in the case of anhydrite cementation. In spite of this uncertainty the geological model represents an improvement on simple independent porosity contouring. The gross trends of porosity distribution, defined by the sedimentological model, have helped enhance confidence in a seismic porosity model and have helped with estimates of gas resources.

Fluid flow and mineral reactions at high temperatures and pressures

Although fluid flows in response to differences in hydraulic head, chemical reactions occur in response to changes in fluid pressure. Depending on the flow direction, cross-sectional area, and hydraulic conductivity of the system, flow of fluids through porous media may lead to increases and/or decreases in fluid pressure along the flow path. At high temperatures corresponding to those in the supercritical region of the system H 2 O, mass transfer among minerals and aqueous fluids in hydrothermal systems is sensitive to changes in fluid pressure as well as temperature. As a consequence, mineral solubilities may both increase and decrease in the direction of fluid flow, causing development of isolated channelways and a heterogeneous distribution of secondary porosity and alteration products. For example, vertical downward flow is accompanied by increasing fluid pressure if the Darcy velocity is less than the hydraulic conductivity of the medium. Hence, for a constant flow rate an abrupt increase in cross-sectional area may lead to dissolution of quartz, but precipitation of sulphides.