Estimates Of Effective Porosity Research Articles

Estimation of effective porosity in large-scale groundwater models by combining particle tracking, auto-calibration and <sup>14</sup>C dating

Abstract. Effective porosity plays an important role in contaminant management. However, the effective porosity is often assumed to be constant in space and hence heterogeneity is either neglected or simplified in transport model calibration. Based on a calibrated highly parametrized flow model, a three-dimensional advective transport model (MODPATH) of a 1300 km2 coastal area of southern Denmark and northern Germany is presented. A detailed voxel model represents the highly heterogeneous geological composition of the area. Inverse modelling of advective transport is used to estimate the effective porosity of 7 spatially distributed units based on apparent groundwater ages inferred from 11 14C measurements in Pleistocene and Miocene aquifers, corrected for the effects of diffusion and geochemical reactions. By calibration of the seven effective porosity units, the match between the observed and simulated ages is improved significantly, resulting in a reduction of ME of 99 % and RMS of 82 % compared to a uniform porosity approach. Groundwater ages range from a few hundred years in the Pleistocene to several thousand years in Miocene aquifers. The advective age distributions derived from particle tracking at each sampling well show unimodal (for younger ages) to multimodal (for older ages) shapes and thus reflect the heterogeneity that particles encounter along their travel path. The estimated effective porosity field, with values ranging between 4.3 % in clay and 45 % in sand formations, is used in a direct simulation of distributed mean groundwater ages. Although the absolute ages are affected by various uncertainties, a unique insight into the complex three-dimensional age distribution pattern and potential advance of young contaminated groundwater in the investigated regional aquifer system is provided, highlighting the importance of estimating effective porosity in groundwater transport modelling and the implications for groundwater quantity and quality assessment and management.

Multiscale characterization of pore structure in carbonate formations: Application to the Scurry Area Canyon Reef Operators Committee Unit

Carbonate formations consist of a wide range of pore types with different shapes, pore-throat sizes, and varying levels of pore-network connectivity. Such heterogeneous pore-network properties affect the fluid flow in the formation. However, characterizing pore-network properties (e.g., effective porosity and permeability) in carbonate formations is challenging due to the heterogeneity at different scales and complex pore structure of carbonate rocks. We have developed an integrated technique for multiscale characterization of carbonate pore structure based on mercury injection capillary pressure (MICP) measurements, X-ray micro-computed tomography (micro-CT) 3D rock images, and well logs. We have determined pore types based on the pore-throat radius distributions obtained from MICP measurements. We developed a new method for improved assessment of effective porosity and permeability in the well-log domain using pore-scale numerical simulations of fluid flow and electric current flow in 3D micro-CT core images obtained in each pore type. Finally, we conducted petrophysical rock classification based on the depth-by-depth estimates of effective porosity, permeability, volumetric concentrations of minerals, and pore types using an unsupervised artificial neural network. We have successfully applied the proposed technique to three wells in the Scurry Area Canyon Reef Operators Committee (SACROC ) Unit. Our results find that electrical resistivity measurements can be used for reliable characterization of pore structure and assessment of effective porosity and permeability in carbonate formations. The estimates of permeability in the well-log domain were cross-validated using the available core measurements. We have observed a 34% improvement in relative errors in well-log-based estimates of permeability, as compared with the core-based porosity-permeability models.

Integrated Interpretation of Wireline and 3D Seismic Data To Delineate Thin Oil-Producing Sands in San Jorge Basin, Argentina

Summary Hydrocarbon-bearing sand units in San Jorge basin, Argentina, exhibit a wide range of spatial variability and thickness. In view of this, a typical well is planned to vertically intersect as many sand units as possible. There are several problems faced by the petrophysicist in assessing whether a given sand unit should be perforated: (a) discrimination between oil- and water-bearing sands is not trivial because of very low-salinity water, (b) there exist substantial vertical variations of effective porosity within an individual sand because of shale laminations, and (c) it is often impossible to assess lateral extent away from the well. We have successfully addressed most of these difficulties with an interpretation procedure centered on the 2D inversion of wireline array-induction data. Two-dimensional inversion of array-induction data is necessary for the accurate estimation of shallow and deep resistivities, as well as the invasion length in light of significant shoulder-bed effects. This procedure has been complemented with the use of borehole nuclear magnetic resonance (NMR) data to provide estimates of effective porosity within individual sand units. Finally, we have made use of geostatistical inversion of 3D post-stack seismic data to estimate the lateral extent of hydrocarbon-bearing sands laterally away from wells. We present several application examples that yield results consistent with borehole testing and production data.

Estimation of Effective Porosity, Tirrawarra Sandstone, Cooper Basin, Australia

Estimation of effective porosity in shaly sand formations is the prime parameter for reserve calculation and for good understanding of the behaviour of a formation during production. The Late Carboniferous to Early Permian, fluvio-deltaic Tirrawarra Sandstone is a kaolinite-bearing sandstone and an important hydrocarbon reservoir in the Cooper Basin. Petrographie point count and image analysis data from 130 samples, together with data from about 650 core samples and wireline log data from 14 wells of the Tirrawarra Sandstone in the Moorari and Fly Lake Fields, have been used to estimate effective porosity from sonic log.Based on integration of all data and with the knowledge of the volume fraction of kaolinite and associated microporosity (20% for kaolinite masses in the Tirrawarra Sandstone in the studied samples) effective porosity can be expressed as: ϕe=ϕsonic−0.2Vkwhere ϕe is effective porosity and ϕsonic is sonic porosity, and Vk is volume fraction of kaolinite.On average, estimated effective porosity is 2 porosity units less than the total porosity and it varies for depositional environments which have different kaolinite contents.

Effective porosity of unconsolidated sand; Estimation and impact on capture zone geometry

Effective and total porosity were measured for three sand samples, and a computer model was used to evaluate the impact of effective porosity (ne) on the geometry of a groundwater capture zone. Estimates of effective porosity ranged from 90 to 94 percent of values obtained for total porosity. While the magnitude of effective porosity is inversely proportional to capture zone area, the upgradient radius of capture is most sensitive to changes in the magnitude ofne. Incremental changes in the magnitude ofne have the greatest impact on capture zone radii for low values ofne. The results of this study suggest that: (1) a simple permeameter and vacuum pump apparatus can facilitate the estimation of effective porosity for unconsolidated sands, (2) the magnitude of effective porosity is close to that of total porosity for unconsolidated sand, and (3) accurate estimates of effective porosity are important for modeling the remediation of contaminated groundwater with capture zones.

Formation Evaluation and Gas Detection in Shallow, Low-Permeability, Shaly Sands of the Northern Great Plains Province

Summary Montana log, core, and production data are combined with geology in a systematic approach to improve log analysis of shallow Upper Cretaceous gas sands having a high silt/clay content. Qualitative (overlay) techniques are reviewed with emphasis on the normalized At-compensated-neutron overlay for gas detection.Log interpretation of the Bowdoin member of the Carlile shale and Eagle sandstone in the area peripheral to the Bowdoin Dome field is discussed in detail. Porosity-tool responses are examined with respect to observed lithology and mineralogy. Lithology crossplots, when compared with predicted log responses, may aid in detecting gas and minerals that could be related to natural fracture systems (e.g., gypsum and pyrite).Quantitative techniques for estimation of effective porosity, volume clay, and water saturation are presented. The total shale relation appears to give useful water saturation values. Log parameters relating to this equation are generalized and refined. Introduction With increasing gas prices, exploration for shallow, low-deliverability resources in the northern Great Plains province is becoming more attractive. Log interpretation always has been a problem in the Upper Cretaceous sands of north-central Montana due to the extreme shaliness of potential reservoirs, very fresh formation water, high water saturations, complex lithology, and indefinite logging assumptions. This paper suggests techniques to improve gas detection and formation evaluation, particularly in marginally economical areas peripheral or adjacent to production in the Bowdoin Dome field (Fig. 1).This report uses a synergetic approach to improve log interpretation. The lithology and mineralogy of the Bowdoin member of the Carlile shale and the Eagle sandstone are described in detail. This information is useful for making predictions of log response and in refining logging assumptions. Suggestions made for crossplot methods on the basis of these log responses may be useful to detect gas and/or to identify mineral components of the complex lithology. Finally, suggestions are presented for improving quantitative log interpretations. Lithology and Mineralogy The Upper Cretaceous clastic section in the Bowdoin Dome area is dominated by shales and clayey siltstones. The rock may carry various amounts of gypsum, pyrite, carbonates, and micas. Reservoir rocks may contain up to 50% clay minerals.The Bowdoin sand (Fig. 2) is a poor-quality reservoir rock except where fractured. Most of the gas storage is in silty laminae less than 1 cm thick. Permeability is low, and artificial stimulation methods are used to make the marginal wells more economical.Table 1 is a summary of the recent X-ray mineralogical analysis of 30 sidewall cores taken from two wells. The mineral composition for each well is expressed as weight percent and is averaged into] a tool analysis and a clay size fraction analysis.The sieve size distribution of Well 2962 cores (Figs. 1) consists of 60% silt (2 to 62 m), 35% clay (greater than 2 m), and 5% sand (greater than 62 m). The analysis shows the rock to be made up dominantly of quartz (52%), with the major clays being illite and kaolinite. Pyrite and gypsum are present in significant amounts.Well 0370 cores (Fig. 1) comprise a similar size distribution with 62% silt, 35% clay, and 30% sand. The mineralogy is nearly identical to Well 2962. JPT P. 1976^