Calculo de porosidad en gas shale

SPWLA Thirtieth Annual Logging Symposium, June 11-14, 1989

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SHALE POROSITY - ITS IMPACT ON WELL LOG MODELING AND INTERPRETATION
R.B. Truman, W.E. Howard, D.L. Luffel ResTech Inc., Houston, Texas ABSTRACT Shale contains porosity which must be accounted for when evaluating well logs in shaly sands. Data is presented showing that in addition to relatively large amounts of total porosity(19-20Yo), shales may also contain significant effective porosity (8-1 2Yo). When the effective porosity is greater than zero, shale can store hydrocarbons. As a result, actual in-place and recoverable hydrocarbons may be significantly higher than would normally be estimated when this effective porosity has not been accounted for. This problem may be particularly acute in a shaly sandstone gasreservoir. An example is shown for core analysis data obtained from both sand and shale in a Frio formation gas reservoir in south Texas. Core data includes porosity measured at 40% relative humidity and 140°F, and total porosity measured at 230”F. Effective porosity is also presented utilizing the methods proposed by Hill, Shirley, and Klein’. A method of log analysis is then shown for modeling thetotal and effective porosity as obtained from core The result is a robust well log porosity interpretation model, which is in analysis. harmony with both the measured physical properties from the core analysis and the well log response. INTRODUCTION The industry has made significant progress in understanding and modeling shaly sandstone reservoirs for formation evaluation, but there are still somemajor problems. It is usually presumed that shale pore volume contains only immobile water, i.e., it cannot contain free hydrocarbons. As as result, many log computation models do not allow effective porosity in shale. If no attempt is made to determine the effective porosity in the shales or very shaly intervals or to compute this effective porosity from log analysis, then hydrocarbon pore volumewill be in error. If effective porosity does exist in a shale, it is capable of storing In addition, if the effective porosity is high enough it will be capable of hydrocarbons. holding hydrocarbons at reasonable capillary pressures or height above the free water level. DETERMINATION OF POROSITY

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Shale may contain effective porosity in addition to significant amounts of total porosity.For the purpose of this paper, total porosity, Ot, is defined as the interconnected pore space occupied by water (both bound and free) and hydrocarbons. Effective porosity, Oe, is defined as the pore space occupied only by free water and hydrocarbons. It should be noted that this differs from the conventional definition of total and effective porosity when dealing with the reservoir rocks.Previously total porosity has been defined as the ratio of the total void space in the rock to the bulk volume of the rock and effective porosity as the
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SPWLA Thirtieth Annual Logging Symposium, June 11-14, 1989

ratio of the interconnected void space in the rock to the bulk volume of the rock, expressed in percent, regardless of the type of fluid in the void space.z As part of alarger study, a conventional whole core was cut in a well in Seeligson Field, Jim Wells County, Texas. Thirty-six feet of whole core was recovered from 5,406’ to 5,442’. Thirty-seven samples were selected for analysis. Two porosity measurements were made on each sample. A summary of these porosity measurements are presented in Table 1. The humidity dried porosity was obtained by drying thesamples at 40° relative humidity and 140°F. This reportedly will leave at least one to two layers of absorbed water The convection dried porosities were determined by drying the on the clay surfaces samples at 230”F to constant weight. In the sandstone interval, samples 1S through 14, there are small differences between the convection dried and humidity dried porosity with the humidity dried being...