Abstract
Log calculations of porosity and permeability from case studies on staged field experiment (SFE) wells sponsored by the Gas Research Institute (GRI) were compared to core. In good hole conditions, log-derived porosity and permeability compared to core analyses were favorable. However, when rugose hole occurred, pad-type tool measurements were questionable. A technique for reconstructing the bulk density reading in poor hole conditions was needed.
The conventional method to estimate the bulk density in rugose hole replaces the questioned density measurements with neutron or acoustic measurements. However, this conventional method may not represent the changes in the true bulk density. The new method employed, possible only if elemental log measurements are available, reconstructs the bulk density in terms of its probable magnitude using mineral information. The elemental measurements allow the derivation of mineralogy, which is valid in spite of rugosity.
The new method of density reconstruction in rugose hole used the log-calculated minerals. Porosity was calculated from the acoustic log and clay minerals. From these two porosity estimates and the matrix density derived from the minerals, an estimated maximum bulk density (bulk density derived from clay porosity) and minimum bulk density (bulk density derived from acoustic porosity) are calculated. In areas of good hole, the recorded bulk density was retained. In areas of rugose hole, the most probable bulk density was between this maximum bulk density and minimum bulk density. A discriminator system involving the caliper was used to determine the rugosity of the borehole.
Using the rugose hole discrimination, the dielectric and microresistivity log measurements were selectively utilized in good hole with log element-derived mineralogy to produce an estimate of variable porosity-cementation exponents, level by level. A "wet" resistivity, Ro ("wet" refers to the assumption of water without hydrocarbons in the pore space), was calculated and compared to measured resistivity where the hole was smooth. An overlay in a wet zone or shale zone verified the mineral modeling of clay type, abundance, water resistivity and porosity. The variable exponents were then used in water saturation calculations. The mineral-based methodology when logs of elemental concentrations were available, was applicable in spite of rugosity that invalidated the bulk density measurement. The calculations implementing the reconstructed density log resulted in favorable correlations with core porosity and, additionally, permeability and saturation on the GRI wells studied.
