Porosity, Permeability, and Pore Structure of the Tight Mesaverde Sandstone, Piceance Basin, Colorado

Abstract

Summary: Special core analyses on 44 tight Mesaverde sandstone samples from the U.S. DOE Multiwell Experiment (MWX) were combined with petrographic investigations to relate the porosity and permeability of the cores to the pore structure of the rocks. Core analysis was performed on 1-in. [2.54-cm] -diameter horizontal plug samples with a computerized steady-state-flow measuring device that routinely measures gas flow rates with a resolution of better than 10−6 std cm3/s. All samples were selected from intervals expected to be gas-productive on the basis of wireline well logs and were taken from the portion of the interval that showed the lowest gamma ray log response. The core plugs were measured for dry permeability to gas, relative permeability at various water saturations, porosity to gas, and PV compressibility. Petrographic samples were taken directly off the plug ends and were analyzed with both an optical microscope and a scanning electron microscope (SEM). The petrographic study was explicitly directed toward observing the flow paths and pore structure deduced from the core analysis data. Petrographic observations revealed that the pore geometry of tight sandstone falls into three general categories: grain-supported primary pores, secondary solution pores connected by narrow intergranular slots (the most common pore structure in the Mesaverde), and matrix-supported grains. Reservoir properties measured from core analysis correlated fairly well with the observed pore geometry and showed trends associated with various depositional environments in the formation.