Connate Water in Oil and Gas Sands

Abstract

Several investigators have reported evidence of the existence of native orconnate water in oil-and-gas-bearing strata. Both water and salt have beendetected in cores of oil sands that yielded oil and gas exclusively whenproduced. Electrical logs often reveal unexpectedly low resistivity through oiland gas sands, indicating the presence of some material that is a considerablybetter conductor of current than oil and gas. Furthermore, calculations of theoil and gas content of reservoirs based upon production and reservoir-pressuredecline data indicate appreciably lower values than are arrived at under theassumption that the pore space is occupied completely by oil and gas. Origin and Occurrence of Connate Water in Oil-and-Gas-Bearing Formations The view has been expressed, 1O–15 and is generally agreed upon, that theexpulsion of oil and gas material from source beds and the subsequent migrationand accumulation occurred in the presence of salt water. As the strata thatusually form reservoirs for oil and gas were laid down by, or in the presenceof, ancient seas, the pore space within the strata must have been saturatedwith salt water before the entrance of the oil and/or gas. Thus, theaccumulation of oil and/or gas in the upper regions of structures suitable forreservoirs must have required displacement of the connate watertherefore. In the absence of any forces other than gravitational, it would be expectedthat the displacement of the connate water out of the zones finally occupied bygas and oil would be complete, and that the vertical distribution 'of gas, oil, and water in a reservoir would be strictly in the order of their densities.Apparently, however, surface and interfacial forces that will not permitcomplete segregation of the fluids are active within the capillary spacescomprising a reservoir. The effects of these forces on both accumulation andproduction of oil and gas have been the subject of study by a number ofinvestigators. T.P. 869