Effect Of Gas Saturation On The Pressure Gradient In The Oil Zone
Authors: Baptist Oren C. , Smith Charles R. 
Published: December 2007

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Abstract
A study of pressure gradients in porous sandstones in the presence of various gas saturations was made in the laboratory to define the proper gradients to use when calculating fluid flow in the SS-1 sand, Elk Hills field, Calif. An accurate pressure gradient was needed to use in calculations of possible downdip migration of oil.
Tests were made on long, upright, artificially consolidated sandstone models which were properly sealed, and had the desired physical properties. The effective oil permeability was about 1.5 darcys. The models contained gas-saturated oil and an irreducible water saturation at the beginning of each series of tests. The pressure was lowered by increments, releasing gas from solution, and allowed to stabilize before reading the pressure differential between the top and bottom of each of four sections of the model.
A gas cap formed rapidly as gas was released. The pressure gradient in the oil zone remained at that due to oil density throughout the tests. The pressure gradient in the gas cap was that due to gas density. The gas saturation at which the oil pressure gradient become less than that of the oil column was not determined, but it nay have been as high as 8 per cent of pore space.
Tests were made on core samples and a long, horizontally-mounted, revolving, artifically-consolidated sandstone model to determine the critical gas saturation. The critical gas saturation, best determined from kg/ko relationships, was found to be about 3 per cent of pore space.

Introduction
The pressure gradient used to convert observed reservoir pressures to pressures at a common datum plane is especially important in the Elk Hills field, Calif.., Naval Petroleum Reserve No. 1. The rate of oil production from the SS-1 sand is small compared to that from other, nearby fields which produce from the same zone. This unequal rate of fluid withdrawal from fields connected by a common aquifer would cause downdip oil migration in the Elk Hills field if it were not prevented from doing so by water injection below the oil-water contact. This attempt to maintain equilibrium pressures so that the oil will neither move upwards into the gas cap, nor downdip into the aquifer requires unusually accurate pressure control.