The Role of Vaporization in High Percentage Oil Recovery by Pressure Maintenance
Authors: Cook Alton B. , Spencer George B. , Bayazeed Abdo F. 
Published: December 2007

Abstract
Gas cycling is generally considered a much less efficient oil recovery mechanism than waterflooding. However, recoveries from some fields have been exceptionally high as a result of gas cycling. Recovery from the Pickton field, for example, was calculated to be 73.5 percent of the stock-tank oil originally in place. In evaluating pressure maintenance projects, determining how much of the recovery is due to displacement by gas and determining how much is due to vaporization of the immobile oil in the flow path of the cycled gas is very difficult. Even though most of the oil is recovered by displacement, the success of a project may depend on the amount of oil vaporized. A limited number of experiments have been performed with a rotating model oil reservoir that simulates gas cycling operations and allows a separation of the oil from the free gas flowing into the laboratory wellbore at reservoir conditions, thus revealing which is displaced oil and which is vaporized oil. It has been determined that the amount of vaporization is significant if proper conditions exist. These experiments show that oil vaporization depends on pressure, temperature, volatility of the oil and amount of gas cycled. Increases it each of these conditions increase the volume of oil vaporized. Data from six experiments affecting vaporization are presented to illustrate reservoir conditions that range from favorable to unfavorable. In these experiments recovery by vaporization ranged from 73.6 to 15.3 percent of the immobile oil (oil not produced by gas displacement).

Introduction
Between 1930 and 1950, gas cycling was a popular oil recovery practice, especially for the deeper reservoirs. Later, with many case history-type studies published for both gas cycling and waterflooding, it was generally believed that waterflooding was far superior to gas cycling. even when gas cycling was conducted as a primary production procedure by complete pressure maintenance. A good example illustrating the advantage of waterflooding over gas cycling is given in a paper by Matthews on the South Burbank unit where gas injection was followed by waterflooding. The author concluded in part that "Early application of water injection, without the intervening period of gas injection, would have recovered as much total oil as ultimately will be recovered by waterflooding following the gas injection, and total operating life would have been shortened". This appears to be a logical conclusion. However, it should not be applied to all fields. Pressure maintenance with gas in the Pickton field, as reported by McGraw and Lohec, will result in a much larger percentage of oil recovery than was obtained in the South Burbank unit. The great success in the Pickton field resulted partly from vaporization of the immobile oil in the flow path of the cycled gas. The amount of vaporization is related to the following conditions: volatility of the oil as reflected by the API gravity of the stock-tank oil; reservoir temperature; reservoir pressure during gas cycling; and the amount of gas cycled. Therefore, the U. S. Bureau of Mines is investigating these effects on vaporization in a research project using a model oil reservoir. Three different stock-tank oils having 22, 35 and 45 degrees API gravities are being used as base stock to synthesize reservoir oils. Experiments are being performed to determine vaporization at 100, 175 and 250F and at 1,100, 2,600 and 4,100 psia. This is a progress report showing the results from six experiments. Other Bureau of Mines reports concerning vaporization are listed.

LABORATORY EQUIPMENT AND PROCEDURES
The equipment consists of an internally chromiumplated steel tube packed with finely sifted Wilcox sand. The tube is approximately 44 in. long and has an ID of 1 3/4 in. The sand section contains approximately 570 ml of voids, has a porosity of 32 percent, and a permeability to air of 4.3 darcies. A unique feature of the laboratory reservoir (Fig. 1) permits the tube part to rotate at 1 rpm while the outlet and inlet heads are held stationary. The outlet end contains diametrically opposed windows to permit observation of the flowing fluids, and two valves, one on the top and the other at the bottom. Oil and free gas, when being produced simultaneously, can be separated by manipulating the two valves to keep a gas-oil interface in view through the windows. Thus, only gas is produced through the top valve and only oil flows through the bottom valve. The laboratory equipment was designed to study vaporization. Therefore, a uniform reservoir was made using dry sifted sand as opposed to using a consolidated sand core with interstitial water. Furthermore, the reservoir was tilted to minimize fingering of gas.