Rudolf J. Novotny*
Production of unwanted water from oil and gas wells has consistently burdened the industry. Many chemical systems have been developed to selectively reduce the flow of water into the wellbore 1 but not appreciably affect the production of hydrocarbons. Although these systems work well in the laboratory, many field applications have proven less than successful. The limited success of these types of systems may not be due to product failure but to poor candidate selection. The functioning mechanism of the chemical systems limits their application to wells where the production of water moves through the formation matrix, not from a channel or fracture.
A methodology has been developed utilizing production data and Darcy's flow equations to evaluate well candidates for these and other matrix applications. This evaluation technique will assist the engineer in deciding the potential mechanism of excessive water influx. Once the source or conduit for water production is determined, the appropriate type of water control application can be designed and applied. Examples of candidate evaluations and field application results are presented.
Many oil and gas wells also produce water. This water production can occur on initial completion or later in the life of the well. A small amount of water production does not create a problem and in some instances is beneficial; however, when water production becomes excessive, then consideration for reducing it should be made. Excessive water adds cost to the production of the well. More water means additional lifting, treatment, and separating cost. It can also reduce the hydrocarbon production, by increasing the head in the wellbore, reducing the drawdown, or in gas wells, higher density water in the production string can lead to early shut-in or the necessity to install artificial lift.
Fluid distribution vertically in a reservoir is important. The relative amounts of oil, water, and gas present at a particular level in the reservoir determine the fluids that will be produced by a well completed at that level, and also influence the relative rates of fluid production. In most "water wet" sandstone formations, the water saturation varies from 100% below the oil zone to progressively lower percentages at points higher in the oil zone. The zone from a point of 100% water saturation to some point above where water saturation is fairly constant is called the transition zone. Relative permeability relations permit both water and oil to be produced from the transition zone.