The success or failure of secondary and tertiary recovery operations can depend in large measure on maintaining the quality of the water chosen to carry the EOR chemicals and to displace and move the oil through the formation matrix toward the producing wells.
Some commonly accepted indicators of poor water quality include the presence of corrosion and its by-products, carbonate and sulfate scale formation and deposition, oxygen and elevated dissolved acid gas levels, high bacterial populations, heavy suspended solids load and oil carryover.
At present, engineers and operators typically rely on periodic, yet relatively infrequent, monitoring of the injection water in order to obtain the data necessary for good water management. Periodic monitoring however may allow occasional but potentially damaging system upsets to occur without detection.
The relative uncertainty of intermittent sampling can be greatly reduced by combining the benefits of continuous in-line filtration with controlled flow rates, accumulated volume, and monitoring of scale and corrosion tendencies. Changes in water quality can therefore be quickly recognized and appropriate, timely measures initiated.
This paper discusses the installation of a Water Quality Control Station, the individual components, and the contribution of each to the definition of water quality problems. A key field case history is presented to show how the concept is applied.
Water quality is a measure of relative degree of suspended solids plugging which occurs when a given volume of water is passed through a membrane filter of a given pore size, usually 0.45 micron. Suspended solids will either be deposited in the tubular goods or carried to the formation where they can act as plugging agents. Poor water quality is most commonly indicated when the amount of the following particulates exceeds flood design specifications:
formation fines and sand from supply wells or producing wells,
corrosion by-products including iron oxides, iron hydroxides, and iron sulfides,
carbonate or sulfate scale particles,
bacteria populations, and
such other organics as oil carryover, plunger pump lubricants, asphaltines, and paraffins.
Any or all of the suspended solids types can be present in a single secondary or tertiary project.
Early definition of water quality problems is extremely important in secondary and tertiary recovery operations to maintain injectivity and help ensure financial success. Tight reservoirs with permeabilities of 3-15 millidarcies, such as Wyoming's Shannon and Sussex sands, benefit especially well through the use of quality injection water.
All too often the injection water "looks good" visually, which delays detection of the water quality problem until expensive damage has occurred to one or more components of the injection system. Early problem detection provides the added benefit of minimizing power requirements, costly equipment repair and replacement, and injection well cleanups and workovers. Also, cost effective chemical treatment programs can be initiated and monitored early in the life of the waterflood.
Intermittent problems with suspended solids can occur more frequently than is generally realized.