Desalination of Soil after Produced Water Releases Using Soil Flushing: Theory and Practice

HICKS, RANDALL T., R.T. Hicks Consultants, Ltd. Albuquerque, New Mexico and KRISTIN FARRIS, Rice Operating Company

A recent study funded by the American Petroleum Institute and Rice Operating Company found that most sudden and accidental produced water releases do not pose an unacceptable threat to ground water quality. However, such releases can effectively sterilize soil and cause temporary or long-term environmental impairment of the land surface. Returning the surface area affected by produced water releases to its original productive capacity can be accomplished by flushing the salt below the root zone with water. Where clay minerals are abundant in the soil horizon, amendments such as gypsum may be required to fully restore the soil.

The protocol for soil flushing is quite simple. First, the operator must add sufficient water to move chloride below the root zone. In sandy, highly permeable soils, this process will occur naturally after precipitation events. Natural restoration, while effective, is slow and a favorable outcome is not always certain. The addition of water immediately after rainfall events accelerates this natural process by increasing the moisture content of the soil and deepening the infiltration profile (causing salt to dissolve and move downward by gravity). At sites where clay swelling occurs due to sodium concentrations in the released water, chemical additives must be incorporated into the program to improve the permeability of the soil before soil flushing can bee effective. The addition of gypsum to the soil causes calcium (from dissolving gypsum) to exchange with sodium (from the produced water release) within the clay minerals – thereby reducing the clay swelling and improving soil permeability. In arid climates, covering the affected area with plastic during this process minimizes evaporation and eliminates the upward movement of soil water.

Periodic soil testing determines when high chloride concentrations are below the root zone and re-vegetation can begin. Re-vegetation is important for two reasons. First, a re-vegetated landscape is generally considered “returned to its productive capacity”. Second, evapo-transpiration is required to minimize infiltration of precipitation and the attendant downward movement of the chloride in the unsaturated zone. In arid climates, a vegetative “cap” can prevent the downward migration of chloride to ground water.

In practice, high winds, caliche layers and other factors can create problems for cost-effective soil flushing programs. The practical experience outlined in this presentation may allow operators to avoid these pitfalls.