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Land Subsidence, Salt Dissolution

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This report summarizes results of an extensive search for examples of land subsidence in central Kansas associated with rock salt dissolution caused by man's activities in exploring for and producing natural resources--oil, gas, water, salt--for nearly a century from the 1880s to 1976. As documented within the report, only 13 such areas are known, leading to the conclusion that such depressions are unusual and that the formation of a noticeable surface subsidence area is a rare event. There are only eight known subsidence areas associated with oil exploration or production, although an estimated 80,000 boreholes penetrated the Hutchinson Salt in Kansas. This is a ratio of one subsidence area for each 10,000 holes drilled through the salt. Only five documented surface subsidence areas are associated with the continuous production of salt in Kansas, largely by solution mining since 1888, or an average of one subsidence area for each 17 years of salt production.

Evidence is presented that solution of salt during modern rotary drilling using fresh water results in borehole enlargement within the salt section to about three times the diameter of the drilled hole, an amount too small to cause surface subsidence.

Evidence is also presented that solution of salt during early rotary drilling in the 1930s using fresh water resulted in borehole enlargement to five feet or more in diameter (bit size commonly nine inches) through the salt section. Gamma-neutron cased hole logs recorded years later are interpreted as indicating, opposite the lower, cleanest portion of the salt, the packing of former void space behind the casing with shale cavings. It is concluded that salt dissolution tends to be self-limiting due to caving shale in such early rotary holes. This conclusion also applies to case histories of two documented holes completed before 1935 which have no surface casing at all, and no plugging except in the cellars 10 to 17 feet below the land surface.

Ordinarily, no salt dissolution occurs after drilling ceases. This important principle is valid if shallow aquifers above the salt are adequately isolated by surface casing and/or by proper hole plugging as required by regulations of the Kansas Corporation Commission. This is the normal situation in the broad ten-county study area. The subject was investigated by a study of the ten principal aquifers, depths to 4000 feet, within the 4,000 square mile area of Russell, Lincoln, Barton, Ellsworth, and Rice Counties. These aquifers are oil reservoirs where hydrocarbon trapping conditions exist. In boreholes with properly isolated shallow aquifers above the salt, the deeper aquifers below the salt-although possessing static fillup levels higher than the top of the salt-will equalize pressure by flowing up or down the borehole from one aquifer into another without flow across the salt face, hence without dissolving the salt. This is true for all such oil and gas test holes regardless of how else the borehole is plugged, if at all. This principle accounts for the scarcity of surface subsidence areas in central Kansas due to salt dissolution.

It is further concluded that where aquifers above the salt are not isolated by casing or hole plugging, flow from them by gravity downward across the salt causes salt dissolution. This situation prevails in the Witt and Crawford Sinks in the Gorham Oil Field. After 20 years of sinking, these two areas are still subsiding, causing damage to Interstate Highway 70.

It is also concluded that subsidence areas around former salt water disposal wells (Panning, Berscheit, Hilton) were caused by casing failure which permitted disposed brine, unsaturated as to chlorides, to circulate across the salt. In such a system, the disposed brine falls down the borehole, across the salt face, and downward into a lower permeable zone by gravity flow, accelerated by increase in brine density as salt is dissolved, but the basic energy source is provided by oil well pumping units which lift oil and brine upward. With the abandonment of the oil wells, the energy input is terminated, brine flow ends, subsidence other than that due to compaction ceases, and the areas become stable.

It is an important observation of this investigation that all surface subsidence areas in Kansas related to salt removal have a common history of slow development in a time frame of months and years, but where near-surface materials consist of water-saturated unconsolidated sands and gravels, and the underlying bedrock is breached, a surface sinkhole formed in a few hours or days.


In June 1975, the author completed an extensive three-part research project concerning "Salt Dissolution in Oil and Gas Test Holes in Central Kansas," supported by the Oak Ridge National Laboratory (ORNL) of the United States Atomic Energy Commission (AEC) under Contract #78X-38283V. Excerpts from that investigation are included in the present report with permission of the successor agency, United States Energy Research and Development Administration (ERDA), Office of Waste Isolation (OWI), Oak Ridge, Tennessee. Dr. William C. McClain, Technical Project Director.

The author assumes full responsibility for the investigation and conclusions of this report, but wishes to acknowledge the indispensable help of the following geologists or engineers, all of whom contributed basic data from personal observation or experience:

Thomas H. Allan F. G. Holl
Heber Beardmore, Jr. R. P. Lehman
Virgil B. Cole William J. Luck
James R. Dilts Millard W. Smith
O. S. Fent Raymond A. Whortan.

In addition, the author thanks:

The author is particularly indebted to Thomas B. Piper for his thoughtful editorial review of early drafts. His knowledge and consideration have improved the accuracy of statements concerning the solution mining of salt and, hopefully, have contributed to the overall readability. In particular, the entire section on "Cause, Mechanism, and Time Framework: 1974 Sinkhole" bears the mark of Piper's skilled editing based on his own practical experience in managing brine wells, and might properly carry his "by-line."

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Kansas Geological Survey, Geology
Placed on web July 24, 2009; originally published February 1978.
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