WorldWide Drilling Resource

Environmental Monitoring by Thomas Kwader, Ph.D., P.G. Well Yields from Carbonate Aquifers Generally, most all water wells in sand aquifers are completed with a screen; or in carbonate strata (limestone or dolomite) which are usually a competent rock that will retain an open hole, without a screen, after it has been drilled. Open hole limestone wells are a bit peculiar and can have a wide range of pumping rates depending upon the type of porosity encountered. On the “low” yielding side, limestones can be soft and have poor permeability due to the lack of inter- connected pore spaces. Clayey limestones (or argillaceous limestones) have clay filling the interconnecting pores which limit the amount of water that can be pumped and may even produce “cloudy or turbid” water which can cause these sediments to settle in the well bore or surface tanks and water heaters. Pumping low yielding wells at a lower rate for longer periods of time may reduce the turbidity. Aggressive well development (surge block) is recommended to remove “loose clay” along the borehole, increase yield, and improve clarity of the water. At the other extreme, hard limestones and dolomites often have high yielding charac- teristics due to the high permeability of the fractures intersecting the borehole. Fractured rock aquifers generally have long interconnected fractures capable of transmitting water quickly due to the low friction coefficients along the fractures. Open hole fractured rock wells also, generally, take less time to develop because of the lack of fine-grained silts and clays in these formations. Well development of carbonate wells often includes the use of hydrochloric acid (15- 30%) which helps remove or dissolve the carbonate smear and compaction created by drilling with roller cone bits. One other characteristic of carbonate wells versus sand aquifer wells is the sand zones tend to be more uniform in permeability through their thickness. However, carbonate wells are less predictable, and may produce 90% of their water from 10% of the borehole. Tom michele@worldwidedrillingresource.com 27 WorldWide Drilling Resource ® NOVEMBER 2015