Geophysical Surveys to Maximize Horizontal Well Yield and Extend Production in Depleting Aquifers

Directionally drilled horizontal wells are an effective way to increase well yield from thin aquifers and extend production in aquifers with declining saturated thickness. The capacity of a horizontal well is a function of the permeability of the formation and submergence of the screen. Selecting a viable horizontal well site requires that the properties of the aquifer be understood along a long linear path. This can be a difficult task in patchy aquifers or aquifers that lie in narrow channels. Geophysical methods offer an economic means to map aquifer properties of large areas to direct test drilling programs and find favorable horizontal well sites efficiently.

One such thin and patchy aquifer is the Ogallala Aquifer in the Texas Panhandle. The Ogallala formation was deposited as an extensive braided stream system that drained the eastern slope of the Rocky Mountains. The thickness of the sand and gravel deposits is highly variable with the thickest deposits filling stream valleys eroded into the underlying land surface. The thickness varies from about 0 to over 500 feet in a few buried channels.

The aquifer has been extensively pumped for decades and significant dewatering has occurred. In portions of the aquifer the saturated thickness is becoming a limiting factor controlling the production of a well and the sustainability of the supply. While the mining is not sustainable, wells in deeper portions of the aquifer with coarser sands will provide higher yields for longer periods. Thinner parts of the aquifer become dewatered but the deeper parts of the aquifer still contain saturated permeable material. While it is not possible to predict the thickness of the Ogallala based on surface topography, there are geophysical methods that can be used to map the thickness of permeable sand and gravel units above the underlying units.

A high resolution electrical resistivity survey conducted in west Texas for an industrial client. The client had an extensive well field to provide cooling water for an industrial facility but regional declines in water levels continues to reduce the capacity of most of the wells. Seven miles of resistivity lines were used to map the deeper portions of the aquifer. Subsequent test borings confirmed the channels, which varied from about 100 to about 200 feet deep, with the coarsest formation in the deepest part of the channels. A horizontal well was constructed in one of the deeper channels with a capacity of about 900 gpm. The additional submergence provided by siting the well in a deeper channel with coarser sand and gravel significantly increased the capacity of the well and extend the usable life of the aquifer in the face of declining water levels. The net effect is to more efficiently mine a depleting aquifer which raises long term sustainability and management issues. This is at best a short term solution but it can keep critical infrastructure in service for several years to a few decades until sustainable solutions can be developed.