ECONOMICAL MONITORING OF PORE PRESSURES IN DEEP SALINE AQUIFER TO ASSIST IN INDUCED SEISMICITY EVALUATIONS

Disposal of brine, from oil and gas operations, within the deep Arbuckle Group saline aquifer in Kansas and Oklahoma has resulted in increased pore pressures, which are suspected to be the source of induced seismicity in the region. Due to the scarcity of monitoring wells that are open to the Arbuckle Group, data is lacking to estimate the regional changes in pore fluid pressures. Information about temporal and spatial pressure trends is necessary to validate geomechanical and fluid-flow models, both of which are necessary to develop technical tools for predicting the likelihood of induced seismicity in the future. It is commonly assumed that gages for monitoring pressure changes in deep aquifers need to be installed at perforation depths, which is typically several thousand feet below ground. Here we present a comprehensive methodology for estimating pressures deep in the injection zone using pressure data acquired near the top of the water column in a well. Such shallow data can be obtained for about one tenth the cost of acquiring deep pressure data, thereby enabling economic monitoring of deep aquifers on a regional scale. The methodology was tested in an experiment conducted in south-central Kansas to demonstrate that both pressure changes and absolute pressures within deep aquifers can be accurately estimated by monitoring pressures at the top of the water column in a well. The shallow gage was located 15 feet (4.5 m) below the top of the water column and the deep gage was installed at the perforation depth of approximately 5,000 ft (1525 m). The results are very encouraging and will be shared along with theoretical details of the methodology.