OPTIMAL MONITORING NETWORK DESIGN FOR GROUNDWATER QUANTITY AND QUALITY: A CASE STUDY

Mexico is one of the top crude oil suppliers to the United States of America, however, in the last years, shale gas has been explored in Northern Mexico as an alternative energy source to conventional oil and gas. Some wells drilled in the Burgos basin by fracking the Eagle Ford formation, are indicative of substantial reserves in this shale play. Development of unconventional gas reservoirs implies the consideration of economic, legal and environmental challenges. Most of the northern Mexico border has a semi-arid condition, groundwater is the main source, then water competition between former users (ecosystems, agriculture, and people) and additional demand water for i) drilling, ii) hydraulic fracturing, iii) maintenance water (brine dilution fluid) and iv) domestic water for the oil services population, can affect local groundwater resources. The understanding of baseline geochemistry and water level distribution with an optimal monitoring network is an essential prerequisite for identifying impacts associated with hydraulic fracturing and for the evaluation of future potential effects of unconventional oil and gas development. In this investigation, we present two cases of monitoring network optimal designs for groundwater levels and multiple groundwater quality parameters. The methodology considers the selection of those wells that minimize the estimate error variance for the selected variable and allows the use of different weights for the error variance within the aquifer. The hydrogeological conceptual model and flow systems definition support the design of the optimal monitoring network, incorporating the combination of geostatistical, Kalman filter and heuristic optimization methods. The optimal design is a cost-effective methodology to avoid redundancy in data acquisition. National agencies in Mexico should prudently consider the implementation of a water-quality monitoring program, before starting hydraulic fracturing and gas production. The effects of hydraulic fracturing on groundwater may be difficult to detect, future assessments to evaluate impacts of development of shale gas require a detailed pre-drill water quality monitoring program, especially within and around gas production areas.