Water Geochemistry as a Tool for Tracing Contaminant Fluids and Calculating Flowback Water Recovery in Saudi Arabian Oil and Gas Fields
Chemical and isotopic fingerprinting techniques provide significant tools for distinguishing formation waters from contaminant fluids (mud filtrate, completion brine, and spent acid) in produced water samples, taken during downhole and drill stem tests from exploration wells. As a practical benefit, these techniques can help to constrain appropriate testing intervals, thereby minimizing the number of poor or inconclusive well tests and avoiding the need for redrilling and retesting. In the present paper, a total of more than 100 produced water samples were taken from the Upper Cretaceous to Middle Ordovician units in exploration wells throughout Saudi Arabia, and analyzed for their major components (Na, Ca, K, Mg, Cl, HCO3 and SO4) as well as (partially) for their minor elemental (Sr, Ba, Br and Fe) and stable isotopic composition (δ18O and δD). The application of specific thresholds for physico-chemical parameters, such as ion balance (>5-10%), elevated K/Cl and δD ratios, and extreme pH conditions (< 3.5 and > 8.5), allowed a pre-selection of samples with the highest mixing proportion of contaminants or with a potential analytical uncertainty. Measured reservoir temperatures from downhole tests (tRes) were compared with calculated subsurface temperatures (tMg), which were reconstructed by the Mg-corrected Na-K-Ca geothermometer for each sampled interval. The detection of discrepancies between tRes and tMg represents a successful geochemical tool for recognizing the formation of condensed water during production (e.g., in deep gas wells), as well as the presence of contaminant fluids (from drilling or intervention). As a practical benefit, the chemical fingerprinting of produced water types allows a quantitative determination of recovery rates for fluids used for drilling, stimulation or hydraulic fracking. A representative and selective sampling of produced water from each reservoir unit and reference fluids (e.g., mud drilling fluid), combined with on-site measurements of physical-chemical parameters, as well as advanced analytical methods (e.g., Li for Mg-Li geothermometer), are essential to develop the most efficient technologies for tracing the composition and origin of produced water.
AAPG Search and Discovery Article #90188 ©GEO-2014, 11th Middle East Geosciences Conference and Exhibition, 10-12 March 2014, Manama, Bahrain