Cation Exchange Capacity (CEC) is commonly used toquantify electrical rock-fluid interfacial interactions andthe influence of clay minerals on electrical conductivityof clay-rich rocks and clay-bound water. Quantificationof CEC is, however, challenging. Conventional methodssuch as wet chemistry, multiple salinity, and membranepotential have limitations associated with theirapplication in in-situ condition, certain type offormations, and use of empirical correlations. Toovercome the aforementioned challenges, we recentlydeveloped a new method for CEC assessment in pureclay minerals by integrating nitrogen adsorptiondesorption,nuclear magnetic resonance (NMR), and Xraydiffraction (XRD) measurements. This method hasyet to be developed for complex rock compositions. Inthis paper, we (a) introduce a new workflow for CECevaluation in rocks with complex mineralogy andpresence of multiple types of clay minerals and (b) crossvalidatethe effective CEC from the new method againstthe directly-measured CEC from the wet chemistrymethod.The introduced workflow for CEC estimation in rocksincorporates quantitative characterization of mineralcomposition as well as XRD measurement, nitrogenadsorption-desorption and NMR measurements. To testthe workflow, we start with synthetic rock samples,which are manually mixed with different minerals withknown compositions. We perform XRD measurementsto obtain basal spacing variations. We performsuccessive controlled nitrogen adsorption-desorptionmeasurements as well as NMR measurements, andpropose an inversion algorithm to estimate surface areaof each mineral and volume of hydration water of eachclay mineral in multiple samples, respectively. Then, weuse these results as inputs for CEC estimation.Meanwhile, for actual rock samples, we start withquantitative XRD measurements to obtain thecomposition of different minerals and then quantifyvolume of hydration water, basal spacing variations, andsurface area for CEC estimation. Finally, we crossvalidateCEC estimation, both for synthetic rock samplesand actual rock samples, with CEC estimates from thewet chemistry method by use of inductively coupledplasma mass spectrometry (ICP-MS) measurements.The proposed workflow is successfully tested on bothsynthetic and actual rock samples. The CEC estimatesfrom the new method are in agreement with those fromwet chemistry method, with less than 3 meq/100gdifference for synthetic rock samples and with less than4 meq/100g difference for actual rock samples, whichincludes three types of clay minerals. The experimentalresults demonstrate reliability of the introduced methodfor CEC quantification. The outcomes of this paper canbe applied to variety of formations with complexlithology and can be considered as an onsite supplementapplication for mud logging.
展开▼
