Evaluation of a Cation Exchanging Tracer to Interrogate Fracture Surface Area in EGS Systems

Abstract

Laboratory tracer experiments were conducted to evaluate the potential use of lithium ion as a cation-exchanging tracer to interrogate relative changes in fracture surface area in EGS reservoirs. Tracer experiments were conducted at two different temperatures (225C and 300C) in high-pressure columns packed with a crushed amphibolite schist from Fenton Hill, NM using a synthetic water as the working fluid. Lithium transport through the columns was compared to that of bromide and 1,5 naphthalenedisulfonate (two conservative tracers with diffusion coeffiecients that vary by a factor of 3-4) to extract surface area estimates. Lithium as a tracer has the advantages that it does not thermally decay, and it adsorbs weakly according to a simple reversible process (cation exchange), which simplifies test interpretations. The use of lithium as a tracer may be very well suited to single-well tracer tests where return concentrations are much higher than in interwell tests (thus avoiding limitations imposed by lithium background concentrations in reservoir water), and it could be especially useful for interrogating additional surface area created by a stimulation procedure. Information on additional surface area could in principle be deduced from relative lithium responses before and after stimulation without having detailed knowledge of lithium sorption behavior onto fracture surfaces (which would be required to obtain absolute estimates of surface area). Cesium ion is discussed as a potential alternative to lithium that would have lower background concentrations and stronger sorption behavior, making it more suitable to interrogating systems with low surface-area-to-volume ratios.