Thermal single-well injection-withdrawal tracer tests for determining fracture-matrix heat transfer area

THERMAL SINGLE-WELL INJECTION-WITHDRAWAL TRACER TESTS FOR DETERMINING FRACTURE-MATRIX HEAT TRANSFER AREA Karsten Pruess and Christine Doughty Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA K_Pruess@lbl.gov ABSTRACT Single-well injection-withdrawal (SWIW) tracer tests involve injection of traced fluid and subsequent tracer recovery from the same well, usually with some quiescent time between the injection and withdrawal periods. SWIW are insensitive to variations in advective processes that arise from formation heterogeneities, because upon withdrawal, fluid parcels tend to retrace the paths taken during injection. However, SWIW are sensitive to diffusive processes, such as diffusive exchange of conservative or reactive solutes between fractures and rock matrix. This paper focuses on SWIW tests in which temperature itself is used as a tracer. Numerical simulations demonstrate the sensitivity of temperature returns to fracture-matrix interaction. We consider thermal SWIW response to the two primary reservoir improvements targeted with stimulation, (1) making additional fractures accessible to injected fluids, and (2) increasing the aperture and permeability of pre-existing fractures. It is found that temperature returns in SWIW tests are insensitive to (2), while providing a strong signal of more rapid temperature recovery during the withdrawal phase for INTRODUCTION Development of enhanced geothermal systems (EGS) in rocks with insufficient natural permeability and fluid content requires stimulation, a treatment that involves injection of aqueous fluids at near-ambient temperatures and high pressures to enhance permeability and improve the flow and heat transfer characteristics of the reservoir. Such improvements arise from a combination of hydraulic effects (pressurization), mechanical changes (shear movement along pre-existing fracture planes that increases fracture permeability and porosity), chemical interactions between rocks and fluids that can dissolve and precipitate rock minerals, and thermal effects, including thermal contraction and thermal stress cracking (Kohl et al., 1995; Kohl and Megel, 2007; Xu et al., 2009). In Iceland it is common practice to finish off drilling of a geothermal well with a stimulation treatment that involves cold water injection over time periods from days to a couple of weeks (Tulinius et al., 1996; Bjornsson, 2004). In order to design and optimize stimulation treatments, methods are needed for characterizing and quantifying the results of stimulation treatments in terms of enhanced permeability of the fracture network, and exposure of additional rock surface for heat transfer to injected fluids circulating in the fractures (Nalla and Shook, 2005). Permeability changes may be evaluated by means of flow tests and repeat pressure transient tests, in which the time dependence of injection pressures in response to applied injection rates is monitored. Enhancements in fracture-matrix heat transfer areas from stimulation treatments cannot be assessed by such flow tests. Additional characterization of reservoir properties, and assessment of the success of stimulation treatments, may be achieved through testing with conservative or reactive tracers. The most common tracer test involves injection of aqueous solutes into one or more injection wells, and monitoring of tracer returns in fluids produced from offset production wells (Shook, 2001; Sanjuan et al., 2006). Interdiffusion of solute tracers between fractures and rock matrix produces characteristic tails in tracer breakthrough curves (BTC), that may permit determination of fracture-matrix interface areas (Pruess, 2002; Pruess et al., 2005; Shan and Pruess, 2005). One disadvantage of interwell tracer tests is that tracer breakthrough at offset observation wells may be weak and slow, requiring fluid sampling over extended time periods of weeks or even months. Also, suitable observation wells may not always be available. A possible alternative involves single-well injection-withdrawal (SWIW) tests, variously referred to as huff and puff, push-pull or injection-backflow tests, in which fluid with tracer is injected into a well and, after some quiescent or rest period, is produced out of the same well (Kocabas and Horne, 1987; Haggerty et al., 2001; Nalla and Shook, 2005; Ghergut et al., 2006, 2009; Neretnieks, 2007). One potentially attractive feature of SWIW tests as compared to interwell tracer tests (ITT) is the typically much shorter test duration, hours to a few days for SWIW as compared to weeks or months for ITT. This holds out the promise of obtaining test results much more quickly, providing significant economic benefits. Another potential advantage is that SWIW are much less affected than ITT by heterogeneities of flow in the fracture