Presenter
Dr. John McLennan
Department of Chemical Engineering
University of Utah
Abstract
The U.S. DOE recognized that new technologies are required to access heat from conduction-dominated, geothermal reservoirs. The concept of an engineered or Enhanced Geothermal System (EGS) in these sparsely fractured reservoirs, was first tested by the Los Alamos Scientific Laboratory in the 1970s, under the U.S. Department of Energy’s (U.S. DOE) Hot Dry Rock program. In its simplest form, an EGS consists of an injection well and a production well interconnected by a network of hydraulic fractures.
Developing an EGS reservoir has proven to be challenging. Despite numerous attempts worldwide over the last four decades, no commercial-scale EGS has been created. In 2014, the U.S. DOE initiated a multi-year initiative, known as the Frontier Observatory for Research in Geothermal Energy (FORGE) to develop technologies for characterizing, creating, and sustaining an EGS. Funds support the drilling, stimulation, and testing of two deep wells that will be drilled at the FORGE field laboratory in south-central Utah. The FORGE project is being led by the University of Utah.
As can be imagined, the propagation of fractures interconnecting these EGS wellbores is essential. The magnitudes of the in situ principal stresses are the most basic element for understanding propagation. In a pilot vertical well, in 2017 and in 2019, injection-based stress assessments were carried out in the openhole toe and in a cased/perforated section. The basic measurement program is described, and the familiar complexities of interpretation are discussed.
Additional discussion is directed to flowback and temperature as diagnostics. In particular, preliminary measurements are described that used flowback rather than shut-in to rapidly identify closure. Flowback is not a new method (Savitski and Dudley, 2011; Raaen et al., 2001). The technique remains very promising and lessons learned are described. Also, in one measurement campaign, bottomhole temperature during shut-in stands out to be a useful corroborative closure stress diagnostic (Whitsitt and Dysart, 1970).
The FORGE project is continuing and a brief update will be provided.
References
Raaen, A.M., Skomedal, E., Kjørholt, H., Markestad, P., and Økland, D. 2001. Stress Determination from Hydraulic Fracturing Tests: The System Stiffness Approach”. Int. J. Rock Mech. Min. Sci., 38 (4), 531–543
Savitski, A., and Dudley, J.W. 2011. Revisiting Microfrac In-situ Stress Measurement via Flow Back - A New Protocol, SPE-147248, SPE Annual Technical Conference and Exhibition, 30 October-2 November, Denver, CO.
Whitsitt, N.F. and Dysart, G.R., 1970. The effect of temperature on stimulation design. Journal of Petroleum Technology, 22(04), pp.493-502.
Xing, P., Goncharov, A., Winkler, D., Rickard, B., Barker, B., Finnila, A., Ghassemi, A., Podgorney, R., Moore, J. and McLennan, J., 2020, September. Flowback Data Evaluation at FORGE. In 54th US Rock Mechanics/Geomechanics Symposium. American Rock Mechanics Association.
