TransGas Limited (TGL) currently owns and operates 24 salt caverns for the storage of natural gas. These storage caverns were developed in an extensive salt bed running through the province of Saskatchewan called the Prairie Evaporite, see figure 1 in the appendix. TransGas was the owner, operator, and user of all our storage caverns until 1988. In 1988 TransGas was split from the provincial crown electric utility and became a separate crown corporation. The new company had two components, a local distribution company (LDC) and a transmission & storage company. Initially the only storage customers were the electric utility and the LDC. In 1990 TransGas started to market our storage services to other customers. This meant increased demand for our storage services. TGL usually produced gas during the winter season and injected gas in the summer months, (one injection/production cycle per year). With the market expansion more frequent cycling of gas in storage was anticipated. The expansion in market also meant maximizing the useable volume of the storage facilities.
TransGas traditionally set the minimum cavern operating pressure of our storage caverns at the expected transmission pipeline pressure. The resultant minimum bottomhole pressures were 0.12 to 0.13 psi/ft of depth to the cavern roof. The minimum cavern pressure was rarely reached prior to 1990. The maximum bottomhole cavern pressure was based on the depth to the cavern roof, 0.70 psi/fool. We had experienced some problems with three of our Regina caverns. Roof falls and side wall falls had resulted in the derating of these caverns.
We decided the time was right to closely examine the design pressure of our caverns. Specifically we wanted answers to the following questions:
Was our minimum design pressure acceptable?
Was our maximum design pressure acceptable.
Could we lower our minimum design pressure?
Could we raise our maximum design pressure?
Could we increase the useable volume of our caverns by changing the operating pressures?
What impact did cycling have on the caverns.
To determine the answers to these questions TransGas hired two consultants to assist with modeling our caverns. The two consultants were Dr. Gabe Fernandez and Murray Forster. Dr Fernandez is a Geotechnical Consultant and professor at University of Illinois Murray Forster is a Geotechnical Consultant in Saskatchewan. Doug Ruse, a local consultant, also reviewed the results of theses studies. This report summarizes the fmdings of these studies.
Salt is a unique material. Salt's material properties depend on how it is loaded. When salt is loaded at low levels it will creep and deform. This creep and deformation ability is what makes salt "self-healing". Potash mines in Saskatchewan have experienced failure at stress levels well below that indicated by lab analysis. Dr. Malcolm Reeves of the University of Saskatchewan published a paper on "Dynamic brittle-rupture stable creep criteria in evaporite mine design". In his paper Dr. Reeves discusses the effect of stress on strain rate.