Well Testing, Methods and Applicability

Abstract: Well testing is widely used today in water wells, oil and gas wells, for geothermal applications, within civil engineering projects, thermal storage and CO2 storage. The overall testing goal is to verify the productivity from a well and/or for characterization of hydraulic and thermal properties, as well as the spatial limitations of an aquifer. There are three major methods for conducting well testing; slug tests, pumping tests and injection tests. The main objective of this thesis is to compare and evaluate the applicability of the three major well testing methods, using experiences gained by active participation in several well testing operations. Testing of shallow and very deep wells as injection tests using a complete tunnel lining are compared and uniquely presented together in the same work. To compare and evaluate the applicability of the three well testing methods, several conditions which govern the selection of hydraulic testing methods are varied and these include, amongst others, the location, depth and diameter of the well. In addition, technical constraints, such as logistics, water handling and external power supply for conducting the testing are also considered. Certain emphasis is also placed on the use and comparison of pneumatic slug testing with other well testing methods, as pneumatic slug testing often complementary to the more expensive and logistically demanding pumping and injection tests. A small diameter slug test equipment was put together for evaluating the applicability of this method and applied in three different geological environments, namely sedimentary deposits, volcanic deposits and the crystalline basement. It was found that knowledge of the applicability of slug tests, pumping tests and injection tests is essential for selecting the most appropriate method. Two of the methods, namely pumping tests and injection tests, are logistically demanding while the third one, slug tests, is easy to execute. Consequently, it is therefore also an inexpensive test method, which is supported by the fact that several tests can be conducted in different wells during the same day. Slug testing using pneumatic initiation has also been demonstrated as a good alternative to pumping tests, obtaining similar transmissivity estimates. An advantage of slug testing is that the transmissivity is estimated from a series of slug tests and not from one single test, which is often the case for pumping or injection tests. Slug testing using pneumatic initiation is often associated with testing in small diameter wells, but in this work it has been shown applicable in large diameter wells at least up to 12". A major advantage with pneumatic slug testing compared with the other methods is that it can be used for transmissivity mapping, simplified by the method’s easy logistics. It is a useful approach if the wells are distributed over a large area. It is suggested that slug testing using pneumatic initiation should be used as a preinvestigation method for civil engineering projects. Further work needs to be carried out for explaining the observed non-linear characteristics in data obtained from some of the slug tested wells. It has also been demonstrated that well testing methods are not only restricted to drilled wells. The methods can also be applied inside a tunnel with injection through the tunnel lining. The testing was performed as step injection and constant head/pressure tests and, in conjunction, the response from the testing was measured in observation wells drilled from ground level. Analytical solutions developed for vertical wells could be used to interpret hydraulic properties such as transmissivity, using injection tests performed at tunnel level and the pressure response measured in two of the observation wells. Well development was found to be of great importance for any type of well testing and, for deep wells, a hydrojetting system using coiled tubing and simultaneous pumping was found to be applicable, successful and time efficient.