Analysing Spinner Measurements from Well Tests Using Computerized Interpretation Techniques

Abstract

The development of reliable spinner tools may help avoid much of the ambiguity which often accompanies well tests in geothermal wells, due to interlayer flows through the well bore. However, the use of both pressure and flow rate changes requires new methods of well test interpretation. The Stanford Geothermal Program has been developing microcomputer-based techniques for the simultaneous analysis of pressure and flow rate measurements. There are two key steps in the procedure. Firstly, the non-linear regression is achieved by calculating the gradients of the response (with respect t o the unknown reservoir parameters) i n Laplace space, and inverting numerically. Secondly. the variable flow rate is represented in terms of a superposition of many step changes - this was found to work better than a spline fit to the data. One problem was encountered when attempting t o analyze data in which the spinner "stalled', causing a jump to zero flow rate. The method shows great promise in that the degrees of freedom on the interpretation are greatly reduced, the well bore storage effect disappears, and inter-feed flows do not affect the results.