Downhole Pressures Derived from Wellhead Measurements during Hydraulic Experiments

Abstract

The availability of reliable downhole pressure data during hydraulic tests is of crucial importance for interpreting the behaviour of the underground system. Especially in the case of EGS systems the high pressure and temperature conditions make downhole measurements rather a challenge. The downhole pressure data collected during recent stimulation experiments at the European EGS project site in Soultz-sous-ForÍts, France, have shown several uncertain values and gaps - especially in the 5 km borehole GPK3.

With the new numerical borehole tool HEX-B the downhole pressure data taken during the stimulation test of GPK3 in May 2003 has been corrected and completed. The input parameters needed are: wellhead data for flow rate, pressure, density of the injected fluid and its temperature. HEX-B takes into account heat exchange processes with the rock mass, buoyancy forces and pipe friction. Deviations between the measured and the calculated downhole temperature in GPK2 have been identified as an upward flow in the annulus of warm water leaking at the casing shoe.
The location and time of occurrence of microseismic signals released by failure on individual fractures indicate where, when and how pressure has been exceeded. The near-borehole events detected during the stimulation of GPK3 have been related to calculated borehole pressures at the corresponding time and depth. The resulting failure pressure profile in the borehole specifies the minimum depth-dependent hydraulic pressure that a stimulation must generate in the host rock at this site to improve its permeability.