Hydraulic observations from a 1 year fluid production test in the 4000 m deep KTB pilot borehole

A long-term pump test was conducted in the KTB pilot borehole (KTB-VB), located in the Oberpfalz area, Germany. It produced 22 300 m(3) of formation fluid. Initially, fluid production rate was 29 l min(-1) for 4 months, but was then raised to an average of 57 l min(-1) for eight more months. The aim of this study was to examine the fluid parameters and hydraulic properties of fractured, crystalline crusts as part of the new KTB programme 'Energy and Fluid Transport in Continental Fault Systems'. KTB-VB has an open-hole section from 3850 to 4000 m depth that is in hydraulic contact with a prominent continental fault system in the area, called SE2. Salinity and temperature of the fluid inside the borehole, and consequently hydrostatic pressure, changed significantly throughout the test. Influence of these quantities on variations in fluid density had to be taken into account for interpretation of the pump test. Modelling of the pressure response related to the pumping was achieved assuming the validity of linear Darcy flow and permeability to be independent of the flow rate. Following the principle 'minimum in model dimension', we first examined whether the pressure response can be explained by an equivalent model where rock properties around the borehole are axially symmetric. Calculations show that the observed pressure data in KTB-VB can in fact be reproduced through such a configuration. For the period of high pumping rate (57 l min(-1)) and the following recovery phase, the resulting parameters are 2.4 x 10(-13) m(3) in hydraulic transmissivity and 3.7 x 10(-9) m Pa-1 in storativity for radial distances up to 187 m, and 4.7 x 10(-14) m(3) and 6.0 x 10(-9) m Pa-1, respectively, for radial distances between 187 and 1200 m. The former pair of values mainly reflect the hydraulic properties of the fault zone SE2. For a more realistic hydraulic study on a greater scale, program FEFLOW was used. Parameter values were obtained by matching the calculated induced pressure signal to fluid-level variations observed in the KTB main hole (KTB-HB) located at 200 m radial distance from KTB-VB. KTB-HB is uncased from 9031 to 9100 m and shows indications of leakage in the casing at depths 5200-5600 m. Analysis of the pressure record and hydraulic modelling suggest the existence of a weak hydraulic communication between the two boreholes, probably at depths around the leakage. Hydraulic modelling of a major slug-test in KTB-HB that was run during the pumping in KTB-VB reveals the effective transmissivity of the connected formation to be 1 1/2 to 2 orders of magnitude lower than the one determined for the SE2 fault zone.