The tight gas formation sandstone in North Germany with a permeability below 0.6 mD is characterized by an unknown Biot's and Skernpton's coefficient, which are the hydro-mechanical coupling parameters to decide whether the under-balanced drilling (UDB) could be used. This two coefficients and the sandstone strength are determined by laboratory experiments. Based on the experimental results and on theory of poroelasticity as well as on the parameter variation analysis, the range of minimum mud density is determined for a vertical and horizontal wellbore in the tight gas formation sandstone.


The gas resources in tight gas formations in Germany in the deep Rotliegendes and Upper-Carboniferous are estimated to be between 300 and 500 billion m3. However, the recovery of this huge reserve potential depends on the application of new technologies - UBD to avoid or mitigate formation damage, or multiple-hydraulic fracturing to improve the recovery. While UBD offers many potential benefits, drilling with a bottom hole pressure less than the reservoir pressure will generally increase the risk of borehole instability due to shear failure of the rock adjacent to the borehole. The objective of this paper is to determine experimentally the Biot's and Skempton's coefficient of the tight gas formation sandstone and to investigate the wellbore stability in conditions of UBD in this formation over 4600 m deep in North Germany as well as to identify the optimal mud densities or drilling fluid pressure, in consideration of the hydro-mechanical interactions.

Multi-failure triaxial compression measurement of the sandstone strength

Experiments have been performed in a triaxial test setup at the Institutc of Petroleum Engineering (Fig. I), Heriot-Watt University, in the framework of the EU program "EUROPEAN INFRASTRUCTURE FOR

(Figure in full paper)

ENERGY RESERVE OPTIMISATION (EIERO)". The samples were cored at a depth of about 4800- 5000 m from North Germany.

There are a total of 10 sandstone samples with a diameter of d ≈ 38 mm and a height of h ≈ 76 rnm which have been tested. In order to get more results, a special test program for each sample was planed in the following sequence, Hou et al. 2004:

  • Measurement of Biot's coefficient in the phase of pre-failure

  • Measurement of Skempton's coefficient in the phase of pre-failure

  • Measurement of the failure strengths under different confining pressures as well as the strength parameters (cohesion c and friction angle Φ) with the multi-failure test method

(Figure in full paper)

  • Measurement of Biot's coefficient in the phase of post-failure

  • Measurement of Skempton's coefficient in the phase of post-failure

In this way we can get the Biot's and Skempton's coefficients before and after failure as well as the strength parameters of each sample. One sample is tested five times; the corresponding experiment duration is about 8 hours.

The results of the failure strengths of sandstone from North Germany are shown in Figure 2 and in Table 1.

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