The In Situ Stress Field of the West Tuna Area, Gippsland Basin: Implications for Natural Fracture Permeability and Wellbore Stability
Nelson, Emma J.
Honours Degree, 2002
University of Adelaide
The in situ stress field and natural fracture occurrence in the West Tuna area of the Gippsland Basin were studied in order to assess the potential for natural fracture enhanced permeability in the deep, tight R- and S-reservoirs.
Ninety-one borehole breakouts, four axial drilling-induced tensile fractures (ADITFs) and two sets of transverse (TDITFs) were observed in image logs from six wells located in the West Tuna area. The breakouts and DITFs indicate the maximum horizontal stress (SHmax) is regionally oriented 138N in the Gippsland Basin. The broad NW-SE SHmax orientation is in agreement with previous studies based only on dipmeter logs.
Analysis of 8 leak-off tests conducted in the Gippsland Basin indicate the minimum horizontal stress (Shmin) is 20 MPa/km. Vertical stress derived from density and sonic log data ranges from 20 MPa/km at 1km depth to 22 MPa/km at 3km depth. The maximum horizontal stress (SHmax) was constrained using the criteria for the formation of breakouts, and DITFs observed on image logs. This involved utilising knowledge of Pore Pressure (Pp), Mud Pressure (Pw), rock strength and calculated values for the minimum horizontal stress (Shmin) and the vertical stress (Sv). The occurrence of TDITFs constrained SHmax to 40 MPa/km (in sands at 2600 metres). The calculated stress tensor suggests that the West Tuna area is on the boundary between strike-slip and compressional stress regimes (Hmax>v hmin).
Natural fractures and faults were observed in the six West Tuna image logs investigated. Fracture orientation, rock strength data and knowledge of the in situ stress field were used to construct structural permeability diagrams which indicate that electrically conductive (Conductive_1 and Conductive_2) fractures and faults are optimally oriented to be hydraulically conductive in the present day stress regime. Electrically resistive fractures were also found to be oriented optimally for reactivation, however these are interpreted as cemented and therefore likely barriers to fluid flow in the West Tuna area.
Although the Conductive_1 and Conductive_2 classes of fractures, and faults have the potential to be hydraulically conductive, fracture density in the wells investigated is low, hence fractures are not likely to greatly effect reservoir permeability. The location of the fractures in highly resistive, cemented rock units suggests the fractures may be important to reservoir connectivity.
Knowledge of the in situ stress tensor in the West Tuna area was used to determine optimal well trajectories for maximum borehole stability, and hydraulically conductive fracture intersection. Horizontal wells drilled in the direction of SHmax (towards 138) will be most stable. To intersect the low angle hydraulically conductive fractures wells should be drilled vertically with high mud weight to prevent breakout. A deviated well through non-reservoir units becoming vertical through the reservoir is considered optimal.