In-situ stress field of the Whicher Range gas field, Perth Basin, Australia: Implications for underbalanced drilling.
Engineering Honours Degree 2009
University of Adelaide
Tight gas fields such as Whicher Range are often characterised by a large gas resource, the commercial recovery of which is problematic due to low permeability. Two key development options for tight gas fields are fracture stimulation and underbalanced drilling. These technologies are strongly influenced by the in-situ stress field. As previous fracture stimulations at Whicher Range were unsuccessful, this project involved determining the in-situ stress field in order to assess the feasibility of underbalanced drilling as a development option.
Using density logs from six wells in the area and check shot velocity survey results, the vertical stress (Sv) in the Whicher Range Field at a reservoir depth of 4000 m was found to be 99.8 MPa which equates to 25 MPa/km. The magnitude of the minimum horizontal stress (Shmin) was determined using formation integrity and minifracture tests obtained from four wells. At reservoir depth this stress was found to have a magnitude of approximately 81.4 MPa or 20.3 MPa/km. As no rock strength data was available, the upper bound to the maximum horizontal stress magnitude (SHmax) was determined using the frictional limits technique. Through this the upper bound of the maximum horizontal stress was determined to be 167.6 MPa or 41.9 MPa/km. The orientation of SHmax was resolved through analysis of a single image log. Through identification of breakouts, the orientation of the maximum horizontal stress was found to be 084°N.
These stress magnitude findings demonstrate that a strike-slip stress regime, where SHmax> Sv >Shmin, prevails in the Whicher Range Field. In such a stress regime the vertical well is the least mechanically stable. Owing to this and having determined a ‘pseudo’ rock strength of 340 MPa (with the knowledge of the stress field and the fact that vertical wells have been successfully drilled), it was demonstrated that wells drilled in the direction of 084°N should be mechanically stable with respect to breakout formation for theoretical mud weights between -14.13 ppg and 8.65 ppg. Therefore underbalanced drilling was found to be a viable option for enhancing the production of the tight gas in the Whicher Range Field.