The reservoir stress path and its implications for water-flooding, Champion Southeast Field, Brunei

Abstract

A geomechanical study of the Champion Southeast (CPSE) field, Brunei was undertaken as part of the planning of a waterflood in the field. The key geomechanical issue addressed determining the change in reservoir pressure that could be sustained without reactivating faults in the reservoir or fracturing intact rock. Differential depletion of reservoirs and fault blocks has resulted in pressure compartmentalisation and variable pore pressure-stress conditions in the CPSE field. Pore pressure and minimum horizontal stress (σh) data recorded during depletion of the two most significantly depleted fault blocks were used to determine the stress path between undepleted and depleted reservoirs. The minimum horizontal stress (σh) is 15.5 MPa/km in hydrostatically pressured reservoirs. In the most depleted reservoirs, pore pressure is currently 6 MPa/km and σh is 12.5 MPa/km. The pore pressure-stress (Pp/σh) coupling ratio with depletion in the two fault blocks is 0.84. Other reservoirs and fault blocks at various stages of depletion in the CPSE field are assumed to lie on the same stress path. The vertical stress magnitude (σv) has been constrained to approximately 22 MPa/km in CPSE and the maximum horizontal stress (σH) has been loosely constrained to ~17 MPa/km. There is no clear evidence for the variation in σH with depletion. The present day stress regime is one of normal faulting where σH is the intermediate stress hence accurate knowledge of σH is less significant to failure analysis than σh and σv. The likelihood of reactivating pre-existing faults and failing intact rock was assessed using failure envelopes of C=0 MPa/km; µ=0.5 and C=1 MPa/km; µ=0.5 respectively. The conservative assumption that the decrease in σh with Pp is non-reversible during re-pressurisation was made. This assumption minimises the pressure increase required to induce fracturing during repressurisation. σH is oriented 128°N across the CPSE field. This orientation is orthogonal to the region’s major extensional deltaic growth faults, hence the orientation of σH has rotated approximately 90° since the faults were active in the Miocene-Pliocene. The orientation of the faults within the in-situ stress field results in the faults being at relatively low risk of reactivation. The faults are capable of sustaining significant re-pressurisation without reactivating, despite assuming an irreversible stress path. • 4.0 MPa/km re-pressurisation can be sustained from reservoirs at 9.8 MPa/km pore pressure (hydrostatic), and; • 8.3 MPa/km re-pressurisation can be sustained from reservoirs at 2.5 MPa/km pore pressure (significantly depleted). Since the pre-existing faults are mis-oriented for re-activation within the in-situ stress regime, the failure of intact rock presents greater risk of fault reactivation in CPSE: • 3.5 MPa/km re-pressurisation can be sustained from reservoirs at 9.8 MPa/km (hydrostatic), and; • 0.1 MPa/km re-pressurisation can be sustained from reservoirs at 0.5 MPa/km (significantly depleted). The amount of re-pressurisation that can be withstood prior to intact rock failure reduces with the amount of prior depletion because of the assumption that the decrease in σh with pore-pressure is non-reversible.