Pressure-Stress Evaluation of Wells Drilled at the Angore Field in the Southern Highlands of Papua New Guinea
Thirteen ExxonMobil operated gas development wells were drilled from 2012 to 2015 at the onshore Hides and Angore fields, Southern Highlands, Papua-New Guinea (PNG), as part of the PNG-LNG drilling program. The fields are delineated by separate northwest-southeast trending, thrust-related, hanging wall anticlines that are part of the present day PNG fold and thrust belt. Following the initial gas discoveries (Hides in 1987, Angore in 1990) and prior to recent PNG-LNG drilling, four wells had penetrated the Hides structure and one well had penetrated Angore. Well pads for the PNG-LNG drilling program were constructed on Miocene age Darai Limestone which outcrops at the surface. Below the Darai Limestone, the wells penetrated a clastic section that consisted of the Cretaceous age Ieru Formation (Haito, Ubea, Giero, Bawia, Juha, and Alene Members), early Cretaceous Toro sandstone reservoir, and Jurassic age Imburu Formation. A complex pore pressure and minimum stress depth profile was typically observed when drilling nearby offset wells. Hydrostatic pressure gradients occur below the water table (which is typically intersected in the Darai Limestone) and continues into the Ubea Member of the Ieru Formation. The clay-rich lower Ubea Member supports the onset of a pore pressure ramp that reaches a maximum excess pore pressure in the Giero Member. Elevated excess pore pressures persist to the lower Alene Member at which point a pore pressure regression is often encountered as drilling proceeds into the Toro Sandstone and upper Imburu Formation. We focus here on the pressure-stress analyses at Angore, using data from the final three wells of the PNG-LNG drilling program. The initial exploration well (Angore 1A drilled by BP in 1990) encountered stratigraphy, pore pressures, and rock stresses similar to those encountered by other wells drilled in the vicinity. However, the first well of the Angore PNG-LNG program (Angore B1) encountered a level of structural complexity together with extreme fluid pressures and rock stresses that were not anticipated prior to drilling. Due to these complexities, the target reservoir was not reached and the well was plugged and suspended. Two subsequent wells (Angore A1 and A2) encountered similar tectonic complexity, but successfully drilled through the extreme conditions to reach the reservoir. The pressure-stress analyses of the PNG-LNG Angore wells incorporated data collected while drilling, post-drill geologic and structural interpretations, and utilized a variety of geomechanical concepts that are constrained by the well data. Post-drill formation pressures were either estimated (e.g. from petrophysical trends, mud-log data, cavings analysis, formation fluid influxes) or directly measured using downhole pressure tools (reservoir only). Rock stresses were estimated or inferred from geomechanical relationships that were constrained by wellbore data (e.g. leak-off tests, mud weights), wellbore geometry (e.g. ovalization related to breakout), cavings analysis, and drilling events (e.g. lost returns, ballooning). The analyses indicate that at Angore: 1) both the formation pressure and stress regime varies with depth and associated lithology; 2) stratigraphic mechanical properties and structural geometry allow for a stress state not well described by Andersonian mechanics (minimum stress exceeded density-derived overburden); 3) stress magnitude may be controlled by a combination of fault geometry/timing and level of formation pressure; and 4) parts of the structure may presently be close to Mohr-Coulomb failure. These interpretations support a geomechanical model that is based on the interplay between the geometry of the Angore structure and the mechanical stratigraphy.
AAPG Datapages/Search and Discovery Article #90371 © 2020 AAPG Asia Pacific Region, The 1st AAPG/EAGE PNG Geosciences Conference, PNG’s Oil and Gas Industry: Maturing Through Exploration and Production, Port Moresby, Papua New Guinea, February 25-27, 2020