Pressure-Stress Evaluation of Wells Drilled at the Angore Field, Western Fold Belt, PNG

Abstract

Thirteen ExxonMobil-operated gas development wells were drilled from 2012 to 2015 at the onshore Hides and Angore gas fields, Papua New Guinea (PNG), as part of the PNG LNG drilling program. Prior to drilling the PNG LNG wells, four wells had penetrated the Hides structure and one well had penetrated Angore. This paper focuses on pressure-stress interpretations at Angore, using drilling observations and data collected from the final three wells of the PNG LNG drilling program. 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 were 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 Angore well pads were constructed on Miocene Darai Limestone. Below the Darai Limestone, the wells penetrated a clastic section consisting of Cretaceous Ieru Formation (Haito, Ubea, Giero, Bawia, Juha, Alene Members), early Cretaceous Toro sandstone reservoir, and Jurassic Imburu Formation. The initial exploration well (Angore 1A drilled by BP in 1990) encountered a complex pressure-stress depth profile that was comparable to nearby offset wells. Hydrostatic pressure occurs below the water table (in the Darai Limestone) and continues into the Ubea Member. The clay-rich lower Ubea Member supports the onset of a pressure ramp that reaches maximum excess pressure in the Giero Member. Elevated excess pressures persist to the lower Alene Member at which point a pressure regression occurs into the Toro Sandstone and upper Imburu Formation. The first Angore well of the PNG LNG program (Angore B1) encountered a level of structural complexity together with extreme pressures and 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 analyses from Angore support a geomechanical model that involves an interplay between structural history and mechanical stratigraphy, insofar as: 1) formation pressure and stress regime varies with depth and lithology; 2) stratigraphic mechanical properties and structural geometry allow for a non-Andersonian stress state (minimum stress exceeded overburden); 3) stress magnitude may be controlled by fault geometry/timing and formation excess pressure; and 4) parts of the structure may be presently close to Mohr-Coulomb failure.