Geomechanical Analysis of Some Potential Shale Reservoir Formations in North Sumatra Basin, Indonesia

Abstract

ABSTRACT: Indonesia, as one of the oil and gas producing countries, has potential shale gas and tight oil resources in some of its basins. However, careful assessment of the geomechanical setting is required because most Indonesian basins lie on active tectonic margins, in contrast to most shale play in the U.S, Canada, and China which lie in stable cratonic settings with normal or strike-slip fault regimes. This study aims to analyze the in-situ stress in the potential shale resources of the Baong, Belumai, and Bampo formations in the North Sumatra Basin, Indonesia, and discuss its implication to hydraulic fracturing strategies. Analyses were conducted on the regional and borehole scales. On the regional scale, the earthquake focal mechanism and surface geological features indicate a reverse fault (RF) environment with the maximum horizontal stress (SHmax) in the northeast-southwest (NE-SW) direction. At the borehole scale, analyses were carried out using wireline logs, drilling reports, leak-off test, and hydraulic fracturing data from a pilot hole in the North Sumatra Basin. Borehole stress analysis suggests a near isotropic stress condition in the RF/SS (strike-slip) faulting regime due to overpressure within the Baong formation. On the other hand, the Belumai and Bampo formations show an anisotropic stress state with a normal pressure interval in the NF (normal fault)/SS faulting regime. These conditions lead to a lower minimum principal stress in the Belumai and Bampo formations which suggests that these formations cannot act as a fracture barrier for hydraulic fracturing in the Baong formation. In addition, considering the RF stress state in the Baong formation and the bedding planes that dip at about 30°, horizontal hydraulic fractures can form propagating in all azimuth directions without any obvious fracture barrier preventing them from penetrating into adjacent formations. Meanwhile, hydraulic fracturing in the Belumai formation will create vertical fractures due to the SS/NF faulting regime that will propagate until it reaches the Baong Formation acting as a fracture barrier. Our study suggests that overpressured formations in this region whose prevailing faulting regime is RF are not favorable for hydraulic fracturing operations.