Integrated Fault Seal Analysis Methods For Step-OutDevelopment Wells: A Case Study; Pematang LantihField, South Sumatra Basin, Jambi-Indonesia

Abstract

Fault seal analysis has been widely applied in hydrocarbon exploration and field development. In exploration, analysis of the sealing capacity of fault could assess the probability of finding hydrocarbons that involve the definition of spill point, column height, juxtaposition, and compartmentalization. In field development, fault can provide seals, baffles and/or conduits to fluid flow. Proposing a step-out development well beyond main producing area separated by major fault carries subsurface uncertainties. The amount of well and reservoir data from nearby main producing field is an advantage. However, associated risks in the new area without well penetration particularly on the fault characteristics need to be assessed. This paper elaborates the study of fault seal characteristics separating main field and new step out area. Pematang Lantih consists of several structures that are bounded by faults. Pematang Lantih field (main structure) has been developed since 2015 and infill development program has reached its effective hydrocarbon drainage radius. Subsurface evaluation of the area suggests that the structure to the southeast of the main field has a potential hydrocarbon. A step-out well was then proposed to be drilled in the southeast block. The NNE-SSW normal fault separated the southeast block from the main field. Although the southeast block is relatively at the downthrown side of the fault, seismic attribute analysis showed considerable hydrocarbon potential. The size of the fault and the magnitude of its displacement suggest that the fault might potentially act as a barrier between the main field and the southeast block. A  combination of methods was performed in this study. The first method was Geomechanical study derived from various data evaluation of well-logs, drilling parameters and regional data. Field Stress model related to fault integrity and reactivation risk were resulted from this first approach. The second method applied the triangle juxtaposition diagram which represents across-fault relationship and sealing capacity from a single well. The fault’s throw and field stratigraphy were used to generate juxtaposition diagram. Membrane seals were assessed by using Shale Gouge Ratio (SGR) where shale material (vshale) was incorporated into the fault as a function of fault throw. The third method involved 3D fault model where stratigraphic juxtaposition on fault plane and seal properties derived from 3D grid model can be displayed in three dimensional. This provided a relatively more accurate view of potential leaking or sealing zone and also guided the well geometry to be drilled in the area adjacent to the fault. The study helped in understanding the fault characteristics (integrity, reactivation risk and seal capacity). As a result, this will help in the more confidence placement of the proposed step-out well location and the determination of subsurface target.