ONE DIMENSION GEOMECHANICAL ANALYSIS AND ITS IMPLICATION FOR FLUID INJECTION PRESSURE LIMITS IN LOWER ARANG FORMATION IN THE WNB AREA, WEST NATUNA BASIN

• Main Author: Gumelar
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/75806
• Institution: Institut Teknologi Bandung

<p align="justify">The Lower Arang Formation is one of the main sandstone reservoirs in the WNB area, West Natuna Basin, which has been producing since 1992. Currently, there is a decline in hydrocarbon production due to reservoir pressure depletion, necessitating fluid injection into the reservoir. The one-dimensional geomechanical analysis in this study aims to determine the safe fluid injection pressure limit that the reservoir can withstand to sustain hydrocarbon production. Understanding the in-situ stress conditions and their relationship with the developed geological structure patterns in the research area is crucial before determining the injection pressure limit. The one-dimensional geomechanical analysis in this study is conducted using wireline log data, formation pressure, hydraulic tests, laboratory rock strength tests, and drilling parameters from seven wells in the research area. Pore pressure analysis using the Eaton method indicates hydrostatic pressure conditions with a gradient of 0.44-0.45 psi/ft for most intervals, except for the long-produced reservoir interval, which has a gradient of 0.39-0.40 psi/ft. The calculated unconfined compressive strength (UCS) values using the Horsrud method average at 2174 psi for the shallow interval (Muda Formation) and 8169 psi for the deeper interval (Belut Formation). The calculated friction angle for the sandstone reservoir provides values between 28-33° or a friction coefficient (?) of 0.53-0.65 for the entire formation. The Borehole breakout analysis shows that the Shmin orientation is generally southwest-northeast with a range of 34.8° to 79.5° E, with an average orientation of 60.4° E, while the SHmax orientation is northwest-southeast or perpendicular to the Shmin direction. The patterns and orientations of structures developed in the WNB area are closely related to the in-situ stress orientation from the analysis results. The dominant inversion faults in the research area generally run perpendicular to the maximum horizontal stress (SHmax) and parallel to the minimum horizontal stress (Shmin) in the southwest-northeast direction. The main strike-slip faults in the research area are estimated to be still actively moving and influencing the in-situ stress in the deeper parts of the research area, confirmed by earthquake data recordings that coincide with the position of the major strike-slip fault. The calculation results based on a 65° breakout width from the image log in BK-5 Well show that the magnitude of SHmax is 8189.3 psi at a depth of 8052 feet. As for the calibration value of Shmin, it is obtained from hydraulic fracturing tests and rock integrity tests (LOT/FIT). The SHmax and Shmin data at several points will be used to calibrate the SHmax and Shmin models calculated using the Poro-elastic Horizontal Strain Model approach. The in-situ stress analysis results show that the stress regime in the WNB research area is generally in a normal stress regime (Sv > SHmax > Shmin) for the shallow interval and a strike-slip stress regime (SHmax > Sv > Shmin) for the deeper interval. A critical failure stress analysis is conducted to determine the safe fluid injection pressure limit for the Lower Arang Formation in Well BD-19, which has experienced a reservoir pressure depletion. Based on the analysis results, the Lower Arang sandstone reservoir has a formation pressure limit of 2826 psi or 19.63 MPa before failure. The faults in the research area are relatively safe because most of them are parallel or oblique to the Shmin orientation, thus having a low risk of moving prematurely due to injection pressures exceeding the specified limit.