--> 4-D Coupled Reservoir Geomechanics To Aid Field Development Planning Of Unconsolidated Sandstone Reservoirs: Fortuna Project Case Study

AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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4-D Coupled Reservoir Geomechanics To Aid Field Development Planning Of Unconsolidated Sandstone Reservoirs: Fortuna Project Case Study

Abstract

Compaction and subsidence were identified as key project risks early in development planning of Fortuna deepwater gas development. These risks are due to the nature of the reservoirs (large lateral extent, shallow depth below mudline and low rock strength) and coupled with the planned production strategy (depletion drive with very weak aquifer support). The negative impacts of compaction and subsidence in similar type of reservoirs are well documented and include deformed/sheared/collapsed well casings or completion strings, and loss of cement integrity and zonal isolation. This case study showcases comprehensive workflow of 4-D coupled reservoir geomechanical modelling in Fortuna field to evaluate reservoir compaction and seabed subsidence, well drillability and long term well integrity risk with reservoir pressure depletion. Traditionally for such analyses, either an analytical method based on elastic deformation and lab data or a numerical method decoupled from the reservoir using relatively simple constitutive models are used. Both these approaches could under predict compaction for such unconsolidated formations. Herein, volumetric failure (pore collapse) has been fully accounted for within the Finite Element Model by way of modified Cam Clay constitutive criterion. Fortuna field 3-D finite element geomechanical model is constructed by incorporating data from the single well geomechanical models, mechanical core test data, seismic, reservoir properties and structural information from 3-D static geological model, and dynamic reservoir model. Each cell is assigned with appropriate mechanical properties by integrating 1-D mechanical properties model, 3-D reservoir properties and seismic inversion results. 4-D coupled reservoir geomechanical simulation involves numerical calculation of in-situ stresses and strains throughout the model at pre-selected production time-steps from dynamic reservoir model. The advantage of such a scheme is that a complex stress state can be calculated in a realistic manner by considering the variations in geomechanical property distributions, geological discontinuities as well as the geometry of the reservoir, overburden and any heterogeneous depletion. The results of the 4-D coupled reservoir geomechanical simulation provide information on the magnitude of reservoir compaction expected in Fortuna field. Most of the reservoir compaction is transmitted to the seabed in the form of subsidence due to the thin and soft overburden. The results are also used to asses well drillability at different stage of field development. Formation breakdown, borehole breakout and fracture gradient 3-D mud weight cubes are generated for mitigation of drilling hazards at field scale to provide information of stable mud weight window. Significant reduction in the stable mud weight window for infill wells are expected due to the large amount of reservoir pressure depletion expected at the time of the drilling. This analysis provides useful insights into the timing plan for future well drilling. To understand the risk of long term well integrity, the 4-D, full filed, coupled reservoir geomechanical simulation results are extracted at the planned well paths to construct 3-D near wellbore model. Well design, completion and cement mechanical properties are also used as input to this model. The results from the 3-D near wellbore model finite element simulation are benchmark against published literature where well failures have been observed. Using the aforementioned as a guide, a set of criteria have been set up to assess the risk of well failure for the wells in Fortuna field. The 4-D coupled reservoir geomechanics simulation workflow offers a platform where field and well scale issues can be evaluated for the drilling and completion team. The technically sound and meaningful results help drive key decisions in the planning of the field development, such as the timing of the infill well drilling, as well as design of the well.