4-D Geomechanics to Predict Compaction and Subsidence for Development of Unconsolidated Sandstone Reservoirs: Fortuna Project Case Study, Offshore Equatorial Guinea
Authors: French Simon , Puspitasari Ratih , Isherwood Alison , Cox Phil , Marsden Rob , Edo Mbang Manuel Ndong , Tan Chee Phuat , John Zachariah 
Published: March 2017

This paper presents a comprehensive workflow for determining reservoir compaction and seabed subsidence through a case study on a deepwater gas development in Equatorial Guinea. It shows how a 4-D coupled reservoir geomechanical model can be used to evaluate well integrity risk with reservoir pressure depletion and outlines how the results and findings drive key decisions in the planning of the field development and design of the wells.

The workflow includes constructing and calibrating single well geomechanical models that are up-scaled and combined with seismic, structural model and reservoir model to create a field wide 3-D geomechanical model. Coupled numerical simulations with this model provided predictions of geomechanical phenomena for the projected field life with modified Cam-Clay method accounting for pore collapse in the reservoir sands. 4-D coupled simulation results on deformations, fracture gradient, breakout and breakdown mud weights were generated for mitigation of drilling hazards at field scale rather than the more traditional well-by-well analysis. Further numerical simulations of the completion well casing and cement provided well operability risk.

The magnitude of compaction in the upper and lower reservoirs was predicted to be several metres, corresponding to maximum and average reservoir strain of 6% and 3%, respectively. The majority of the reservoir compaction was transmitted to the seabed in the form of subsidence due to the thin and soft overburden. The 4-D simulation results were used effectively to visualize well performance risks in different areas of the field which allowed optimisation of well locations and timing of drilling. The mechanical integrity of the completions, well casing and cement were assessed by incorporating the output from the full field model into a near-wellbore model where material strains and plastic deformations were calculated. Impact on well operability was determined by referencing to data from published literatures and other projects. A telescopic contraction joint will be incorporated into the open hole gravel pack lower completion design to protect the completion from damage due to the high axial strain predicted in the string. Loss of production casing annulus cment integrity is also identified to be a risk which is mitigated by careful cement placement to ensure long term barrier integrity.

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 model. In this study, utilisation of the latest techniques in advanced 4-D coupled reservoir geomechanical modelling reduce the study time and costs significantly, making it affordable for in-time solutions suitable for decision making to the drilling and completion team.