--> Depletion-Induced Reservoir Compaction, An Overview Of Analytical And Numerical Approaches

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

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Depletion-Induced Reservoir Compaction, An Overview Of Analytical And Numerical Approaches

Abstract

The magnitude of reservoir compaction is more considerable in thick and highly porous reservoir layers where a large amount of depletion is expected. The most common risk associated with reservoir compaction is casing collapse in the producing wells. The reservoir compaction may also induce a significant surface subsidence above the reservoir where the production infrastructures are usually located. Geomechanical analysis can be implemented to evaluate and predict the amount of possible reservoir compaction and surface subsidence associated with the planned production. These types of analysis may be conducted by means of analytical methods. In general, the analytical approaches are based on simplified material models, e.g. linear poro-elastic models, and the reservoirs are assumed to have a simple geometries, e.g. a disc-shaped with constant thickness. This may lead to uncertain results, but in most of the cases they are sufficient to make engineering decisions.

A more comprehensive modelling approach can be achieved using numerical methods. A finite element model, referred to as 4D geomechanical model, can be prepared from the structural geology model, surface seismic data, offset well geomechanical models and laboratory core tests. In this model the geometry of the reservoir and its overburden layers are captured more realistically. The constitutive equations used to simulate the behaviour of the reservoir rocks are more comprehensive compared to the analytical methods, e.g. it is possible to model the plastic deformation beyond the pore collapse in highly depleted zones. The geomechanical model is one-way coupled with a reservoir dynamic model, i.e. the expected reservoir pressures from the reservoir dynamic model are transferred to the geomechanical model. The life span of the field is discretised to time steps. At the end of each time step, the tensor of stress and strain, and the resulted compaction and subsidence are calculated for the entire field. These results can be used to conduct a detailed numerical wellbore-scale simulation for casing collapse analysis.

The impact of reservoir compaction is not limited to casing collapse and surface subsidence. The results of the 4D geomechanical model can be used to investigate the impact of the changes in reservoir porosity and permeability due to compaction on production performance throughout the life of the field. This can be achieved by conducting a two-way coupled geomechanical-reservoir numerical simulation. For this purpose, the results of the 4D geomechanical model are transferred back to the reservoir simulator at the end of each geomechanical step. The impact of the reservoir compaction on the reservoir performance can be evaluated by comparing the results of the coupled reservoir dynamic simulation against the uncoupled simulation. The obtained results are particularly important for decision making in the field development planning stage.

In this presentation, the workflow, advantages and disadvantages of the analytical and numerical methods for compaction analysis are discussed. The required data to conduct such analyses are listed with an emphasis on the importance of the rock mechanical laboratory tests. Moreover, the conditions in which each of these analyses are more suitable are elaborated. The output and interpretation of such analyses are elaborated by presenting real examples from South East Asia.