Understanding and predicting multiple fracture propagation behaviour whilst injecting water in the layered reservoir of Field A, offshore Nigeria, presents several major challenges.

The water flood efficiency will be affected by the behaviour of each independent fracture during the injection process. For this reason an accurate estimation of the behaviour of each individual fracture and their interdependence are essential for field development.

The principle objective of the study was to find the main factors that controlled the multiple fracture growth behaviour using a 3D fracture simulation model. A water injection well was planned in a reservoir with 5 moderate to high permeability sand layers varying from 6 to 25 feet in true vertical thickness. A further consideration was that the sand layers in the reservoir had depleted from 2600 psi to 1900 psi. A comprehensive lithological and geomechanical model of the reservoir section was constructed for simulating the injection process. As a mixture of seawater and formation water was planned for injection possible chemical effects were also considered in the study. The 3D simulation model of multiple fracture propagation considered thermal and pore pressure effects. In addition, reservoir pressure depletion and particle plugging were also considered.

The behaviour of each individual fracture was shown to be interdependent on the other fractures in the model. The results of the simulations demonstrated that over the first 5 years of planned water injection, the behaviour of the fractures was mainly dependant on the pressure depletion effects at the time of water injection and particle plugging. Critically, the depletion effects were the decisive element in fracture containment i.e. to guarantee vertical growth into only the selected sand sequences. Some scenarios that were modelled showed that for the planned injection flow rates, fracture containment was not possible after one year of injection. However, in these cases, if the injection rate was reduced by around 20% for a limited period, the fracture growth could be controlled. Full rates could be resumed after this reduced injection period. The introduction of such an injection flow rate control (IFC) during the injection period would then allow the fracture containment and water flood confinement into the selected reservoir sequence.

Understanding the variables that influence multiple fracture behaviour allows timely decisions on surface equipment requirements. In addition the operational conditions associated with the injection process are defined. The injection strategy can then be targeted to promote an optimum water flood profile and water placement into only the selected sand intervals.

It is through the process of modelling the multiple fracture interaction and behaviour that the water injection efficiency can be optimised.

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