Depletion Effects on a Currently Active Fault and Existing Subsidence Bowl: Geomechanics Assessment - Giant Field Northern Oman
Authors: Qobi Latifa , Ita Joel Jacob , Von Winterfeld Claus , Shukaili Jalal , Qusseimi Issa 
Published: May 2016

This paper discusses the Geomechanical interaction between vertically stacked reservoirs, two of which are currently producing and deeper reservoirs that are being considered for development. The shallowest Natih reservoir is a highly compacting carbonate gas reservoir under depletion whereas the intermediate Shuaiba reservoir is an oil-bearing reservoir under water flood. The deeper reservoirs are oil and gas bearing located in the Sudair and Khuff formations.

Interpretation of 3D seismic data shows a major NE/SW and NW/SE fault system separately developed in all reservoirs. Depletion in the shallow gas reservoir (Natih) has induced a subsidence bowl and has caused a subset of the faults (NE/SW) to reactivate causing seismic tremors that are often felt at surface. Water injection in the oil-bearing reservoir (Shuaiba) is believed to cause some additional localized micro seismic activity around the NE/SW fault system (5 to 10% of observations).

Consideration is being given to developing the deeper (Sudair and Khuff) reservoirs under natural depletion. A geomechanics assessment study has been initiated to assess whether such a development would significantly add to the geomechanics related risks and if so how these risks could be mitigated. This paper presents data inputs, workflow, calibration, results and preliminary conclusions related to the geomechanics assessment study. The impact of depletion and injection on fault reactivation and subsidence bowl of all reservoir layers are investigated using the in-house proprietary finite element modeling software tool. An extensive dataset [logs, laboratory tests, field measurements, well and reservoir monitoring data (compaction monitoring instrument, microseismic) as well as surface deformation data (GPS and InSar)] is utilized to constrain and derive the stress state as well as the rock mechanical properties of each reservoir layer and the various overburden layers.

Introduction

Given the exisiting observations of subsidence and localized fault reactivation, the study focuses on: 1. Safety and environment: · Effects of Sudair and Khuff depletion on the currently reactivating faults: magnitude and frequency of tremors and potential gas leakage2. Well and reservoir management (WRM) and urban planning of the new development faciltities · Assess additional compaction damage to the existing wells, especially given that recent surface deformation surveillance tools (GPS and InSar) reported a noticeable acceleration in the measured subsidence· Decide on new well placement and trajectory relative to the seismic tremor belt and appropriate casing design· Decide on facility placement relative to the seismic tremor belt and the subsidence bowl

A previous geomechanical study [1] has focused on understanding and predicting future compaction/subsidence caused by the Natih reservoir. The incentive was to to assess what damage this compaction could have on the 450+ (Natih and Shuaiba) wells drilled through this compacting reservoir. The current study includes deeper reservoirs in the modeling, assesses the effect of depletion on fault reactivation and uses non-linear computations for all scenarios investigated.

A typical geomechanic workflow consists of three components (Tripod approach): Laboratory data,Field data andModeling efforts.