Fault Seal Assessment of a Fractured Carbonate Reservoir Using 3D Geomechanical Characterisation, Abu Dhabi
Authors: Sirat Manhal , Shinde Ashok Laxmanrao , Al Naeimi Reem , Perumalla Satya V 
Published: March 2015

ADCO's oil and gas concessions onshore Abu Dhabi have several faulted structures including 3-way closure with fault seals within Cretaceous carbonate reservoir sequences. The prolific reservoir units have around 20–25% matrix porosity with tenths of mD permeability. The cap-rock of these reservoir units is mainly composed of a very dense carbonate with less than 5% porosity.

A Geomechanical study was initiated with exploration objectives: (1) to understand the Geomechanical and geological rock properties variation closer to the fault (2) to evaluate the fault seal assessment for this complex faulted structure including Hydrocarbon Column (HC) height at exploration scale (3) uncertainties and its impact on fault seal assessment and (4) its application to exploration potential of the prospect. This paper discusses the sensitivity of different assumptions, Geomechanical parameters and hydrocarbon column height on structure and fault seal integrity, which influence exploration portfolio.

A dedicated well was drilled and cored especially at the vicinity of few seismic faults to characterize the rock mechanical and petrological properties. Seismic semblance, core data and image log analyses across the fault have been carried out, together with Rock Mechanics Testing (RMT), which was very challenging due to selection of identical plug samples from similar depths. 1D Geomechanical models, which include in-situ stresses, pore pressure and rock strength and elastic properties for offset wells were built by integrating various data sets from this field. These calibrated 1D Geomechanical models are used to build 3D Geomechanical models and are further used for fault seal assessment.

Results from core examination and thin section analysis show intense fracturing at the vicinity of the fault zones. Furthermore, there is micro-porosity loss clearly seen in thin sections as a result of cementation and digenesis. The seismic semblance, core data and image log analyses across the fault suggests that there is a significant uncertainty with dip magnitude of faults and this dip may range from 60° to 90°. The strike of these faults varies from N45W to N75W, typical fault orientations in Cretaceous formations of Abu Dhabi. While high porous reservoir rock is experiencing normal-stress setting, very dense cap rock is experiencing strike-slip stress setting. The strain along the fault-seal planes varied substantially depending on their dips and strikes and demonstrated different strengths under both predominant stress settings.

Introduction

The exploration portfolio of ADCO (Sirat et al., 2012) involves numerous leads and prospects and 42% of these plays represent three way closures with fault seals as shown in Figure 1. The built-up pressure from the height of the hydrocarbon column (HC), together with the pore-pressure and the in-situ stress magnitude control the integrity of the fault seals capacity.

Fault slippage and reactivation was first recognized as a possible cause of fluid flow in faulted areas by Sibson (1985) and is fundamentally governed by the Mohr-Coulomb criterion. Analytical approaches usually simplify the problem to homogeneous and regular shaped reservoirs submitted to constant pore pressure and make use of closed-form solutions that range from simple Mohr-Coulomb failure relationships to provide fundamental solutions of elastic inclusions embedded in a halfspace (Geertsma, 1966, 1973; Segall, 1989, 1992; Zoback, 2007; Soltanzadeh and Hawkes, 2009). Zoback (2007) has extensively investigated this phenomenon in a detailed review.