Geomechanical study of fractured carbonate reservoir -part I, rock testing data

D. Wang, P. A. Nawrocki, A. Jebbouri, M. Hozayen, E. S. Radwan, X. Zhang

    Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review

    Abstract

    A major geomechanical study was conducted for one of the UAE onshore oil fields with the purpose of studying reservoir rock properties and stress distribution, based on which a 3D coupled reservoir geomechanical model was built. This model was later used for wellbore stability analysis and for analyzing the effect of fractures on future reservoir production performance, including fracture deformation and permeability evolution occurring due to reservoir depletion and resulting changes in local stress field. The subject of this study was a complex fractured Upper Cretaceous carbonate reservoir characterized by high degree of heterogeneity, both vertically and laterally. Wellbore instability related drilling problems were encountered while drilling the wells in this field. The problems were mainly in drilling the horizontal wells, including breakouts and mud losses. Experimental results of laboratory tests are summarized in this paper whereas wellbore stability analysis and formulation of a 3D geomechanical model that was built for that purpose is summarized in a sister paper (Part II). A suite of tests reported includes tensile strength, uniaxial and triaxial compression and permeability under stress at simulated reservoir conditions. All tested rocks have been grouped into twelve test units and reservoir geology, rock types and challenges related to designing a viable sampling and testing program for a big reservoir are discussed first followed by sample selection criteria and details of the testing program. All test units showed brittle behavior at zero confinement and some samples showed similar behavior at low confining stresses. With confining stress increase, the behavior gradually turned to plastic but some samples showed features of ductile behavior even at low confinement. Obtained experimental data have been summarized in the form of strength criteria and correlations. The Mohr-Coulomb strength equations were obtained for each test unit and also for the averaged values. It was noticed that triaxial compressive strength (TCS) increases with decreasing porosity or increasing density but their influence on TCS is getting smaller with confining pressure increase. Good correlation of porosity and density with Young's modulus, Poisson's ratio, unconfined compressive strength (UCS), friction angle, cohesion and tensile strength has been noticed and there was an increasing trend between the confining pressure and TCS. Rock type (mud or grain supported) was not influencing the TCS as much as porosity and density and the two lithologies encountered affected TCS approximately in the same way. These results and correlations are summarized and explained in this paper. They were further used in the numerical modelling part (Part II) of this paper, Hozayen et al. (2015). It is also concluded that rock lithology is not a decisive factor and mechanical properties of tested rocks should be rather considered dependent on porosity and permeability. This helped greatly in reducing the number of tests that had to be performed.

    Original languageBritish English
    Title of host publication13th ISRM International Congress of Rock Mechanics
    Editors Hassani, Hadjigeorgiou, Archibald
    ISBN (Electronic)9781926872254
    StatePublished - 2015
    Event13th ISRM International Congress of Rock Mechanics 2015 - Montreal, Canada
    Duration: 10 May 201513 May 2015

    Publication series

    Name13th ISRM International Congress of Rock Mechanics
    Volume2015- MAY

    Conference

    Conference13th ISRM International Congress of Rock Mechanics 2015
    Country/TerritoryCanada
    CityMontreal
    Period10/05/1513/05/15

    Keywords

    • Correlations
    • Elastic properties
    • Failure criteria
    • Rock strength
    • Rock testing

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