a comparative study of profile permeability otosigbo, et. al., permeability versus air permeability

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  • Correspondence: E-mail: gloriaotosigbo@gmail.com Website: www.fjst.org

    FUNAI Journal of Science and Technology

    3 (1), 2017, 110-123

    A COMPARATIVE STUDY OF PROFILE PERMEABILITY AND AIR PERMEABILITY IN RESERVOIR CHARACTERIZATION: THE CASE OF GX 2

    WELL IN WESTERN NIGER DELTA BASIN

    Otosigbo, Gloria Ogochukwu

    1 , Nzekwe, Kenneth Emeka

    2 , Eluwa, Ndidiamaka Nchedo

    1

    1. Department Of Physics/Geology/Geophysics, Federal University, Ndufu-Alike, Ikwo,

    2. Delta Terratek Laboratories Limited, Ajah, Lagos State

    (Received 4 February, 2017; Revised 19 June 2017; Accepted:23 June, 2017)

    Abstract

    Profile permeability has been compared to air permeability to ascertain the prospect of using it as a cost

    effective method determining core permeability in reservoir characterization. The study was carried out

    using pressure decay permeameter (PDPK-300™). Plugs were extracted of oil prior to measurements in

    the case of permeability while in profile permeability the oil was not extracted from the cores. Plugs

    dimensions, Length (L) and diameter (D) were taken to be imputed in the Darcy’s equation. In probe

    permeability, the ratio of the external radii, ro to internal radii, ri of the probe tip was used to calculate a

    dimensionless geometrical factor, Gf with several limiting factors. During air permeability

    measurements, the cylindrical plugs were confined one at a time in Hassler cell while in probe

    permeability, the slabbed cores were not confined. In GX well, the results of profile permeability (Kp)

    were compared with that of air permeability (Ka) at 27 points. Air permeability (Ka) values ranged from

    121md to 2870md while probe permeability ranged from 33.9 to 2190mD. The mean values calculated

    were 956 and 1001mD respectively for Ka and Kp. A plot of Ka and Kp(log10) versus Depth (ft) revealed

    that major deviations were observed at depth between 7265ft(2421.6m) and 7272ft(2424.0m) and also

    between the depth 7253ft(2417.6m) and 7256ft(2418.6), which corresponds to Silty Sandstones (high

    permeable zone) and Shaly sand (low permeable zone) intervals respectively. A linear graph of profile

    permeability versus air permeability reveals that the general regression R 2 is 0.212 while the linear

    equation is Y = 0.840X, which is equivalent to Kp = 0.840Ka. The deviation of probe permeability values

    from air permeability could be caused by coarse surface of sand grains resulting to imperfect sealing of

    the probe tip against the core surfaces, heterogeneity caused by bioturbations and diagenetic features

    which were accounted for by close samplings and dimensionless geometrical factor lapses.

    Keywords: core analysis, profile perm, geometric factor, Darcy

  • A comparative study of profile permeability... Otosigbo, et. al.,

    FUNAI Journal of Science and Technology, 3(1), 2017 Page 111

    1. Introduction

    Petrophysical properties of sedimentary rocks

    are decisive parameters for the quantitative

    and qualitative evaluation of reservoir

    rocks.Permeability (K) is one of the most

    important quantitative parameter for reservoir

    which describes the magnitude of flow

    through porous rock. Reservoir engineers are

    faced with challenges of the equipments and

    methods of obtaining accurate permeability

    results. More often, different equipments are

    used in combination to obtain dependable

    results. Reliable permeability values are a

    prerequisite for the assessment and modelling

    of hydrocarbon reservoirs (Li et al., 1995;

    Branets et al., 2009), their economy and

    sustainable production (Davies and Davies,

    2001; Dutton et al., 1991). They are also

    crucial for hydrological studies (Huysmans et

    al., 2008; Todd and Mays, 2005). In core

    samples, permeability is determined on core

    plugs or slabbed cores using gas or liquid

    phase. The experiment could be carried out

    under steady state or unsteady state. Steady

    state flow is where the external temperature

    and pressure remains constant throughout the

    experiment. The pressure fall-off or pulse

    decay flow could be run under steady state.

    The flow could be applied axially, transverse

    or radially on the core plugs or slabbed cores.

    Each method has its advantages and

    limitations. For the purpose of this study, gas

    phase under steady state have been chosen for

    both probe permeameter and air permeameter

    method. Both approach are normally

    corrected for Klinkenberg(1941) effect .

    Klinkenberg effect is as a result of the

    slippage of gases along the pore walls which

    gives rise to an apparent dependence of

    permeability on pressure because gas does not

    adhere to the pore walls as liquid does. Often

    times, reservoir engineers use more than one

    method in combination to achieve accurate

    results. Economy and time are also crucial to

    the management in preparation to production

    of crude oil. Probe permeameter is a fast, easy,

    and non destructive equipment used to

    measure the permeability at very closed

    interval of 1cm. It has the advantage of

    obtaining 3D permeability image where cores

    from 3 wells are available with their

    geographical coordinates. The air permeability

    using core plugs is a time consuming method

    but it has the advantage of obtaining oil/water

    saturation, pore volume/porosity, and easily

    solve the problem of geometry resolution

    since plugs have definite shapes. This study

    critically evaluated the accuracy and reliability

    of the profile permeability method compared

    to that of air permeability results. Also, to see

    the possibility of using probe permeability

  • A comparative study of profile permeability... Otosigbo, et. al.,

    FUNAI Journal of Science and Technology, 3(1), 2017 Page 112

    results independently for reservoir

    characterization.

    1.1. Geology of the Study Area

    Study cores were taken from GX2 well in

    Western Niger Delta Basin (Fig 1). The well is

    situated within the coastal swamps depobelts.

    From bottom to top, the Niger Delta consists

    of three formations; Akata Formation, Agbada

    Formation and Benin Formation. The three

    diachronous Formations (Akata, Agbada and

    Benin) in the Niger Delta were deposited in

    each of the five depobelts shown in Fig. 1.

    The depobelts are 30–60 km with the oldest

    northward while progradation is 250 km

    south-westward over oceanic crust into the

    Gulf of Guinea (Stacher, 1995). The

    interaction of sagging and rate of sediment

    supply caused deposition of each depobelt

    (Doust and Omatsola, 1990). These depobelts

    are in separate unit and represent a break in

    regional dip of the delta and is confined

    landward by growth faults and basin-ward by

    large counter regional faults or the growth

    fault of the adjacent basin-ward belt (Evamy

    et al., 1978; Doust and Omatsola, 1990).

    These depobelts are characterized by distinct

    sedimentation, deformation and petroleum

    history (Michele et al., 1999). The Akata

    Formation which comprises at least 6500m of

    marine clays with silty and sandy interbeds

    (Whiteman,1982). The Agbada Formation

    (petroleum bearing unit), which is

    characterized by paralic to marine coastal and

    fluvial-marine deposits mainly composed of

    sandstones and shale organized into

    coarsening upward off-lap cycles (Weber,

    1987).

    The Benin Formation consists of continental

    and fluvial sands, gravel, and back swamp

    deposits (2500 m) thick (Reijers (2011). The

    sedimentation in each depobelts was caused

    by deposition and sedimentation rate with syn-

    sedimentary growth fault upsetting the balance

    (Evamy et al., 1978). The local stratigraphy

    section of the studied well is shown in fig 2.

    Total depth of core ranged from 7250ft

    (2416.7m) to 7273ft (2440.3m).

    Fig. 1: Location of the studied well, GX2

    within Coastal Swamp Depobelts.

  • A comparative study of profile permeability... Otosigbo, et. al.,

    FUNAI Journal of Science and Technology, 3(1), 2017 Page 113

    Dark grey mudstone to fissile shale with sand pinchout.

    Depth(m)

    2416.7

    2418.4

    2419.1

    2419.5

    2421.6

    2424

    2425

    2440.3

    Parallel to symmetrical ripple lamination heterolithics sand = 50%, shale/siltstone = 50% folded along laminars. Also cross

    bedded with synsedimentary faulting. Presence of slump and load structure at

    basal contact with shale.

    Ripple laminated silty sand. Syndepositional fault.

    Sof

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