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Pore Pressure Prediction in Challenging Areas Reducing Uncertainty by Understanding Rock Behaviour*Sam Green1, Richard Swarbrick1, Stephen O'Connor1, Phill Clegg1, David T. Scott1, and Bitrus Pindar1 Search and Discovery Article #40650 (2010)Posted November 30, 2010

*Adapted from poster presentation at AAPG Annual Convention, New Orleans, Louisiana, April 11-14, 20101

GeoPressure Technology, Durham, United Kingdom (r.e.swarbrick@geopressure.co.uk; s.a.oconnor@geopressure.co.uk)

Abstract In extensional Tertiary deltaic systems where compaction disequilibrium is the main source of overpressure generation, porosity/effective stress relationships can be utilised to predict pore pressures. These techniques rely on measurement of abnormally high porosities in shales at depth, referenced to those expected in a normally compacting sequence. These approaches tend to underestimate pore pressure in situations where cementation occurs, where temperatures (>100120oC), deep fluid expansion mechanisms (e.g., gas generation) are present or chemical compaction (porosity no longer related exclusively to effective stress) occurs. Also, in carbonate lithologies, pore pressure prediction using effective stress/porosity relationships is invalid, and therefore other approaches are required. In areas where there is increased tectonic stress, pore pressures will be underestimated if vertical stress loading is assumed the primary source of overpressure generation. Pressures in reservoirs/sands and shales also may be outofequilibrium, depending on sequence net/gross and lateral extent. Finally, subsalt is problematic for several reasons, including the definition of a normal compaction curve. Therefore, a series of nonstandard approaches are presented based on worldwide experiences, whereby mechanisms of overpressure generation are identified and quantified in shales; e.g., using velocity/density crossplots, carbonate pressure prediction is discussed and facies controls on pressure buildup and retention are detailed (e.g., laterallydrained reservoirs). Also, understanding of rock properties adds confidence to pressure prediction in areas of limited data. By first using standard techniques related to vertical stress, then using these nonstandard approaches allows more robust geological pressure models to be constructed.

References Hoesni, M.J., 2004, Origins of overpressure in the Malay Basin and its Influence on petroleum systems: Ph.D. Thesis, University of Durham. Lahann, R.W., D.K. McCarty, and J.C.C. Hsieh, J.C.C., 2001, Influence of clay diagenesis on shale velocities and fluid pressure: Offshore Technology Conference paper 13046. OConnor, S.A., R.W. Lahann, and R.E. Swarbrick, 2007, Mid Norway pressure study: GeoPressure Technology & IHS Energy.

Pore Pressure Prediction in Challenging AreasReducing uncertainty by understanding rock behaviourSam Green, Richard E. Swarbrick, Stephen A. OConnor, Phill Clegg, David T. Scott & Bitrus Pindar

Pore Pressure Prediction: A User GuidePore pressure prediction revolves around the compaction state of shales relative to their con ning stress, usually the vertical stress (also known as the overburden). Pore pressure prediction uses the Terzaghi stress relationship between total stress (S), e ective stress (s ) and the pore pressure (PP) in the simpli ed equation;

S = ' + PPAssumptions:> Source of overpressure is disequilibrium compaction (ine ective de-watering) > Porosity can be imaged (e.g. seismic internal velocity, ITT, Resistivity, Dx-Exponent) > Shale is at its maximum e ective stress (i.e. no recent uplift or in ationary pressure increase) The magnitude of pore pressure can be determined using either an empirical function (e.g. Eaton Ratio Method) or deterministic solution (e.g. Equivalent Depth Method). Both methods require development of a normal compaction trend (NCT) which acts as a reference porosity-e ective stress relationship. Determination of a NCT is itself a challenge in many areas, or a source of uncertainty in results.

Devastating rig re due to a blowout believed to result from inaccurate pore pressure prediction.East Timor Sea

Finally, shale-based pressure prediction can be compared with direct reservoir pressures, but care must be taken as lateral transfer along inclined but con ned reservoirs, and lateral drainage, uid and pressure escape to surface, can generate poor compliance between shale and reservoir pressures.

Accurate pore pressure prediction results when thick shales are well imaged by wireline / LWD tools interspersed by thin, con ned reservoirs which have been pressure tested.

Panel 2 > What makes for Challenging Areas

Panel 3 > How to recognise Challenging Areas

What makes for Challenging AreasThe challenge for pore pressure prediction can be usefully summarised as: (1) Poor data areas - either poor wireline / LWD logs and/or no direct pressure tests. (2) Shales not at their maximum e ective stress, e.g. uplift or uid expansion (primarily gas generation). (3) Shales which have undergone chemical alteration, e.g. lithi cation processes or mineral transformations (smectite - illite diagenesis).(1) Typical di culties with pore pressure prediction arise in deep-water settings where logs are not run top-hole (e.g. shallow top OP) as well as where shales vary widely within a single vertical succession (e.g. Central North Sea).Porosity Porosity NCT to correctly identify shallow overpressure Top OP Top OP

(3) Since pore pressure prediction in shales relates porosity to e ective stress, chemically-driven changes in porosity will lead to erroneous prediction.12000

Tertiary DeltaIllitisation with loading

Central North Sea Glacial Low Smectite High Smectite Sand / Silt Chalk

Deep Water Gulf of Mexico Sand / Mud Silt Mud Reservoir Mud ReservoirVelocity (ft/s)

11000

10000

9000

Depth

Depth

8000 Velocity (ft/s)

Illitisation with unloading7000

Smectite

With increasing depth smectite changes to illite. Once the illitisation reaches a critical level there must be a porosity reduction due to pore collapse or uid generation inceasing overpressure and unloading the grains. Unloading breaks the porosity e ective stress relationship inherent in pore pressure prediction methods.

6000

Best t NCT to normally pressured data

Best t NCT to overpressured data

Source Rock Reservoir

Modi ed after Lahann (2001)5000 1.60 1.70 1.80 1.90 2.00 2.10 Density (g/cc) Depth = 1300-7600 Depth = 7600-11400 Depth = 11400-13700 Depth = 13700-15500 2.20 2.30 2.40 2.50 2.60

(2) Making a successful pore pressure prediction onshore is often frustrated by uplift and sediment removal such that the porosity of the sediments relates to earlier, deeper burial.Depth

Pressure

Porosity

Mid NorwayInitial burial leads to overpressure generation. Cessation in sedimentation allows pressures to equilibrate to hydrostatic conditions. Recent burial (A to B) leads to overpressure (2900 psi) including a hydrostatic gradient parallel section at depth. Deep reservoir pressure test data demonstrate a lithostatic gradient convergent trend as a result of chemical compaction - an extra 3300 psi of overpressure at 14800 ftTVDss.

PressureModi ed after OConnor (2007)Depth Time Normal Pressure

A BRecent Burial

ADepth

Log

Pore pressure prediction will reveal higher e ective stress than valid for the current depth of burial. The same e ect results from uid expansion and leads to under-estimation of the pore pressure.

B

Lithostatic gradient convergent trend

Pressure due to disequilibrium compaction

Pressure due to chemical compaction - up to 3300 psi

How to recognise Challenging Areas?Di cult pressure prediction resulting from uid expansion (due to gas generation) and chemical compaction (with or without pressure generation) can be anticipated by applying the following techniques(1) Determine mechanism using velocity and density15000

(2) Temperature > 100C - 120C

Malay Basin, S.E. Asia120C 110C

Modi ed after Hoesni (2004) Onset of Overpressure 130C 150C 140C

90C

100C

160C

80C

5000 1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.7

2.8

Conclusions Porosity based pore pressure prediction techniques work best where a normal compaction trend can be reliably developed, the lithology is moderately constant, and the overpressure is due to disequilibrium compaction. In low temperature, non-mechanical regimes challenges relate to lateral transfer / drainage, poor data and lithological variability. Fluid expansion and uplift generate reduction in eective stress without being revealed in higher porosity (compaction is irreversible) leading to under-estimation of pore pressure. Chemical changes in shales modfiy porosity and destroy the link between porosity and eective stress associated with normal compaction behaviour and challenge traditional porosity-based pore pressure prediction techniques.References: Hoesni, M.J., 2004, Origins of Overpressure in the Malay Basin and its In uence on Petroleum Systems (PhD Thesis), University of Durham. Lahann, R.W., McCarty, D.K. & Hsieh, J.C.C., 2001, In uence of Clay Diagenesis on Shale Velocities and Fluid Pressure, O shore Technology Conference. OConnor, S.A., Lahann, R.W. & Swarbrick, R.E., 2007, Mid Norway Pressure Study, GeoPressure Technology & IHS Energy.