Res Eng Pp Ch2

Heriot -Watt University Heriot-Watt DEPARTMENT OF PETROLEUM ENGINEERING

Reservoir Pressures Adrian C Todd

Reservoir Pressures z

Magnitude and variation of pressures in a reservoir are an important aspect of reservoir understanding during exploration and production phase

Reservoir Pressures

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Oil and gas occur at a range of sub-surface depths. At these depths pressure exists as a result of: – the depositional process – the fluids contained.

Lithostatic Pressures & Fluid Pressures z

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Lithostatic pressure grain to grain transmission of weight of rock sometimes termed geostatic or overburden pressure.

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Function of depth, density

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1 psi./ ft

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Pov at depth D = 1.0 x D psi.

Lithostatic Pressures & Fluid Pressures z

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Lithostatic pressure is balanced in part by the pressure of fluids within pores, pore pressure and by grains of rock under compaction. Unconsolidated sands, overburden totally supported by fluid pressure. In deposited rocks, like reservoirs, fluid pressure is not supporting the rocks but arises from the continuity of the aqueous phase from surface to the depth. Termed hydrostatic pressure.

Hydrostatic Pressure z

Imposed by a column of fluid at rest.

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Value depends on the density of fluid.

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Water - salinity

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0.433 psi/ft - fresh water

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0.45 psi/ft for saline water 55,000ppm.

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0.465 psi for 88,000ppm

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Pfluid = ρfluidDg

g=acceleration due to gravity

Lithostatic Pressures & Fluid Pressures

Hydrostatic pressure Lithostatic pressure

Hydrodynamic Pressure

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Arises as a result of fluid movement. This is the fluid potential pressure gradient which is caused by fluid flow

Fluid Pressure ⎛ ⎛ dP ⎞ ⎞ PW = ⎜ ⎜ D ⎟ + 14.7 psia ⎟ ⎝ ⎝ dD ⎠ water ⎠

Dictated by prevailing water pressure in vicinity of reservoir. Normal situation dP/dD is the hydrostatic gradient

Assumes continuity of water pressure from surface and constant salinity If pressure extrapoloted to zero depth is atmospheric pressure - normal pressured reservoir

Fluid Pressure-Normal Pressure Atmos. Pressure 0 psig. 14.7psia.

Normal pressured reservoir

Fluid Pressure-Abnormal Pressure z

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Under certain conditions fluid pressures are not normal. Overpressured reservoirs. Hydrostatic pressure greater than normal pressure

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Underpressured reservoirs

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Hydrostatic pressure below normal pressure

Abnormal Pressure

Overpressured reservoir

Underpressured reservoir

Abnormal Pressure Pressure Overpressured

Depth

0.45psi/ft. Water-normal 0.45psi’ft.

1000-2000psi N. Viking Graben-N.Sea

Abnormal Pressure ⎛ ⎛ dP ⎞ ⎞ PW = ⎜ ⎜ D ⎟ + 14.7 + Cpsia ⎟ ⎝ ⎝ dD ⎠ water ⎠ C - constant positive - overpressured C - constant negative - underpressured

Causes of Abnormal Pressure

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Thermal effects-expansion or contraction of water

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Rapid burial of sediments

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Geological changes.

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Osmotic effects via salinity differences

Causes of Abnormal Pressure Geological changes

Abnormal Pressure Regional Trends

North Sea Examples

Fluid Pressures-Hydrocarbon Systems z

Hydrocarbon pressure regimes different since densities of oil and gas are less than water.

⎛ dP ⎞ = 0.45 psi / ft ⎜ ⎟ ⎝ dD ⎠ water

Depth

0

Pressure

⎛ dP ⎞ ⎜ ⎟ = 0.35 psi / ft ⎝ dD ⎠oil ⎛ dP ⎞ ⎜ ⎟ gas = 0.08 psi / ft ⎝ dD ⎠

Pressure distribution for an oil reservoir with a gas-cap and oil water contact. Pressure Path of well

Impermeable bed

Gradient in gas column Gradient in oil column

Gradient in aquifer Over pressured reservoir

Pressure distribution for an oil reservoir with a gas-cap and oil water contact.

Hydrocarbon Pressure Regimes z

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Nature and magnitude of pressures and the position of fluid contacts important to the reservoir engineer. Data for fluid contacts from:

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Pressure surveys

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Equilibrium pressures from well tests

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Fluid flow from minimum and maximum depth

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Fluid densities from samples

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Saturation data from logs

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Capillary pressure from cores

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Fluid saturation from cores.

Techniques for Pressure Measurement z

Earlier tests for pressure logging have been replaced by open-hole testing devices which measure vertical pressure distribution in a well.

Examples of Pressure Measurement z

Pressure distributions before and after production provide important reservoir description information. Pressure survey after production Production from here Original pressure profile

Examples of Pressure Measurement After subsequent production

Evidence of layering

Examples of Pressure Measurement z

Can also be used to indicate lack of hydrodynamic continuity.

Examples of Pressure Measurement

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As an interference test can indicate flow behaviour between wells.

Reservoir Temperature z

Earth temperature increases from surface to centre

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Heatflow outwards generates a geothermal gradient.

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Conforms to local and regional gradients as influenced by lithology, and more massive phenomena. Obtained from wellbore temperature surveys. Reservoir geothermal gradients around 1.6oF/100ft ( 0.029K/m). Because of large thermal capacity and surface area of porous reservoir, flow processes in a reservoir occur at constant temperature. Local conditions , eg around the well can be influenced by transient cooling or heating effects of injected fluids.