Reservoir PressuresAdrian C ToddHeriot-Watt UniversityDEPARTMENT OF PETROLEUM ENGINEERING

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

Reservoir PressuresOil 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 PressuresLithostatic pressure grain to grain transmission of weight of rock sometimes termed geostatic or overburden pressure. Function of depth, density 1 psi./ ft Pov at depth D = 1.0 x D psi.

Lithostatic Pressures & Fluid PressuresLithostatic 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 PressureImposed by a column of fluid at rest.Value depends on the density of fluid.Water - salinity 0.433 psi/ft - fresh water0.45 psi/ft for saline water 55,000ppm.0.465 psi for 88,000ppmPfluid = rfluidDg g=acceleration due to gravity

Lithostatic Pressures & Fluid PressuresHydrostatic pressureLithostatic pressure

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

Fluid PressureDictated by prevailing water pressure in vicinity of reservoir.Normal situationdP/dD is the hydrostatic gradientAssumes continuity of water pressure from surface and constant salinityIf pressure extrapoloted to zero depth is atmospheric pressure - normal pressured reservoir

Fluid Pressure-Normal Pressure

Fluid Pressure-Abnormal PressureUnder certain conditions fluid pressures are not normal.Overpressured reservoirs. Hydrostatic pressure greater than normal pressureUnderpressured reservoirs Hydrostatic pressure below normal pressure

Abnormal PressureOverpressuredreservoirUnderpressuredreservoir

Abnormal PressureWater-normal0.45psift.

Abnormal PressureC - constant positive - overpressured C - constant negative - underpressured

Causes of Abnormal PressureThermal effects-expansion or contraction of waterRapid burial of sedimentsGeological changes.Osmotic effects via salinity differences

Causes of Abnormal PressureGeological changes

Abnormal Pressure Regional Trends

North Sea Examples

Fluid Pressures-Hydrocarbon SystemsHydrocarbon pressure regimes different since densities of oil and gas are less than water.PressureDepth0

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

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

Hydrocarbon Pressure RegimesNature and magnitude of pressures and the position of fluid contacts important to the reservoir engineer.Data for fluid contacts from: Pressure surveys Equilibrium pressures from well tests Fluid flow from minimum and maximum depth Fluid densities from samples Saturation data from logs Capillary pressure from cores Fluid saturation from cores.

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

Examples of Pressure MeasurementPressure distributions before and after production provide important reservoir description information.Production from hereOriginal pressureprofile

Examples of Pressure MeasurementAfter subsequent productionEvidence of layering

Examples of Pressure MeasurementCan also be used to indicate lack of hydrodynamic continuity.

Examples of Pressure MeasurementAs an interference test can indicate flow behaviour between wells.

Reservoir TemperatureEarth temperature increases from surface to centreHeatflow outwards generates a geothermal gradient.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.