Importance of Abnormal Formation Pressures (includes associated paper 6560 )
Authors: Fertl W.H. , Chilingarian G.V. 
Published: December 2007

Abnormally high pressures occur worldwide informations as old as theCambrian age. Better knowledge and the ability to locate and evaluate overpressured formations are critical in drilling operations, completiontechniques, and development of exploratory and reservoir engineering concepts.

Introduction

Detection and quantitative evaluation of overpressuredformations are critical to exploration, drilling, andproduction operations involving hydrocarbon resources.Worldwide experience indicates a significantcorrelation between the presence and magnitude of formationpressures and the shale/sand ratio of sedimentarysections. Distribution of oil and gas is related to regionaland local subsurface pressure and temperatureenvironments. Knowledge of the expected pore pressureand fracture gradients is the basis for (1)efficientlydrilling wells with correct mud weights, (2) properlyengineered casing programs, and (3) proper completionsthat are effective and safe, and allow for killing the wellwithout excessive formation damage. In reservoir engineering, formation pressures influence compressibilityand the failure of reservoir rocks, and can be responsiblefor water influx from adjacent overpressured shalesections as an additional driving mechanism in hydrocarbon production.

Abnormally high pore-fluid pressures are encounteredin formations ranging from the Cenozoic era (Pleistoceneage) to as old as the Paleozoic era (Cambrian age).Such pressures may occur from a few hundred feet below the surface to depths exceeding 20,000 ft and can bepresent in shale/sand sequences and/or massivecarbonate-evaporite sections.

Pressure Concepts

Overburden pressure originates from the combinedweight of the formation matrix (rock) and the fluids(water, oil, and gas) in the pore space overlying theformation of interest.

Generally, it is assumed that overburden pressureincreases uniformly with depth. For example, averageTertiary formations on the U.S. Gulf Coast and elsewhereexert an overburden pressure gradient of 1.0 psi per foot of depth (4231 kg cm m). This corresponds to a forceexerted by a formation with an average bulk density of2.31 gm/cc. Worldwide experience also indicates that theprobable maximum overburden gradient in clastic rocksmay be as high as 1.35 psi/ft (0.312 kg cm m).Furthermore, field observations over the last few years haveresulted in the concept of a varying (not constant) overburden gradient for fracture gradient predictions used indrilling and completion operations.

Hydrostatic pressure is caused by the weight ofinterstitial fluids and is equal to the vertical height of a fluidcolumn times the unit weight of fluid. Size and shape ofthis fluid column have no effect on the magnitude of thispressure.

The hydrostatic pressure gradient is affected by theconcentration of dissolved solids (salts) and gases in thefluid column and the magnitude of varying temperaturegradients. An increase in dissolved solids (higher saltconcentration) tends to increase the pressure gradient, whereas increasing amounts of gases in solution andhigher temperatures would decrease the hydrostaticpressure gradient.

JPT

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