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IV OVERPRESSURE

1.0 OVERPRESSURE DETECTION

Overpressure detection is another service performed at wellsite by Continental Labs. This service is performed in part with the standard hydrocarbon detection equipment and analysis, but also with the use of auxillary equipment and methods. These auxillary services include Shale Density, Dc Exponents and Temperature Monitoring. The following section discusses the theory of abnormal formation pressures as well as some of the equipment and methods used to detect these overpressures.

1.1 INTRODUCTION TO ABNORMAL FORMATION PRESSURES

1.11 General

Abnormally high pore fluid pressure are encountered worldwide in formations ranging in age from the Cenozoic era (Pleistocene age) to as old as the Paleoxoic era (Cambrian age). Such pressures may occur as shallow as a few hundred metres below the surface or at depths exceeding 6100 meters and can be present in shale/sand sequences and/or carbonate-evaporite sections.

Detection and quantitative evaluation of over-pressured formations are critical to exploration, drilling and production operations involving hydrocarbon resources. Distribution of oil and gas is related to regional and local subsurface pressure and temperature environments. Knowledge of the expected pore pressure and fracture gradients is the basis for:

a. the efficient drilling of wells, with correct mud densities; b. the proper engineering of casing programs; c. the proper completion's which must be effective, safe and allow for the killing of the well without excessive formation damage.

1.12 Pressure Concepts

1.121 Hydrostatic Pressure:

The Hydrostatic Pressure is the pressure executed by a column of fluid. Hydrostatic pressure varies with the density of the fluid and the height of the column, but is independent of the shape and size of the column. Expressed mathematically the Hydrostatic Pressure (Phy) is as follows:

Phy = 0.098 X MD X D
where MD = Mud Density
and D = Vertical Fluid Column (i.e. depth)

As the calculation shows where the density of the fluid is greater than that of water, the density used in the equation is equal to the mud density. The down hole hydrostatic Pressure will change when parameters such as the amount of dissolved solids and gases and the temperature of fluid which affects the mud density vary. A pressure gradient is defined by the rate of change of pressure with depth e.g. Formation pore pressure gradient is the formations pressure at a given depth divided by the height at the fluid columns at that depth ( Px/D ).

1.122 Formation Pressure

Within a lithified formation there will be a number of pressures which individually tend to either lend support to, or attempt to further compact the formation. The main pressures involved in this system are:

1.123 Overburden Pressure

The overburden pressure in an area is due to the combined weight of the rock matrix itself and the fluids (water, oil and gas) which occupy the pore spares within the rock of the formation of interest. Generally as compaction increases with depth, so will the overburden pressure gradient until it reaches the critical limit where porosity is virtually non existent. Mathematically the overburden pressure (Po) can be expressed as follows:

Po = weight (rock matrix & fluid) = D (1-0) Pma & OPfl

               area

where D = vertical height of a geological column in feet and meters

O = porosity of a formation expressed as a fraction

Pma = density of rock matrix in lb/ft3 or kg/dm3

Pfl = density of fluid in lb/ft3 or kg/dm3

a. Rock Grain Pressure: Since individual grains often do not exist within a rock formation the rock grain pressure refers to a theoretical fraction of the overburden pressure which is supported by the rock matrix of the formation. Since a rock mass is not homogeneous, pressures will not be exerted equally in all directions as is the case with fluid pressures.

b. Pore Pressure (Formation Pressure): The pore pressure of a formation refers to that portion of the overburden pressure which is not supported by the rock matrix, but rather by the fluids or gases which exist in the pore spaces of the formation. Normal pore pressure is equal to the hydrostatic pressure of a water column from that depth to the surface. If for some reason communication between fluids contained at depth and surface fluids is interrupted, fluids will be unable to flow and normally equalize the pressures within the system. Thus fluids become entrapped within the formation and, in the case of over pressured formation, the grain to grain pressure decreases as the fluids within the interstices effectively "floats" the overburden. If the pore pressure is less than normal hydrostatic pressure the formation is said to be subnormally pressured. If the pore pressure at that depth exceeds the expected hydrostatic pressure for that depth the zone is termed abnormally pressured. Other terms which apply to the same system are sub-pressure, under-pressure, sur-pressure and over-pressure respectively.

1.124 Geo-Pressure vs. Hydro-Pressure

Formations which are abnormally pressured are considered to be either geo-pressured or hydro-pressured, depending on the conditions of the origin of the pressure. A hydro-pressured zone is a sequence wherein hydraulic communication to the surface is maintained and the excessive pressure is due to the weight of the overlying fluids.

As a sediment is buried it becomes more compacted with depth and a reduction of the porosity occurs. As the water trapped in the sediments is being forced out it encounters a resistant force which essentially maintains the over-pressure.

As the sediments become further buried and compacted the system maintains it's pressure equilibrium by bleeding off the pore fluids in response to the reduced porosity. As a result there is a limited depth to which hydropressures are sufficiently strong and porosity and permeability are sufficiently large to create an effective reservoir.

Geo-pressures are found in units of rock in which flow of fluids has been effectively prevented by either the insitu formation of an impermeable barrier bed, or the appropriate placement of such a barrier through tectonic activities such as: folding, falting, or diapiric action of shale or salts. A common source of pressure in a geo-pressured form are fluids which have been trapped within the formation and which, as compression proceeds, are forced to support the increasing overburden. Since the zone is effectively sealed with respect to explusion of fluids, normal compaction does not continue with depth. As a result, geopressured formations are not depth dependent and have been encountered at depths up to 20,000 feet (6100m). Thus these geopressured units are of great interest in the petroleum industry.

1.13 Occurrence and Origin of Over-Pressured Formation

Over pressured formations are much more common than subpressures and are encountered world wide in sediments varying in age from Pleistocene to Cambrian. Whereas normally pressured formations are considered "open systems" permitting hydraulic communication of interstitial fluids with the surface; abnormally pressured formations of interest are usually found to be "closed systems" which have been geologically pressured. In this case by forcing the formation to maintain it's fluid content and in doing so, cause it to become abnormally pressured, the permeability barrier acts as a pressure seal. In a geo-pressured sequence of shales and sands the shales composed primarily of 'platy clay minerals" fill the role of the permeability barrier. In such a sequence the ratio of shale to sand must be fairly high in order to increase the possibility of a sand unit being completely isolated and encapsulated by the surrounding shales. The creation of an over pressured formation is related to many physical, geochemical and mechanical processes and their order. "Genesis" of over pressure during geologic time is controlled by the environment of deposition on the paleo-continental shelf, and slope, the geometry and lithology of the sediments, regional and local faulting, basin hinge lines, burial and compaction, and subsequent structural deformation.

Generally for a formation to be producing reservoir it must be porous. Potential reservoirs are either geologically or hydrologically pressured. Of those that are geo pressured systems the pressuring of the reservoir may be either primary (originates at reservoir) or secondary (transmitted to reservoir from a deeper overpressured zone). Factors which affect the formation of overpressured zones include: (1) the natural occurrence of reservoir structures, (2) the rate of deposition of sediments and the depositional environment, (3) the amount of uplift and erosion, (4) the tectonic activity in the area and (5) diagenetic processes.

Other minor factors which are thought to contribute to the formation of an over pressured zone are, osmotic phenomenom, massive salt bed deposition, and permafrost encroachment.

For a hydro pressured system, these conditions can occur only at relatively shallow depth, where the degree of compaction is not such as will cause the formation to nonporous. In this case the porous and permeable aquifers are structurally situated between two impermeable beds and the aquifer is structurally deformed so that the necessary hydraulic "head" may be generated to pressure the formation.

The natural structure of a reservoir may lead to pressuring of the capped area. Water pressures which may be normal at the base of the zone are transmitted to over lying oil and gas pockets thereby creating an overpressured gas formation at the top of the layer. Most of normally pressured formations, however, are located within shale sequences who's initial sediments were deposited at a fast rate causing a premature development of a permeability barrier which restricts fluid expulsion before compaction is complete. (Factors other than the rate of sedimentation which effect formation of an overpressured zone are: (a) total thickness of sediments, (b) presence of clay rocks, (c) shale to sand ratio in interbedded sequenced, (d) slope of sedimentary basin.) In this situation the water will, instead of moving vertically, be squeezed into the adjacent sand sequences creating an overpressured sand formation. Shales and sands in this type of sequence will illustrate a high porosity and low bulk density.

Enclosed porous rocks which have been stabilized with respect to pressure at great depths and then through the processes of up lift and erosion being transported to shallower depths will show evidence of abnormally high pore pressures.

Tectonic activity within the area of interest may contribute to secondary charging of a reservoir. Through tectonic action such as local and regional folding, faulting, sliding and slipping, earthquakes and diapiric shale and salt movements, it is possible that the vertical geologic sequence in an area may be rearranged and an impermeable barrier situated in an appropriate position. It is also possible that a fault plane may cause a pressure link between an overpressured formation and an upper normally pressured formation. Shale and salt diapirs are formed through the upward migration of substances of relatively low density through layers of material of relatively higher density. Massive salt and shale deposits gradually accumulate and move upward penetrating over lying sediments, creating impermeable barriers and causing associated faults. Less commonly, overpressured formations may result from diagenitic processes. This post depositional alteration of sediments and the constituent minerals may result in the formation of minerals which occupy a greater amount of space which increases the volume occupied by the rock matrix. For example, the hydration of anhydrite to gypsum increases the bulk volume of the mineral by 40%. Occasionally thin shale sequences respond as semipermeable membranes in the presence of excessive salt solutions, contributing to the development of an osmotic pressure differential between the two formations on either side of the shale. Osmotic pressures are most commonly associated with evaporate deposits and have been known to create pressures sufficient to rupture the reservoir.

1.131 Occurrence and Orgin of Under Pressured Formations

Sub pressures, although not as common as sur pressures, do occur frequently and may be the result of one of several conditions which cause formations to become underpressured. Most commonly, sub pressures are subsequently developed when a reservoir is depleted of all its fluids. They may also result from surface exposure of the permeable bed at a depth greater than where it is penetrated by the bit or by the burial of a fully enclosed and compacted shallow formation.

1.14 Pressure Detection Theory

Special structural characteristics or series of traits are unique to formation which contain high pore pressures. These characteristics may include evidence of a pressure barrier of fault plane associated with the abnormally charged formation. It is this evidence which must be detected to enable an operator to be prepared for the pressures to be encountered. Various operations performed both before and during the drilling operation are utilized to aid in location of possible over pressured zones.

Prior to spudding the well, the operator will use geophysical techniques, such as sonic logs, to search for any possible pressure anomalies as well as to indicate the location of possible formations tops. During the drilling of the well both drilling and drilling fluid parameters are observed and the rock cuttings circulated up from the bottom of the hole are carefully examined. These observations which are a particular interest to the mud logger, include drilling rates, mud weight, flow line temperature, torque and bulk shell density. With an understanding of the interrelationships between these various factors and normally and over pressured zones, trained personnel may interpret the data to optimize drilling programs and detect abnormal pore pressures while drilling. Other techniques which involve the usage of direct pressure measuring devices and wire line tools may be utilized upon completion of the drilling of the well to obtain more information in reference to potential production zones.

As an overpressured formation is approached marked differences in degree of compactions, porosity and minerals composition of interstitial fluids occurs. Also the formation pressure may begin to rise and approach the bottom hole pressure thus decreasing the bottom hole pressure differential. If properties which are affected by such factors are closely monitored and plotted with respect to depth, an abnormally pressured zone may be suspected when a distinct deviation from an average line trending with depth is observed.

It must be remembered though, that such trends may also vary slightly with change in the lithologic composition of the unit and will be interrupted totally by a formation change. Such minor deviations must be recognized and interpreted as such to distinguish them from more significant anomalies associated with a possibly abnormally pressured formation.

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This page was last updated on May 11, 1999