Glossary of terms
Bubblepoint
The bubble point is the pressure and temperature at which gas comes out of solution from its host oil. At discovery, all petroleum reservoir oils contain some natural gas in solution. When an oil is below the bubble point gas is partially exsolved from the oil and will concentrate as a separate phase (e.g. natural gas cap above an oil accumulation). When the oil is above the bubble point it is undersaturated and all the gas is in solution. When the pressure is lowered below the bubble point gas exsolves from the oil.
Fluid pressure/ Formation pressure/ Pore pressure
The fluid or formation pressure is the pressure exerted by the fluid within the pore space of a rock. Typically this is measured within permeable horizons (normally reservoir rocks) using a wireline formation testing tool. There is a wide range of formation testing tools manufactured by different companies. Those in common use include:
RFT: Repeat Formation Tester
FMT: Formation Multi-Tester
MDT: Modular Dynamic Tester
RCI: Reservoir Characterisation Instrument
FIT: Formation Interval Tester (Not to be confused with a Formation Integrity Test)
Stethoscope
GeoTap
Fluid Retention Depth (FRD)
The ‘Fluid Retention Depth’ (FRD) is a mathematical construct from a lithostatic parallel gradient in fine-grained rocks, and its intersection with the hydrostatic gradient. The depth of the FRD below the seabed is governed principally by the permeability and rate of loading. The FRD is shallow when sedimentation rate is high and/or when sediment permeability is reduced to nanoDarcy levels (10-9 mD; 10-21 m2) early in the burial history of the rock (usually controlled by the clay-sized fraction of the rock). Conversely the FRD is deep for coarser-grained or more slowly buried sediments.
Formation Integrity Test/ Limit Test
A Formation Integrity Test (FIT), also referred to as a limit test (LT) is taken during drilling operations to test the pressure integrity of the casing shoe and cement. Pressure in the borehole is increased to a predetermined value to ensure the casing, shoe and cement meet required standards.
Fracture Gradient
The fracture gradient is a pressure gradient which reflects the pressures at which tensile failure will initiate in a borehole. This condition may happen naturally. Fracture gradient determination is based on Leak Off Pressure (LOP) measurements conducted routinely in boreholes immediately after casing has been cemented in.
Free Water Level (FWL)
The level at which the water has zero capillary pressure. The height of the Oil Water Contact (OWC) above the FWL is a function of the capillary forces within the rock, and is typically <1 m. When dealing with long transition zones in low permeability reservoirs, such as in the North Sea chalk, the separation can be much greater.
Hydrodynamic flow
Hydrodynamic flow is the lateral movement of groundwater through an aquifer. Hydrodynamic flow is driven by fluid overpressure (differences in overpressure) in a connected aquifer.
Hydrostatic pressure
The hydrostatic pressure gradient is defined as the increase in pressure with depth in a static column of water or brine. The gradient is controlled by the salinity of the water, which may vary with depth. There is therefore a direct correlation between salinity and pressure gradient, and the hydrostatic gradient is not necessarily a single, straight line.
WARNING: The term “hydrostatic” is also applied to the mud pressure gradient of a static column of mud, and appears on RFT data logs, for example. Hydrostatic is also used sometimes to mean a water fluid gradient, whether under normal pressure or overpressure conditions.
GIS
Geographical Information System/Software. GIS is a computer system for capturing, storing, checking, integrating, manipulating analysing and displaying spatial data i.e. data related to positions on the Earth’s surface.
Lateral drainage
Where continuous reservoirs are connected either directly to, or via a network of faults to, shallower levels or the seabed, fluid leakage and pressure dissipation occurs. When this happens where all the fluids, have not equilibrated, the reservoirs have lower pressure than the associated shales and other fine-grained rocks (which are more highly overpressured). The recognition of laterally-drained reservoirs has implications for fluid flow, hydrodynamic traps and long petroleum columns.
Lateral transfer
In inclined reservoirs under burial conditions, fluid rapidly reaches equilibrium along a fluid gradient (water plus associated hydrocarbons) but the surrounding shales do not. This leads to higher overpressure in the sand body at the apex of the reservoir and lower overpressure than the shales at the deepest point. The pressure differences lead to further influx from the shales in the down-dip area and fluid loss from sands into shales in the updip area. The equilibrium point (where the overpressure in the sands equals the corresponding shales) is termed the centroid.
Leak Off Test
A Leak Off Test (LOT) is taken during drilling operations to test the pressure integrity of the casing shoe and cement and in addition provides information regarding the fracture strength of the rock. Unlike a Formation Integrity Test, pressure in the borehole is increased until fracturing is induced in the surrounding rocks. The pressure at which fracturing is induced is an estimate of the minimum stress.
Leak Point
A leak point is the location where fluids are actively transferring from one reservoir horizon to another, often associated with hydraulic failure of the seal. Leak points are sometimes recognised by overpressure anomalies. They may be coincident with the lowest effective stress within a pressure compartment.
Lithostatic pressure
The lithostatic pressure is the combined weight of rock and fluids at a specified depth. The lithostatic (pressure) gradient, which begins at the sea-bed (mudline), is the changing pressure with increasing depth. Since many sedimentary rocks compact with increasing depth (stress) becoming denser, the lithostatic pressure gradient therefore varies with depth, often changing to higher gradients with increasing depth of burial.
WARNING: The lithostatic gradient is often depicted as a straight line with a gradient of 1.0 psi/ft (22.66 MPa/km or 2.31 g/cc), or 0.94 psi/ft (21.26 MPa/km; 2.168 g/cc) in the deep water environment. Using a single gradient is a poor approximation, and a regional default should be found where possible for multi-well plots.
Mudweight
Mudweight is the average density of the mud used to drill a well, and it is likely to change many times before the well reaches TD. It is often expressed in units of ppg (pounds per US gallon) or g/cc (Specific Gravity). Note that when the mud is circulated through the borehole during drilling operations the mudweight is elevated, and termed Equivalent Circulating Density (ECD), which is higher than the static mud weight. The reference datum for mud weight is KB.
Overpressure
Overpressure is defined as the difference between the pore pressure and the hydrostatic pressure at the same depth:
∆P = Pp – Phydr
Overpressure residual map
A map showing the difference in overpressure at well locations (or a grid surface) between two map horizons. These residual maps can be used to infer preferred vertical flow directions, and to help in the recognition of leak points.