What is 4D siesmic ?

4D seismic survey is simply a series of three dimensional (3D) seismic surveys over the same area of interest at different times. Here "time" is considered as the 4th dimension.

As a field produces oil or gas, the reservoir fluid distribution and pressure changes. This variation is reflected in seismic surveys,. The difference of surveys taken before and after production can us a clearer picture of remaining oil/gas un-swept or remained trapped and help engineers plan/redesign the depletion strategy like new infill drilling, injection optimization etc.

What is MWD & LWD ?

MWD stands for Measurements While Drilling.
LWD stands for Logging While Drilling.

As a well is drilled to a subsurface target, some instruments are attached to drilling string just above the drilling bit. The MWD tools measure the well trajectory parameters like inclination, azimuth etc, while LWD tools measure rock properties like Gamma Ray, Neutron & Sonic Density etc.

In modern highly complicated reservoirs, these tools play vital roles in steering the well along the most favorable reservoir body. It also saves time and money eliminating need for wireline logs and round trips of the drilling bit. 

Another very interesting aspects of these tools is that they communicate with the engineer sitting on the surface via mud-telemetry, that is, they uses pulses in the mud or the fluid system present inside the drill string to send data to the surface and also to receive instructions from the surface.

What is STOIIP or OOIP ?

The full form of these two abbreviations are as follows:

• STOIIP = Stock Tank Oil Initially In Place
• OOIP = Original Oil In Place, or
• STOOIP = Stock Tank Original Oil In Place

All of the above, essentially mean the volume of oil originally or initially present in a hydrocarbon reservoir expressed in equivalent volume that it would occupy in a stock tank at standard temperature and pressure.

Mathematically, it is expressed as:

STOIIP = BV *ϕ * (1-Swi) / Bo

where,
BV = Bulk Volume of the Reservoir
ϕ = Porosity (need to be Effective Porosity)
Swi = Initial Water Saturation
Bo = Formation Volume Factor

Capillary Pressure

Due to interfacial tension (IFT), a fluid experiences a force of pressure known as "Capillary Pressure" through the narrow capillaries of a porous medium and rises up or down from the free fluid level. In Figure-1, the left side of figure shows water rising above the Free Water Level (FWL) differently in capillary tubes of different diameters, whereas the right side of the figure shows the same phenomena in a porous medium with different pore sizes.

Figure-1

Capillary Pressure is often observed in oil-water hydrocarbon system with a varied thickness of a "Transition Zone". In some situations, the entire reserve may be within the "Transition Zone".