Fluid Contacts Definition

 

At the time of discovery, reservoir fluids are in hydraulic equilibrium, and they are vertically distributed  according to their density at reservoir pressure and temperature .The interface  between these fluids is horizontal and therefore , if the reservoir is hydraulically connected , all the wells will encounter these fluid  contacts  at the same depth . As a consequence, if different wells drilled in the same reservoir  encounter fluid contacts at different depths , the reservoir  is likely to be compartmentalized .

It should  be noted  that the existence of a common fluid contact in all the wells drilled during the appraisal phase does not  guarantee  in itself  reservoir  continuity . In some cases , barriers  to fluid flow  may have  been generated  only after the hydrocarbon migration phase , as a consequence   of  diagenetic  effects  relate  to circulation  of fluids in  the reservoir . In  this case , reservoir barriers  are normally detected  only after  the beginning of the exploitation , observing  for example  different rises of the fluid contacts in different blocks , as  a result of  reservoir fluids withdrawal .

In the majority  of cases , however , the general rule holds  and differences in the contacts depth can be interpreted as evidences of  a degree of reservoir  compartmentalization .   

Several types of data can be used  to locate fluid contacts , from wireline  logs , to routine  core analysis , to pressure measurements .Without  going into  further  detail on this basic   issue , it should be  appreciated that WFT  (Wireline  Formation Tester )  pressure measurements   are  one of the most effective  way  to identify  fluid contacts , at any  stage  of field life .

The precise knowledge of the contact of fluids (gas-oil, gas-water or oil-water) makes it possible to define the useful height of the reservoir as well as the surface of the reservoir.

Methods for determining fluid contact:

·         Resistivity log

·         MDT -Modular Formation Dynamics Tester

·         DST- Drill stem Testing

·         Sw Cut off evaluation

·         Structural closure:

WOC  determined by  SW evaluation

 

WOC  determined by Resistivity log 

 

WOC  determined by  MDT

WOC  determined by DST 

Spill point in the absence of WOC, ODT and WUT, the surface of the deposit can be approximated by the curve of structural closure (Spill point).This assumes a load factor of the structure of 100% (Optimistic surface and reserves).

Spilling  Point (La fermeture Structurale ) 

The ODT is the base of the hydrocarbon column that does not rest directly on an aquifer. It can be taken as a pessimistic interval  of water for estimating the surface area of the deposit (and the useful height).In a well, there is no evidence of the existence or absence of hydrocarbons lower than this base. ODT as a "pessimistic interval  of water".

Example: HC column resting on a compact or clay level

ODT Contact 

The WUT is the highest recognized water grade in a well. Also, nothing proves the existence or non-existence of water higher than this level .

The WUT can be taken as an optimistic body of water to estimate reserves in the absence of an obvious body of water. The WUT as an optimistic 'water interval '

WUT Contact 

 

 

Imaging Techniques for Geological Modeling

 Maps and Cross Sections : 

Most reservoir maps in the world use m.s.l.  as the reference. Depths of the layer increases away from 

the crest of the structure.

The reference is needed because the drilling rig can be on top of  a mountain or an offshore platform.

 In each case the measured depth of the same layer is different as the drilling reference is different.

Piratically any type of geologic data  can be represented on a map .Some of the  most useful maps are 

those that present clear pictures  of the distribution of geologic parameters .

A geologic  map is  an  example of this because  it shows the distribution of  individual rock 

formations   over the area of the map . 

Contour maps can illustrate thickness , facies , percentages , topography , and structure .They  show 

variations that are useful in interpreting the complete geology of an area or individual characteristics 

within the entire data structure . 

Contour  Maps : 

Data upon which numerical values can be placed  can be contoured , since contour  lines connect 

points  of equal value .Contour maps are important interpretive aids and can represent anything  from 

sequential geologic events to absolute values of individual parameters within single rock units .

1. Contour interval should adequately represent the data .Too large an interval overlooks some of the data and too small an interval clutters the map . 
2. Contour lines should honor the data and be properly spaced relative to them .
3. Contour lines should  be drawn smoothly and as parallel to each others as the data will allow .
4. Contour lines should never cross .Crossing contour lines are an impossibility. 
5. Contour lines  should be close together where gradients are steep and farther apart where gradients are shallow .
6. Contour lines  should be labeled . 

Geological Maps :

Geological maps can include as much or as little data as desired .Usually they include  formations and 

their contacts and the most prominent faults .However , geologic maps can also include topographic  

and structural  contours , as well as structural features .

Most geologic maps are colored .But data also can be represented by the use of symbols or different 

tones of gray . Geographic features are essential for purposes of location .

Cross Sections :

Structural , stratigraphic  and topographic information  can be portrayed on cross-sections  that

 reproduce horizontally represented map information in vertical section .

Maps represent information  in the plan  view and provide a graphic view of distribution .Cross-

sections present the same information in the vertical  view and illustrate vertical relationships such as 

depth , thickness , superposition , and lateral and vertical changes of geologic features .

Raw data for cross-sections  come from stratigraphic sections , structural data , well sample logs , 

cores , electric logs  , and structural , stratigraphic and topographic  maps . Datum for the cross-

sections is sea level .