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 .

Formation Evaluation & Objectives

Formation Evaluation is the systematic way of recording  the information required to evaluate 

formation Characteristics of a well being drilled .

Objective: To establish the existence of producible hydrocarbon reservoirs (oil & gas).

Type of information obtained from formation Evaluation in Petroleum Geology :

·         Presence of Hydrocarbon

·         Pore pressure analysis

·         Caliper for hole size

·         Core and fluid samples

·         Cuttings

·         Reservoir characteristics

·         Formation strength

Methods to obtain information :

·         Wireline logging : GR , Laterolog , Litho-density tool ,Neutron tool,

·         Logging while drilling

·         Coring

·         Mud logging

·         Well testing

Basic logging Tools measurements:

1.   Wireline Logging tools : 

Electrical Logs:  measure the electrical properties of the 

Example of a Gamma Ray Log

formation along with the formation fluids. 

Gamma Ray Logs: measure the natural radioactivity of the 

formation. It reflects the shale content of 

the formation.    

        GR Logs are used as:

• Correlation tool
• Shale indicator
• Shoulder bed delineation
• Differentiate reservoir rocks from non-reservoir rocks.

Spontaneous -Potential Log: measures the potential 

difference in milli-volts between an electrode in 

the borehole and a grounded electrode at surface. 

The SP is  a passive measurement of very small electrical 

voltages resulting from electrical currents in 

the borehole caused by the differences in the salinities 

(resistivity) of the formation connate  water 

(R_w)  and the drilling mud filtrate (R_mf) , and by the presence 

of ion selective  shale beds .

The tool can be run :
Open hole	centered
Cased hole	eccentered
In a borehole fluid of :
Water or water –based mud
Logging speed : the logging speed is constrained by other measurements in the toolstring
Comments : Usually run with induction logs and old electric logs , the SP can also be run with laterologs , sonic, micrologs , dipmeters , and sidewall cores .There usually is no separate SP tool .

Spontaneous Polarization

Density Logs LITHO-DENSITY TOOL (LDT): measure electron density of the formation which is 

related to formation density. The density value is used to determine the porosity of the formation LDT 

measurement are made by emitting medium energy gamma-rays into the formation and measuring the 

number, and the energy of the gamma-rays returning to the tool.

Neutron logs:  measure hydrogen index of the formation. This log is used primarily for delineation of 

porous formations and determination of porosity.

A combination of neutron log with one or more other porosity logs provides more accurate porosity 

values and lithology identification.

Laterolog Tools: require current to be passed through the borehole and into the formation in order to 

measure fluid resistivity. It consists of resistivity and conductivity tool.

-          RESISTIVITY TOOL :  Send current into the formation and measures the ease of electrical 

flow through the formation.

-          CONDUCTIVITY TOOL: Induces an electric current into the formation and measures how 

large the formation is Rt is measured by both tools and is used to calculate hydrocarbon reserves.

Sonic Logs: measure the elastic or (sound) wave properties of the formation. It records interval transit 

time (dt) in us/ft. It is the time taken for a sound pulse emitted from a transmitter to transverse 1 ft of 

formation.The interval transit time for a  given formation depends on its  lithology and porosity. When 

the lithology is known, sonic log is a useful porosity tool

Caliper Logs: measure the size or geometry of the hole.

2.   LOGGING WHILE DRILLING (LWD):

LOGGING WHILE DRILLING (LWD) is a method used to perform formation evaluation similar to 

electric wireline logging .However , the difference  between the tow is that the the log results from   

LWD   are obtained Real –Time while drilling the hole section .  

3.   CORING :

Is a process used to recover cylindrical rock sample from the wellbore by using specific coring tools . 

The main objective of taking core is to gain  an understanding of the composition of the reservoir rock,

physical rock properties , inter-reservoir seals  and the reservoir pore system.

-        Advantage of coring - more accurate and reliable data of porosity and permeability.

-         Disadvantage - additional cost required for the extra BHA trip to cut the core, late information in place 

du   to the core need to be analyzed and studied in laboratory.

-        Coring is performed  between drilling operations once the formation for which a core is required has  

       been identified on the mud log or LWD log , the drilling assembly is pulled out of  hole .

-        A special assembly   is run on drill pipe  comprising  of a core bit and core barrel.

-        The core is cut free and prevented from falling out of the barrel by a core catcher while being brought 

up to surface.

Sample of Core 

3.   MUD LOGGING :

Is the recording of information derived from examination and analysis of formation cuttings  

circulated  out of the hole for Oil shows and Gas reading through mud circulation?

    Cuttings are taken across the full width of the shakers to be representative of the interval drilled.

   Cutting logs shall be known at all times. Lithological analysis of the cuttings normally is performed 

  on  the washed samples indicating fluorescence after treatment with solvents to indicate presence of hydrocarbons.

4.   WELL TESTING :

The main objectives of running a well test are to make an indirect determination of the reservoir 

physical characteristics and to get physical measurement of well performance and representative fluid 

samples .

•          It is performed after the presence of hydrocarbon is confirmed from the logs.

•          Well test results are very important for development plan.

 By performing a well test, we gather information on:

1-      PERMEABILITY

2-      SKIN

3-      RESERVOIR MODEL

4-      FLOW EFFICIENCY

5-      FLOW RATE

6-      GOR

7-    FLUID COMPOSITION

8 -   PVT SAMPLES

 

 

Uses of Logs by variety of Petroleum People .

 

             A set of logs run on a well  usually mean different things to different people .                 In this blog , we examine the questions asked –and/or answers sought by variety of petroleum people .

The Geophysicist :

•  Are the tops where you predicted ?
•  Are the potential zones porous as you have assumed from seismic data ?
• What does a synthetic seismic section show ?

The Geologist :

• What depths are the formation tops ?
• Is the environment suitable for accumulation of hydrocarbons?
• Is there evidence of hydrocarbons in this well ?
• What type of hydrocarbons ?
• Are hydrocarbons present in commercial quantities ?
• How a well is it ?
•  What are the reserves ?
• Could the formation be commercial in an offset well ?

The Drilling Engineer :

• What is the hole volume for cementing ?
•  Are there any key seats of severe doglegs in the well ,
• Where can you get a good packer seat for testing ?
• Where is the best place to set a whipstock ?

 

The Reservoir Engineer :

• How thick is the pay zone ? 
• How homogeneous is the section ?
• What is the volume of hydrocarbons per cubic meter ?
• Will the well pay-out?
• How long will it take?

The Production Engineer :

•  Where should the well be completed (in what  zone(s))?
• What kind of production rate can be expected ?
•  Will there be any water production ?
•  How should the well be completed ?
• Is the potential pay zone hydraulically isolated ?
• Will the well require any stimulation ?
• What kind of stimulation would be best ?

Geological Considerations and Types of Wells designed in Oilfields

Geological Considerations and Types of Wells designed in Oilfields