Reservoir Characteristics and Petroleum

It is note quite sufficient   to say that to be  a reservoir , a rock requires porosity  and permeability . Reservoir behavior relative to oil  and gas accumulation and production certainly involves  porosity and permeability , but its performance is based  upon several  important engineering  factors . 

Porosity : 

Porosity represents the amount of void space in a rock and is measured as a percentage of the rock  volume. Connected  porosity where void space  has flow-through potential is called effective porosity .

Noneffective porosity is isolated .Summation of effective and noneffective  porosity produces  total porosity , which  represents all of the void space  in a rock . 

Pore space in rocks at the time of deposition  is original , or primary porosity . It is usually a function  of the amount of space between rock -forming grains . Original porosity is reduced by compaction and groundwater -related diagenetic processes .

Grownd water solution , recrystallization , and fracturing cause secondary porosity , which develops after sediments are deposited . 

Effective , noneffective , and total porosity

 

Permeability : 

A rock that contains connected porosity and allows the passage of fluids through it is permeable .Some rocks are more permeable than others because their intergranular  porosity , or fracture porosity , allows fluids to pass through them easily .

Permeability is measured in darcies . A rock that has a permeability  of 1 Darcy permits 1 cc of fluid with a viscosity  of  1 centipoise (viscosity  of water  at 68⁰ F) to flow through one square centimeter of its surface for a distance of 1 centimeter in 1 second with  a pressure drop of 14.7 pounds per sequare inch . 

Permeability is usually expressed in millidarcies since few rocks have a permeability of 1 Darcy . Intergranular material  in a rock , such as clay  minerals or cement , can reduce permeability and diminish  its reservoir potential . It is evident , however that mineral grains must  be cemented to some degree to form coherent rock and that permeability will reduce to some extent in the process . 

Fluid flow through Permeable Sand 

 

Clay cement and porosity and permeability 

Relative Permeability : 

When water , oil , and gas are flowing through  permeable reservoirs , their rates of flow will be altered by the presence  of the other fluids . One  of the fluids will flow through a rock at  a certain rate by itself . However , in the presence of one or both of the other fluids , its rate of flow can be changed .

The flow rate of each fluid is affected by the amounts of the other fluids , how they reduce pore space , and to what extent they saturate the rock. Comparison of the flow rate of a single fluid through a rock its that same flow rate , at the same pressure drop , in the presence of another  fluid determines the relative permeability of the system .

When rock pores decrease in size , the surface tension of fluids in the rock increases . 

If there are several fluids in the rock , each has a different surface tension , which exercises a pressure variation between them .This pressure is called capillary pressure and is often sufficient to prevent the flow of one fluid in the pressure of another .    

Relative permeability From Clark , 1969. Copyright 1969, SPE-AIME

 

Core Analysis

Objective :

The  objective of every coring operation is to gather information that leads to more efficient oil or gas production . 

some specific tasks might include the :

Geological objectives : 

• Geologic maps 
• Fracture orientation 
• Lithological information  :

• Rock type
• Depositional environment  
• Pore type  
• Mineralogy/ Geochemistry  

  Petrophysical  and reservoir engineering :

• Capillary pressure data  
• Permeability information :  

• Permeability / Porosity  correlation 
• Relative Permeability

•  Data for refining log calculations :

• Electrical properties 

• Grain density

• Core Gamma Log 

• Mineralogy and cation exchange capacity   

• Enhanced oil recovery studies
• Reserve estimate : 

• Porosity

• Fluid saturations

 Drilling and completions : 

• Fluid/ formation compatibility studies 
• Grain size  data for gravel pack design 
• Rock mechanics data 

 Why Core Analysis is important for Development Plan ? 

 Exploration :

• Exploration of structures  and determination of their physical  characteristics. 

•  Estimate of production  possibilities  for wildcats, extension wells  and edge wells .

Well completion and workover operations : 

• Selection of intervals for testing . 

• Interpretation of tests during drilling -Comparison results -Explanation of test anomalies etc.  

• Determination of the best combinations for order of completions when there are several horizons . 

• Selection of intervals and choice of depths if plugs , packers , cement plugs  etc.  are installed to keep out water and gas influxes . 

• Selection of intervals for perforations or acidizing . 

• Estimation of completion efficiency . 

• Selection of intervals for recompletion . 

Field Development : 

• Determination of optimal spacing . 

• Determination of the location  of new wells . 

• Definition of field boundaries . 

• Estimate of production for determination of field equipment . 

• Definition of contact zones for the various fluids . 

• Structural  and stratigraphic correlations . 

• Sampling and bases of interpretation for other well logging . 

• Selection of intervals for optimum completion . 

Well and reservoir evaluation :

• Determination of net pay zone . 

• Estimate of initial productivity 

• Estimate of water production rates and injection pressures . 

• Estimate of decompression zones invaded bay water or gas , and simultaneous  production of various  zones . 

• Estimate of probable recovery . 

• Estimate of oil or gas  reserves in place . 

• Estimates for equitable shares in unitization operations . 

• Reservoir engineering and programming for maintaining pressure or secondary recovery . 

• Forecasts for optimum well completion and maximum future recovery .  

 

 Core Analysis Tests : 

The  experiments done on core sample are as following :

• Routine Test
• special test
• Geo-mechanics tests 
• Formation damage tests

1. Routine Test : 

• Porosity measurements 
• Permeability measurements
• Saturation's measurements 
• Grain density 

These measurements are made to correct the well logs results and use this correction in non-cored  intervals , as example :

• Tyne the Archie's equation  constants (a, m ,n) until having matching to core saturation.
• Get a correlation between porosity and permeability .
• Rock Typing .

2. Special Test : 

•  Relative permeability : used for multiphase modelling in case of having saturation less than 100 % .

• Capillary pressure : used to determine the thickness of transition zone and the saturation distribution in this interval .

• Wettability : used a criteria in EOR applications screening .

• Rock compressibility : used especially for material balance calculations to estimate oil recovery and also in geomechanically applications . 

• Acoustic properties : used to tune the Archie's equation constants to estimate the saturation and porosity from ell logs.

• Other tests like XRD  and SEM . 

3. Geomechanics Test : 

Geomechanics has an important role to play in assessing formation integrity during well construction and completion and in the response of the reservoir  to oil production , water injection and depletion .
The tests most commonly used to determine  fundamental rock mechanics parameters are described including : 

• Unconfined compressive strength 

• Thick Wall Cylinder .

• Tensile Strength.

• Triaxial Tests .

• Pore Volume Compressibility .

• Elastic Moduli.

• Particle Size Analysis . 

Application of Core Analysis Data: 

 

• Permeability Modelling .  

• Facies distribution modelling .

• Production prediction . 

• Residual Oil Saturation targeted by EOR . 

•  STOIP calculation . 

•  Sand control . 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

Introduction to the Petroleum Industry

The oil and Gas Industry is divided into 3 categories : 

•  Upstream
• Midstream
• Downstream 

 

What does Upstream , Midstream and Downstream mean in the oil Industry ? 

Upstream Oil and Gas : 

is E&P (Exploration and Production ) , involves the drilling off exploration wells and drilling into established wells to recover oil and gas .

• Exploration 
• Field Development 
• Production Operation 

 

Midstream : 

involves  the transportation , storage , and processing of oil and gas . Once resources  are recovered , it has to be transported to a refinery , which is often in completely different geographic region compared to the oil and gas reserves . Transportation can include anything from tanker ships to pipelines and trucking fleets .

• Transportation 
• Storage  and distribution 

 

Downstream : 

Refers to the filtering of the raw materials obtained  during the upstream phase . The marketing and commercial distribution  of these products to consumers and end users in a number of forms including natural gas , diesel oil , petrol , gasoline , lubricants  , kerosene , jet fuel , asphalt , heating oil , LPG ( Liquefied Petroleum Gas ) as well as a number of other types of petrochemicals .

• Refining 
• Marketing