Chapter 4_Formation Evaluation_Dr. Adel Salem

Formation Evaluation [PEP 437]

Dr. Adel Salem Asst. Prof. of Petroleum Engineering Faculty of Petroleum and Min. Eng. Suez Canal University Spring Semester 2010-2011

Course Outlines

Chapter one: Methods of Gathering Formation Evaluation Data Chapter Two: Mud Logging Chapter Three: Measurements While Drilling Chapter Four: Coring Chapter Five: Wireline Logging Operations Chapter six: Open-Hole Logging Measurements Chapter Seven: Analysis of Logs and Cores Chapter Eight: Formation Testing (DST) Chapter Nine: Integrated Formation Evaluation

Formation Evaluation-PEP437_Dr. Adel Salem May 1, 2011 Chapter 4: 2

Coring Operations


Dr. Adel Salem Spring 2011

Chapter 4: Outlines 1. Objectives

1.1. Engineering Objectives 1.2. Geological Objectives

2. Coring Operations A. Wireline Coring

A-1. Conventional Sidewall Cores A-2. Rotary Sidewall Coring

B. Conventional Coring 3. Coring Fluids 4. Special Core Handling 5. Coring Program Considerations 6. References


1. Objectives 1. The objectives of coring are to bring a sample of the

formation and its pore fluids to the surface in an unaltered state, preserve it, and transport it to a laboratory for analysis.

These objectives are hard to meet since the very act of cutting a core will, to some extent, alter both the sample properties and the saturation of the fluids in its pores.

There are a number of techniques for minimizing damage to formation samples. These will be discussed in this chapter. Other methods may also be used at the time the core is analyzed, aimed at restoring the original state of the sample under reservoir conditions.


1.1. Engineering Objectives 1. Defining areal changes in porosity, permeability, and lithology 2. Defining reservoir water saturation 3. Assisting in the definition of reservoir net pay; 4. Providing information for calibrating downhole logs as well as

the measured values of electrical properties that will be used to improve log calculated water saturations;

5. Acquiring data on the magnitude and distribution of reservoir residual oil saturation;

6. Providing core material from which petrographic studies can be made to define clay type and distribution;

7. Acquiring rook samples for special core analysis studies, including relative permeability, capillary pressure, and formation wettability tests;

8. Providing data on porosity, as well as on horizontal and vertical permeability distributions.


1.2. Geological Objectives 1. Defining gas-oil and oil-water contacts, formation limits, and

type of production expected; 2. Providing core data from which the depositional environment can

be deduced, including grain size and grain size sequences; vertical sequence of facies; sedimentary structures (ripples, cross beading); biogenic structures (root zones, burrows); diagenetic alterations (cementing, secondary porosity, secondary mineralization);

3. Permitting a visual study of the frequency, size, strike, and dip of fractures. This requires that fracture studies be undertaken and may require the availability of an oriented core;

4. Retrieval of oriented cores so that directional permeability trends can be ascertained; this applies to both fractured and to non-fractured samples;

5. Acquisition of samples of non-reservoir rook in exploration areas so that source bed studies can be made.


2. Coring Operations

Coring Operations may include:

A. Side Wall Coring

(Wireline Coring) B. Conventional Coring


A. Wireline Coring Two wireline tools are currently in use for retrieving formation samples: 1. The conventional Sidewall Core Gun and 2. The newer Rotary Sidewall Coring Tool.


A-1. Conventional Sidewall Cores

The Figure illustrates a sidewall core gun. The body of the gun carries a number of hollow steel bullets which can be fired individually into the formation by means of explosive charges.

Once lodged in the rock, the bullet is retrieved by flexible steel cables attached to the bullet.

Raising the gun in the borehole pulls on the wires, and is usually sufficient to dislodge the bullet with the formation sample inside.


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A-1. … Note that the gun is equipped with an SP electrode. This

allows correlation to other logs and correct depth positioning prior to sampling. A GR is also available.

These guns come in a variety of shapes and sizes. On average they can retrieve 60 samples per trip in the hole. The diameter of the core barrel may be anywhere between 3/4 in. and 1-l/8 in.

The length of the core retrieved is a function of many variables. Depending on the strength of the explosive charge, the type of core barrel, and the hardness of the formation, the lengths of the recovered samples may vary from 0 to 2 in. Sometimes the retainer wires used to retrieve the core barrel will break and the bullet will be lost in the hole.


A-1. … Once all the samples have been collected, the gun is raised to

the surface and each core plug stored in a glass jar or special airtight bag that is marked with the well name and the depth at which it was cut. These types of cores may be analyzed for porosity, permeability and hydrocarbon content.

There are limitations to the data that can be obtained from sidewall cores. 1. In the first place, the sample is taken from a part of the

formation that has been flushed with mud filtrate. 2. Second, the act of shooting the core barrel into the formation

may alter the porosity and permeability of the rock sample. 3. Last, the trip to the surface through the mud column may

cause additional flushing of fluids from the sample. Despite these drawbacks, sidewall cores are still good quick-look indicators of formation properties.


A-2. Rotary Sidewall Coring The Rotary Sidewall Coring Tool

(RSCT) uses a motorized bit to drill into the borehole wall to retrieve its samples (Figure).

It is capable of cutting 30 core samples per trip in the hole. Core size is 15/16 in. in diameter and 1- 3/4 in. long.

Each core takes about 5to 10 minutes to cut in hard formations, and slightly longer in softer rocks.

This device works better than the conventional sidewall core gun in consolidated formations and causes very little physical damage to the sample. Recovery in good hole conditions averages 80 to 90%.


B. Conventional Coring When a point is reached to cut a conventional core (a drilling break, a pre-planned depth, etc.), the drill pipe is removed from the hole. It is then dressed with a hollow core bit and a hollow barrel equipped with a non-rotating inner barrel as shown in Figure .


Cores can be from 10 to 60ft long and from 1-7/8 in. to 4-l/2 in. in diameter. If unconsolidated formations are cored, a rubber sleeve is used to hold the friable material more securely. For some applications, a special pressure core can be cut. The core barrel is designed so that after the core is cut, it is maintained at original reservoir pressure until it arrives at the laboratory. The Figure illustrates a pressure core barrel. Pressure cores are normally frozen at the wellsite for transportation.


4. Coring Fluids Depending on the coring fluid, the saturations of gas, oil and water in the core may be increased, decreased, or unchanged Water-base muds, for example, produce a water filtrate that invades the core and displaces hydrocarbons. Oil-base muds with an oil filtrate may replace reservoir oil by flushing, but do not substantially alter the original oil saturation. The Table summarizes the effects of various coring fluids.


For some applications, it is of no consequence if fluid saturations change (i.e, if the core is being cut only for porosity and permeability estimates, the change of water saturation is not important). The Table below lists the most suitable coring fluid for a given core analysis objective.


Fluid Saturations !!!! Saturation changes in cores can take place at three different times: 1. When the core is cut, as it travels in the core barrel

from the reservoir to the surface, and at the surface during transportation and storage.

2. It is worthwhile to review the causes and approximate magnitudes of these saturation changes.

3. Once the core is retrieved at surface, precautions must be taken to prevent further saturation changes.

4. The NEXT Table documents ten different cases covering a range of initial reservoir saturations and coring fluids.



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4. Special Core Handling At the surface, the core is exposed to the atmosphere, where evaporation of water and light hydrocarbons can occur. It is normal practice to store the core immediately in a protective environment. Storage methods used include: Freezing the core with dry ice; Wrapping cores in plastic bags; Wrapping cores in plastic wrap and/ or aluminum foil and

sealing them with paraffin wax; Submergence under deaerated water; Submergence under non-oxidizing crude

Rubber-sleeve cores are normally left in the sleeve, although they may be sliced into shorter lengths to ease handling


Coring Program Considerations 1. Core Alteration during Recovery

1. Filtrate Invasion 2. Fluid Expansion and Expulsion

2. Selection of Coring Fluid 1. Coring While the Well Is Being Drilled

CONVENTIONAL CORING THE RUBBER SLEEVE THE PLASTIC LINER THE FIBERGLASS INNER BARREL THE PRESSURE CORE BARREL THE SPONGE (FOAM-LINED) CORE BARREL

2. Cores Cut After the Well Is Drilled THE PERCUSSION SIDEWALL CORER


Coring Program Considerations, … 3. Core Handling, Sampling, and Preservation 3. 1. Core Handling at the Wellsite

4.1.1. Handling Conventional Cores 4.1.2. Handling Sidewall Cores 4.1.3. Other Coring Device (Rubber, plastic, and fiberglass cores , OR Pressure core

Sponge core)

3.2. Core Preservation Techniques 4.2.1. A General Discussion 4.2.2. Sealing in Plastic Bags 4.2.3. Sealing in Strippable Plastic or Wax

3.3. Core Sampling 4.3.1. Conventional (Plug) Analysis 4.3.2. Full Diameter Analysis 4.3.3. Side wall Cores 4.3.4. Rubber, Plastic, and Fiberglass Cores 4.3.5. Pressure Cores 4.3.6. Sponge Cores


Saturation changes that occur during coring and recovery

Water – Based coring fluid


Oil – Based coring fluid





POROSITY

PERMEABILITY

CONVENTIONAL

POR

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UNDER STRESSES

CAP CURVES

INTERFACIAL TENSION

WETTABILITY

CAP PRESSURE

HARDNESS

STRESSES

YOUNG MODULUS

POSSION'S RATIO

FRICTION ANGLE

HOLE STABILITY

ROCK MECHANICS COPATABILITY

SPECIAL

CORE ANALYSIS

References 1. Richard M. Bateman : “Open-hole Log Analysis and Formation

Evaluation,” International Human Resources Development Corporation, Boston, ISBN 0-88746-060-7 (U.S.), 1985.

2. Halliburton : “Formation Evaluation Manual,” HLS 3. D. G. Bowen : “Formation Evaluation and Petrophysics,” Core

Laboratories, Jakarta, Indonesia, March 2003. 4. Heriot-Watt University “Formation Evaluation,” Institute of

Petroleum Engineering, 5. Toby Darling : “Well Logging and Formation Evaluation,” Gulf

Professional Publishing is an imprint of Elsevier Science, Elsevier, 2005.

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May 1, 2011 Formation Evaluation-PEP437_Dr. Adel Salem Chapter 4: 29

Documents

Chapter 4_Formation Evaluation_Dr. Adel Salem