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Contents A1.0 INTRODUCTION TO OPENHOLE LOG INTERPRETATION ...................................1 A.1 USES OF LOGS.......................................................................................................................1 A.2 BASIC PETROLEUM GEOLOGY..............................................................................................2 A.3 BASIC LOG INTERPRETATION CONCEPTS...........................................................................4 A.4 RESISTIVITY AS A BASIS FOR INTERPRETATION—THE ARCHIE EQUATION.......................5 A.5 DEFINITIONS ...........................................................................................................................7 a) Formation Porosity (φ)...........................................................................................................8 b) Formation Resistivity (R)........................................................................................................8 c) Formation Factor (F)..............................................................................................................8 d) Water Saturation: Sw ...........................................................................................................8 e) Hydrocarbons Saturation (Shy )...............................................................................................9 f) Clean Formations ..................................................................................................................9 g) Shaly Formations..................................................................................................................9 h) Key Formulas ....................................................................................................................11 i) Key Symbols........................................................................................................................11 A.6 LOG SCALES AND PRESENTATIONS ..................................................................................12
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Introduction to Openhole Logging
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A1.0 Introduction to Openhole Log Interpretation A.1 USES OF LOGS A set of logs run on a well will usually mean different things to different people. Let us examine the questions asked—and/or answers sought by a variety of 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 good 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 keyseats or 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?
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Log evaluation can be many things to many people. As the answers are sought each individual will possibly use the available data in a different manner. The common approach will be in reading the logs and understanding the various reactions produced by formation characteristics on our logging devices. The factors influencing log reading and the information they provide are what we wish to introduce to you in this course.
deposition is such that crossbedding structures, channel patterns and gradational rock types are common. In areas of freshwater deposition coal beds may occur, indicating non-marine conditions.
A.2 BASIC PETROLEUM GEOLOGY In order to better understand log responses, we should first review the types of rocks that are found in the boreholes.
Carbonate Deposition Carbonate deposition occurs in marine conditions by the precipitation of limestone from organisms as fine particles, shells or massive growths. Limestones are deposited either as flat-lying beds on the ocean floor or as mounds or pinnacle reefs.
Common sedimentary rocks are sandstone, siltstone, shale, limestone, dolomite and anhydrite In general, sedimentary rocks are deposited as either clastic sequences containing sandstone, siltstones and shales or carbonate sequences of limestone, dolomite, anhydrite and shale. (Figure A1). Clastic Deposition Clastic rocks are formed from rock fragments and weathered particles of preexisting rocks. These sediments are transported by wind and water and are usually deposited in rivers, lakes and oceans as relatively flat-lying beds. Current and wave action later sorts the sediments such that in high-energy environments coarse-grained sands are deposited and in low energy environments fine-grained silts and clays are deposited. The nature of the
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After deposition and with deeper burial of the sequence, compaction occurs and the clastic grains can become cemented together to form sedimentary rock.
Barrier reef chains that grow in this manner may form restricted ocean basins landward, in which dolomite and anhydrite are precipitated by the evaporation of seawater. When limestones form near shore, there may be mixing of limestone and eroded clastic material. In deeper ocean basins, limestone and shale mixtures are common. After deposition, later burial may cause dolomitization of the limestone in which the actual composition of the rock is changed to dolomite. Because of their brittle nature compared with other sediments, limestones tend to fracture with deformation, which increases permeability and helps in the dolomitization process.
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Figure A1: Clastic Deposition vs. Carbonate Deposition
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In many parts of the world multiple sequences of clastic rocks overlie older carbonate sequences. Between each of the clastic and carbonate groups, erosional inconformities are common and the nature of deposition within each group is unique.
d.
e.
RW = water resistivity: the electrical resistance of the water filling the pore space in the rock. This value varies with water salinity and temperature. k = permeability: the ability of the rock to pass fluids through it.
A.3 BASIC LOG INTERPRETATION CONCEPTS Any given rock formation has numerous unique physical properties associated with it. Only those that can be measured and are useful will be considered in this course. They are
Consider the following unit cubes (Figure A2):
φ = porosity: the void space between grains that is generally filled with liquids or gases. b. Sw = water saturation: the percentage of the pore space filled with water (as opposed to hydrocarbons or air). c. R = resistivity: the resistance to electrical current flow presented by a unit volume of rock.
Cube B If the porosity φ is 70% filled with water and 30% hydrocarbons, then, the water saturation
a.
Cube A If the porosity (φ) is filled with water then, by definition, the water saturation SW = 100%.
70 SW = 70 + 30 and hydrocarbons saturation
Cube “A”: porosity = waterfilled φ SW = 100%
Cube “B”: porosity = hydrocarbons and water in φ SW = 70%
Figure A2
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% = 70%
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Shy = 1 - Sw = 30% Therefore the percentage volume of water saturation = φ × Sw
The usefulness of resistivity logging rests on the facts that - water is a conductor (low resistivity) - hydrocarbons and rocks are insulators (high resistivity) Consider the following unit cubes (Figure A3):
For example: if φ = 20% and Sw = 70%, then 14% of the bulk volume is water and 70% of the pore space is water filled.
A.4 RESISTIVITY AS A BASIS FOR INTERPRETATION—THE ARCHIE EQUATION In the previous section we introduced a number of parameters used to evaluate rock formations. If we could build on the effects of resistivity in conjunction with the other parameters to develop a mathematical relationship, we would have an extremely useful tool for our work with potential hydrocarbon zones.
Cube C The resistivity Rt of the cube will vary with water resistivity Rw (i.e. as Rw increases, Rt increases and vice versa). Therefore: Rt α Rw.
(1)
Cube D Replace 25% of the cube with rock (hence φ = 75%) but maintain a constant Rw. Resistivity Rt increases with decreasing porosity φ (i.e. as φ decreases, Rt increases).
The remainder of this section is devoted to developing such a formula.
S
Cube “C” - Constant Current - Porosity = 100% - Sw = 100%
Cube “D” - Constant Current - Porosity = 75% - Sw = 100%
Cube “E” - Constant Current - Porosity = 75% - Sw = 70%
Figure A3
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Therefore: Rt α 1/φ.
(2)
Cube E Replace 30% of remaining porosity φ with hydrocarbons. Resistivity Rt increases with decreasing water saturation Sw (i.e. as Sw decreases, Rt increases). Therefore: Rt α 1/Sw.
Ro α
Rw (5) φ
Now, let φ = 1, then Ro α Rw . Now, let F = constant of proportionality defined as the formation factor. Therefore: Ro = FRw
(3) Ro
By combining the above observations (1, 2 and 3), we can say Rt α Rw ×
1 φ
×
1 Sw
Fα Rw (4) φ Sw
To solve for the constants of proportionality let us first limit the equation as follows: Let Sw = 100% (i.e. there is no hydrocarbon present and the porosity is 100% water filled). Then, define Ro = Rt (ie: Ro is the wet resistivity of the formation for the condition Sw = 100%):
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(6) Rw
Returning to Equation 5 and introducing porosity as a variable, it is clear that
or Rt α
or F =
1 φ
This is intuitively obvious as the relationship between Ro and Rw is related to that particular unit cube of rock and its porosity characteristics. Through empirical measurements, it was determined that a F=
φ
(7) m
where a = constant m = cementation factor
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The cementation factor m relates to the porosity type and how it will transmit electrical current to the actual rock (also called tortuosity).
aRw or S
n w
=
(9) φ Rt m
Using the above equations Recall Ro = FRw (Equation 6) aRw when Sw = 100% m φ
Rt = Ro = if Sw ≠ 100%, then
1
aRw
Rt α
φ
× Sw
m
or Rt α Ro ×
1
a) Formation Porosity (φ) Defined as the fraction of total volume occupied by pores or voids, where
Ro (8) Rt
Through laboratory measurements, it was found that this relationship (8) is dependent on the saturation exponent n as Ro S
=
n w
Rt FR w or S
n w
=
The remainder of this course is dedicated to measuring, evaluating and using porosity and resistivity to calculate water saturation and hence hydrocarbons reserves using the concepts of this equation. A.5 DEFINITIONS
Sw or Sw α
Equation 9 forms the Archie relationship that is the basis for all conventional log interpretation techniques. Enhancements and refinements may be applied for the more complicated rock types.
φ=
pore volume
× 100%
total volume When the pore space is intergranular it is known as primary porosity. When the porosity is due to void space created after deposition, (e.g., vugs or fractures in carbonates), the porosity is known as secondary porosity. When shale is present, the pore space occupied by the water in the shale is included with the pore space in the rock to give total porosity (φ T ). If only the rock pore space is considered in a shaly formation, the pore space is called effective porosity (φe ).
Rt
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b) Formation Resistivity (R) Defined as the resistance offered by a formation to the flow of electrical current. It is expressed in ohm-meter2/meter. We use several terms to describe formation resistivity under various circumstances of fluid content. Rt : Describes the resistivity of a formation undisturbed by the drilling process. Ro: Describes a special form of Rt . It is the resistivity of a clean formation when all pore space is filled with connate water (Rw). Rw: Is the symbol for the resistivity of formation (connate) water.
For Porosity In a 1942 paper Gus Archie proposed that the relationship between formation factor and porosity could be described by the formula a F=
where a = empirical constant. m = cementation factor. Some recommended F and φ relationships are 0.62 F=
c) Formation Factor (F)
Ro F= Rw F is a constant for the formation under consideration. The value of F for any particular formation depends on: - formation porosity - pore distribution - pore size - pore structure.
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φ
(for sands) 2.15
0.81 F=
For Resistivity An important relationship exists between the resistivity of a fully water saturated formation and the resistivity of the contained water. The ratio of these two values is called formation resistivity factor (or more commonly, formation factor) where:
φm
φ
(for sands) 2
1 F=
φ
(for carbonates) 2
Chart Por-1 (figure A4) in the Log Interpretation Chart book is based on several different F-φ relationships. d) Water Saturation (Sw) Defined as the fraction of pore volume filled with water where water filled pore volume sw = total pore volume
× 100%
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e) Hydrocarbons Saturation (Sh y) Defined as the fraction of pore volume filled with hydrocarbons where: hydrocarbon-filled pore volume Shy =
× 100%
total pore volume or
Shy = 1 – Sw.
g) Shaly Formations This describes formations where some of the formation void space (porosity) is filled with shale. Shale distribution is considered to be: - Laminated: The formation is built up of thin laminae of sand and shale. - Dispersed: The shale particles are dispersed in the pore space. - Structural: The shale replaces matrix.
f) Clean Formations The term clean formation refers to those that are shale free.
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Formation Resistivity Factor versus Porosity 50 2.5
5
10
20
50
100
200
500
1000
2000
5000
10,000
40 30 25 20 15
φ, porosity (p.u.)
FR = 10 9 8 7
1 φ2
m Vugs or spherical pores
6 5
FR =
4
0.62 φ2.15
FR =
1 φm
2.8 2.5
Fractures 2.2
3 1.8
2.0
2 FR =
0.81 φ2
1.6 1.4
1 2.5
5
10
20
50
100
200
500
1000
2000
5000
10,000
FR, formation resistivity factor
This chart gives a variety of formation resistivity factor-to-porosity conversions. The proper choice is best determined by laboratory measurement or experience in the area. In the absence of this knowledge, recommended relationships are the following: 0.62 For Soft Formations: Humble Formula: Fr =
φ2.15
0.81 or Fr =
φ2
0.62 For Hard Formations: Fr =
φm
with appropriate cementation factor, m.
EXAMPLE: φ is 6% in a carbonate in which a cementation factor, m of 2 is appropriate Therefore, from chart, Fr = 280. Chart Por-1
Figure A4 (05/96) A-10
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h) Key Formulas FRw Archie’s formula: Sw =
Formation Factor: Ro
n
Rt
a. From deep resistivity
F = Rw Rxo
where n is usually taken as 2 b. From shallow resistivity
F = Rmf a
c. From porosity
F =
φm
i) Key Symbols BHT
-
di
-
h RIDPH
-
RIMPH
-
RSFL
-
Rm Rmf Rmc Rw Rwa
-
Rt
-
Ro
-
Rxo
-
Rsh F φ Sw
-
bottom hole temperature in degrees Celsius average diameter of invaded zone (Di) bed thickness in meters resistivity from the deep phasor induction resistivity from the medium Phasor induction resistivity from the Spherically Focused Log resistivity of the mud resistivity of the mud filtrate resistivity of the mudcake resistivity of the formation water apparent resistivity of the formation water resistivity of the formation (uncontaminated zone) resistivity of the formation when 100% water filled resistivity of the flushed zone (close to borehole) resistivity of the shales formation resistivity factor porosity in percent water saturation, percent of pore space occupied by water in uncontaminated zone
Sxo
-
S hc
-
K SSP
-
PSP
-
k
-
φ
-
φS φD φN
-
φT
-
φe φ2 Vsh Pe
-
water saturation, as above, in flushed zone hydrocarbons saturation as percent of pore space occupied by water coefficient in the sp formula static spontaneous potential - the maximum possible for a particular Rmf / Rw pseudostatic spontaneous potential—the SP found in a thick shaly sand permeability in millidarcies pore volume porosity = ×100%. total volume sonic porosity density porosity neutron porosity φN + φ D total porosity ≅ 2 effective porosity secondary porosity volume of shale photoelectric index
A complete list of symbols and subscripts is included in Section J (Miscellaneous).
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A.6 LOG SCALES AND PRESENTATIONS a) Well logs provide a continuous graph of formation parameters versus depth. Normal depth scales are - 1:240—1 m of log per 240 m of measured hole depth. Each line is 1 m, with heavy lines every 5 m, and heavier lines every 25 m for ease of reading. Depths are indicated every 25 m (Figures A5 and A6). - 1:600—1 m of log per 600 m of measured hole depth. Each line is 5 m, with heavy lines every 25 m. Depths are indicated every 25 m (Figure A7). - Other scales are available. These include 1:1200, 1:120, 1:48 and 1:5. - Log grids may be either logarithmic (resistivity logs—Figure A6) or linear (porosity logs—Figure A5). b) If a caliper device is present or the log being generated is a type of sonic log, event markers are placed on each side of the depth track integrating the quantity of hole volume or transit time recorded. 1. Integrated hole volume—requires caliper device (Figure A5) - placed on the left side of the depth track 3 - small marks indicate 0.1 m whereas large marks represent 3 1.0 m . 2. Integrated cement volume—Requires caliper device plus future casing size - placed on the right side of the depth track when space permits— and if sonic not present 3 - small marks indicate 0.1 m while large marks represent 3 1.0 m .
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3. Integrated transit time—Requires sonic tool (Figure A5) - placed on the right side of the depth track - small marks indicate 1 msec whereas large marks represent 10 msec of time. If the log is recorded using logging-whiledrilling methods, event markers on both sides of the depth track (Figure A6) represent the conversion from time-based sampling to a depth-based presentation. The markers therefore indicate the number of data samples per unit depth. In other words, the larger the concentration of markers over a depth interval, the greater the number of data samples used to make the log. c) Logs also have headings and inserts. - Log headings provide such information as well depth, casing depth, mud params, maximum temperature and other comments pertinent to the evaluation of log data (Figures A8 and A9). - Inserts provide such information as curve scaling, coding, date/time of acquisition, data curve first-reading points and constants pertinent to the logging run following the insert. Curve coding on the log data indicates the deepest reading primary measurement (long dashed) to the shallowest reading primary measurement (solid) when two or more measurements are combined (Figure A10).
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Figure A5: Linear Grid 1/240 Scale (05/96) A-13
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Logarithmic Grid 1/240 Scale Data Sample Event Markers for LWD Curves Figure A6
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Figure A7: Linear Grid 1/600 Scale
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Figure A8: Log Heading (page 1) (05/96) A-16
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Figure A9: Log Heading (page 2) and Log Tail
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Figure A10: Log Insert (05/96) A-18
