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Coring and Core Analysis Reading Assignment

(Last Updated 11 April 2016)

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Table of Contents

Summary ....................................................................................................................................................... 1

Why do we need cores? ................................................................................................................................ 1

Petrophysics .............................................................................................................................................. 1

Geology ..................................................................................................................................................... 1

Reservoir engineering ............................................................................................................................... 2

Production technology .............................................................................................................................. 2

Core analysis planning .................................................................................................................................. 2

Coring / Core Analysis Planning Flow Sheet ................................................................................................. 3

Core Acquisition ............................................................................................................................................ 6

Coring methods ......................................................................................................................................... 6

Core handling .......................................................................................................................................... 11

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Summary

In this chapter the following subjects are discussed:

Coring methods Core handling procedures Depth and quality control Routine core analysis for petrophysical properties (porosity, permeability, grain density) Averaging of porosities and permeabilities Special Core Analysis (SCAL) for petrophysics (especially resistivity parameters, i.e. Archie’s m

& n and Waxman-Smits parameters, and compaction)

Why do we need cores?

In several stages of log evaluation parameters are required which are not available from the logs, e.g. the matrix density, Archie's cementation factor (m) and saturation exponent (n), similar parameters for more advanced saturation models used in shaly sands, permeability, rock compressibility, strength, etc. Moreover, cores are important for geological descriptions and evaluations as well as for other disciplines (e.g. reservoir engineering: relative permeability, capillary pressure; Production Technology: compatibility tests, strength). Also, core measurements may give an independent estimate of the formation hydrocarbon content, e.g. via measurement of the capillary pressure curve.

The Value of Information (VOI) in core analysis in many cases is sufficiently high to justify it. An extreme example is the case of the rapid subsidence (caused by reservoir compaction) of the Phillips Ekofisk field. Proper core analysis (i.e. laboratory compaction measurements) prior to production would have cost about 1 million US $. Because of lack of such high quality tests (some low quality test data were available which didn’t highlight the potential compaction danger), Phillips only noticed the rapid subsidence of the field when it was already too late. They had to jack up a production platform with about 6 metres (which required stopping production for several months), costing them in total about 1 billion (1000 million) US $, hence a thousand times more than a proper core analysis study would have cost them. Not all cases will be this dramatic, but core analysis will pay off in many cases very easily.

The different discipline needs for core data can be summarised as follows:

Petrophysics Basic rock properties (porosity, permeability, grain density) Saturation from capillary pressure Effect of stress and reservoir (production induced) compaction / subsidence Electrical properties (m, n) and Cation Exchange Capacity (CEC) Acoustic properties

Geology Core description

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Facies analysis (also for Special Core Analysis [SCAL] sampling) Mineral identification Diagenesis and clay typing Depositional information Formation age Microscopic and X-ray analysis

Reservoir engineering Relative permeability (not discussed here: see courses on Reservoir Engineering) Capillary pressure curves Critical gas saturation Pore volume compressibility Flooding tests

Production technology Well injectivity Sand control parameters Rock mechanical data Mineralogy for acid stimulation

Core analysis planning

As coring is expensive it has to be carefully planned. Different types of coring jobs are required in different circumstances (lithologies, fluid/pressure regimes, objectives). Once the core has been obtained careful planning is again required to get the most out of it: sample handling and the measurement sequence have to be defined in advance and agreed by all disciplines involved.

The following is an example list of things to consider.

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Coring / Core Analysis Planning Flow Sheet

Prior planning

Company SCAL focal point available? And/or: Consult Company manuals, guidelines, strategies Contact Company specialists if scope is outside that of standard contractors

Define core objectives

Dependent on reservoir and well objectives. To be defined by multi-discipline planning.

Multi-disciplinary planning

Define what is needed for each discipline & how many routine / SCAL samples (also very much dependent on lithology & wettability).

Decide which sections/intervals to be cored. Discuss with all parties involved (drilling contractor, mud engineer, coring and core analysis

companies).

Discipline requirements

See section above; The petrophysicist normally coordinates coring and core analysis. Drilling input: Identify & advise on possible well problems (shale, over-pressure, fractures, sticking hole), bit type, mud type, cased sections, etc.: see also Wellsite planning (below).

Calculate costs & VOI of core

May go for SWS, rotary coring tool, etc. (adv. / disadv.) In all of the below: cost aspects drive the decisions Decide whether core will be taken. If yes:

Make a justification for the core: Get it approved

Prepare budget for all the below: Including budget for QA/QC, post-mortem review

Select coring & CA contractors: Tender; criteria: audit reports --> reliability vs. time/ costs

Put contract in place

Wellsite planning

Choice of drill bit, ROP, WOB,

Roller cone, PDC or diamond bit: dependent on lithology etc. Best core quality if well is drilled slowly Clean consolidated: standard Unconsolidated: special bits & barrels, core freezing? stress Shaly sandstones: fluids that don't cause swelling, drying process

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Choice of drilling mud, overbalance

Dependent on presence of swelling clays, overpressure

Choice of core barrels: Dependent on lithology, temperature, consolidation, fractures

Choice of core types

ROS requirements? (sponge, or pressurised coring): depends on pressure/consolidation/deviation, costs, safety Oriented cores? (if special geological considerations)

Wellsite handling

Bringing the core to surface: Running Out of Hole (ROH): preferentially: slow!!

Core handling / preservation: Freezing, wax sealing required? etc.

Transportation to the lab: Avoid shocks & vibrations

Laboratory handling

Define requirements: Reporting: raw data should be kept (tell contractor)

Reassemble core; Core screening / preparation: Depth matching (Gramper, Cat- scan)

SCAL screening: Preserve SCAL sections

Whole core required? If not: slab, UV/normal photo; geol. descr.; take plugs (Slabbing: slicing off part of the core parallel to its axis).

Preservation & storage

Don't neglect these aspects: core & samples can deteriorate pretty quickly

Plug taking # horizontal & vertical plugs (see discipline requirements above)

Routine: every ft; SCAL every 20 - 100 ft Be aware of impact of well deviation & laminae Core disturbance observed?: may lead to biased sampling Take into account: facies description, critical parameters, number of samples required per

lithological unit

Core sampling / Sample preparation: Cleaning & drying (wettability & clay considerations)

Core analysis measurements

See discipline requirements defined above Choice of CA/SCAL method dependent on: plug conditions, lithology, required stress level,

need for simulated in-situ conditions, accuracy vs. costs/time

Evaluation / analysis: Corrections, averaging

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Reconciliation with log data. Discrepancies can be due to:

problems with the core data: biased sampling, core disturbance, vugs, incorrect measurement equipment or procedure, scale / averaging effects, improper correction for clay effects, stress, temperature, wettability, fluid type problems with the log data: incorrect interpretation model and/or parameters, thin bed and/or invasion effects, clay/lithology effects, use of wrong tool type, tool failure / malfunctioning

Repeat measurements? May be required dependent on CA / SCAL results

QA/QC

Check tables & figures for anomalies. If possible: check consistency against available data for similar wells / fields.

Report and review: May be input for annual audit report

Discuss with contractor: Possible improvements in their procedures

Store data in database

Close out. Return core; pay the bills

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Core Acquisition

Coring methods Coring has to be done using special drilling bits and core barrels (Figure 1 and Figure 2). Special attention also has to be paid to the coring fluid used (WBM, OBM etc.).

Conventionally one used a fixed steel inner barrel. However, core damage was quite common, especially in unconsolidated sands. Therefore, nowadays containerised coring methods are used. They make use of a disposable inner sleeve or liner. This gives improved handling, less core exposure and easy stabilisation (e.g., using resin, or freezing).

Figure 1 – Coring Process

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Figure 2 – Core Barrel

Common inner barrels are:

fibreglass (standard) aluminium ( used at high temperature) plastic (soft liner: used in very unconsolidated sands) (chrome plated) steel (used in fractured formations, high temperatures, deep wells or at high

vibrations)

Another coring technique is wireline coring. In this technique, a drill bit can be transformed to a coring bit by removing the centre of the bit downhole using a wireline removable plug.

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Yet another technique is oriented coring. In this method a groove is cut in the core during the coring process. This groove is aligned to the EMS survey tool. However, this process can damage / destroy the core (e.g. due to jamming) and is, therefore, not very popular.

The fluid saturation in the core at surface can be entirely different from the in situ saturation, because of flushing by the mud and because of fluid (gas) expansion while the core is transported to surface (pressure release). For instance, if the in- situ oil saturation is 70% (hence brine saturation 30%), the oil saturation in the core down-hole after mud filtrate invasion might for instance be reduced to 30% (brine / mud filtrate saturation 70%). During the trip to surface, solution gas will expand, driving oil and water out, such that at surface we might have 12% oil, 40% gas and 48% brine (just as an example: actual numbers will depend on many factors).In case of a gas reservoir, if the in-situ gas saturation is 70% (hence brine saturation 30%), the gas saturation in the core down-hole after mud filtrate invasion might for instance be reduced to 30% (brine / mud filtrate saturation 70%). During the trip to surface, the gas will expand, driving water out, and partly form condensate, such that at surface we might have 1% oil, 49% gas and 50% brine (again: just as an example: actual numbers will depend on many factors).

Therefore, in general, fluid saturations obtained from cores (e.g. using the Dean & Stark technique, see further below) are not very reliable.

If one still wants to preserve the in situ fluids, two techniques are available, which are especially used in Residual Oil Saturation (ROS) studies (and are, therefore, not very popular at these current times of low oil prices):

1. Sponge coring: the fluids that leak out of the core because of the pressure release are captured by an oil-wet sponge which surrounds the core. In the laboratory the total amount of fluids (those in the core and those in the sponge) are analysed: together they should come closer to the in situ saturation. Sponge coring is about twice as expensive as conventional coring. It requires two trips. It has a high chance of success (about 95%).

2. Pressure coring: the core is kept at in situ pressure by using a special valve / closing system. Hence, at surface the core and the fluids in the core are still contained in a pressurised barrel which is at in situ pressure (though not at in situ temperature). This process is obviously dangerous (high pressure!) and therefore safety measures have to be taken. The method can only be applied for consolidated reservoirs which are at ROS, with reservoir pressure less than 5000 psi. Also the borehole deviation should not be very strong. Pressure coring is more than ten times as expensive as conventional coring. It requires four trips. The success rate is rather limited (65 – 75%).

A new coring method is gel coring in which a protective gel is put around the core during the coring process.

Alternatives to taking cores are taking Sidewall Samples (SWS) (Figure 3) or using a rotary coring tool (Figure 4).

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Figure 3 – Sidewall Sample Gun

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Figure 4 – Sidewall Coring Tool

Sidewall samples have the advantage that they are fast and cheap, available up to 500 deg. F, one can individually select the sample depths, and a large number of samples can be acquired in a single trip (up to 90). However, the sample quality can be low (because of the impact of the bullet), such that petrophysical parameters cannot be trusted. Also, the samples are small, and they cannot easily be obtained in large holes. SWS are normally used in a qualitative sense, namely for identification of lithology and fluid type.

Another alternative to coring is using a wireline rotary coring tool, which drills samples out of the borehole wall (rather than cutting them out with the SWS bullet). Because of this process the sample quality is better, especially in hard rock. Again, the sample depths can be selected individually. However, the number of samples per run is more limited (maximum 30). Furthermore, the tool is

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ineffective in unconsolidated formations and in washouts. Finally, the costs are substantially higher than costs for SWS.

Core handling The standard core handling scheme after the core has been gotten out of the hole and laid on the well site surface is depicted in Figure 5. A standard colour coding is used to identify top and bottom, namely two lines are drawn on the core surface from top to bottom: a blue and a red one, where the red one is to the right of the black one.

Figure 5 – Core handling on the Wellsite (Containerised Coring)