PAB2084 Well Site Geology_Drill Cuttings

7/31/2019 PAB2084 Well Site Geology_Drill Cuttings

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PAB2084

WELL LOGGING &

FORMATION

EVALUATION

Assoc Prof Dr Abdul Hadi Abd Rahman

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http://en.wikipedia.org/wiki/File:West_Texas_Pumpjack.JPG





Well Site Geology

Cuttings Analysis;

Hydrocarbon shows detection

Pressure Detection



Drill Cuttings

Source of emissions (the cuttings)

When a hole is drilled in wood by a domestic drill,sawdust is produced

Similarly, small pieces of rock – called cuttings – arecreated when a well is drilled through rock toreach an oil or gas reservoir. These cuttings vary insize and texture, ranging from fine sand to gravel,

depending on the type of rock being drilled andthe type of drill being used.





Drill cuttings



Drill cuttings

To prevent the well being clogged, the cuttings are carriedback to the surface by a special fluid which is pumpeddown the well to keep it clean and also to lubricate thedrill bit and to control pressure within the well. This fluid is

known as ‘mud’ because of its appearance and consistency.

On the drilling rig the cuttings are separated from the mud;the mud is recycled to be used again and the cuttings are

either; discharged to the seabed, re-injected into a well ortaken ashore for treatment and disposal. The choice of disposal route depends on the type of drilling mud beingused and the location of the



Drill Cuttings

Drill cuttings contained in the "mud" that

comes up when an oil well is drilled, can

provide information that may lead to

significant savings for oil companies.



Drill Cutings

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Introduction

When no core or sidewall samples are available, it may stillbe possible to extract valuable information from drill cuttings. Amicroscope geologist will prepare and select drill cuttings, using abinoculair.

Mineralogical Analysis

For petrography usually a small amount of rock is sufficient forXRD/SEM/Particle size or thin section description.

Porosity and permeability

- High pressure mercury injection can be used on small core chips ordrill cuttings to determine porosity, capillary pressure, pore throat size

distribtion and theoretical permeability.

- Darcylog (IFP) can be used as a non-destructive method todetermine porosity and permeability (SPE 77563). Permeabilities areoften accurate within an order of a magnitude .



Introduction

Experience Breadth: Formation Evaluation Surface logging (mud logging) entails detailed analysis of formation

cuttings and fluid entrapped in drilling mud to determine thesubsurface nature and presence of oil and gas.

Formation cuttings analysisThe analysis of formation cuttings carried to the surface in drillingmud is a fundamental component of surface logging. On full-serviceoperations, our experienced crews examine cuttings to detecthydrocarbon shows, to assess a formation’s porosity and to

characterize its lithology. Coupled with wellsite laboratory-basedmineralogical analysis, these data enhance reservoir understandingto inform well placement and completion operations.



Introduction



Introduction



Introduction

Drill cuttings[1] are the broken bits of solid materialremoved from a borehole drilled by rotary,percussion, or auger methods. Boreholes drilled inthis way include oil or gas wells, water wells, andholes drilled for geotechnical investigations ormineral exploration.

The drill cuttings are commonly examined to make a

record (a well log) of the subsurface materialspenetrated at various depths. In the oil industry, thisis often called a mud log.



Introduction

Drill cuttings are produced as the rock is broken by thedrill bit advancing through the rock or soil; the cuttingsare usually carried to the surface by drilling fluidcirculating up from the drill bit. Drill cuttings are

separated from the drilling fluid by shale shakers (forliquid drilling fluid), or by cyclone separators (for airdrilling). In cable-tool drilling, the drill cuttings areperiodically bailed out of the bottom of the hole. Inauger drilling, cuttings are carried to the surface on the

auger flights. One drilling method that does not produce drill cuttings

is core drilling, which instead produces solid cylinders ofrock or soil.



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Drill Cuttings Analysis

Accurate sample description is basic and important geologictask – it is the foundation to the interpretation of thestructure, conditions and visualisation / imaging of thesubsurface

The cores and cuttings obtained from drilling a well are theONLY direct data from the remote subsurface

Well logs, pressure data, seismic sections are indirect data – signals concerning the conditions and structure of the

subsurface

The models on your work stations and PCs are to a largeextent your ‘imaginations’ of the subsurface – subsurfaceimaging




- Methods and Procedures

Log forms, symbols and legends



Example of Sample Log /Mud Log



SampleLog / Mud Log




Sample description

Porosity and Permeability estimation

Hydrocarbon detection Problems in interpreting drill cuttings



I Sample Description

The quality of the sample log often reflects the

quality of the samples

Clean, good quality samples are exception rather than

the ruleGeologist logging these samples (mud logger) must

learn to make description and interpretations from

samples of varying quality

Cavings and other contaminants must be identified andseparated from original cuttings



I Sample Description Procedure

Recommended:

Samples should be laind out inthin layers in trays, with

depths marked

Separate the cavings by either sieving ordry panning Samples are examined for




Description under transmitted light

Properties examined and described:

Rock types – sandstone, limestone, dolomite, etc

Color – described on cleaned set samples Texture – size, shape and arrangement/sorting of

component grains of the rock

Cement and matrix

Fossils and accessories – microfossils

Sedimentary structures – often not obvious

Porosity and permeability – modern methods

Hydrocarbon shows




Testing methods: Test with dilute HCl – carbonates from non-carbonates

Hardness – using hardness scale

Parting - shale

Swelling in water – montmorillonites Thin Sections

Staining for carbonate rocks

Insoluble residues – chert, sand, silt in carbonates

Heavy mineral studies Identification of clay, chert, evaporites, salts, phosphate,

siderite, feldspar





II Porosity and Permeability estimation

Detection and Evaluation of porosity, and inferred

presence of permeability in important

Often requires magnification of 10X or higher –

microscopes, via thin section, or more modernsophisticated equipments (mercury porosity meter)

Recognition of the types of porosity is important; in

sandstones – intergranular, moldic and fracture; incarbonates – interparticle, intercrystal, vuggy,

moldic and fracture





III Hydrocarbon detection

Petrophysical analyses may give conclusive

evidence of the presence of commercial quantities

of oil

However, the well site geologist is resposible toreport and log all oil shows – goo shows

Positive indications of hydrocarbons in cuttings –

strong factor in the petrophysicist’s eva;uation of thewell




Routine Hydrocarbon Detection Methods

Odor – oil odor or condensate odor – reported as good,fair and faint odor –

Staining and bleeding – oil stain on ditch cuttings and cores

a function of porosity and oil distribution in the pores- Thecolor of the stain or bleeding must also be reported

Reaction in acid of oil-bearing rock fragments – in diluteHCl – large bubbles in the presence of oil

Fluorescence – hydrocarbon fluorescence under ultravioletlight

Other tests






IV Problems in interpreting

drill cuttings

1 Contamination from previously penetrated beds Cavings May be easily recognised as rock materials identical to layers that

have been identified and recorded from a shallower level/depthin the well

Mixing of previously drilled/penetrated rocks into the ascendingmud stream is common and pronounced after trips of the drillstem for bit changes, drill stem tests, coring operations and/orother rig activities

Soft shales, thinly bedded brittle shales (mudstones), bentonitereadily caves

These may be found in cuttings samples representing depthshundreds of feet below the normal stratigraphic position of thecaved layers

Recirculation …

IV P bl




drill cuttings

1 Contamination from previously penetrated beds

Recirculation

These are sand grains and microfossils from previously drilled

rocks which re-enter the hole with the mud stream and

contimanate the rising sample

IV P bl




drill cuttings

2 Other Contaminants

Lost circulation material

‘Foreign’ substances introduced into the hole to combat lostcirculation difficulties

Feathers, leather, burlap sacking,cotton seed hulls, cellophane,perlite, coarse mica – allwhich may be mistaken and erroneouslyinterpreted as formation cuttings

Requires careful observation and recording

Cement

Cement fragments may be mistaken for sandy, silty of chalkycarbonates

Often, they look different and unusual and easily identified andremoved

IV P bl i i i




drill cuttings

2 Other Contaminants

Drilling mud

Oil contaminants

Pipe acale and bit shavings – metals, remove bymagnets

IV P bl i i i




drill cuttings



Thank you, read further in the reference given

Documents

PAB2084 Well Site Geology_Drill Cuttings