WHAT IS WELL LOGGING: WELL LOG IS A CONTINUOUS RECORD OF MEASUREMENT MADE IN BORE HOLE RESPOND TO VARIATION IN SOME PHYSICAL PROPERTIES OF ROCKS THROUGH WHICH THE BORE HOLE IS DRILLED TRADITIONALLY LOGS ARE DISPLAY ON GIRDED PAPERS SHOWN IN FIGURE NOW A DAYS THE LOG MAY BE TAKEN AS FILMS, IMAGES, AND IN DIGITAL FORMAT WIRELINE LOGGING IS PERFORMED WITH A SONDE LOWERED INTO THE BOREHOLE OR WELL TYPES OF WIRELINE LOGGING : OPEN HOLE LOGGING CASED HOLE LOGGING INTERPRETATION METHODS QUALITATIVE QUANTITATIVE MANUAL COMPUTERIZED LITHOLOGY TOP SAND SAND THICKNESS SAND PROSITY PERMEABILITY FLUID SATURATIONS IS A PROCESS OF USING WELL LOGS TO EVALUATE THE CHARACTERISTIC OF FORMATION : • STORAGE CAPACITY å porosity, • GEOLOGICAL SETTING å fluid saturations and net pay thickness • FLUID PROPERTIES å density, fluid type, fluid contacts, API gravity, water resistivity & salinity, temperature, GOR structural/dip/fracture, geologic environtment, facies characteristic, top/bottom reservoir, heterogeneities, distribution • PRODUCTIVITY : permeability, water cut, GOR and rate (estimated) LOG INTERPRETATION Log interpretation should provide answers to questions on: IS PART OF RESERVOIR CHARACTERIZATION PROCESS WHICH SHOULD BE INTEGRATED WITH THE FOLLOWING SURVEY AND ANALYSIS: – DRILLING OPERATION LOGS: • CUTTING ANALYSIS, MUD ANALYSIS, DRILLING DATA COLLECTION (PRESSURE, GAS READING, PENETRATION RATE ETC.) AND ANALYSIS – CORRING & CORE ANALYSIS : • SIDE WALL CORE & FULL HOLE CORE • VISUAL LITHOLOGY DESCRIPTION, HYDROCARBON SHOWS, POROSITY, PERMEABILITY, FORMATION FACTOR, SATURATION ETC – PRODUCTIVITY TEST : • RFT, MDT, DST, PRODUCTION TESTS – GEOLOGY & GEOPHYSICAL : • SURFACE GEOLOGY, SEISMIC SURVEY & INTERPRETATION ETC LOGGING UNIT CONTAINS: • logging cable • winch to raise and lower the cable in the well • self-contained 120-volt AC generator • set of surface control panels • set of downhole tools (sondes and cartridges) • digital recording system Open Hole Logging : The traditional wireline logging Logging While Drilling Logging on drill pipe Logging Job Sequences : î Rig-up logging unit î î î LOGGING UNIT î Tool run in hole î WIRELINE î The system is on but never be used for log interpretation Pull-out and logging î î SONDE / TOOL î Check Tool and system Wellsite Geologist (WG) will perform system & tool quality control Safety meeting WG is the witness, checks the logging speed and quality WG has authority to stop, refuse and re-logging when necessary Rig-down the logging unit î î Print the result WG signs the services ticket containing type of services and charges SAMPLE : OPEN HOLE LOG SP, GR, AIT, SONIC, DENSITY & NEUTRON RHOZ AIT NPHI SP GR DT SP å SPONTANEOUS POTENTIAL LOG GR å GAMMA RAY LOG ELECTRICAL LOG å INDUCTION, LATERAL, SPHERICAL FOCCUSS, MICRO LATERAL ETC NEUTRON LOG å CNL, SNP DENSITY LOG å LDT SONIC LOG å BHC OTHERS : FMI (DIPMETER & IMAGING), NMRI (Nuclear Magnetic Resonance Immaging, TEMPERATURE LOG, CALLIPER LOG, ETC • • • • Rw m n a = resistivity of connate water = "cementation factor", set to in the simple case = "saturation exponent", set to in the simple case = constant, set to in the simple case All the constants have to be set In clastics the values are usually measured for each reservoir Values could be m = 1.8 n = 2, a=1 An often quoted old formula, the Humble Equation uses: m = 2.15, n = 2, a = 0.62 • Rw is an important parameter • Sources include: – Formation water analysis – Local tables / knowledge – SP – Resistivity plus porosity in water zone – RFT sample – From Rxo and Rt tools • If Sw = 1, the saturation equation can become: R w = φ R t • Assuming simple values for a, m, n • Procedure is to: • Compute an Rwa (Rw apparent) using this relationship • Read the lowest value over a porous zone which • This is the method employed by all computer based interpretation systems • In a water zone Sw = 1, thus the alternative saturation equation becomes: • The value of Rmf is measured; • Rxo and Rt are measured, the value of Rw can be calculated Example of variations in the Archie parameters S n w = a φ m Rw Rt The following are measurements POR = 25%, Rt = ohm-m, Rw = 02 ohm-m Assuming a simple formation with a = 1, m = 2, n = Sw = 25% Changing n to 2.5, changes the Sw to 33% Changing m to changes Sw to 50% Hence the choice of these constants is important © Schlumberger 1999 Porosity Clean formation Structural shale Porosity Porosity Matrix Matrix Laminar shale Dispersed shale Porosity Matrix Shale Shale Porosity Shale Shale Matrix Matrix • • • • • • • • Shales have properties that have important influences on log readings: They have porosity The porosity is filled with salted water They are often radioactive Resistivity logs exhibit shales as low resistivity zones Neutron porosity logs exhibit shales as high porosity Density and sonic logs react to the porosity and matrix changes Gamma ray logs react to shale radioactivity • The volume of shale must be computed to correct the tool readings • This is achieved using simple equations such as: V V cl cl = = GR GR SP SP log max log max − GR − GR − SP − SP min min • The Archie equation has to be changed to take account of the shale effect • The shale looks like low resistivity so another term is added to the equations • The result is an equation which will can be used to compute water saturation in shaly sands • All these equations return to Archies equation if there is no shale present •Indonesia Equation S w = V V cl ⎛ ⎜⎜ − ⎝ cl R •Nigeria Equation R t ⎛ = ⎜ ⎝ •Waxman-Smits Equation R ⎞ ⎟⎟ ⎠ cl cl V R = t + φ F * R m + e w w BQ S v F ⎞ ⎟ ⎠ aR w S w * •Dual Water Equation C t = φ m t a S n wt ⎡ C ⎢⎣ w + S S wb wt (C wb t w φ cl S e R + R * − C w ⎤ )⎥ ⎦ n w EXAMPLE : PROCESSED LOG POROSITY & SATURATION CALCULATION RESULTS OPEN HOLE LOG PROCESSED LOG FLUID ANALYSIS SATURATION VOLUME hydrocarbon total porosity φt fluids unit volume far water bound water dry clay solids clean matrix φ hy effective porosity φ wf φe = φ wf+ φ hy φ wb Vdcl Vcl wet clay φt SAND Matrix Far Water φt Matrix φt Matrix Dry Colloid φt SHALE Dry Colloid Bound water [...]... Average: Δ t = φ Δ t log φ = Δ t Δ t log f f + (1 − Δ t − Δ t ma ma − φ )Δ t ma • Raymer Gardner Hunt • This formula tries to take into account some irregularities seen in the field • The basic equation is: 1 Δ t = (1 c − φ Δ t ma 2 )+ φ Δ t f • A simplified version used on the Maxis is: φ = C Δ t log − Δ t Δ t log C is a constant, usually taken as 0.67 ma DETECTING OVERPRESSURED ZONE Lithology & Porosity... Ti – From these the exact mineralogy can be computed Crossplot Solution Porosity and Lithology Determination from Litho-Density* Log and CNL* (Compensated Neutron Log) Schlumberger Chart • The plot is a straight line from the matrix point to the 100% porosity, water point It is scaled in porosity 2.48 Porosity 13 % 75% sand & 25% limestone 12 ELECTRICAL RESISTIVITY LOGS • The resistivity of a substance... Schlumberger 1999 • Most tools react to lithology - usually in conjunction with the porosity • Major lithology tools are: – Neutron - reacts to fluid and matrix – Density - reacts to matrix and fluid – Sonic - reacts to a mixture of matrix and fluid, complicated by seeing only primary porosity – NGT - identifies shale types and special minerals – Geochemical logging, identifies 10 elements; K, U, Th,... slowness by measuring the first arrival transit times Used for: – – – – Correlation Porosity Lithology Seismic tie in / time-to-depth conversion • Multi-spacing digital tool • First to use STC processing • Able to measure shear waves and Stoneley waves in hard formations • Used for: – Porosity – Lithology – Seismic tie in / time-to-depth conversion – Mechanical properties (from shear and compressional)... from impermeable clays • Define bed boundaries, top & bottom of the layer • For geological correlation • Give an indication of shaliness (maximum deflection is clean; minimum is shale) • Indicate vertical grain size distribution • Determine Rw (formation water resistivity) in both salt and fresh muds R mfe SSP = − k log R we SHALE BASE LINE SP scale + • Baseline shifts: These can occur when there are... reference electrode Power lines, electric trains, electric welding, close radio transmitters: All these create ground currents which disrupt he "fish“ reference causing a poor, sometimes useless, log • The Gamma Ray log is a measurement of the formation's natural radioactivity • Gamma ray emission is produced by three radioactive series found in the Earth's crust – Potassium (K40) series – Uranium series... Neutron Tool (CNT) The latest tool is the Accelerator Porosity Sonde (APS), using an electronic source for the neutrons and measuring in the epithermal region NEUTRON USES • POROSITY & LITHOLOGY å with density log • HYROCARBON INDICATION å The tool measures hydrogen index • The Density Tools use a chemical gamma ray source and two or three gamma ray detectors • The number of gamma rays returning to... the number of electrons per atom and A is the atomic weight • The density tool is extremely useful as it has high accuracy and exhibits small borehole effects • Major uses include: – Porosity – Lithology (in combination with the neutron tool) • Mechanical properties (in combination with the sonic tool) • Acoustic properties (in combination with the sonic tool) • Gas identification (in combination ... recording system Open Hole Logging : The traditional wireline logging Logging While Drilling Logging on drill pipe Logging Job Sequences : î Rig-up logging unit î î î LOGGING UNIT î Tool run in... LITHOLOGY DESCRIPTION, HYDROCARBON SHOWS, POROSITY, PERMEABILITY, FORMATION FACTOR, SATURATION ETC – PRODUCTIVITY TEST : • RFT, MDT, DST, PRODUCTION TESTS – GEOLOGY & GEOPHYSICAL : • SURFACE GEOLOGY, ... geologic environtment, facies characteristic, top/bottom reservoir, heterogeneities, distribution • PRODUCTIVITY : permeability, water cut, GOR and rate (estimated) LOG INTERPRETATION Log interpretation