An Introduction to Well Logging Pardeep Kumar DGM(Geophy-Wells) Prog. No.:GT-Batch,ONGC,Academy Dec. 17th, 2014 INTEG, GEOPIC ONGC, DEHRADUN SOURCES OF INFORMATION IN SUBSURFACE Field Geological Survey: Exposures of Sedimentary rocks. Surface Geophysical Survey: Seismic(2D,3D,4D) @ Non-Seismic(Gravity, Magnetic, sea bed logging ) Mud Logging - Hydrocarbon shows during drilling, ROP , Gas Counts Cores & Cuttings - SWC/CC Geological : Megascopic Study, Thin Sections, XRD, SEM, XRF, EDX Reservoir : Porosity, Permeability, wettability, Bulk/Grain density, Swirr, Electrical Petrophysical parameters. Borehole Geophysical Survey : Well Logs, Well seismic (VSP) Well tests : RFT/MDT, DST, Conventional Test WELL LOGGING TECHNOLOGY Logging - “ Eye ” of E&P Industry. Well Log - Record of Physical properties measured with respect to depth. Well Logging involves• Data Acquisition • Data Processing • Interpretation of Results WELL Logging set up at drill site FEATURES OF WELL LOGGING INDUSTRY High Tech Industry Capital Intensive Rapid Technological Obsolescence. Globally dominated by only 2 or 3 MNC players India among the select few countries with inhouse logging 5 capabilities LOGGING EQUIPMENT • SURFACE EQUIPMENT (LOGGING UNIT) Truck/Offshore Skid • DOWN HOLE TOOLS – SOPHISTICATED SENSORS & ELECTRONICS • ELECTRO-MECHANICAL CABLE Tool Conveyance Open - Cased Hole Measurements : • The Traditional Wire-line Logging. • LWD (Logging While Drilling) • Logging on Drill Pipe (TLC) • TRACTOR Real time Monitoring Importance of well logging Usefulness of Well Logging Well Birth of Logging September 5th, 1927 when H. Doll and Schlumberger brothers, Conard & Marcel made semi continuous resistivity measurement in 1600 ft deep France`s Pechelbronn field First Log Recorded in Petroleum Industry Semi-continuous resistivity measurement at discrete depth points by H.D. Doll, in in 1600 ft deep well in Pechelbronn field of France Logging was thought to be replacement for Coring and therefore was known as Electrical Coring. Later on well Logging became a popular and most Useful technique for formation evaluation History of Well Logging Technology Cont… History of Well Logging Technology WELL LOGGING SERVICES IN ONGC Departmental services cater to 95% of the data acquisition in onshore areas. Conform to International Quality Standards. In house Processing & Interpretation Cost effective vis-à-vis Contract 14 services ECONOMICS OF DEPT. LOGGING SERVICES Cost of Logging Equipment • • • • 15 OHU & Tools CH/PLU & tools Expected Life Pay Back Period :Rs. 25 – 30 Crores :Rs. 15 - 16 Crores :8 - 10 Years < 3 Yrs. Well Logging set up in ONGC Logging Bases Nazira, Jorhat, Agartala, Rajahmundry, Karaikal, Ahmedabad, Ankleshwar, Mehsana, Kolkata, Mumbai Logging Institute Centre for Excellence in Well Log Technology, Vadodara MAP Material Planning & Acquisition, Logging Services Logging Manpower also available in Institutes • Keshva Deva Malviya Institute of Petroleum Exploration, Dehradun • Geo Data Processing & Interpretation Centre, Dehradun • Institute of Reservoir Studies, Ahmedabad Assets MH Asset, N&H Asset, B&S Asset, Ahmedabad Asset, Mehsana Asset, Ankleshwar Asset, Assam Asset, Tripura Asset, Rajahmundry and Cauvery Asset Basins Western Offshore, Western Onshore, A&AA Basin, Cauvery and KG Basin, Frontier Basin, MBA Basin Others E&D Directorate, Dehradun, CEC, ONGCA, D(E) office, Nhava Material Acquisition & Planning Centralized procurement for: Logging Units Logging Tools Equipment, Logging cables Perforation Material Hiring of Contractual Services Wireline Services LWD Services Mud Logging Services LWD Contractual Services Resistivity-GR : ARC resistivity, Geo-vision & GR Porosity : Neutron & Density with Imaging EcoScope : Resistivity-GR & Neutron-Density StethoScope : Formation Pressure while Drilling PeriScope : Bed Boundary Detection Wireline Open Hole Services Service Departmental Contractual Resistivity DLL, HDIL, HRI, DFL, MSFL, DLL, AIT, HRI, HRLA, MSFL MLL Porosity SDLT, DSNT, BCDT, DAL LDL, CNL, BCS, DSI,DAL Dipmeter FED, SED, STAR FMI, SED, STAR, XRMI Formation Tester SFT, SFTT, RCI MDT, XPT, SFTT, RCI Coring SWC CST, MSCT Other Tools GR, NGRT,FIAC GR, NGS, CALI,CMR, ECS Cased Hole Services Service Departmental Contractual Cement Evaluation CBL-VDL, CAST CBL-VDL, CAST, USIT, SCMT Depth Correlation N-GR-CCL N-GR-CCL Perforation Gun Perforation, TTP Gun Perforation, TTP, TCP Back-Off Services FPIT- String Shot FPIT- String Shot Cutters Tubing/ Casing/ Split Cutters Tubing/ Casing Cutters, Colloidal Bridge Plug Setting BP/ CR BP/ CR/ MPBT Analysis Behind Casing CHFR/ CHFD/ CHFP/ RST LOG OF WELL – CAMBAY-1 Date 01.11.1958 Unit -Russian OKC-56 TYPES OF LOGGING Depending upon the condition of bore hole, logging operations are classified as; Open Hole Logging : Reservoir Identification & Evaluation ( Reservoir Characterisation /Formation Evaluation) Cased Hole Logging : Well Completion, Well Integrity. Production Logging : Production Problems & Reservoir Monitoring ROLE OF OPEN HOLE LOGGING • Among the whole set of geo scientific data the log data is most precise, depth controlled, well resolved and most representative of the in-situ reservoir condition. • Well log data plays an important role in decision making for casing lowering, initial testing of prospective zones, hydrocarbon reserve estimation and future reservoir monitoring. Contd. ROLE OF OPEN HOLE LOGGING Helps Assest and Basin managers for regular prospect evaluation for making investment decisions and regularly updating the remaining reserves and accretion Helps geologists to correlate different sedimentary sequences across the field, deduce the depositional environment, stratigraphic and structural features of the subsurface formation to firm up a geological model. ROLE OF OPEN HOLE LOGGING Helps drillers to identify the zones of lost circulation, know the free point of a stuck pipe and to calculate the amount of cement required for efficient primary cementation job. Knowledge of fracture pressure gradient, stress anisotropy for break out analysis, mud weight optimisation. WELL LOG IS HOROSCOPE OF WELL IT IS USED SINCE DRILLING TO ABANDONMENT OF THE WELL. Wireline Open Hole Services SERVICE RESISTIVITY FORMATION DENSITY LOGGING TOOLS DLL, HRLA, MSFL, MCFL, MLL, DFL, DIL, AIT, HDIL, HRI, HRAI, RTEx, RTScanner, 3DEx LDL, SDLT, NEUTRON POROSITY CNL DSNT SONIC BCS, BCDT, DAL FORMATION TESTER MDT, XPT, SFT , SFTT, RDT,RCI SIDE WALL CORING SWC CST, MSCT GEOMETRICAL RESPONSE PARAMETERS OF LOGGING TOOLS Subsurface Lithology & Bore hole Environment Different Saturation States of Formation around Bore hole Role of Cased Hole Services • CEMENT EVALUATION : TO ASSESS SEALING BEHIND CASING AND ISOLATION OF ZONES. • PERFORATION : MEANS TO ESTABLISH COMMUNICATION BETWEEN FORMATION AND BOREHOLE CRITICAL FOR IMPROVING PRODUCTIVITY, RECOVERY FACTOR, SAND CONTROL AND REDUCE TIME FOR ACTIVATION. • PLUG SETTING : ISOLATION OF ZONES. • CUTTER : HELPS RETRIEVE TUBINGS / CASING/ DRILL PIPE ETC. SALVAGE OPERATION SAVES WASTEFUL EXPENDITURE. • BACK OFF SERVICES : HELPFUL IN LIQUIDATING STUCK UPS. Cased Hole Services SERVICE LOGGING TOOLS CEMENT EVALUATION CBL-VDL, CAST, USIT, SCMT DEPTH CORRELATION N-GR-CCL PERFORATION GUN PERFORATION, TTP, TCP BACK-OFF SERVICES FPIT- STRING SHOT CUTTERS TUBING/ CASING CUTTERS, COLLOIDAL BRIDGE PLUG SETTING BP/ CR/ MPBT Role of Production Logging HOW MUCH OF WHAT IS COMING FROM WHERE ??? HOW MUCH IS GOING WHERE ??? • Analyse problems of sick wells & suggest remedial measures in aged & depleted fields. • Can be done under dynamic conditions without loss of production Deployment of rigs not essential • Monitor profile of flowing wells & injection wells for better reservoir management. • Should be done at the start of production life of a well , which will act as a base log for future analysis. Production Logging Services 1 TEMPERATURE 2 FLUID DENSITY 3 FLOW METER: SPINNERS 4 PRESSURE: STRAIN GAUGE/ QUARTZ GAUGE 5 HOLD UP: GAS / WATER 6 CASING INSPECTION: MULTI ARM CALIPER Schematic Diagram of Completed Well Casing Tubing Packer Tubing Shoe Perforations Gas Zone Gas Zone Oil Zone-1 Oil Zone-1 Shale Shale Oil Zone-2 Oil Zone-2 Water Zone Water Zone PLUG Grease Injection Schematic Pack Off Assembly Grease Seal Flow Line Grease Line Riser Pipe Grease Drum Wire Line BOP ROLE OF CASED HOLE / PRODUCTION LOGGING Helps surface and subsurface managers to diagnose well/reservoir problems such as movement of oil water contacts, gas water contacts, development of secondary gas cap, water coning, gas cusping, water fingering, preferential break through of injected water, water /polymer injection profiling, casing leakage, channeling behind casing, cement quality for optimal recovery and efficient reservoir management. TYPICAL LOG PRESENTATION TYPICAL LOG PRESENTATION TYPICAL LOG PRESENTATION Detection and location of reservoirs from well logs Hydrocarbon Reservoir-rock Seal-rock and/or source-rock Hydrocarbon Hydrocarbon Hydrocarbon Detection and location of reservoirs from well logs ELAN Processed Results along with Log Data IDENTIFICATION OF OWC & GOC Shale Shaly/sand G.B. Shale GOC OWC Shale Tools Nomenclature DLL Dual Latero Log HRI High Resolution Induction Log HDIL High Resolution Dual Induction log AIT Array Induction Tool HRLA High resolution Lateral Array Resistivity MSFL Micro Spherical Focused Resistivity Log MLL Micro Latero Log LDL Litho Density Log SDLT Spectral Density Litho Tool CNL Compensated Neutron Log DSNT Dual Spaced Neutron Tool BCS Borehole Compensated Sonic BCDT Borehole Compensated Sonic Tool (DITS) DAL Digital Acoustic Log Tools Nomenclature DSI Dipole Shear Sonic Imager FED Four Arm Electrode Dipmeter SED Six Arm Electrode Dipmeter FMI Formation Micro Imager STAR Simultaneous Acoustic & Resistivity Imager XRMI Extended Reach Micro Imager XMAC Cross Multi Pole Acoustic Tool SFT Selective Formation Tester MDT Modular Dynamic Tester MDT-LFA Modular Dynamic Tester-Live Fluid Analyzer MDT-CFA Modular Dynamic Tester-Chromatographic Fluid Analyzer XPT Extreme Pressure Express RCI Reservoir Characterization Tool RDT Reservoir Description Tool Tools Nomenclature SWC Side Wall Coring CST Sequential Core Sampler Tool MSCT Motorized Sidewall Coring Tool GR Gamma Ray NGS Natural Gamma Ray Spectral CMR Combinable Magnetic Resonance MRIL Magnetic Resonance Imaging Log ECS Elemental Capture Spectroscopy CALI Caliper PEX Platform Express CBL-VDL Cement Bond Log – Variable Density Log USIT Ultra Sonic Imaging Tool SCMT Slim hole Cement Mapping Tool CAST Circumferential Acoustic Scanning Tool RST Reservoir Saturation Tool Tools Nomenclature CHFR Cased Hole Formation Resistivity CHFD Cased Hole Formation Density CHFP Cased Hole Formation Porosity FPIT Free Pint Indicator Tool BP / CR Bridge Plug / Cement Retainer TCP Tubing Conveyed Perforation TTP Through Tubing Perforation TLC Tough Logging Conditions LWD Logging While Drilling MPBT Mechanical Plug Back Setting Tool VSP Vertical Seismic Profiling CPLT Compact Product Logging Tool Max Track Tractor Conveyance FSI Flow Scan Imager Tools Nomenclature 3DRI 3D Resistivity Imager / RT Scanner TWSS Multi level Tri-axial Well Seismic Service PSP Production Service Platform WSTPL Well Shuttle Services FPWD Formation Pressure While Drilling (StethoScope) DBBI Directional Bed Boundary Imaging Tool (PeriScope) ELAN PROCESSING RESULTS ALONGWITH OPEN HOLE LOGS & OVERLAYS ESTIMATION OF RESERVES For oil, the number of barrels in Place is given by N=7758 × ø (1-Sw) × h × A where ø is the porosity, Sw is water saturation, h is the pay thickness in feet and A is the areal extent in acres. For gas, the number of cubic feet in situ is given byG=43560 × ø (1-Sw) × h × A SPONTANEOUS POTENTIAL (SP LOG) SPONTANEOUS POTENTIAL LOG Simplest Log from Recording Point of View But Very Useful from application point of view. SP is naturally occurring electrical potentials opposite each lithological unit penetrated in a well bore with respect to local potential of earth surface. Measured potential between a movable electrode & a fixed surface electrode SP is generally recorded along with resistivity by including a ring type electrode in the resistivity tool body or a cylindrical electrode in the cable armour and presented in first track SP is generated due to electrochemical and electro kinetic phenomenon Membrane potential, Liquid junction potential and Electro-kinetic potential REQUIREMENTS FOR OCCURRENCE OF SP •ELECTRICALLY CONDUCTIVE BOREHOLE FLUID (BRINE, WATER BASED MUD) •A SANDWICH OF A POROUS AND PERMEABLE BED BETWEEN LOW POROSITY AND IMPERMEABLE FORMATIONS. (SAND SHALE SEQUENCE) •SALINITY CONTRAST BETWEEN THE BOREHOLE FLUID AND THE FORMATION WATE •PRESSURE DIFFERENTIAL BETWEEN BOREHOLE AND FORMATION ( NOT ESSENTIAL IF SALINITY CONTRAST EXISTS) SPONTANEOUS POTENTIAL STATIC SP (SSP) Electro-kinetic Potential The electro-kinetic potential also known as streaming or filtration potential is caused by filtration of mud through the mud cake, shale and sometimes low permeability reservoir sections. This potential is generally regarded as very small and is neglected in practice. Potential developed across shale and mud cake is considered to cancel each other. EK = D ΔP Rmf, 4 Where, , D, P, Rmf & are Zeta potential of rock surface in relation to saturating brine, dielectric constant of mud filtrate, pressure differential, resistivity and viscosity of the mud filtrate respectively. This potential is denoted by EH also as the relation was given by Helmholtz. As , D, Rmf and depends upon the salinity of the mud filtrate and nature of ions , EK strongly depends upon the chemistry of invading solution. IDEALISED AND REAL SP LOG EXAMPLES FIELD EXAMPLES NaCl Mud, Rmf>Rw KCl Mud, Rmf<Rw PSEUDO STATIC SP (PSP) SP DEFLECTION (AMPLITUDE) AGAINST SHALY SANDS MEASURED FROM SHALE BASELINE . PRESENCE OF SHALES REDUCE SP AMPLITUDE. THIS FACT IS USED TO COMPUTE VOLUME OF SHALE IN A SHALY SAND VSH = 1- PSP/SSP OR VSH= (SPSh – SPZone)/ (SPSh-SP Sd) Applications of SP Log Identification porous permeable reservoir zones Define bed boundaries Well to well correlation Information about depositional environment Determination of formation water resistivity Estimation of shale content in a shaly reservoir. Identification of hydrocarbon zones Computation of realistic water saturation in LRLC reservoirs NATURAL GAMMA RAY LOG NATURAL GAMMA RAY LOG GR IS MEASUREMENT OF NATURAL RADIOACTIVITY OF THE FORMATION RADIOACTIVE ELEMENTS TEND TO CONCENTRATE IN SHALES. – POTASSIUM SERIES , K 40(1.46 MEV – URANIUM SERIES , TH232 (2.61 MEV ) – THORIUM SERIES, U 238(1.76 MEV) DOLOMITES MAY HAVE SOME URANIUM CONTENT NATURAL GR SPECTRUM Radioactive potassium Isotope (K40) with half life 1.3x109 years Argon 40 Uranium238 (U238) with half-life of 4.4 x 109 years Thalium 208 Thorium232 (Th232) with half-life of 1.4 x1010 years Bismuth 214 GAMMA RAY LOG CLEAN SANDS AND CARBONATES EXHIBIT LOW GR SHALES AND SHALY SANDS ARE HIGHER IN RADIOACTIVITY FELDSPARS AND MICAS RICH IN RADIOACTIVITY AMONG CLAY MINERALS ILLITE HAS MAXIMUM GR SOMETIMES SILTS ARE MORE RADIOACTIVE THAN SHALES HOT SHALES EXHIBIT TREMENDOUSLY HIGH GR ( THOUSANDS API) SOURCES OF RADIOACTIVITY IN SHALES SHALES ARE FINE GRAINED CLAY RICH ROCKS CONTAINING MICA, FELDSPAR, CALCITE, DOLOMITE , QUARTZ & ORGANIC MATTER. 1. RADIOACTIVE ELEMENT (K-40) IS FOUND ONLY IN CRYSTAL LATTICE ILLITE CLAY. EVEN MORE CONCENTRATION IS FOUND IN MICA & FELDSPAR. 2. THORIUM IS ABSORBED ON CLAY PARTICLE SURFACES. THORIUM IS PRESENT IN MICAS & FELDSPARS. 3. URANIUM ASSOCIATED WITH ORGANIC SHALES PROVIDES HIGH GR GR LEVELS AGAINST SEDIMENTARY ROCKS GAMMA RAY TOOL • GR ENTERING TOOL BODY AFTER PASSING THROUGH FORMATION & BOREHOLE ARE COUNTED WITH SCINTILLATION /G.M. COUNTER • GAMMA RAYS PASSING THROUGH ROCKS ARE SLOWED AND ABSORBED AT A RATE WHICH DEPENDS ON THE FORMATION DENSITY. • COUNT RATE (CPS) IS CONVERTED INTO API UNITS AND PRESENTED ON LOG GENERALLY IN FIRST TRACK. Estimation of Clay Volume from GR O il Examples of GR variation across Formation P y rit e B io t it e Q u a rt z ILD ILD @ M IN C O M 1 MD 1 : 200 150 m G R C O . W E _ D S _ f in 0 6 ( gAPI ) C A LI C A LI@ M IN C -1 0 ( mV ) 100 ( ohm .m ) M SF L.D F M SF L@ D 0.45 1 100 ( ohm .m ) R T @ A S C IILoad;1 1.95 70 T EN S 0 0) (4 5m0V 1 ( ohm .m ) 100 ( m 3 / m 3 ) -0 . 1 5 R H O B . W E _ D S _ F in 2.95 ( g/c m 3 ) D T . W E _ D S _ F in a l 140 ( us /ft ) B M S _ to p 3450 3250 3475 3500 0.5 ( m 3/m 3 ) Sw 0 1 ( m 3/m 3 ) 0 N P H I . W E _ D S _ F in 1 T VDS S 16 ( in ) S P S P @ M IN C O M 2; 100 ( ohm .m ) ILM ILM @ M IN C O M PH IE 40 B ound W ater R F _ c ly s t n SW C la y E LA N _V O LU M E S E PH IE 1 ( V/V ) 0 APPLICATIONS OF GR LOG • Determination of lithology (sand , shale, coal, limestone etc.) for identification reservoir layers, seals. • Estimatation of shale or clay volume in shaly reservoirs • Stratigraphic correlation of depositional sequences and formations • Determination of depositional environment • Determination of source rock potential Depositional Environment from GR Log CALIPER LOG CALIPER LOG EXAMPLE OF HEAVING/SWELLING CLAY APPLICATIONS OF CALIPER LOG Identification of porous and permeable zones. Used for applying environmental corrections to various log measurements Helps in decision making for formation testers and side wall Coring. It provides volume of cement slurry required for efficient cementation job with knowledge of casing outer diameter Annular Volume={Cali2 -(Casing OD) 2 }* Casing Length RESISTIVITY LOGS RESISTIVITY LOGS Resistivity of formation rock is the key parameter in determining hydrocarbon saturation. Resistivity resistivity. devices record values of apparent Interpretation depends upon assumption that the logging sonde is surrounded by an Infinite, homogeneous and isotropic medium. The capability of measuring true formation resistivity as close as possible is an ultimate objective Determination of Resistivity Ohms law: V = I*R where: V = Voltage, Volts, R = Resistance, Ohms, I = Current, Amps. R r A l = *l/A = Resistivity, Ohm.m, = Resistance, Ohms, = Area, m2, = Length, m. A simplified schematic diagram of the borehole environment and the effects produced by the invasion of drilling fluids Types of Resistivity Logs DIFFERENCE BETWEEN INDUCTION & LATERO LOG MEASUREMENTS Effective Resistance, Ra Ra = Rm + Rxo + Rt Highest resistantion element dominates the signal 1/Ra = 1/Rm+ 1/Rxo+ 1/Rt Lowest resistance element dominates the signal CHOICE OF APPROPRIATE RESISTIVITY TOOL Choice of better tool for Rt determination depending upon Rmf/Rw ratio and porosity. At lower right use both logs in area under appropriate Rw curve. Choice of better tool for Rt determination based on Rmf/Rw ratio and Sw for step profile of invasion and worst case of Annulus. 6FF40 is configuration for deep induction in DIL tool. Relationship Between Resistivity & Hydrocarbon Saturation Un-invaded Zone Flushed Zone RESISTIVITY LOG EXAMPLE Interpretation of Resistivity Log Neutron Tool Neutron Porosity Log •Neutron logs respond to the fundamental formation property of hydrogen richness. •If all of the formation's hydrogen is contained in the form of liquids, and if these liquids completely occupy the total pore volume, hydrogen richness is an index to porosity. •Hence, a neutron log is used to determine Porosity through Hydrogen Index. HYDROGEN INDEX OF FLUIDS Hydrogen Index is defined as the ratio of amount of hydrogen present in a given volume of rock or any substance to the amount of hydrogen present in equal volume of water HI of water : 1 HI of light H.C (h <0.25): 0.2h HI of heavier H.C (h >0.25): h +0.3 NEUTRON RADIATION • Neutrons are uncharged particles having mass approx. equal to that of protons They are classified by the amount of energy they possess. This energy is directly related to their speed of travel. 1. Fast neutrons > 100 KeV of energy 2. Intermediate neutrons -100 KeV to 100 ev 3. Slow neutrons < 100 ev of energy • Epithermal neutrons - between .1 and 100 ev • Thermal neutrons - < 0.025 ev at room temp. RELATING COUNT RATE TO POROSITY • In high porosity, liquid-filled formations, the neutron flux is thermalised close to the source, so that relatively few neutrons or capture gamma rays are detected. – Thus high porosity is indicated by a low neutron counting rate. – The reverse reasoning will relate low porosity to a high count rate. FIELD EXAMPLES FORMATION DENSITY TOOL • Most useful of all porosity tools. • Measures formation bulk density (RHOB) and photoelectric absorption factor (Pe) • Based upon principle of Compton scattering and photo-electric absorption of gamma ray • Pe is dependent on Z, the atomic number, and is used to identify lithology. Density Tool utilizes Cesium 137 (Cs137) gamma ray source and two Sodium Iodide scintillation detectors mounted on an articulated pad. INTERACTIONS OF GAMMA RAYS WITH MATTER In Compton scattering Gamma photons collide with the orbital electrons If the energy given to the electron is greater than the binding energy of the electron, the electron will be ejected from the atom and the photon is "scattered," at an angle to its original path. The energy reduction is proportional to the scattering angle θ hν′ = hυ / {1+ (1-Cosθ) ε} , Where, ε = hυ/meC2 Photoelectric effect occurs when the incident gamma ray is completely absorbed by the electron. It is favoured in low photon energy & high Z targets RELATIVE IMPORTANCE OF INTERACTIONS FORMATION DENSITY TOOL Measures formation porosity based upon mixing law; In fully water bearing formation ρb = Φ Φ *ρf + (1- Φ) *ρma = (ρma - ρb)/(ρma – ρf ) APPLICATIONS OF DENSITY TOOL • Measurement of bulk density & hence porosity • Lithology determination using Pe. • Lithology determination using density-neutron Overlay & cross plots. • Synthetic seismogram and acoustic impedence logs for calibration of bore hole gravimetry and well seismic surveys. RELATING GAMMA INTERACTIONS TO LOG VALUES Gamma rays lose their energy when they collide with electrons of formation material By measuring the number of gamma rays and their energy levels at a given distance from the source, the electron density of the formation can be predicted. Denser formation gives low count rate Understanding the relationship between electron density and bulk density is an essential part of the density measurement e = b * (2 Z / A) Most cases, 2 Z / A = 1 e = b RELATING GAMMA INTERACTIONS TO LOG VALUES energy spectrum divided into four energy windows SOFT, SFT2, HRD1 and HRD2. Total counts occurring in windows HRD1 and HRD2 are due to Compton Scattering, and are used in combination with the SS detector count rate to determine bulk density. Pe is derived from the ratio of the counts in the SOFT window to HRD2 window. CURVE ENERGY WINDOW DOMINANT INTERACTION SOFT 60 - 100 keV Photoelectric Absorption SFT2 100 - 140 keV Photo & Comp for low Z HRD1 140 - 200 keV Scattering HRD2 200 - 540 keV Scattering LOG EXAMPLE Figure-34: Sandstone Reservoir from Western Onshore Basin, Upper Part (1200-1272 m.) is shale from GR, Cali, SP(+ve), Neutron(CNCF), Density(ZDEN).Interval 1272-1306 m is reservoir with Gas Oil Contact at 1284 m and Oil Water Contact at 1 296 m. Zone below 1306 is again Shale. See hydrocarbon effect on Resistivity and SP. Gas Effect on Neutron. SONIC LOG(ACOUSTIC VELOCITY) • The Sonic Log is a porosity log that measures interval transit time (Δt) of compressional sound wave travelling through the formation along the axis of the borehole. • Acoustic transit time Δt and is usually expressed in microseconds per foot. • Velocity, v and transit time Δt are related by: Δt (μsec/ft) = 106 / v(ft/sec) • Sonic tools consist of transmitters and receivers arranged according to the goals of the measurement. • The tool response is affected by the formation matrix, fluid and porosity Acoustic wave Propagation from a Monopole source ACOUSTIC WAVE PROPAGATION IN FLUID FILLED BOREHOLE SONIC VELOCITY OF FLUIDS Formation Fluid Sonic velocity(ft./sec) 5,300 Travel Time(μsec/ft) 189 5,000 200 4,800 208 4,600 218 Oil 4,200 238 Methane 1,600 626 Air 1,100 910 Water with 20% NaCl Water with 15% NaCl Water with 10% NaCl Water (pure) SONIC VELOCITY OF EARTH FORMATIONS Formation Sandstone: Unconsolidated Semi-consolidated Consolidated Limestone Dolomite Shale Calcite Anhydrite Granite Gypsum Quartz Salt Sonic velocity(ft./sec) Travel Time(μsec/ft) ³17,000 18,000 19,000 21,000 23,000 6,000-16,000 22,000 20,000 20,000 19,000 18,000 15,000 58.8 or more 55.6 52.6 47.6 43.5 167-62.5 45.5 50.0 50.0 52.6 55.6 66.7 BASIC SONIC TOOL PRINCIPAL ACOUSTIC LOGGING APPLICATIONS SONIC LOG EXAMPLE SONIC POROSITY ESTIMATION Wyllie time-average equation ROCK SAMPLING(SIDE WALL CORING) Chronological Sample Taker Mechanical sidewall coring These types of tools allow the sampling of rock at a desired depth. These rock samples allow a better evaluation of the porosity and permeability, as well as their mineralogical composition, the clay type and distribution, the grain and pore size. (Schlumberger’s courtesy) FORMATION TESTER PRINCIPLE DOWN HOLE FLUID SAMPLING Pressure Invaded Zone Quartzdyne Gauge (L) Time Drawdown pump Piston Position Sensor Packer Back-up arm Sample Isolation Valve CASED HOLE SERVICES CEMENT QUALITY EVALUATION CEMENT BOD LOG-CBL CBL/VDL The CBL measurement is the amplitude in millivolts of the first arrival E1 at the 3 –foot receiver. No Cement It is a function of the attenuation due to the shear coupling of the cement sheath to the casing. The attenuation rate depends on the cement compressive strength, the casing diameter, the pipe thickness, and the percentage of bonded circumference Good Bond Cement Quality Evaluation – Cont. Cement Quality Evaluation – Cont. • Basic interpretation 5 – Free pipe • No cement to casing bond • No attenuation of the signal Free Pipe Signal 3 E1 T0 Threshold 2 TT T • Cement Quality Evaluation – Cont. Good cement to casing bond – If casing is well bonded, soundwave will be attenuated. – The received CBL amplitude will be low. CBL: Free Pipe 5 3 2 CBL: Good Bond T Cement Quality Evaluation – Cont. • Variable density log – 5 ft Receiver for VDL Analysis – Allows easy differentiation between casing and formation arrivals Tx R3 R5 5 ft Cement Quality Evaluation – Cont. • Factors affecting CBL Free Pipe Signal E1 – Good cement – Stretch T0 Good Bond Signal Threshold TTTT’ DT – Good cement – Cycle skipping Threshold E1 E3 T0 TT TT’ E2 Cement Quality Evaluation – Cont. • Basic interpretation 5 – Free pipe • No cement to casing bond • No attenuation of the signal Free Pipe Signal 3 E1 T0 Threshold 2 TT T Good Cement Example Weak casing arrival CBL flat, low Strong formation arrival Cement Quality Evaluation X X X Good casing to cement to formation bond CASED HOLE SERVICES CEMENT QUALITY EVALUATION CEMENT BOD LOG-CBL CBL/VDL The CBL measurement is the amplitude in millivolts of the first arrival E1 at the 3 –foot receiver. No Cement It is a function of the attenuation due to the shear coupling of the cement sheath to the casing. The attenuation rate depends on the cement compressive strength, the casing diameter, the pipe thickness, and the percentage of bonded circumference Good Bond Cement Quality Evaluation – Cont. • Variable density log – 5 ft Receiver for VDL Analysis – Allows easy differentiation between casing and formation arrivals Tx R3 R5 5 ft Good Cement Example Weak casing arrival CBL flat, low Strong formation arrival Cement Quality Evaluation X X X Good casing to cement to formation bond LOG INTERPRETATION OF CLEAN FORMATION Can you tell where to complete this well? Gamma Radiation Electrical Porosity Resistivity good porosity 200’ poor resistivity, probably water poor porosity good porosity 500’ good resistivity, may have oil or gas Looks like good sand quality poor porosity good porosity poor porosity poor resistivity, probably water 3000’ good porosity }Right here! This shows a clean sand, with good porosity and resistivity. CLEAN FORMATIONS Chances of encountering clean limestone are more than sandstone Ortho-quartzites may be considered as clean formation. These sands are very porous and permeable Feldspar, mica are very rare They have also the characteristics of winnowed sand deposited on submarine rises or of Aeolian sands Evaluation of clean formation is basically a question of inserting the right values of the parameters Rw, Rt & Ø with appropriate a, m, n values in Archie water saturation equation: Sw = (a Rw /Øm Rt)1/n Highly saline formation waters make shaly reservoirs suitable for evaluation with Archie’s equation as maximum current flows pore spaces INTERPRETATION OF CLEAN FORMATIONS Clean Zones: Shale/Clay free from GR, SP, Caliper,N-D. Clastics : Sandstone, volcaniclastics Carbonates Clastics : Carbonates: : Limestone, Dolomite Check ResistivityFirst Check PorosityFirst Tight Zones : Low DT, High RHOB, Low PHIN Water Zones : Zone of Low Resistivity, Good SP, Separation between deep & shallow Resistivity Interest : High porosity & Resistivity Fluid Contact : Cross over porosity/Resistivity Drop in resistivity Drop in porosity Detailed Analysis : Computation of ,SW &He POROSITY COMPUTAION IN CLEAN RESERVOIRS SONIC POROSITY DENSITY POROSITY BEST ESTIMATE IN OIL/WATER ZONES BEST ESTIMATE IN GAS ZONES s Dtlog Dtma Dt f Dtma ma b d ma f 7D 2N S 9 e 2 SATURATION DETERMINATION Archie gave an expression for computation of water saturation In 1942, which is still being used with some modifications. Electrical efficiency theory (EET) was given by D.C. Herrick & David Kennedy in 1994., which has not been well taken by The industry. Basic Information Needed Lithology Many log analyses require knowledge of lithology for calculations (e.g., porosity logs require matrix information, and Sw calculations use formation factor, F, which depends on lithology) Formation Temperature Formation Temperature is important because mud, mud filtrate and formation water resistivities vary with temperature. FORMULATION OF ARCHIE’s EQUATION Swn xmxCw = Ct Swn = Ct / mxCw C= x Cw = Cw Change from conductivity to resistivity Swn = Rw / mxRt Archie used a=1, m=n =2 C= mxCw = C0 C= Sw Sw2 = Rw / 2xRt Later on Humble suggested A=0.62, m=2.15, n=2 n xmxCw = Ct Final expression in use today is ARCHIES’s Equation : Swn = a Rw m.R t ARCHIE’ Parameters ‘a’ , ‘m’ , ‘n’ ARCHIE’ Parameters For Limestone : a=1, m=2, n=2 For sandstones a=0.62, m=2.15, n=2, a=0.81, m=2, n=2 a=1, m=1.96,n=2 How a, m & n are calculated 1. In laboratory 2. From logs. What are m* & n*. Courage to Explore Knowledge to Exceed Technology to Excel Thank You