Chapter 09 Formation Evaluation

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Information about Chapter 09 Formation Evaluation

Published on February 2, 2009

Author: 3210651


Chapter 9 : Chapter 9 Introduction to Formation Evaluation Slide 2: GOALS OF FORMATION EVALUATION To evaluate the presence or absence of commercial quantities of hydrocarbons in formations penetrated by, or lying near, the wellbore. To determine the static and dynamic characteristics of productive reservoirs. To detect small quantities of hydrocarbon which nevertheless may be very significant from an exploration standpoint. To provide a comparison of an interval in one well to the correlative interval in another well Slide 3: Methods Mud logging Coring Wireline Logging Testing Sampling Mud Log : Mud Log Immediate interpretation of what the drill bit has penetrated and whether there are any hydrocarbons present (a show). Making maps of the subsurface geology. Coring - Conventional : Coring - Conventional Taking a core requires that the regular drill bit be removed from the hole. It is replaced with a "core bit", which is capable of grinding out and retrieving the heavy cylinder of rock. The core bit is usually coated with small, sharp diamonds that can grind through the hardest rock. A core bit cuts very slowly. A core is a solid cylinder of rock about 4-5 inches in diameter, and a single core will usually be about 30 feet long. Coring - Conventional : Coring - Conventional Whole Core Slab Core Coring - Sidewall : Coring - Sidewall This method is cheaper than the conventional coring. Cores can be taken in hours, instead of days. In sidewall coring, a slim wireline coring tool is run into the hole. The tool may be of two general types; either "rotary sidewall" or "percussion". Typically, cores about 1" in diameter and 1" to 2" long can be retrieved with this method. Coring - Sidewall : Coring - Sidewall Coring - Sidewall : Coring - Sidewall INTRODUCTION - WHAT IS LOGGING? : INTRODUCTION - WHAT IS LOGGING? In situ meas. (vs. depth) of Rock properties Fluid properties When Openhole (before casing) While drilling (LWD / MWD) After drilling (wireline) Cased hole Interpretation for Geological properties Petrophysical properties Production properties Baker-Atlas Casing Open hole Slide 11: VALUE AND LIMITATIONS OF WELL LOG DATA Strengths Provides remotely sensed values of reservoir properties and fluids Among the most abundant reservoir data Presentation results fairly well standardized Allows evaluation of lateral (map) and vertical (cross section) changes in reservoir properties and fluids Limitations Indirect measurements Vertical resolution Depth of investigation OPEN HOLE LOGGING MEASUREMENTS : OPEN HOLE LOGGING MEASUREMENTS Passive Caliper Gamma Ray Spontaneous Potential (SP) Active Acoustic ?tc, ?ts, Ac, As Nuclear ?b, ?N, Pe, ?1, ?2 Electromagnetic R, tPL, EATT LOGGING TOOL CASED HOLE LOGGING MEASUREMENTS : CASED HOLE LOGGING MEASUREMENTS Passive Gamma Ray Temperature Flow Velocity Caliper Active Acoustic Nuclear Electromagnetic Mechanical SOME QUESTIONS ADDRESSED BYLOG INTERPRETATION : SOME QUESTIONS ADDRESSED BYLOG INTERPRETATION Geophysicist / Geologist Are the tops as predicted? Are potential zones porous? Formation intervals? Lithology? Hydrocarbons? What type of hydrocarbons? Commercial quantities? Reservoir Engineer How thick is the pay zone? How homogeneous is the zone? Porosity? Permeability? Production Engineer Which zone(s) to complete? What production rates? Any water production? Is zone hydraulically isolated? Will well need stimulation? What stimulation would be best? WHAT DOES AN OPEN HOLE LOG COST?IT DEPENDS ON... : WHAT DOES AN OPEN HOLE LOG COST?IT DEPENDS ON... Well type Vertical/Deviated Deep/Shallow Hot/Normal Measurements Depth charge Survey charge Time / location / special procedures Land/offshore Service charge Equipment availability Rig time Wireline/LWD TYPICAL OPEN HOLE WIRELINE COSTS : TYPICAL OPEN HOLE WIRELINE COSTS LOGGING IS COMPARITIVELY INEXPENSIVE! : LOGGING IS COMPARITIVELY INEXPENSIVE! Total cost to drill a well: $75 to $200 per foot! WIRELINELOGGINGEQUIPMENT : WIRELINELOGGINGEQUIPMENT Slide 20: DETAILS OF WIRELINE LOGGING RIGUP Modified from Halliburton (EL-1007) LOGGING CABLE : LOGGING CABLE LOG PRESENTATION - THE HEADING : LOG PRESENTATION - THE HEADING Well location Depth references Well depth Date of log Casing shoe depth Bit size Mud data Type Properties Resistivities Max. Temperature LOG PRESENTATION - LINEAR GRID : LOG PRESENTATION - LINEAR GRID LOG PRESENTATION - LOG GRID : LOG PRESENTATION - LOG GRID Track 1 Depth track Track 2 Track 3 2x10n 2x10n+4 LOG PRESENTATION - HYBRID GRID : LOG PRESENTATION - HYBRID GRID Track 1 Depth track Track 2 Track 3 2x10n 2x10n+2 LOG PRESENTATION - COMMON DEPTH SCALES : LOG PRESENTATION - COMMON DEPTH SCALES Correlation 1:500 or 1:1000 2 in. (1:600) or 1 in. (1:1200) Heavy lines every 100 ft. or 50m Light lines each 10ft or 5m Routine 1:200 or 1:240 (5 in) Heavy lines every 50 ft. or 5 m Medium lines each 10 ft. or 5 m Light lines each 2 ft or 1 m CHOOSING A LOGGING TOOL : CHOOSING A LOGGING TOOL It is necessary to choose the right tool to get the desired measurement. Considerations: Type of well ( wildcat or development ) Hole conditions ( depth, deviation, hole size, mud type ) Examples: Oil based mud : Induction tool Water based salty mud : Laterolog Tool Formation fluid content (fresh/salt connate water) Economics (cost of the job, rig time involved) TYPES OF LOGS TO BE RUN : TYPES OF LOGS TO BE RUN Logging suites generally include one resistivity and one porosity device The logging string will also have other tools like the gamma ray, SP and caliper tools However, logging suites usually have two porosity devices to give more information about rock type, hydrocarbon type and porosity Other considerations – to estimate permeability or to take fluid samples – require other special tools like the formation testers Slide 29: NOMENCLATURE FOR ZONES IN AND AROUND THE BOREHOLE Modified from Halliburton (EL-1007) TOOL CALIBRATIONS : TOOL CALIBRATIONS A logging tool collects data that are converted to porosity, resistivity, and other values Each tool is calibrated to an industry standard This ensures that each tool, irrespective of the type of tool or tool history or service company, reads the same value when logging the same formation (normalization may still be required between log) Check tool calibrations before and after a logging job to ensure good quality log data LOG QUALITY CONTROL : LOG QUALITY CONTROL Check all calibrations before and after job Record a repeat section of about 200 ft to ensure validity of data and to explain abnormal curve response Compare log response with offset well logs Keep hole conditions (hole size, mud type, tool centralization) in mind when interpreting log data Ensure that logging speeds are as recommended by the service company. DRILLING DISTURBS FORMATION : DRILLING DISTURBS FORMATION Drilling and rock crushing Damage zone Mud systems and invasion Oil based mud Small conductivity mud Shallow invasion Thin cake Water based mud Moderate to very conductive mud Shallow to deep invasion Thin to thick cake MUD FILTRATE INVASION : MUD FILTRATE INVASION Slide 34: Borehole Rm : Borehole mud resistivity Rmc : Mudcake resistivity Invaded zone Rmf : Mud filtrate resistivity Rxo : Invaded zone resistivity Sxo : Invaded zone water saturation Uninvaded zone Rw : Interstitial water resistivity Rt : Uninvaded zone resistivity Sw : Uninvaded zone water saturation COMMON TERMINOLOGY PASSIVE MEASUREMENTS : PASSIVE MEASUREMENTS Caliper Spontaneous Potential Gamma Ray Natural Spectral CALIPERS : CALIPERS Uses Hole volume Mudcake (permeability) Tool corrections Crude lithology indicator Properties two, three, or four arms linked or independent Calipers may disagree (limitations) non-circular hole deviated wells Slide 37: One electrode Insulators on either side Surface ground electrode – at a stable potential THE SP TOOL SHALE SHALE SAND Slide 38: TYPICAL SP RESPONSES – BASED ON THE DIFFERENCE BETWEEN Rw and Rmf. Rmf >> Rw - Amplitude large and negative Rmf > Rw - Amplitude negative but not large Rmf = Rw - No SP deflection Rmf < Rw - Amplitude positive but not large 5. Rmf << Rw - Amplitude large and positive GAMMA RAY LOGS : GAMMA RAY LOGS Uses Correlation Lithology indicator; exploration for radioactive materials Evaluation of shale content Paleoenvironmental indicator Open or cased hole; any fluids Fracture detection Properties Measures natural gamma radiation random fluctuations Rock Formations GR RESPONSE IN COMMON FORMATIONS : GR RESPONSE IN COMMON FORMATIONS Shales often radioactive Clays Trace and heavy minerals Sandstones may be radio- active Non-clay minerals, e.g., mica, feldspar Clays Units GR calibrated to standard Response in “mid-continent shale” equals 200 API units Calibration pits PASSIVE LOGCORRELATION : PASSIVE LOGCORRELATION GR, SP, and CAL often correlate different measurements different reasons Correlation helps GR instead of SP in OBM Easier detection of shales Facilitates “zonation” POROSITY TOOLS : POROSITY TOOLS Sonic (acoustic) Density Neutron SONIC PRINCIPLE : SONIC PRINCIPLE Slide 44: Ray, 2002 FAMILY OF NUCLEAR TOOLS : FAMILY OF NUCLEAR TOOLS Gas Oil Slide 46: From Halliburton (EL – 1007) Uses Density Porosity Lithology Curves Bulk density (rb and Dr) Pe DENSITY & POROSITY MEASUREMENTS DENSITY PRINCIPLE : DENSITY PRINCIPLE Detect GR’s from the source which have been scattered back by the formation PRINCIPLE : PRINCIPLE Gamma rays emitted from radioactive source Gamma rays collide with electrons in formation, losing energy Detectors measure intensity of backscattered gamma rays High energy GR relate to - Density Low energy GR relate to - Lithology Slide 49: NEUTRON LOGS Uses of neutron logs Identify porous zones Determine porosity Identify gas in porous zones Where neutron logs can be used Any borehole Open or cased Liquid- or air-filled Depth of investigation 6-12 inches for CN NEUTRON MEASUREMENT : NEUTRON MEASUREMENT Uses Lithology Porosity Curve fN Pe rb Dr fN NEUTRON TOOL PRINCIPLE : NEUTRON TOOL PRINCIPLE Detects neutrons from the source which have been scattered back by the formation Source AmBe 15-20Cu 5MeV Slide 52: The neutron tool employs a dual detector design to compensate for mudcake, lithology, etc. Still, corrections are required for the NPHI values NOTE : The tool is pressed against the borehole wall to minimize mud effects LIFE OF A NEUTRON - 1 : LIFE OF A NEUTRON - 1 Neutrons emitted from source Neutrons interact with Hydrogen in formation Neutrons loose energy Neutrons are absorbed or reflected back to detectors High counts = Low porosity Low counts = High porosity LIFE OF A NEUTRON - 2 : LIFE OF A NEUTRON - 2 Source AmBe 15-20Cu 5MeV neutrons Collisions cause neutrons to lose energy Energy loss due mainly to hydrogen Therefore tool measures amount of hydrogen in formation, ie., water, oil NEUTRON SCATTERING : NEUTRON SCATTERING Energy transfer to the nucleus is a maximum if the collision is head-on and the nucleus has the same mass as the neutron. The only atom that has the same mass as a neutron is hydrogen. Thermal Neutrons : Thermal Neutrons The neutron tool responds primarily to the presence of hydrogen The more hydrogen, more neutrons slowed to the thermal level and captured by the formation Other minerals also have a small effect on the neutron tool, which requires compensation Slide 57: RESISTIVITY Resistivity The voltage required to cause one amp to pass through a cube having a face area of one square meter Units are ohm-m / m; usually ohm-m (?.m) 2 Slide 58: RESISTIVITY – DEFINITION OF THE OHM-METER From Halliburton (EL 1007) RESISTIVITY OF EARTH MATERIALS : RESISTIVITY OF EARTH MATERIALS Increasing Resistivity (1) Rock (2) Gas (3) Oil (4) Fresh Water (5) Salt Water Increasing Conductivity FACTOR AFFECTING RESISTIVITY : Resistivity of water Porosity of the formation, Pore geometry - tortuosity Lithology of the formation Degree of cementation, and Type and amount of clay in the rock FACTOR AFFECTING RESISTIVITY From J. Jensen, PETE 321 Lecture Notes Slide 61: Rt Ro Rw Resistivity Cube of water having resistivity, Rw Non-shaly rock, 100% saturated with water having resistivity, Rw Rock containing pores saturated with water and hydrocarbons = 100% Sw = 100% = 20% Sw = 100% = 20% Sw = 20%

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