Well LogsGeol472-2009What do we need to find out?• Rock type?– Sandstone? Shale? Limestone?• Rock Properties– Porosity– Permeability– Bedding Orientation– Fractures?–Temperature• Fluids– Type (water, oil, gas)– Saturation–Salinity– Pressure• Engineering– Well trajectory (coord)– Shape of hole– Casing Joints– Quality of cement–etcWhat for? (Log applications)• Stratigraphic correlation• Formation Tops• Quantitative Oil, Gas , Water saturations• Porosity• Correlation with seismic data• Sedimentological studies• Reservoir modeling• Structural studies•etcWhat can we measure?• Electrical Properties• Natural radioactivity• Induced radioactivity• Acoustic Properties (velocity)• Shape of hole•Noise• Temperature•Depth• Orientation of hole•…Log Types• Lithologic Logs– Spontaneous Potential (SP)– Gamma Ray (GR)• Porosity Logs–Neutron–Density– Sonic• Resistivity Logs (Fluid Type)– Resistivity– Induction•Other– Dipmeter– Caliper– Temperature– Acoustic–FMI–Many more …Spontaneous Potential (SP)• One of the Oldest Logging Measurements– Used Commercially in 1931• Discovered as Noise in Resistivity • Found to be Related to Presence of SandstoneSurfaceDownholeThe earth well works like a batteryE = -K log (aw/amf)E is in millivoltsaw= water salinityaw= mud salinityIon flow is easier in permeable sandstonesTypical SP logTypical SP logSP has PoorSP has PoorLimestoneLimestoneResponseResponseSpontaneous Potential (SP) DriftGamma Ray Log• Lithology log• Measures natural radioactivity• Uses a scintilometer (Gaiger counter)• Potassium (K), Uranim, Thorium, Phosphorous•K → abundant in clay → shales• Unaffected by fluidsSP and Gamma Ray•Gamma Ray in API Units•Shales swing rightSpectral Gamma RayDifferentiates the different sources of gamma raysResistivity Log ApplicationsResistivity Log Applications•“True”RtFormation Resistivity• Fluid Saturation Swfrom Archie’s Equation • Determination of Hydrocarbon-Bearing vs. Water-Bearing Zones• Geopressure Detection• Diameter of Invasion• Porosity• CorrelationResistivity Tool BackgroundResistivity Tool Background• Three Classes– Electrode Logs– Laterologs• Focused Electrodes– Induction• Measure Resistivity in OhmsResistivityResistivityResistivity of Water - RwResistivity of Water and FormationResistivity of Water, Hydrocarbons,and FormationResistivity of FormationIncreasing ResistivityNormal Resistivity ToolNormal Resistivity ToolSpacing of electrodes determines penetrationInduction (Conductivity) ToolInduction (Conductivity) ToolReceiver coil measures the induced electrical field created in the rocks by the transmitter coilGuard or Guard or LaterologLaterologToolToolThe guard electrodes focus the current in a narrow diskLog Presentation and ScalesLog Presentation and ScalesID, IM, LL8 measure resistivity at different distances form the borehole. ID= deep inductionIM= medium inductionLL8= shallow inductionResistivity logs : spherically-focussed (SFL), medium induction (ILM), and deep induction (ILD) from KGS Jones #1The Borehole EnvironmentThe Borehole EnvironmentCalculating Fluid Saturation(Rxo/Rt)5/8Sw=(Rmf/Rw)Rxo= Resistivity of flushed zoneRw= Resistivity of formation waterRmf= Resistivity of mud filtrateRt= Resistivity of uninvaded zoneWellbore EnvironmentDepth, Diameter, Temperature,Mud ParametersHeaderHeaderSonic (Acoustic) LogsSonic (Acoustic) LogsMeasures of acoustic travel time in formation∆∆t = 1/Vt = 1/V••∆∆ttis related to Porosity, Fluid Content and Lithology• Used to generate Synthetic Seismic traces to link wells logs to seismic dataAcoustic impedancefrom Sonic logSeismicReflectivityWaveletSyntheticSeismicCreating synthetic seismic data:Creating synthetic seismic data:ConvolutionGEOL 553 Lecture 3; Subsurface AnalysisDensity LogDensity Log• Measures of Density -ρρ• Tied to Porosity, Fluid Content and Lithology• Tool emits gamma rays• Detects returning scattered gamma rays• Gamma ray absorption is proportional to rock densityNeutron Log (CNL)• Tool has a neutron source• H absorbs neutrons and emits gamma rays• Tool detects the emitted gamma rays• H is mostly in formation fluids (water and hydrocarbons)• Can be run through casing• Reads low in gas zones• Cannot distinguish oil from waterNeutron-density cross over indicates gas!High resistivityPorosity calibration• If lithology is known, neutron and density logs can be calibrated for porosityApplications of logs• Stratigraphic studies – Sedimentary facies• Well correlation• Reservoir models• Structural interpretation– Fault recognitionRelating log character to sedimentary Relating log character to sedimentary faciesfaciesBuilding a reservoir Building a reservoir model modelmodel model1. Define facies in core2. Relate facies to log3. Predict facies in wells without core, but with good logs4. Fill the gaps between wells28 cores1600 wells108 Million CellsLog Datum TerminologyLog Datum Terminology••KB KB --Kelly Bushing elevationKelly Bushing elevation..••MD MD --Measured Depth along the wellbore Measured Depth along the wellbore from the Kelly bushingfrom the Kelly bushing(usually)(usually)••SS SS --Depth Relative to Depth Relative to SealevelSealevel••TVD TVD ––True Vertical Depth, (important for True Vertical Depth, (important for nonnon--vertical wells)vertical wells)••SSTVD SSTVD --SubSub--Sea True Vertical DepthSea True Vertical DepthCorrelationCorrelationExampleExampleMajor SandsMajor Sandson SPon