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Subsurface geophysical methods


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Subsurface geophysical methods

  1. 1. Subsurface Geophysical Methods WELL LOGGING FOR DELINEATION OF AQUIFER AND ESTIMATION OF WATER QUALITY Mohit Kumar Integrated B.Sc. (Hons.) Geology M.Sc. Geology Roll no. 3071, Semester 9th Department of Geology Hansraj College, Delhi University
  2. 2. Subsurface geophysical methods • It is a detailed & comprehensive study of groundwater and conditions under which it occurs. • It provides information about location, thickness, composition, permeability and yield of the aquifer. • It also provides information about the location, movement & quality of groundwater. Advantages • Data from geophysical log can be digitized and stored in storage devices. • Graphic display permit rapid visual interpretation. Disadvantages • It is costly, so a few %age of new wells drilled each year are logged by geophysical equipment
  3. 3. Subsurface geophysical methods Delineation of aquifer simply means to draw or trace an outline of aquifers, which can infer by the determination of lithology and stratigraphic correlation of aquifers and associated rocks. • It can be done by several methods such that resistivity, sonic, caliper log which operated in open holes and also radiation log which operated either in open or cased holes. Estimation of water quality includes chemical and physical characteristics of water, including salinity, temperature, density and viscosity. • It can be done by calibrated fluid conductivity or resistivity, temperature logs and resistivity logs.
  4. 4. Resistivity Logging • It is also called Electric logging. • Within an uncased well, current & potential electrodes can be lowered in borehole to measure electric resistivity of the surrounding media and to obtain a trace of their variation with depth. • Resistivity log affected by several components – Fluid within a well – Well diameter – Character of surrounding strata – Groundwater • Uses of multielectrode can minimize the effect of drilling fluid and well diameter. • Recorded curves are termed as normal or lateral depending on the electrode arrangement.
  5. 5. Resistivity Logging • In normal arrangement, effective spacing is considered to be distance AM and recorded curves is designated as AM. • Boundaries of formation having different resistivities are located most readily with a short electrode spacings. Normal Arrangement
  6. 6. Resistivity Logging • Sometimes, a long normal curve is recorded based on the same electrode arrangement as the normal but with a larger AM distance. • Information on fluids in thick permeable formation can be obtained best with long spacings. Long Normal Arrangement
  7. 7. Resistivity Logging • Spacing for lateral curve (AO) is taken as distance AO, measured b/w electrode M & N. • Lateral measures the resistivity of the formation beyond the zone of invasion. Lateral Arrangement
  8. 8. Resistivity Logging • Resistivity of unconsolidated aquifer controlled by – Porosity – Packing – Water resistivity – Degree of saturation – Temperature • Resistivity range of different formations – Shale, Clay & Saltwater –> Low value – Freshwater sand –> Moderate to high value – Cemented sandstone & Nonporous limestone –> High value • Resistivity of groundwater depends on – Ionic conc. of salt solution – Mobility of salt solution Applications  Physical and chemical characteristics of fluids,  Formation resistivity  Porosity  Mud resistivity
  9. 9. Resistivity Logging Applicability of resistivity logs to the estimation of groundwater quality (Given by Jones and Buford and later by Turcan) A field formation factor F for an aquifer is determined by 𝐹 = 𝜌0 𝜌 𝑤 where, 𝜌0 = resistivity of saturated aquifer 𝜌 𝑤 = resistivity of groundwater in aquifer Since, 𝜌 𝑤 ∝ 1 𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑎𝑛𝑐𝑒 and specific conductance ∝ chloride content (or TDS values). Resistivity of sediments below water table is a function of the salinity of water filling pore spaces and how those pores spaces ae interconnected.
  10. 10. Spontaneous Potential [SP] • It measures natural electrical potential found within the earth. • S.P. in a hole is due to electrochemical and elctrokinetic or streaming potentials. • Electrochemical potentials are due to differences in conc. of activities of the formation water and mud filtrate called liquid junction potential. • Membrane potential is due to presence of shale layers. • The streaming potential is due to electro-filtration of the mud through the mud cake. • Chemical activity is proportional/related to the salt content and hence to the resistivity.
  11. 11. Spontaneous Potential [SP] • If the permeable formation is not shaly, SP is 𝑆𝑃 = −𝐾𝑙𝑜𝑔 𝑅 𝑚𝑓 𝑅 𝑤 where, K = coeff. proportional to absolute temp. of formation 𝑅 𝑚𝑓 = resistivity of mud fluid 𝑅 𝑤 = resistivity of formation water • SP log is obtained by recording potential differences against depth, b/w a fixed surface electrode and a moveable electrode in the borehole. • Potentials associated with shales and clays are normally the least negative, the SP curve is a straight line called the shale baseline. • Opposite the permeable formations, the SP curve shifts either to the left (-ve) or to the right (+ve) depending on the relative salinities of the formation water and the mud filtrate.
  12. 12. Spontaneous Potential [SP] Applications • To calculate formation water resistivity • To locate bed boundaries. • To distinguish b/w shales and sandstone or limestone in comination with other logs. • For stratigraphic correlation Factors affected SP log • Hole diameter • Bed thickness • Water or mud resistivity • Density • Chemical compostion • Cake thickness • Mud filtrate invasion well temperature
  13. 13. Radioactivity Logging • Also known as nuclear or radiation logging. • It involves the measurement of fundamental particles emitted from unstable radioactive isotope. • Radioactive logs can be used in cased as well as in open holes. [Advantage] • Radioactive logs are of two general types – those which measure the natural radioactivity of formations (gamma ray log) and – those which detect radiation reflected from or induced in the formation from an artificial from an artificial source (neutron logs) • Since, Gamma ray log are recorded in two ways – Natural Gamma log – Gamma – Gamma log
  14. 14. Natural Gamma Logging • It is the measure of naturally emitted gamma radiation from unstable isotopes i.e., K, U & Th. • All rocks emit natural gamma radiation from unstable isotopes. • The minerals in shale and clay emit more gamma rays than in gravels and sands. Application Identification of lithology (i.e., sands, shale and clay)
  15. 15. Gamma – Gamma logging • Gamma rays originates from a source in the probe and diffuse through the formation. Part of the scattered gamma rays re – enter the hole and are measured by an detector. • The higher the bulk density of the formation, the smaller the number of gamma – gamma rays that reach the detector. • The count rate plotted on a gamma – gamma log is an exponential function of bulk density. Hence, porosity of the formation can be determined. 𝜂 = 𝜌 𝑔 − 𝜌 𝑏 𝜌 𝑔 − 𝜌 𝑓 where, 𝜌 𝑔 = grain density 𝜌 𝑏 = bulk density 𝜌 𝑓 = fluid density
  16. 16. Gamma – Gamma Logging Applications • Identification of lithology • Measurement of bulk density and porosity.
  17. 17. Neutron Logging • It produces a record related to the H+ content of the borehole environment. • A fast neutron source is used to bombard the rock. When any individual neutron collides with a H+ ion, some of the neutron’s energy is lost and it slow down. • A large number of slow neutrons recorded indicates a large amount of fluid i.e., high porosity.
  18. 18. Neutron Logging Application • It can measure moisture content above water table and porosity below water table. • By measuring moisture contents above and below the water table, specific yield of unconfined aquifer can be determined. Neutron log results are influenced by hole size. *The gamma ray does not indicate casing or presence of fluid while the neutron log is sensitive to both casing and fluid in the hole as well as in the formation.
  19. 19. Temperature Logging • A vertical traverse measurement of groundwater T in a well can be obtained with a recording resistance thermometer. • The rate of increase of T with depth (geothermal gradient) depends on the locality and heat conductivity of the formations. • T encountered in drill holes are dependent not only on the natural geothermal gradient but also on the circulation of the mud. • Higher T are usually recorded in caved sections where greater volume of cement are deposited. • Lower T may indicate the presence of gas or in deep wells may suggest recharge from ground surfaces.
  20. 20. Temperature Logging Applications • to identifying rock types & aquifers. • To verify that the cement on the outside of the casing has formed a proper bond because cement generates great amount of heat as it sets. • to identify source of recharge or injected waste water (recharge water shows low T, while waste water shows high T).
  21. 21. Induction Logging • It measures the conductivity (reciprocal of resistivity) of formation by means of induced alternation currents. • Insulated coils rather than electrolytes are used to energise the formation. • Borehole may contain any fluid or be empty but the hole must be uncaved. • It is specially used to investigate thin beds because of its focusing abilities and its greater radius of investigation. • It is a superior method for surveying empty holes and holes drilled with oilbased mud.
  22. 22. Fluid Resistivity logging • It is the measurement of resistivity of the fluid (water quality) between two closed spaced electrodes in the hole. • The resistivity of the fluid column is also important in interpreting SP, resistivity and neutron log which may be affected by salinity changes. • Temperature logs should be made in conjunction with fluid – conductivity logs so that values can be corrected to standard temperature. Application • to locate points of influx of waters of different quality • to locate the interface between salt & fresh water • to correct head measurement for fluid density differneces • to locate waste water • to follow the movement of saline tracers.
  23. 23. Fluid – Velocity Logging • It is measurement of vertical fluid movement within a borehole constitute a fluid – velocity log. • Fluid movement from one aquifer to another, within a well, can be measured by an impeller flow meter which records the number of impeller revolutions against time. • Speed & direction of groundwater flow can be detected by the use of dyes, soluble salts, etc. • Devices used to measure vertical flow in water wells include – Impeller flow meter – Radioactive tracer ejector – detector and – Brine ejector – detector.
  24. 24. Caliper Logging • A caliper log provides a record of the average diameter of a borehole. • Caliper tools are designed either with arms hinged at the upper end and pressed against the hole wall by springs or with bow springs fastened at both ends. • The hole diameter will be equal to the size of drilling bit, when a hard sandstone or limestone is traversed. • Well bore becomes enlarged in shale beds because the shale becomes wet with the mud fluid, slough off and cave into the hole. • It can determine enlarged hole up to the maximum spread of the caliper arm [Limit]
  25. 25. Caliper Logging Applications • Identification of lithology and stratigraphic correlation. • To locate fractures and other rock openings. • To measuring casing diameter in old wells. • To locating swelling and caving zones.
  26. 26. Sonic Logging • Sonic log records the time required for a sound wave to travel through a specific length of formation. • Such travel times are recorded continuously against depth as the sonde is pulled up the borehole. • The sonic log is recorded as transit time in microseconds per meter, with zero on the right. • The speed of sound in subsurface formations depends on – Elastic properties of the rocks – Porosity of formation and – Their fluid content and pressure.
  27. 27. Sonic Logging Sonic log enables the accurate determination of porosity of the formation. 𝜂 = 1 𝑉 − 1 𝑉 𝑚 1 𝑉 𝑓 − 1 𝑉 𝑚 where, 𝑉 𝑚 = velocity matrix 𝑉 𝑓 = velocity fluid 𝑉 = velocity formation Since, transit time Δ𝑡 = 1/V 𝜂 = Δ𝑡𝑙𝑜𝑔 − Δ𝑡 𝑚𝑎𝑡𝑟𝑖𝑥 Δ𝑡 𝑓𝑙𝑢𝑖𝑑 − Δ𝑡 𝑚𝑎𝑡𝑟𝑖𝑥 This log will also give an indication of rock type and fracturing.
  28. 28. Downhole Photography • It can provide immediate and continuous visual inspection of a borehole wall – live and in color. • By means of a camera, pendulum and compass all fitted in a probe, inclination and direction of drill hole deviation (drift) can be determined. Applications • To identify geologic formation in open holes, • To check damaged walls, • To aid in removing foreign matter from a well, and • To assist development or well cleaning.
  29. 29. Thank You