Subsurface Geophysical MethodsWELL LOGGING FOR DELINEATION OF AQUIFER AND ESTIMATION OF WATER QUALITY

Mohit KumarIntegrated B .Sc . (Hons . ) Geo logy

M.Sc . Geo logyRol l no . 3071 , Semester 9th

Department o f Geo logyHansra j Co l lege , De lh i Univers i ty

Subsurface geophysical methods

• It is a detailed & comprehensive study of groundwater and conditions under which itoccurs.

• It provides information about location, thickness, composition, permeability and yieldof 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 geophysicalequipment

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 correlationof aquifers and associated rocks.

• It can be done by several methods such that resistivity, sonic, caliper logwhich operated in open holes and also radiation log which operated either inopen or cased holes.

Estimation of water quality includes chemical and physical characteristicsof water, including salinity, temperature, density and viscosity.

• It can be done by calibrated fluid conductivity or resistivity, temperature logsand resistivity logs.

Resistivity Logging

• It is also called Electric logging.

• Within an uncased well, current & potential electrodes can be lowered inborehole to measure electric resistivity of the surrounding media and toobtain 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 welldiameter.

• Recorded curves are termed as normal or lateral depending on the electrodearrangement.

Resistivity Logging

• In normal arrangement, effective spacing isconsidered to be distance AM and recordedcurves is designated as AM.

• Boundaries of formation having differentresistivities are located most readily with ashort electrode spacings.

Normal Arrangement

Resistivity Logging

• Sometimes, a long normal curve is recordedbased on the same electrode arrangement as thenormal but with a larger AM distance.

• Information on fluids in thick permeableformation can be obtained best with longspacings.

Long Normal Arrangement

Resistivity Logging

• Spacing for lateral curve (AO) is taken asdistance AO, measured b/w electrode M & N.

• Lateral measures the resistivity of the formationbeyond the zone of invasion.

Lateral Arrangement

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

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 =𝜌0𝜌𝑤

where, 𝜌0 = resistivity of saturated aquifer

𝜌𝑤 = resistivity of groundwater in aquifer

Since, 𝜌𝑤 ∝1

𝑠𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑎𝑛𝑐𝑒and 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.

Spontaneous Potential [SP]

• It measures natural electrical potential found within the earth.

• S.P. in a hole is due to electrochemical and elctrokinetic orstreaming potentials.

• Electrochemical potentials are due to differences in conc. ofactivities of the formation water and mud filtrate called liquidjunction potential.

• Membrane potential is due to presence of shale layers.

• The streaming potential is due to electro-filtration of the mudthrough the mud cake.

• Chemical activity is proportional/related to the salt content andhence to the resistivity.

Spontaneous Potential [SP]

• If the permeable formation is not shaly, SP is

𝑆𝑃 = −𝐾𝑙𝑜𝑔𝑅𝑚𝑓

𝑅𝑤where, K = coeff. proportional to absolute temp. of formation

𝑅𝑚𝑓 = resistivity of mud fluid

𝑅𝑤 = resistivity of formation water

• SP log is obtained by recording potential differences against depth, b/w a fixed surfaceelectrode and a moveable electrode in the borehole.

• Potentials associated with shales and clays are normally the least negative, the SPcurve is a straight line called the shale baseline.

• Opposite the permeable formations, the SP curve shifts either to the left (-ve) or to theright (+ve) depending on the relative salinities of the formation water and the mudfiltrate.

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

Radioactivity Logging

• Also known as nuclear or radiation logging.

• It involves the measurement of fundamental particles emitted from unstableradioactive 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 artificialfrom an artificial source (neutron logs)

• Since, Gamma ray log are recorded in two ways

– Natural Gamma log

– Gamma – Gamma log

Natural Gamma Logging

• It is the measure of naturally emitted gamma radiation fromunstable 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 ingravels and sands.

Application

Identification of lithology (i.e., sands, shale and clay)

Gamma – Gamma logging

• Gamma rays originates from a source in the probe anddiffuse through the formation. Part of the scattered gammarays re – enter the hole and are measured by an detector.

• The higher the bulk density of the formation, the smallerthe number of gamma – gamma rays that reach thedetector.

• The count rate plotted on a gamma – gamma log is anexponential function of bulk density. Hence, porosity of theformation can be determined.

𝜂 =𝜌𝑔 − 𝜌𝑏𝜌𝑔 − 𝜌𝑓

where, 𝜌𝑔 = grain density

𝜌𝑏 = bulk density

𝜌𝑓 = fluid density

Gamma –Gamma Logging

Applications

• Identification of lithology

• Measurement of bulk density and porosity.

Neutron Logging

• It produces a record related to the H+ content ofthe 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 itslow down.

• A large number of slow neutrons recordedindicates a large amount of fluid i.e., highporosity.

Neutron Logging

Application

• It can measure moisture content above water table andporosity below water table.

• By measuring moisture contents above and below the watertable, 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 fluidwhile the neutron log is sensitive to both casing and fluid inthe hole as well as in the formation.

Temperature Logging

• A vertical traverse measurement of groundwater T in a well can beobtained with a recording resistance thermometer.

• The rate of increase of T with depth (geothermal gradient) dependson the locality and heat conductivity of the formations.

• T encountered in drill holes are dependent not only on the naturalgeothermal gradient but also on the circulation of the mud.

• Higher T are usually recorded in caved sections where greatervolume of cement are deposited.

• Lower T may indicate the presence of gas or in deep wells maysuggest recharge from ground surfaces.

Temperature Logging

Applications

• to identifying rock types & aquifers.

• To verify that the cement on the outside of the casing hasformed a proper bond because cement generates greatamount of heat as it sets.

• to identify source of recharge or injected waste water(recharge water shows low T, while waste water showshigh T).

Induction Logging

• It measures the conductivity (reciprocal of resistivity) of formation by meansof 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 abilitiesand its greater radius of investigation.

• It is a superior method for surveying empty holes and holes drilled withoilbased mud.

Fluid Resistivity logging

• It is the measurement of resistivity of the fluid (water quality) between twoclosed 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 logsso 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.

Fluid – Velocity Logging

• It is measurement of vertical fluid movement within aborehole constitute a fluid – velocity log.

• Fluid movement from one aquifer to another, within awell, can be measured by an impeller flow meter whichrecords the number of impeller revolutions against time.

• Speed & direction of groundwater flow can be detectedby the use of dyes, soluble salts, etc.

• Devices used to measure vertical flow in water wellsinclude

– Impeller flow meter

– Radioactive tracer ejector – detector and

– Brine ejector – detector.

Caliper Logging

• A caliper log provides a record of the average diameter of aborehole.

• Caliper tools are designed either with arms hinged at theupper end and pressed against the hole wall by springs orwith 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 shalebecomes wet with the mud fluid, slough off and cave intothe hole.

• It can determine enlarged hole up to the maximum spreadof the caliper arm [Limit]

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.

Sonic Logging

• Sonic log records the time required for a sound wave to travel through aspecific length of formation.

• Such travel times are recorded continuously against depth as the sonde ispulled up the borehole.

• The sonic log is recorded as transit time in microseconds per meter, with zeroon 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.

Sonic Logging

Sonic log enables the accurate determination of porosity of the formation.

𝜂 =

1𝑉−

1𝑉𝑚

1𝑉𝑓

−1𝑉𝑚

where, 𝑉𝑚 = velocity matrix

𝑉𝑓 = velocity fluid

𝑉 = velocity formation

Since, transit time Δ𝑡 = 1/V

𝜂 =Δ𝑡𝑙𝑜𝑔 − Δ𝑡𝑚𝑎𝑡𝑟𝑖𝑥Δ𝑡𝑓𝑙𝑢𝑖𝑑 − Δ𝑡𝑚𝑎𝑡𝑟𝑖𝑥

This log will also give an indication of rock type and fracturing.

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.

Thank You