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Module 4: Caliper and temperature logging
Tamla Springeron 20 September 2012
Transcript of Module 4: Caliper and temperature logging
Calipers contacting three points around the borehole are often thought to be more accurate than those contacting two points, but quite often, the spring actuation of acoustic caliper devices is fouled by drilling mud or cuttings clogging the sliding mechanism in the caliper mandrel.
Two-point density calipers generally show little or no mudcake buildup on the borehole wall because the skid design and pad force tend to wipe the filter cake away from the wall.
Two-point microresistivity calipers generally provide the best indications of mudcake thickness across permeable strata because pad force is only about 15 psi. Hole volume can be integrated from caliper data and is very useful to the completion engineer.
If casing is to be run, the engineer knows the volume of borehole that the casing will use, and by subtracting that value from the integrated hole volume, the engineer can determine the amount of annular space available for cementing. Several different types of calipers are available for use with well logging downhole hardware. Caliper measurement Formation temperature and heat conductivity are important to formation evaluation because all resistivity data are temperature dependant.
Heat conductivity decreases hyperbolically with temperature.
Thermal conductivity of water does not change appreciably with increasing salt concentration, and the effects of pore fluids on gross conductivity is relatively small for rocks of low to moderate porosity.
Thermal conductivity of clays tends to vary inversely with the water content.
In overpressured zones, the higher pore pressure causes higher porosity that accounts for more fluid volume. As a result, geothermal gradients are typically larger in massive shale formations that overlay reservoir rocks, and gradients are usually reduced considerably in aquifers.
Overpressured, high-porosity shales represent a geothermal anomaly, and because of this circumstance, flowline temperature measurements are used as a supplementary pressure indicator by rig personnel. GEOTHERMAL HOT SPOTS AND OIL OCCURRENCES OVER TRINIDAD Assume:
Y = bottom hole temperature (BHT) = 250 F
X = total depth (TD) = 15,000 ft
C= surface temperature = 70 F
Solve for m (i.e. slope or temperature gradient)
M = (Y – C)/ X
M = (250 – 70)/15,000
M = 0.012 /ft or 1.2 /100 ft Temperature Gradient Calculation Chart used for estimating geothermal gradient The geothermal gradient is a function of the thermal conductivity of the rocks in the subsurface The formula allows an estimate of formation temperature if the geothermal gradient and mean surface temperature are known.
Mean surface temperature data are usually provided by governmental agencies
Extreme cold at the surface will affect temperatures at very shallow depths (< 1,000 ft), but extreme heat at the surface will also affect the temperature gradient in very shallow wells. Effects of Temperature
Subsurface temperatures normally increase with depth, and the rate of increase with depth is called the geothermal gradient, defined as
GG = 100 (Tf – Tm) / D
where GG = geothermal gradient (°F/100 ft),
Tf = formation temperature (°F),
Tm = mean surface temperature for a given area (°F),
and D = depth of formation of interest (ft).
This equation can also be written as
Tf = Tm + GG (D/100) , Resistivities of different fluids must be converted to a common temperature for log analysis Resistivity varies with temperature.
When comparing resistivities, it is therefore very important that the temperature be equal, or that resistivities be converted to a common temperature Dual Calipers recorded with a four-arm device Swing-arm pad section Dipmeter tools provide diameter measurements from opposite pads with a four-pad device or distance measurements of radii for each individual pad with six-arm devices or four-pad devices with independent arm actuation.
Acoustic pulse-echo imaging tools provide complete 360º circumferential coverage of the borehole size and shape. Standard Diplog® pad assembly with gauge ring calibrator Micro-resistivity devices make a distance measurement between two pads that are opened electrically to make contact with the borehole wall Caliper type that is run with microresistivity devices Caliper run with acoustic devices
Acoustic devices typically employ a three-arm, spring actuated caliper that also serves as a tool centralizer Comparison of temperature gradient steepness and lithology Assume:
M = temperature gradient = 0.012 /ft
X = formation depth = 8,000 ft
C = surface temperature = 70
Recall : Y = MX +C
Y = (0.012) *(8,000) + 70
Y = 166 formation temperature at 8,000 ft Formation Temperature Calculation Caliper and Temperature Logging Module 4 Density instruments measure the distance between the skid face containing the radiation source and detectors and the backup shoe that forces the skid face against the borehole wall with relatively high pad pressure Caliper run with density devices Mean surface temperature = 80 F
Bottom hole temperature(BHT) = 180 F
Total depth(TD) = 10,000 feet
Formation depth = 6,000 feet
Locate BHT (180 F) on scale (bottom of chart: surface temperature = 80 F)
Follow BHT(180 F) vertically up until it intersects 10,000ft (TD) line. This intersection defines the temperature gradient.
Follow the temperature gradient line up to 6,000 ft (formation depth)
Formation temperature (140 F) is read on the bottom scale vertically down from the point where the 6,000 ft line intersects the temperature gradient. Estimation of Formation Temperature