methods and principles.pptx

7/29/2019 Methods and Principles.pptx

http://slidepdf.com/reader/full/methods-and-principlespptx 1/33



Logging Equipment: Log measurements are made using a measuring sonde

(with electronic cartridge) lowered on a cable from a winch, which is mounted on a logging truck oroffshore unit.



Logging Truck and Offshore Unit The main winch which may hold as much as 26,000 ft

(8,000 m) of multi-conductor cable with a pullingcapacity of several tons.

The surface logging panels which power and controlthe downhole tools, process the incoming informationand transmit the information to recording equipment.

Depth measuring system. Electrical generator.



Dark room for development of film.

Printer for making log prints.

Recent years have seen the introduction of fully computerized logging unit which not only handle thedata acquisition but permit well site dataprocessing/evaluations.

Offshore units are mounted on skids and bolted (or welded) to the deck of drilling vessel or platform.



A Logging Truck


http://slidepdf.com/reader/full/methods-and-principlespptx 7/33An Offshore Logging Unit



Logging Cables The logging cables fulfill three functions:

Running in and pulling out the tool and control of tool

speed. Electrical interface between the downhole logging tool

and the surface processing and recording equipment.

Depth measurement.



The Logging Tool Logging tools vary in complexity from a simple electrode

carrying to a sophisticated system of electronic circuits,enclosed in a pressure resistant metal housing and capable

of operating at high temperatures. The sonde is generally attached below an electronic

cartridge, which carries in a protective housing theelectronic modules or hardware for the downholeinstrument.

Where several tools are being run in combination each of the sondes and cartridges in the tool string has a passthrough facility for the signals to or from tools lower in thestring.



Each sonde-cartridge set can be connected electrically and mechanically to the cable head by a quick connectsystem consisting of pins and sockets and a threadring.

Logging tools vary in sizes and shapes. Typically the wireline tools are cylindrical in shape, usually from 1.5to 5 inches in diameter and 35 ft long.

Most tools are built to withstand 20,000 psi pressure

and 350oF to 400oF temp. Modern tools are modularized to allow combination

tool strings by appropriate mixing and matching.



Log Presentation The values of the parameter measured in the well are

plotted continuously against depth.

Hard copies of well logs are in standard API (AmericanPetroleum Institute) log format.

The overall log width is 8.25 in., with three tracks of 2.5 in. wide each.

A column 0.75 in. wide separates tracks 1 and 2 wherethe depths are indicated.



Track 1 is always linear, with ten division of 0.25 in. while tracks 2 and 3 may have a linear scale similar totrack 1, a 4-cycle logarithmic scale, or a combination of logarithmic scale in track 2 and linear scale in track 3.

For most well logs, the common vertical scales usedare 1:240 and 1:600 but for image logs(microresistivity) it is usually 1:20 and 1:40.

Every log is preceded by a header. It shows pertinent

information for proper interpretation of the log and inaddition, some details of the well and the log run



1. The title indicates the services that are associated with the data that appear on this log.

2. Basic well name and location information. 3. More detailed information about the physical

surface location of the well.

4. Other services that were run at the same time

(during the same trip to the well) as the services in thislog.



5. Information about location and elevation from which the well depths are measured. K.B. = Kelly bushing elevation,

D.F. = drill floor elevation,

G.L. = ground level elevation,

T.K.B. = top of Kelly bushing

6. Environmental information about the well. The drillingmud and borehole size values are especially important inapplying the proper environmental corrections andinterpretation parameters to the data.

7. General information about the logging equipment, theengineer, and any clients who witnessed the logging job.More detailed information about the specific logging toolsis listed in the pages that usually follow this one and intables that detail the calibration techniques and results



The Logging EnvironmentPressure

Formation pressure – the pressure under which the

subsurface formation fluids and gases are confined. Hydrostatic pressure – the pressure exerted by a

column of fluid. In the borehole, it is due to thecolumn of drilling mud and is:

Ph (psi) = 0.052 x height of f luid column (ft.) x density (ppg)

Overpressure – any pressure above the hydrostatic(or normal) pressure



Temperature

Geothermal gradientG=100(Tf-Ts)/D

Formation temperature

Tf = Ts + G(D/100)

G = geothermal gradient, °F/100 ft.

Tf = formation temperature, °F Ts = surface temperature (80°F)

D = depth of formation, ft.



Borehole Geometry

Obtained from caliper log.

Gauged hole – diameter of hole is about equal to the bitsize

Increased borehole diameter Washout – general drilling wear, especially in shaly zones and

dipping beds.

Keyseat – asymmetric oval holes, formed by wear against thedrill string at points where the borehole inclination changes(doglegs).

Breakout – similar to keyseat but not due to doglegs, smallbrittle fractures (spalling) due to existing stress regime of thecountry rock.



Decreased borehole diameter

Generally due to formation of mud cake

Mud cake formation indicates permeability.



BOREHOLE ENVIRONMENT

Where a hole is drilled into a formation, the rock plusthe fluids in it (the rock-fluid system) are altered inthe vicinity of the borehole.

The borehole and the rock surrounding it arecontaminated by the drilling mud, which affectslogging measurements.



Hole Diameter (dh)

The borehole size is determined by the outsidediameter of the drill bit.

But, the diameter of the borehole may be larger thanthe bit size because of washout and/or collapse of

shale and poorly cemented porous rocks. Or smaller than the bit size because of a build up of

mud cake on porous and permeable formations.

Common borehole sizes normally vary from 7 in. to 12

in., and modern logging tools are designed to operate within these size ranges.

The size of the borehole is measured by a caliper log



Drilling mud

Today, most wells are drilled with rotary bits and theuse of a special f luid, called drilling mud, as acirculating fluid.

The mud helps remove cuttings from the wellbore,lubricate and cool the drill bit during drilling.

Maintain an excess of borehole pressure overformation pressure.

The excess of borehole pressure over formationpressure prevents blowouts.

The density of the mud is usually kept high enough sothat hydrostatic pressure in the mud column is greaterthan formation pressure.



This pressure difference forces some of the drillingfluid to invade porous and permeable formations.

As invasion occurs, many of the solid particles (i.e.,clay minerals from the drilling mud) are trapped on

the side of the borehole and form mud cake (having aresistivity of Rmc.

Fluid that filters into the formation during invasion iscalled mud filtrate (with a resistivity of Rmf .

The resistivity values for drilling mud, mud cake, andmud filtrate are recorded on a log’s header and areused in interpretation.



Drilling mud Drilling fluids provide hydrostatic pressure to

prevent formation fluids from entering into the well bore.

Carrying out drill cuttings.

Brings traces of formation fluids to the surface Mud filtrate into formation

Build up of mud cake on borehole walls

The drilling fluid avoid formation damage and to limit

corrosion.



Invaded Zone

The zone in which much of the original fluid isreplaced by mud filtrate is called the invaded zone. Itconsists of a flushed zone (of resistivity Rxo) and atransition or annulus zone (of resistivity Ri).

The flushed zone occurs close to the borehole wherethe mud filtrate has almost completely flushed out aformation’s hydrocarbons and/or water (Rw).

The transition or annulus zone, where a formation’s

fluids and mud filtrate are mixed, occurs between theflushed zone and the uninvaded zone (of resistivity Rt).



The uninvaded zone is defined as the area beyond the

invaded zone where a formation’s fluids areuncontaminated by mud filtrate.

The depth of mud-filtrate invasion into the invadedzone is referred to as diameter of invasion (di and dj).

The solid particles in the drilling muds join togetherand form an impermeable mud cake.

The mud cake then acts as a barrier to furtherinvasion.



Flushed zone Resistivity (Rxo) The flushed zone extends only a few inches from the

wellbore and is part of the invaded zone.

If invasion is deep or moderate, most often the flushedzone is completely cleared of its formation water by mudfiltrate (of resistivity Rmf ).

When oil is present in the flushed zone, the degree of flushing by mud filtrate can be determined from thedifference between water saturations in the flushed (Sxo)zone and the uninvaded (Sw) zone.

Usually, about 70% to 95% of the oil is flushed out; theremaining oil is called residual oil

Sr o = (1.0 - Sxo), where Sro is the residual oil saturation, (ROS).



Uninvaded zone Resistivity (Rt)

The uninvaded zone is located beyond the invadedzone.

Pores in the uninvaded zone are uncontaminated by mud filtrate; instead, they are saturated with

formation water (Rw), oil, and/or gas. Even in hydrocarbon-bearing reservoirs, there is

always a layer of formation water on grain surfaces.

Water saturation (Sw) of the uninvaded zone is an

important factor in reservoir evaluation because, by using water saturation data, a geologist can determinea reservoir’s hydrocarbon saturation.



Sh = 1 – Sw

where: Sh = hydrocarbon saturation (i.e., the fraction of pore

volume filled with hydrocarbons).

Sw = water saturation of the uninvaded zone (i.e., thefraction of pore volume filled with water).

The ratio of the uninvaded zone’s water saturation

(Sw) to the flushed zone’s water saturation (Sxo) is anindex of hydrocarbon moveability.



Factors Influencing Logs Density of mud

The density of the drilling mud (mud weight) effects thesignal.

The denser the mud used, the greater the underestimation will be

Caving Problems may arise if the borehole diameter varies, leading to

varying amounts of drilling-fluid between the formation andthe sensor with depth.

In caved holes in radioactive formations there is usually noeffect observed as the caving effectively replaces a radioactiveformation with radioactive drilling f luid.

Invasion

methods and principles.pptx

Documents