Basic well Logging Analysis Hsieh, Bieng-Zih Fall 2009

Outlines Introduction Borehole Environment Invaded Zone, Flushed Zone, Uninvaded Zone Invasion and Resistivity Profiles Basic Information Needed in Log Interpretation Exercises (#1A, #1B, #2A, #2B)

Introduction Well log, Wireline Log, Geophysical well logging, Log A continuous measurement of formation properties with electrically powered instruments to infer properties and make decisions about drilling and production operations.

Introduction (Cont.) The record of the measurements, typically a long strip of paper, is also called a log.

Introduction (Cont.) In wireline measurements, the logging tool (or sonde) is lowered into the open wellbore on a multiple conductor, contra- helically (反螺旋) armored wireline. Once lowered to the bottom of the interval of interest, the measurements are taken on the way out of the wellbore.

Introduction (Cont.) This is done in an attempt to maintain tension on the cable (which stretches) as constant as possible for depth correlation purposes. (The exception to this practice is in certain hostile environments in which the tool electronics might not survive the temperatures on bottom for the amount of time it takes to lower the tool and then record measurements while pulling the tool up the hole. In this case, "down log" measurements might actually be conducted on the way into the well, and repeated on the way out if possible.)

Introduction (Cont.) Most wireline measurements are recorded continuously even though the sonde is moving. Measurements include electrical properties (resistivity and conductivity at various frequencies), sonic properties, active and passive nuclear measurements, dimensional measurements of the wellbore, formation fluid sampling, formation pressure measurement, wireline-conveyed sidewall coring tools, and others.

Introduction (Cont.) Certain fluid sampling and pressure-measuring tools require that the sonde be stopped, increasing the chance that the sonde or the cable might become stuck. Logging while drilling (LWD) tools take measurements in much the same way as wireline-logging tools, except that the measurements are taken by a self-contained tool near the bottom of the bottomhole assembly and are recorded downward (as the well is deepened) rather than upward from the bottom of the hole (as wireline logs are recorded).

Borehole Environment

Borehole Environment Where a hole is drilled into a formation, the rock plus the fluids in it (rock-fluid system) are altered in the vicinity of the borehole. A well’s borehole and the rock surrounding it are contaminated by the drilling mud, which affects logging measurements. Fig. 1 is a schematic illustration of a porous and permeable formation which is penetrated by a borehole filled with drilling mud.

Fig. 1 Exercise: You have 15 min. to fill in your answer

Fig. 1

The definition of symbols used in Fig. 1

Diameter dh – hole diameter di – diameter of invaded zone (inner boundary, flushed zone) dj – diameter of invaded zone (outer boundary, invaded zone) Δrj – radius of invaded zone (outer boundary)

Hole diameter A well’s borehole size is described by the outside diameter of the drill bit. But, the diameter of the borehole may be larger or smaller than the bit diameter because of (1) wash out and/or collapse of shall and poorly cemented porous rocks (2) build-up of mudcake on porous and permeable formation

Hole diameter (Cont.) Borehole sizes normally vary from 7 7/8 inches to 12 inches, and modern logging tools are designed to operate within these size ranges. The size of the borehole is measured by a CALIPER LOG.

Mud hmc – thickness of mudcake Rm – resistivity of the drilling mud Rmc – resistivity of the mudcake Rm – resistivity of mud filtrate

Drilling mud Today, most wells are drilled with rotary bits and use special mud as a circulating fluid. The mud helps remove cuttings from the well bore, lubricate (潤滑) and cool the drill bit, and maintain an excess of borehole pressure over formation pressure. The excess of borehole pressure over formation pressure prevents blow-outs.

Blow-out

Drilling mud (Cont.) The density of the mud is kept high enough so that hydrostatic pressure in the mud column is always greater than formation pressure. This pressure difference forces some of the drilling fluid 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 MUDCAKE.

Drilling mud (Cont.) Fluid that filters into the formation during invasion is called MUD FILTRATE. The resistivity values for drilling mud, mudcake, and mud filtrate are recorded on a LOG HEADER.

Log header

Resistivity Rw – resistivity of formation water Rs – resistivity of shale Rt – resistivity of uninvaded zone (true resistivity) Rxo – resistivity of flushed zone

Saturation Sw – water saturation of uninvaded zone Sxo – water saturation of flushed zone

Invaded zone

Invaded zone The zone which is invaded by mud filtrate is called the invaded zone. It consists of a flushed zone (Rxo) and a transition or annulus (Ri) zone. The flushed zone occurs close to the borehole where the mud filtrate has almost completely flushed out a formation’s hydrocarbon and/or water (Rw).

Invaded zone (Cont.) The transition or annulus zone, where a formation’s fluids and mud filtrate are mixed, occurs between the flushed (Rxo) zone and the uninvaded (Rt) zone. The depth of mud filtrate invasion into the invaded zone is referred to as the diameter of invasion (dj).

Invaded zone (Cont.) The diameter of invasion is measured in inches or expressed as a ratio: dj/dh where dh = borehole diameter

Question -- General invasion diameters are: dj/dh = 2 for ？ porosity rocks dj/dh = 5 for intermediate porosity rocks dj/dh = 10 for ？ porosity rocks High or Low porosity? And why?

Invaded zone (Cont.) The amount of invasion which takes place is dependent upon the permeability of the mudcake and not upon the porosity of the rock. In general, an equal volume of mud filtrate can invade low porosity and high porosity rocks if the drilling muds have equal amounts of solid particles. The solid particle in the drilling muds coalesce (結合) and form an impermeable mudcake.

Invaded zone (Cont.) The mudcake then acts as a barrier to further invasion. Because an equal volume of fluid can be invaded before an impermeable mudcake barrier forms, the diameter of invasion will be greatest in low porosity rocks. This occurs because low porosity rocks have less storage capacity or pore volume to fill with the invading fluid, and, as a result, pores throughout a greater volume of rock will be affected.

Invaded zone (Cont.) General invasion diameters are: dj/dh = 2 for high porosity rocks dj/dh = 5 for intermediate porosity rocks dj/dh = 10 for low porosity rocks

Flushed zone

Flushed zone The flushed zone extends only a few inches from the well bore and is part of the invaded zone. If invasion is deep, most often the flushed zone is completely cleared of its formation water (Rw) by mud filtrate (Rmf). When oil is present in the flushed zone, you can determine the degree of flushing by mud filtrate from the difference between water saturations in the flushed (Sxo) zone and the uninvaded (Sw) zone.

Flushed zone (Cont.) Usually, about 70 to 95% of the oil is flushed out. The remaining oil is called RESIDUAL OIL. Sro = 1.0 – Sxo where Sro = residual oil saturation (ROS)

Uninvaded zone

Uninvaded zone The uninvaded zone is located beyond the invaded zone. Pores in the uninvaded zone are uncontaminated by mud filtrate; instead, they are saturated with formation water (Rw), oil, 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.

Uninvaded zone (Cont.) By using water saturation (Sw) data, a geologist can determine a reservoir’s hydrocarbon saturation. Sh = 1.0 – Sw where Sh = hydrocarbon saturation (i.e., the fraction of pore volume filled with hydrocarbons) The ratio between the uninvaded zone’s water saturation (Sw) and the flushed zone’s water saturation (Sxo) is an index of HYDROCARBON MOVEABILITY.

Invasion and Resistivity Profiles

Invasion and Resistivity Profiles

Invasion and Resistivity Profiles

Transition Profile – Water Zone

Annulus Profile – Hydrocarbon Zone

Basic information needed in log interpretation

Basic information needed in log interpretation Lithology – from cutting Temperature of formation – Because the resistivities of the drilling mud (Rm), the mud filtrate (Rmf), and the formation water (Rw) vary with temperature. (Resistivities information can be read from LOG HEADER)

Log header

Formation Temperature Calculation Given: Surface temp. = 80 F Bottom hole temp. = 180 F Total depth (TD) = 10000 ft Formation depth = 6000 ft

Exercise # 1a Calculate Formation 1A temperature Given: Surface temp. = 60 F Formation 1A depth = 5500 ft

Exercise # 1B Calculate Formation 1B temperature Given: Surface temp. = 75 F Formation 1B depth = 7600 ft

Correct the resistivities to formation temperature Given: Rm = 1.2 at 75 F, Formation temp. = 160 F Rm=0.56 at 160F START HERE

Exercise # 2a Correct SIX resistivities (Rm, Rmf, and Rmc, in RUN-1 and RUN-2) to surface temperature Given: Surface temp. = 75 F Rm, Rmf, Rmc => from log header RUN-1 and RUN-2

Exercise # 2B Correct the resistivities (Rm, Rmf, Rmc) to Formation 1B temperature Given: Formation 1B temp. => From your answer of Ex. #1B Rm, Rmf, Rmc => From log header RUN-2

End of Chapter 1