WELLOG The Borehole

Part 1, Page 2

THE BOREHOLE ENVIRONMENT:

The borehole environment begins with the fluid within the borehole which is usually drilling mud but can be air or water. The resistivity of the borehole fluid is referred to as Rm.

As the borehole fluid is forced into the surrounding formation, a mud cake having resistivity (Rmc) and thickness (hmc) is formed on the wall of the borehole.

Fluid from the borehole that enters into the immediate surrounding rock formation, and which flushes that part of the formation has resistivity (Rxo) and also is called mud filtrate (Rmf) resulting in saturation of the flushed zone (Sxo).

Continuing outward from the borehole, the invaded zone having resistivity (Ri) and (Rz) is saturated with water and is defined as (Swi).

Beyond the invaded zone is a zone that is not invaded by borehole fluid. This zone is called the uninvaded zone. The uninvaded zone contains fluid (water) having resistivity (Rw) and total resistivity (Rt) with water saturation (Sw).

HYDROSTATIC PRESSURE:

Hydrostatic pressure is a measure of the pressure at any given point in a well. The pressure is a function of the weight of the fluid in the well and the depth. Water has a weight of approximately .4 lbs. PSI per foot.

A water filled well would have a pressure of 400 PSI at 1000 feet. Drilling fluid may have heavy chemicals added to increase the hydrostatic pressure. If the hydrostatic pressure is greater than the pressure exerted by the surrounding formations, then positive hydrostatic pressure occurs. When a well is drilled using rotary drilling methods, it is customary to maintain the weight of the drilling fluid at a weight that gives positive hydrostatic pressure.

The advantage of positive hydrostatic pressure is that the formation fluids do not escape. It is also an advantage that the pressure is greater in the well that fluid flows from the well into the surrounding formation. As fluid flows into a formation, a mud cake is formed on the sides of the well.

WATER LOSS OF MUD:

A well having positive hydrostatic pressure will have water loss as water within the well invades into the surrounding formations. The higher the water loss, the deeper the invasion will be.

DIFFERENTIAL PRESSURE:

Invasion is a function of Differential Pressure. As the Differential Pressure increases, the amount of invasion also increases. Drilling fluids are designed to minimize water loss through the process of creating a mud cake that limits invasion of fluids into the formation.

TIME:

The length of time that a formation is exposed to the forces of a mud column in a well also affects the amount of invasion. It is important to know that the longer a formation is exposed to invasion, the deeper the invasion.

POROSITY:

Under given conditions, a formation having greater porosity will invade LESS deeply than a formation having lower porosity.

PERMEABILITY:

Normally, the permeability of the mud cake is low, and it controls the amount of invasion. When a formation is highly permeable, it may have greater control over the amount of invasion than the mud cake. It is interesting to note in a given well, the differences in filtrate invasion and resulting thickness of mud cake from one formation to another.

BOREHOLE TEMPERATURE:

Borehole temperature increases from a surface average temperature (Tsurf) to a maximum borehole temperature (TTD) assumed to be at the termination depth or TD. This differential is usually measured in degrees per 1000 feet and is referred to as geothermal temperature gradient.

Often times it is necessary to calculate the temperature at an intermediate depth (Tf) in the well.

If the average surface temperature is 70 degrees F and the Temperature at TD of 10000 feet is 170 degrees,

What is the temperature at 3500 feet?

The temperature differential is 100 degrees over 10000 feet or 10 degrees F per 1000 feet.

Tf = (TTD-Tsurf)/TD * Fd + TSurf)

Tf = (170 -70 )/10000 * 3500 + 70 = 105 degrees at 3500 feet.

Temperature versus depth is often obtained from a chart.

SYMBOLS:

A few symbols used in Well Drilling, Well logging, and Well Log Analysis: (this is not a complete list)

d = diameter of the hole f = Porosity

di = diameter of invasion r = density

h = thickness of a formation bed Dt = Delta T

hmc = thickness of the mud cake

Ra = Apparent Resistivity

Ri = Resistivity of the invaded zone

Rm = Resistivity of the mud

Rmc = Resistivity of the mud cake

Rmf = Resistivity of the mud filtrate

Rt = True Formation Resistivity

Rw = Resistivity of the Water

Rwa = Apparent Water Resistivity

Rwe = Equivalent Water Resistivity

Rxo = Resistivity of the flushed zone

Sw = Water Saturation

Sxo = Water saturation of the flushed zone

F = Formation factor

m = Cementation factor

n = Saturation exponent

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