Borehole Cross Sections
Borehole cross sections are measured to assure that logging measurements are valid, to correct logging measurements
calibrations for downhole
conditions, and to compute hole volumes for cement design.
Borehole size or gauge has been measured with caliper logs for many years. The caliper logs used on different tools respond differently in the same non-cylindrical borehole.
Borehole cross sections are often described as circles and ellipses because only these shapes can be defined from the one or two dimensions usually available from one logging run. Studies of multi-arm calipers indicate that borehole
elongation is preferentially in one direction while the section at right angles tends to stay in gauge. The borehole also tends to be more rugose in the direction of maximum elongation.
Standard Caliper Log Configurations
1. One arm calipers also serves as an eccentering device.
o Tend to seek the longest dimension of the borehole cross section, especially if the long axis is in a vertical plane.
o If the contact with borehole is steel it is considered to cut through mudcakes. If the contact is rubber, it reads borehole minus one mudcake thickness.
2. Two arm calipers, extend equidistant from a centralized tool body.
o Tend to record the long axis of out-of-round holes.
o All borehole contacts are rubber and measurement is considered as borehole minus two mudcake thickness.
3. Three arm calipers, center the tool body.
o Maintain their arms equidistant from the body of the tool and measure only one diameter,somewhere between the minimum and maximum of the noncircular section.
4. Four arm calipers, consisting of two calipers at right angles to each other.
o Four-arm calipers typically use two pairs of arms that extend independently of each other. One pair seeks the long dimension of an out of round hole, the other measures the dimension at right angles.
5. Six-arm devices, which use six independent arms, spaced at 60o angles, allowing the characterization of irregular shaped boreholes.
o Six-arm calipers have each arm independent, allowing the arms to characterize the hole shape regardless of the relative position of the tool body. An advantage to this design is that significant pressure is not required to make a measurement, thereby reducing tool drag and irregular tool motion.
Tool Contact
In addition to the number of arms, the nature of the tool contact also affects the caliper response when a hole is not cylindrical or has mudcake. Devices that have small contact area can detect smaller borehole irregularities. Contact pressure is usually high enough to cut through any mudcake (steel pads). Pad type devices have somewhat larger pad contact area and when operated at lower contact pressures will override mudcake (rubber pads).
Changes in hole shape may not be sensed if the borehole irregularities are changing rapidly and are smaller than the pad dimensions, depending on how the tool contacts the borehole wall.
Invasion
Drilling muds are typically designed so the hydrostatic pressure of the mud column exceeds formation pressure. This pressure overbalance causes mud to
enter permeable formations while at the same time depositing solid particles from the mud system on the borehole wall, forming a filter cake (hmc). The time
required to build up sufficient mudcake is a function of specific formation properties and drilling fluid properties, especially solid particles within the mud system. Formation of the filter cake prevents further filtrate invasion and formation damage while maintaining wellbore stability.
In most mud systems, invasion is expected. These invading mud particles alter formation composition, and invading mud filtrate alters formation salinity and saturation. As a result of this invasion, some logging measurements reflect drilling altered properties rather than true formation properties. Separating the part of the logging
response that comes from the invasion altered
region from the part derived from unaltered formation is a major task in well log interpretation.
The control of the mud surge and particle migration is primarily dependent on two things:
1. Maintaining a good size distribution of solid particles in the mud
2. Keeping the drilling fluid-formation pressure overbalance as low as possible.
The porosity of a formation needs to be considered in predicting invasion depth.
Given the same filtrate losses into equally thick intervals:
Invasion will be deeper in the formation with a lower porosity; high filtration and low porosity cause "deep invasion".
Low filtration and high porosity cause shallow invasion.
For most realistic conditions, invasion cannot be eliminated, only slowed. So, prospective intervals should be evaluated as soon as possible.
The depth of investigation of a logging tool determines how much the measurement is affected by invasion. Evaluation of water saturation from electrical properties requires an accurate determination of uninvaded formation resistivity or conductivity. Ideally, a deep sensing resistivity (or conductivity) log (RLD) is designed to respond to unaltered formation resistivity (Rt) without being influenced by any of the following:
Mud column (Rm)
Mudcake (Rmc)
Mud impregnated zone (Rim)
Flushed zone (Rxo); immediately adjacent to the borehole wall and essentially contains only mud filtrate (Rmf) and the deep resistivity log (RLD) is responding
Hydrocarbon saturation will be underestimated when Rxo < Rt
Hydrocarbon saturation may be underestimated if RLD is significantly affected by a low resistivity annulus.
Some formations may be so deeply invaded that saturation evaluation is not possible
Corrections for invasion and determination of depth of invasion require an accurate flushed zone resistivity for even the simplest cases. For more complex and deep alterations, additional measurements with intermediate depths of investigation are required.
Key Points
The pressure overbalance in the borehole causes mud and mud filtrate to "invade" the borehole wall.
Mud cake slows fluid and solid invasion into the formation; some muds contain material which affects log readings.
Mudcake is formed from the solids in the drilling mud.
Ideally mudcake should form quickly and have low permeability to reduce invasion.
Deeper invasion occurs in lower porosity.
Prospective intervals should be evaluated as soon as possible after drilling.
The depth of investigation of a logging tool determines how much the measurement is affected by invasion.
Spontaneous Potential The Spontaneous Potential, commonly abbreviated SP, is a measurement of the naturally occurring electrical potentials in the wellbore as a function of depth. It is one of the oldest logging
measurements and in today's environment one of the most under utilized measurements. It is sensitive to grain size, permeability and fluid content. SP is somewhat less quantitative than other
measurements, however if used carefully it can provide a wealth of information.
Basic Measurement Principles
The recording of the SP is the measured potential difference between a single passive moving electrode in the wellbore and a reference electrode, usually located at the surface in the mud pit, or attached to the casing head, or in sea water. There are three possible sources of the electrical potential which contribute to the SP; they are:
1. The electrochemical, Ec potential ,made up of the.membrane and liquid junction potentials
2. The electrokinetic, Ek. potential. (sometimes called streaming potential)
The sum of these different potentials results in a measurement that is not absolute but relative. The potential sensed by the SP electrode is the voltage drop across the mud in the borehole and is typically reported in mv. Since the SP requires a current path in the mud it will not function in an oil based mud. There also be little or no signal if there is no potential difference between the borehole and the formation i.e. where Rmf=Rw.
The maximum normally encountered SP is called the static SP (SSP). The SSP is the amount of deflection observed when the SP electrode passes from a position inside a very thick, porous, permeable, clean water sand to a point well within a thick uniform shale. The SSP is the value of the SP that is predicted by the following equation: SP = -Klog (aw/amf) ; where:
aw = the activity of the formation water amf = is the activity of the mud filtrate K = constant
Several factors can contribute to less than maximum deflection
1. Insufficient bed thickness causes the effective resistance of the sand to increase because of the corresponding reduction in the cross sectional area of the sand.
2. Increased borehole diameter, the effective resistance of the mud decreases because of the increase of the cross sectional area of the borehole.
3. Deep invasion the interface between the liquid junction and the membrane junction is moved deeper into the formation; which increases the effective resistance of the sand because of the increased path length to the borehole.
4. Presence of hydrocarbons increases the effective resistance of the sand because oil and/or gas have a much higher resistivity than water resulting in a greater drop of potential across the sand, resulting in a suppression of the SP deflection
5. Presence of clay restricts the migration of Cl- ions and assists the migration of Na+ ions due to the predominant negative charge of the clay
6. Significantly reduced porosity and permeability
The shape of the SP curve approaching or leaving the sand/shale boundary is controlled by the relative resistivities of the mud, sand, and shale, an inflection point is observed at the bed boundary interface. This inflection point may be shifted to closure to one formation or another depending on relative resistivities but the inflection point represents the bed boundary.
Applications
differentiate permeable from non-permeable formations
determine bed boundaries and bed thickness
determine formation water resistivity, Rw
can be used to calculate Rw in wet zones
estimate the volume of shale, Vsh
Borehole and Quality Considerations
1. SP's are very sensitive to extraneous electrical fields which can be caused by welding or other rig electrical equipment, residual magnetism from the cable drum, or atmospheric electrical charges.
2. Unresponsive SP's can be caused by poor grounding of the surface electrode
3. Streaming potentials can caused by under or overbalanced mud columns with differential pressure into or out of the formation.
4. The SP is a relative measurement and drifts with salinity and temperature changes, practice in older logs was for the field engineer to manually bring the SP back on scale.
These scale changes are generally obvious but may confuse interpretation.
5. Hydrocarbon causes suppression of the SP signal
6. Thin beds affect SP development how much depends on the resistivity of the formation and the contrast between Rw and Rmf
7. SPs are often base adjusted to remove shifts and drift this needs to be done carefully so as not to introduce anomalous readings
Key Points
1. Variations in SP are the result of the electric potential between the wellbore and the formation as result of the difference is the Rmf and Rw
2. In most wellbore environments, where salinity of the formation water is greater than the salinity of the mud or mud filtrate(Rw<Rmf). The result of this relationship is that the
expected SP development opposite relatively high salinity formations is negative. The deflection will be positive if Rw>Rmf.
3. The SP requires a conductive fluid in the borehole, therefore cannot the SP can not be run in non-conductive mud systems or air or gas drilled wells.
4. The SP response of shales is relatively constant and follows a straight line, known as the shale baseline. SP deflection is measured from the shale baseline.
5. If Rmf Rw the SP will not deflect from the shale baseline.