Understanding the logging environment is crucial to log analysis when drilling horizontally.

Rotary steerable systems are revolutionizing the drilling of horizontal and extended-reach wellbores. Typically, advanced directional drilling has been performed with steerable mud-motor systems. However, drilling with a steerable mud motor results in a rough and tortuous wellbore due to the motor's geometry and operational behavior. A rough wellbore may affect the performance of various logging sensors deployed with the system. Different logging sensors are affected differently, and so the ability to compensate for borehole effects varies from sensor to sensor. The result may be a log that suffers in quality.
Rotary steerable systems drill smoother and less tortuous wellbores. As a consequence, typical borehole effects visible on various formation evaluation (FE) logs may not be apparent when drilling with rotary steerable systems. Knowledge of the logging environment in which the data were obtained is important when analyzing the log.
Borehole geometry effects
Several nongeological borehole effects may be generated in the course of the drilling process. These nongeological effects may not always be easily detected and separable from the true geological effects. As directional drilling and FE measurement-while-drilling (FE-MWD) technology develops, it is anticipated that more wells will be logged with FE-MWD systems during the drilling process. This becomes especially true as more wells are drilled at high inclinations and with horizontal profiles.
The nongeological borehole effects can, in general, be separated into two areas. First are those that are a result of the physical dimensions and properties of the borehole generated during the drilling process. These can occur during tripping, drilling, washing, reaming or other activities in the hole. These observations can best be described as defining the mechanical properties of the borehole.
Second, the near-borehole environment, such as the fluid-invaded zone, must be considered. This may be affected by the time taken during the drilling process and the fluid properties used. Fluid invasion also may generate nongeological effects that would likely affect the output from the logging sensors, and may be looked upon as nongeological formation effects beyond the hole wall.
Measured depth control
The basic technique of depth measurement when logging with FE-MWD tools is to measure and add the length of each section of drill pipe as it is run in hole, and then measure the position of the top drive or other drilling machinery at the drill floor to determine the position of the drill bit. It is possible to correct some of the depth variables, such as temperature and pressure effects on drill pipe length, for example. However, the most significant error source may be related to the varying degree of pipe compression seen at the bottom of the hole as a result of drillstring friction.
Variable and cyclic hole size
With a steerable motor system in the rotary mode, a variable hole size is often generated. The bit is off-center with respect to the axis of the motor due to the bent housing on the motor. Rotation inevitably will result in a hole that is larger than the bit. As soon as the near-bit stabilizer enters this overgauge hole section, the bottomhole assembly (BHA) loses support, and side-forces develop on the bit and the top stabilizer behind the motor. Subsequently, an in-gauge hole then will be drilled with support on the bit and the top stabilizer, while the near-bit stabilizer rotates in the overgauge hole. As soon as the near-bit stabilizer enters the gauge section, it provides increased support, and the bit will recommence this whirling motion and drill an overgauge hole again.
Spiral hole
In addition to creating a variable, cyclic-shaped hole, a steerable motor may generate a spiral hole, again as a result of varying side force on the bit and near-bit stabilizer during rotation. This is typical and nearly always present, depending on the size of the bend in the motor, bit profile, distance between load points and formation characteristics. Another factor that determines the extent to which a spiral hole is generated is the bit's gauge length. A short gauge length, which is the length of the section of the bit that has a full diameter, will result in aggressive side-cutting and little stabilization. Consequently, this will increase the tendency to generate a spiral hole. This effect is more pronounced at high side forces. On the other hand, a bit with a long gauge will be more stable and drill a straighter hole.
When logging in a spiral hole, the density sensor, which includes a stabilizer, will follow the hole and generally keep the distance to the formation. The resistivity sensor, which is destabilized, will alternate between a centered and an off-centered position in the hole. Its position is determined by the action of the stabilizers above and below.
Washed-out hole
A washed-out hole may occur in many places along the well path as a result of hole condition, formation characteristics, flow rate, drill bit profile or nozzle size. In this context, it is worth considering this problem as a result of being stationary in the hole and circulating. This happens, for instance, when taking a directional survey or when steering is to be initiated with a steerable motor. The string must be oriented to the correct tool face whenever sliding is to be performed. This is not always easy, especially in a horizontal hole, and may involve pumping up numerous tool face values from the MWD tool and rotating the drillstring in order to line up the motor, potentially causing washout of the hole at the bit and around stabilizers.
Ledging in the hole
Ledges in the wellbore also may result in washed-out holes. Ledges typically are seen in association with steerable motor systems when changing from rotary to steering mode, or whenever formation changes are encountered that inhibit drilling from proceeding smoothly, as when hitting a hard stringer. As drilling proceeds past the ledge, stabilizers in the BHA may hang up on the ledges. This may cause further washing out at the bit since it is stationary while attempts are made to push the stabilizer past the ledge. Often, the end result is that drilling has to resume in rotary mode until the ledge is passed, and a new procedure of lining up the string to start steering again must be initiated. Although ledges may
not appear on the log other than as an enlarged hole, they may become filled with cuttings, and they tend to cause further washouts and an increase in drilling-associated problems.
Cuttings beds buildup
Poor hole cleaning and cuttings buildup often are associated with ledges. Also, when drilling in sliding mode, hole cleaning may be insufficient. Apart from drilling-related problems, this also may affect FE-MWD sensors, as cuttings may build up around the sensors, for instance, around the density stabilizer.
Hole rugosity
The creation of a rugose borehole generally is a result of the hydrostatic pressures within the borehole that inhibit proper stabilization of the formation. This negative effect may be further accelerated if equivalent circulating density (ECD) fluctuations occur and the borehole wall becomes brittle and unstable. The latter is mostly a result of hole-cleaning limitations associated with steerable motor systems.
On the other hand, when drilling with rotary steerable systems, the mud weight and ECD can be kept more stable and closer to the formation pressure. If the ECD is too low, the formation may break out and fall into the hole. A pitted borehole also may be observed on some shallow-reading FE-MWD logs. However, in most cases the formation pressure is well balanced, and a smooth hole will be generated with a rotary steerable system. It has been observed from performing wireline formation pressure measurements that a significantly higher success rate is obtained in holes drilled with rotary steerable systems.
Elliptical hole
When drilling a horizontal section, the rotation of the drillstring against the low side of the hole can result in abnormal hole wear and enlargement. The erosion effect from the drillstring will depend on the number of revolutions, and the effect will be more pronounced in the upper part of the hole section. The scraping of the stabilizer blades against the low side of the hole also will add to this effect.
Invasion
Invasion of mud filtrate must happen to a certain extent to build a mud cake. Further invasion will take place if the mud cake is removed. This could be the case during a reaming operation, as the stabilizer blades tend to scrape off the mud cake. This being the case, less reaming should lead to reduced invasion.
The risks associated with fluid invasion are more easily managed if the system is run in conjunction with some form of real-time annular pressure sensor. A real-time annular pressure sensor can be used to optimize the ECD regime and monitor the hole-cleaning process more closely.
Another benefit with rotary steerable systems is that FE-MWD measurements generally are taken much sooner after drilling virgin formation. This is due to near-bit sensors and a more efficient drilling process. Some integrated drilling and evaluation systems position the resistivity, gamma, neutron porosity and density sensors within 53ft (16m) of the drill-bit cutting face. On some other systems that are not fully integrated (some point-the-bit rotary systems and traditional steerable motor systems), the density and other FE-MWD sensors may be as far as 95ft or 100ft (25m or 30m) behind the bit.
In summary, measured depth control is inaccurate when drilling with a steerable motor system in sliding mode, as an unknown amount of static friction between the drillstring and borehole wall have to be overcome. Changing between sliding and rotary modes will have a negative impact on log quality. These problems are avoided by using a rotary steerable system.
The smoother borehole that is created with a rotary steerable system has a positive impact on the quality of the FE-MWD logs, especially the nuclear measurements, density and neutron porosity.