BIBLIOGRAFÍA

Various definitions exist for extended-reach drilling (ERD)-class wells. One common definition is a departure of twice or more of the TVD of the well, (a reach-to-TVD ratio of two or more). This definition separates conventional directional wells from wells that could require ERD technology. However, since state-of-the-art ERD can involve wells with reach-to-TVD ratios of four to five or even higher, it should be recognized that distinct classes of ERD wells do exist, as shown in Figure 2-30.

Figure 2-30 ERD classifications and currently achievable envelope

These definitions are listed as follows:

• Reach-to-TVD Ratio < 2 Conventional Directional Drilling (Non-ERD)

• Reach-to-TVD Ratio 2 to 3 Extended-Reach Drilling

• Reach-to-TVD Ratio >3 Severe Extended-Reach Drilling These classifications are somewhat arbitrary, and drilling a well with a reach-to-TVD less than two can be quite challenging, depending on the formations involved and the rig available. Engineers must understand the technical interactions between the rig and equipment and the well being attempted so that constraints and risks can be identified and resolutions sought as early as possible. Various technologies must be applied for the successful planning and execution of an ERD well. These technologies include

• Optimized directional trajectory design

• Refined mechanical wellbore stability estimates for mud-weight windows

• Properly engineered drilling fluids for optimization of chemical wellbore stability, cased and openhole lubricity, and cuttings transport and suspension

• Calibrated torque-and-drag models to allow projection of field loads and diagnosis of deviations from predictions during field execution

• Sizing of rig components to provide adequate rotary, hydraulic, hoisting, solids control, and power capabilities

• Properly designed drillstring that minimizes drillpipe pressure loss (allows higher flow rates for given surface pressure limitations)

• Other technologies such as (1) soft-torque top drives, (2) special running systems and techniques for casings, liners, and completion strings, and (3) rotary-steerable directional-drilling systems

The unique issues associated with ERD wells largely stem from the high inclination of the well that is required to reach the objective departures.

Tangent "sail" angles in some ERD wells have been 80° or higher. "Average"

angles from surface to TD have been 70° or higher. Some "ERD" wells have in fact been drilled as extremely long horizontal wells where the bulk of the well has been drilled horizontally (defined as an inclination of 88° or more).

At such high inclinations, the transport of cuttings from the well is more difficult than in vertical wells. As a result, higher flow rates, tighter control of drilling-fluid rheology, and the use of nonconventional mechanical means to assist hole cleaning must be used. Such mechanical means might include high drillstring rotation (off-bottom) or the use of special, bladed drillpipe to stir cuttings beds mechanically. High rotary speeds and backreaming help clean the hole but can increase drilling shocks and cause fatigue or backoff of motor housings. Therefore, such practices should be viewed as secondary hole-cleaning methods and used only if primary hole cleaning is inadequate.

A common method of removing cuttings beds is the use of low-viscosity sweeps that scour the cuttings off the low side of the hole; high-viscosity

sweeps are then made that carry the dislodged cuttings to the surface. The fundamental driver of hole cleaning, however, is flow rate, and high flow rates are strongly recommended throughout high-inclination sections if ERD allows. Flow rates of 1000 to 1100 gal/min in a 12 ¼-in. hole and 500 to 600 gal/min in an 8 ½-in. hole are not uncommon. For this reason, the rig's pump and piping capacities with regard to flow rate, pressure, and the drillstring design are critical.

Other implications of high inclination include the greater likelihood of mechanical wellbore instability and hence the need for careful mud-weight planning. Since more formation is exposed for longer periods, chemical stability also becomes more critical.

Aside from hydraulics and formation stability, another potential ERD constraint is the ability to sustain drilling torque and run tubulars in the well.

Both of these processes are impacted by optimizing the well trajectory and field control of variations from this directional plan. In terms of design optimization, the ERD trajectory will be subject to similar geologic constraints as discussed in Section 2-2.2. Additionally, the trajectory should be designed to minimize the induced torque during drilling and to maximize the available weight while casing is run. Satisfaction of these objectives varies depending on (1) the TVD and departure of the target and (2) the frictional behaviors of the various hole sections and the location of those sections. One observation from several studies (Banks et al., 1992; Dawson and Paslay, 1984), however, is that multiple build rates should be used to initiate inclination into the well gradually. Increasing the build rates in several steps minimizes near-surface doglegs and the associated torque and drag.

The final tangent angle and shape of the trajectory through the reservoir can be determined on the basis of several considerations. If the well design is close to the limit of wireline intervention capabilities, (65 to 70°), it may be best to keep the KOP high and limit the inclination so that wireline operations are feasible. If the well angle is more severe and the inclination will already exceed wireline operating limits, it may be appropriate to slightly deepen the KOP to increase formation stability and allow a more gradual build section. In such cases, although a higher final inclination angle will result, the higher inclination may in fact be optimal since it inhibits buckling of the drillstring and coiled tubing, which could be critical controlling factors in severe ERD wells.

In addition to an optimized trajectory plan suited for the specific well application, directional drilling must be performed well in the field to minimize unexpected doglegs and variations from plan. Such doglegs will increase drilling torque, make tubular running more difficult, and provide potential trouble spots for excessive casing wear, which can lead to failures.

These serious detriments have been recognized previously (Sheppard et al., 1987; Mueller et al., 1991), and a specific measure is available for defining how closely the well is or has been drilled according to plan. This measure is known as tortuosity, which is the sum of all the increments of curvature along the section of interest, subtracted from the planned curvature, and divided by the footage drilled. In the well-planning phase, specific tortuosity

guidelines should be generated for each section of the well, based on torque-and-drag modeling. Tortuousity limitations should generally be the most stringent in the upper portion of the well and in the build section, where high drillstring tension causes high side loading, which results in torque and drag, keyseating, and worn tool joints and casing. Reducing tortuosity requires an integrated engineering approach as described:

• The rotary drilling directional tendency of BHAs should be tracked, quantified, and refined to yield rotary behaviors that are as close as possible to planned build rates. This process minimizes for oriented drilling as a means of inclination control.

• When steering is required, it should be executed in a controlled manner. For example, doglegs can be minimized by sliding for only part of a joint, or a single joint or stand; then, rotary drilling can be used for the remaining footage. This process can then be repeated as required to offset the build, drop, or turn behavior of the rotary BHA that is causing the problem.

• Sliding repeatedly in short intervals is greatly preferred over executing one continuous, long, sliding section, which will cause a significant and sharp dogleg.

Systematic analysis and planning of BHAs and careful field execution will allow good control of well tortuosity and will ensure that predrill torque-and-drag projections are not exceeded. Projecting requirements for ERD wells, properly sizing the drilling rig and equipment, and optimizing the trajectory are all subject to detailed, site-specific engineering. The key to success is to recognize those technologies that are critical and those tradeoffs that must be examined to engineer the best possible approach for a given well.