The Validus system was designed not only to outperform today's rotary steerable systems in extended-reach applications, but also to replace much of the conventional downhole motor market.

Validus International has completed the design and laboratory testing of a stand-alone rotary steerable system and plans to field test a 63/4-in. prototype in 2003. This tool will produce 0 to 15°/100 ft curvature rates in 81/2-in. holes with bit weights up to 50,000 lb.

The Validus system records near-bit directional surveys and transmits them to the surface using two revolutionary technologies. The downhole directional tool uses a patented Nonrotating Adjustable Stabilizer (NAS) design that provides precise control of curvature rate. The directional drilling operations are controlled automatically by a downhole computer code called the Directional Solution (DS), which computes the position of the borehole after each survey and determines the optimum three-dimensional circular arc trajectory needed to hit directional or horizontal targets. Each subsequent trajectory plan and the expected targeting precision are displayed at the surface immediately after each survey. The combination of these two technologies allows the driller to optimize the performance of the drill bit while the NAS and DS optimize the directional operations.

The Validus system can be run from the kickoff point or even above that depth to the total depth of the well. Utilizing a single bottomhole assembly for all directional operations eliminates the trips required to modify the directional performance of conventional assemblies. This, combined with the opportunity to select the optimum bit and use the most effective weight, rotary speed and hydraulics, should make the Validus system the industry's most efficient drilling system.

The freedom to use best drilling practices will significantly improve directional and horizontal drilling performance in medium- and hard-rock applications. The optimized trajectory control provided by the DS will deliver smoother boreholes than conventional operations by eliminating the need to redirect the path back to the original plan. The automated control will provide such an extraordinary improvement in steering precision it will allow operators to target optimum positions in reservoirs, thereby increasing production rates and ultimate recoveries.

The directional system

The NAS/DS Autoguide System consists of a nonrotating stabilizer unit located immediately above the bit, a non-magnetic flex joint, a communications link, a conventional stabilizer, and optional logging while drilling (LWD) or measurement while drilling (MWD) if needed for formation evaluation measurements (Figure 1). The nonrotating stabilizer unit includes both the fully adjustable stabilizer located near the bit and a fixed undergauge stabilizer located at the upper end of the unit. The adjustable stabilizer houses five individually adjustable blades automatically positioned to provide precise control over the full range of useful curvature rates.

The NAS control system positions the adjustable blades on the low side of the hole to enable full-gauge contact while providing free sliding clearances for the blades located on the top portion of the hole. Utilizing five blades provides stable support of the tool regardless of the orientation of the stabilizer.
The design allows the use of well established three-point geometry solutions to obtain the desired curvature rates. As shown in Figure 2, the well bore curvature is defined by the contact between the gauge surface of the bit and the hole and the contact points between the hole and the two stabilizers located immediately above the bit. The misalignment of the three contact points describes a circular arc that closely matches the drilling performance of the assembly. The NAS utilizes a screw jack driven by a servo motor on an inclined ramp to position each individual blade. By tracking the revolutions of each motor, the radial position of each blade can be controlled within .0001 in.

The DS software includes a routine that significantly improves targeting precision. This capability utilizes the difference between the planned and actual trajectories to compute adjustment factors to offset the observed differences. The goal of this process is to correct for the effect on curvature rate performance due to anomalies like gauge wear, manufacturing tolerances and formation effects.

The DS and the directional instrument package are housed in the NAS unit. The DS will use the target specifications, directional constraints and anticipated survey depths that will have been loaded into the NAS software at the surface. Following each survey, the DS will calculate the coordinates of the survey position, the estimated position of the bit, the optimum circular arc trajectory required to reach the horizontal or directional targets and the optimum trajectory required to drill the next joint.

The NAS unit also includes a computer module that calculates the adjustable stabilizer blade positions and uses the directional sensors to provide the data for determining the orientation of the five adjustable blades. The NAS software computes the required position for each blade based on the directional requirements with the orientation of each blade is determined from the accelerometer or magnetometer data.

Communications link

The NAS communicates to the surface through the communications link (CL) using short-hop communications technology. The NAS-DS unit sends the survey measurements, selected DS parameters and NAS operational data to the CL after each survey and DS calculation step. The CL sends this information to the surface using mud pulse technology immediately after each survey. Potential interference between the CL and MWD pulsers is prevented by simple programming that separates time intervals between the two.

The surface system additionally includes a pressure-based down-link data transmission system that communicates with the CL unit. The CL sends the down-linked data received from the surface to the NAS using short-hop transmission technology. The down-link communication process allows changes to target specifications or directional parameters whenever needed.

Laboratory model

A lab model has been constructed to prove the stabilizer adjustment technology and demonstrate its targeting precision. The lab model uses a full-scale design of the adjustable stabilizer unit for an 81/2-in. hole. The adjustable stabilizer was short-coupled to an eccentric pivot point that represents the fixed stabilizer contact. The target resolution was evaluated by observing a laser pointer aligned with the center of the stabilizer unit that illuminated a target placed about 4 ft from the end of the tool. The model readily demonstrated the efficiency of the motor control unit and its ability to track the motor revolutions used to control blade placement, as well as the precision in positioning the laser pointer.

The Directional Solution

The DS uses a novel solution technique (patent pending) that permits direct calculation of the optimum 3-D circular arc trajectory required to hit directional or horizontal targets. In most applications the trajectory required to hit a directional target requires only a single circular arc segment followed by a straight tangent interval. In more complex applications the trajectory required to hit deeper multiple targets will require that the kickoff point of the deeper circular arcs be moved above the depth of the shallower targets. The DS uses a process that optimizes the trajectory for up to three subsequent directional targets. The final path is further optimized to produce a trajectory that minimizes the torque required to rotate the drill string at total depth.

The trajectory calculations for horizontal targets are actually simpler than those needed for directional targets. In most applications the trajectory needed to parallel a dipping plane in space requires only a singular circular arc. In all other cases the appropriate trajectory consists of two circular arc segments.
The DS also includes a comprehensive curvature rate error tracking and error correction procedure. The process begins with a detailed comparison of the calculated curvature rates between surveys with the optimum trajectories computed in the DS. After each survey the curvature rate errors are calculated. Next, the adjustment factors are calculated, based on a weighted running average of the errors, and those are used for blade positioning instructions in the NAS tool. The running average calculations are designed to minimize the influence of the random errors included in the survey measurements and provide a reasonably rapid response to a systematic change in tool performance.

Directional drilling error simulation

The trajectory control and error correction performance of the DS has been effectively tested using a specially developed directional drilling error simulator. The simulator essentially drills the well on a joint-by-joint basis, accessing the DS to calculate the required trajectory and its error correction routine, which then corrects the performance of the NAS tool. The simulator allows the input of a realistic curvature rate performance error for the NAS tool.

Extensive testing indicates that the DS will provide extraordinary targeting precision. The combination of the trajectory planning technology and an error correction technology is far superior to conventional directional drilling operations. Simulator modeling efforts indicate that in normal applications where the tool has the curvature rate capability of reaching the target, the NAS and DS will routinely land within a few feet of the desired targets.

Multi-target directional example

Included below are the results of an example calculation for a directional well with four targets located at vertical depths ranging from 4,000 ft to 7,000 ft. Table 1 summarizes the input data for this directional simulation. Targets are defined by their true vertical depth, the north and east coordinates, the radius of the target, the target preference specification, and optional fixed inclination and azimuth entry angles. The tie-in survey data comprise the deepest existing surveys above the kickoff point. Table 1 also lists the curvature limits that are used by the DS to compute trajectories. The table specifies the desired design build rate as well as the maximum allowable build rate as a function of true vertical depth.

The initial plan that is generated at the kickoff point indicates that the first target requires two build intervals separated by a straight tangent section. The second target requires a short build interval followed by a long straight tangent and the third target requires a single dropping trajectory. The second trajectory plan included in this table is the plan that would be produced after having drilled through the first target. The incredible efficiency of the targeting and error correction routines is evidenced by a comparison of the target entry position for the second target based on the initial trajectory and then after drilling to the first target. The measured depth of the intersection has only changed by 2.6 ft, the entry angle by 1.7 degrees, and the azimuth by 0.5 degrees. Table 3 directly compares the four target specifications with the actual entry coordinates. Incredibly, the targeting errors are all less than 0.2 ft. Table 3 shows the results of changing the error specifications on the final targeting precision values for this case. Doubling the NAS curvature rate, the toolface error and the random error specifications only increased the maximum targeting error to 0.6 ft.

Multi-target horizontal example

The targeting performance of the directional simulator when used to reach horizontal targets has also been tested (Table 4). The horizontal example is similar to an Austin Chalk well where the target specification is changed six times while drilling the horizontal interval. The first of these changes occurs while drilling the build curve and represents a typical geologic targeting correction. The next five are examples of the required target changes after crossing a fault. In an actual horizontal well, the target changes are determined from geologic data obtained while drilling the build curve or horizontal interval. In the directional drilling simulator that process is modeled by defining the starting depth for each target change.

The directional specifications for the horizontal case are also included in Table 4. The curvature specifications are much simpler than are needed for the directional targets. It is only necessary to specify a maximum and minimum allowable curvature rate in the build curve and a maximum and minimum of curvature rate for near target adjustments. The values selected are appropriate for drilling 81/2-in. holes using 5-in. drill pipe.

Figure 3 shows the initial targeting plan that starts at the kickoff point and is designed to land on the initial target specification. Figure 4 shows the revised targeting plan after drilling to the starting depth of the first change in target specifications. This change is typical of the target adjustments that can be defined by the position of geologic markers observed while drilling the build curve.

Similar changes in design trajectory would result from geosteering interpretations of the target displacement after crossing a fault. The DS re-establishes the target in lateral displacements of 150 to 200 ft, and the actual path is quite close to the planned path of the target correction.

Reaching the goals

The Validus rotary steerable system promises to fulfill many of the industry's long-standing directional goals:

• precise targeting performance with the least amount of operator input;
• optimum trajectory control for smoother well bores that result in lower drilling costs; and
• of a single BHA for all drilling operations that minimizes trips

when configured with optimum bit types, weight, RPM and system hydraulics.
The NAS/DS can be combined with any oilfield service company's MWD or LWD system to provides the operator with the latest trajectory plan and targeting precision after each survey for optimum well control.

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