A new family of MWD tools has been designed specifically for the land-based unconventional drilling market. These tools were designed to meet the technical and operational requirements and also fit the existing cost structure associated with drilling unconventional reservoirs on land. The AccuSteer MWD system includes a surface system that eliminates the need for dedicated MWD personnel at the well site, enabling remote monitoring and remote directional drilling.

The measurements provided by these downhole systems are focused on providing the information specifically required for efficient wellbore placement, which includes drilling dynamics data and annulus pressure for drilling efficiency and formation measurements for geosteering.

MWD 2 Figure 1

FIGURE 1. This cross section of the new MWD system illustrates the placement of the gamma detectors. Mud flow is through the center (white circle). (Images courtesy of Ryan Directional Services)

System cost kept to minimum

All of the measurements required for an MWD system designed for drilling unconventionals are well proven and available from many different companies. However, most of the systems currently being used to drill the horizontal wells were either developed for offshore applications or for more basic directional drilling applications. In the case of the systems designed for offshore use, the cost and packaging are not ideal for land-drilling operations where the daily operation spread rate can be 10% of the cost of an offshore operation. In the case of the systems originally designed for more basic directional drilling applications, the reliability and measurement capability is lacking.

The key to providing a solution for these land-based unconventional drilling operations is to develop a system that meets the needs of the customer from a measurement and reliability standpoint and also has a very low cost of ownership, thereby enabling these services to be provided at an affordable rate.

The drivers that control the cost of ownership for MWD systems are capital cost, reliability, power consumption, repair, and maintenance. One of the biggest contributors to cost of ownership is system length. Since most MWD systems are manufactured using nonmagnetic material such as beryllium copper and austenitic steels, increased length equals increased capital cost. Additionally, increased length typically requires additional electromechanical connectors, and these can add to the cost and system complexity.

When designing a reliable system, simplicity is critical; therefore, the number one driver in this system is keeping the entire system length less than 12.1 m (40 ft) but as close to 9.1 m (30 ft) as possible. This length constraint also has a practical aspect in terms of logistics. There are transportation and handling issues associated with assemblies that are greater than 12.1 m in length.

Efficient wellbore placement

The primary application of an MWD/LWD system used in drilling an unconventional well is to ensure proper and efficient wellbore placement. Ryan Directional Services defined efficient wellbore placement as the ability to place the wellbore precisely within the geological model developed by the operator in a cost-efficient manner. To achieve this, as with all MWD systems, the applications of drilling and geology must be addressed. This includes ensuring that the measurements required for directional drilling and geosteering are included. Additionally, the need was identified for a system that could provide drilling efficiency measurements in a cost-effective manner. This application has been severely overlooked and, if properly addressed, will provide significant benefits to the drilling process.

The precise positioning of a horizontal well requires not only accurate sensors but also changes in survey practices. As an example, accurate and continuous inclination measurements provided while rotating have been in use and proven effective in the offshore environment. It was critical to design this feature into a new system. The company decided to position this measurement at the lowest possible point in the MWD assembly since the closer the measurement of inclination is to the drill bit, the more useful the measurement becomes.

Geosteering

MWD 2 Figure 2

FIGURE 2. The AccuSteer MWD system includes survey measurements, drilling dynamics, pressure, and gamma imaging for geosteering.

The large majority of unconventional wells drilled today are being drilled with a nonazimuthal gamma-ray measurement. However, most of these applications would benefit from an azimuthal gamma-ray measurement that provides images indicating the direction of the approaching beds. Therefore, Ryan concentrated a large part of its efforts on developing a state-of-the-art gamma imaging system. Several different gamma detector packaging concepts were modeled to determine which concept would be best suited for this application.

Drilling efficiency

Many drilling practices that have become accepted and practiced on land for years may not be considered best practices for unconventionals. The challenge remains to be able to make informed decisions on changes to drilling practices and parameters based on combined surface and downhole data. It is not reasonable to expect that measurements of rpm, weight on bit (WOB), and torque on bit (TOB) at the surface are good indications of what is actually occurring downhole.

Although these measurements have been available for decades, they are rarely run in the land environment as they cannot be provided at a low enough cost by the service companies. Additionally, the challenge remains in developing a system that can take the downhole drilling dynamics data and present them in a fashion where decisions can be made in real time.

Real-time drilling optimization requires real-time monitoring of near-bit measurements to maximize ROP while not exceeding the mechanical limits of the bottomhole assembly (BHA), which includes MWD systems. Adjustment of drilling parameters such as WOB, rpm, and flow rate requires real-time monitoring of the dynamic loads in the BHA to achieve the objective of wellbore construction in the shortest duration of time. In addition to mechanical measurements of downhole WOB and rpm, measurements of TOB, bending on bit (BOB), shock, and vibration have been included.

There are several applications and benefits for measuring pressure downhole while drilling. The primary application is the direct measurement of equivalent circulating density (ECD). The measurement of annulus pressure while drilling provides a real-time measurement of ECD, which can alert the driller to ineffective hole cleaning and prevent issues related to seriously over-balanced conditions. The availability of these data in real time eliminates the guesswork normally associated with this process.

With the advent of managed pressure drilling (MPD) systems the need for an accurate measurement of annulus pressure to determine ECD has increased significantly. In an MPD application the annulus pressure measurement has proven to be beneficial when used to calibrate the MPD models. A measurement of internal bore pressure also was included which, when compared to the annulus pressure, will yield a direct measurement of differential pressure. This measurement can sometimes be used to diagnose issues with the drilling motor and the drill bit.