Broadband drillstring system integrated for offshore ERD wells

High-angle offshore wells in Trinidad and Tobago are characterized by hole-cleaning challenges, well-bore instability, and downhole vibrations that damage both tools and drill bits. To reduce operational risks and improve project economics, an operator selected a multiwell campaign to implement a wired pipe or broadband networked drillstring system. The broadband network system provided high-resolution data from downhole tools and along-string annular pressure evaluation in real time for approximately 95% of the 22,875 m (75,000 ft) drilled during the trial period.

The project achieved a significant decrease in wellbore instability related to nonproductive time (NPT) as well as higher tool reliability through downhole diagnostics, improved hole cleaning, and stick-slip vibration mitigation.

The ERD campaign

Extended-reach drilling (ERD) projects are faced with innumerable challenges in today’s environment, including high rig rates and more complex well geometries in deeper and hotter reservoirs at increasing water depths. Overcoming these challenges safely while minimizing impact on project metrics requires real-time understanding and control of subsurface conditions in which the wells are drilled.

Wells drilled from platforms are at high angle and dissect multiple faults. With an approximate 3,000-m (10,000-ft) step-out, the wells are characterized by well-bore instability that fuels the hole-cleaning challenge in a tight pressure window.

A field test was conducted to evaluate the reduction in trouble time that is associated with these challenges. First, the networked drillstring provides high-definition density image logs to determine wellbore instability mechanisms in real time. Second, the along-string annular pressure sensors provide evaluation of hole-cleaning efficiency and prevent pack-offs in an environment where lost circulation is frequently experienced.

High-definition downhole and subsurface information telemetry

Broadband and bi-directional data communication with downhole tools is currently available through a wired or networked drill-string. This high-speed two-way information transfer is important as it more quickly updates geology and geophysics information, reduces geological uncertainty, and allows better control of the bottomhole assembly (BHA) tools. Further, the ability to command downhole tools provides efficient steering, real-time diagnostics and troubleshooting, and more efficient downlink abilities. Real-time transfer of high-definition information also ensures improved well placement.

Networked drillstring components are similar to conventional tubulars in terms of functionality, handling, and specifications, but they are modified to convey a broadband signal from the BHA to surface.

The technology consists of three major components: 1) a stainless steel armored coaxial cable running between the pin and box of each tubular, 2) induction coils at both the pin and the box of each connection, and 3) booster and measurement assemblies – electronic elements that prevent the signal from degrading as it travels the length of the pipe – to measure annular pressure all along the drillstring. Data are transmitted by means of an electromagnetic field associated with the alternating current signal transmitted along the cable. Induction transfers data from one tubular to the next. Figure 1 provides an overview of the system that was field-tested.

FIGURE 1. The networked drillstring and along-string pressure measurement components were field-tested to reduce trouble time associated with wellbore instability and hole cleaning on offshore ERD wells. (Images courtesy of NOV IntelliServ)

Field test objectives

MWD/LWD was the primary subsurface data acquisition and, therefore, the objectives set for the networked drillstring system were:

Providing high-definition subsurface data in real time to make better

decisions while drilling; Evaluating the along-string annular pressure measurements to improve hole-cleaning practices and equivalent circulating density management;

Identifying and preventing poor hole-cleaning events such as pack-offs, stuckpipe, and lost circulation; and

Improving project economics. The field test consisted of three different phases. The planning phases consisted of rig inspection, data swivel installation in the top drive, downhole interface sub, integrating third parties in customer organization, and associated IT infrastructure. The execution phase consisted of three steps. First, the drillstring components were mechanically and electrically qualified through connectivity testing as part of an outgoing systems test prior to the wellsite deployment. Second, hundreds of components were deployed for drilling and completing five wells. Third, drillstring components were regularly inspected for repair and maintenance. The final phase was the evaluation of results.

Evaluation and results

Memory-quality and high-definition data in real time. High-definition density and gamma-ray image logs were available to support decision-making, even in large boreholes and at high ROP (60 m/hr or 200 ft/hr) by data transmission via the networked drillstring. Memory-quality azimuthal density image logs allowed for wellbore instability mechanisms to be identified and facilitated more accurate interpretations of failure modes. Previously unmapped structural features such as faults, unconformities, and breakouts were identified and allowed for updates to geological models while simultaneously highlighting problematic zones. The networked drillstring also enabled time-lapse logging of problematic zones to determine deteriorating conditions and adjust drilling practices without agitating the zones.

FIGURE 2. In examining the uptime and availability of the network for each well drilled in the campaign, it was determined that 95% of the footage was drilled with high-definition data.

Evaluation of annular pressure data all along string. High-frequency along-string annular pressure data while drilling was acquired from six distributed sensors. This marked the first commercial deployment of a full set of annular pressure measurement assemblies and provided learning for the optimal placement within a tight stability-fracture window.

Prevention of poor hole-cleaning events. Through the distributed along-string annular pressure sensors, the required flow rate for hole cleaning and for the first time the actual removal cutting and cavings efficiency was directly measured. A number of hole-cleaning practices were adjusted to maximize efficiency, including the sweep strategy and mud weight selection based on time-lapse logging.

Project economics. The NPT decreased from 47% and 48% on the first two wells to 10% on the third well. Wellbore instability NPT on the subsequent wells was reduced to vanishingly small levels, and trouble time was limited to mainly equipment.

Figure 2 summarizes the overall broadband network performance, expressed in uptime that is defined as a ratio of telemetry time to the total time that telemetry was required. Over five wells, the uptime exceeded 85%. Approximately 95% of the subsurface drilled was with the networked drillstring transmitting high-definition data.