DRILLING TECHNOLOGY: Fresh look at pre-drilling using template, tieback engineering

Sept. 1, 2001
Mitigating interface scheduling, delayed first production
Nine-slot drilling template, designed and installed by Claxton Engineering Services Ltd.

During the late 1970s and 1980s, pre-drilling was increasingly utilized to enhance project economics. For both oil and gas reservoirs, drilling wells while jacket and topsides structures were under construction reaped significant rewards. Once wells were tied back and completed, platform production could often be achieved shortly after first production.

With the increased demand for pre-drilling, the major wellhead companies enhanced the mudline tieback equipment for both jackup and semisubmersible drilling. Surface wellhead equipment was developed to allow space-out with mandrel type hangers. Additionally, pre-drilling template designs were optimized for installation by mobile drilling vessels, making available major cost savings in pre-drilling economics.

In recent years, with the development of smaller hydrocarbon accumulations and the advent of substantially lighter platforms, there has been an increased tendency to drill through jackets once in position, rather than using a pre-drilling template methodology. These small platforms have a much-reduced fabrication cycle.

The perceived complexity and increased costs associated with well tiebacks had encouraged a number of operators to use platform-drilling options. These preferences were fueled by bad experiences from early pre-drilling and tieback campaigns, when issues were less understood and planning was less extensive.

Cost overruns

With many technical operations that occur infrequently, limited knowledge and experience often leads to budget overruns. This is particularly the case for template and tiebacks, where many aspects of the equipment and operations occur at the interface between drilling, structural, and mechanical engineering.

In such instances, the services of a specialist provider can add significant value to the project by identifying risks and remaining uncertainties, as well as optimizing processes based on other operators' experiences. An individual operator may carry out a template and tieback program in a region every five years or so. In a similar period, a specialist provider experiences involvement in more than 20 times that work scope.

Once again, however, operators are evaluating the pre-drilling option for their developments. Successful tieback of wells is now normal, where there is adequate planning and manufacture of cost effective drilling templates. There are many such examples around the world. With the benefit of experience, a typical rig-installed drilling template is in place within 24 hours and the average well tieback takes less than 48 hours.

Marginal developments

As the oil sector has matured, field developments are increasingly marginal. Even the larger accumulations have substantial technical challenges and hence reduced economics. Operators are therefore increasingly looking to innovative use of known technologies to reduce costs. The focus on marginal fields is now on the earlier re-use of appraisal wells in order to acquire more reservoir data, and the decoupling of drilling and project schedules in order to mitigate risk and cost over-runs.

The increased cost of template and tieback hardware is often offset by time savings afforded by batch drilling or completion processes. Refining economic models and replacing annual historical coarse inputs with monthly reports, makes the "time value" of money (through discounting) penalty less onerous in advancing drilling for compressed field development schedules. This scenario is entirely appropriate for short development schedules.

Egypt case study

Rashpetco in the Nile Delta executed one of many such projects that have taken place in recent years. This typical, successful pre-drilling project was on the Rosetta Field during 1999-2000. As with other accumulations in this area, the development of the Rosetta gas field required completion of the project on time to meet contractual gas delivery dates.

With this in mind, it was important to ensure an adequate number of wells could be completed and made available at first production to meet nominations. After careful consideration of fabrication schedules for jackets and topsides, availability of lift barges in the region and the necessary time to comfortably complete drilling operations, it was decided to proceed with a pre-drilling and tieback scenario.

A nine-slot casing supported template was located over the first development well by the drilling rig allowing wells to be suitably spaced out on the unstable seabed. Using the jackup drilling rig King Singapore, a total of four wells were pre-drilled along with two 30-in. conductors set for use as docking piles. In order to prevent the bottom bracing and docking pile locating guides of the jacket from clashing with the template, the template structure was removed. The rig, using only remotely operated vehicle (ROV) intervention, accomplished this after completion of the pre-drilling program.

Angular and horizontal alignment tolerances were successfully achieved for the jacket installation. This, coupled with careful inclination control on the pre-drilled wells, allowed a standard structure and operational tieback design. As with the template installation, extensive planning for operation and contingencies allowed the full tieback program to be executed within seven days for the four wells, which was significantly under the budgeted allowance.

The project demonstrated that new pre-drilling and tieback of wells can be cost effectively executed, while at the same time removing drilling activity from the project critical path, mitigating interface scheduling and delayed first production risks. The key to this success is the careful evaluation of historical experience, the appropriate selection of proven technological methodologies, and the optimisation of the process through planning by personnel with the necessary competencies.