To help satisfy the demands for petroleum products, operators are being forced to drill in increasingly challenging environments. Combine cold winters, some of the strongest winds in the world, and a very remote location in the waters off the coast of Tierra del Fuego certainly could be considered challenging. In this case study, the operator successfully constructed extended reach wells through low fracture gradients under the previously described harsh environment.
Attempting to circulate long columns of cement slurry when constructing extended reach drilling (ERD) wells under these conditions stretches the limits of current technology. Nowhere else in the world has success required more preplanning and implementation of the latest technologies. Failure to achieve a seal could have a large impact on the cost of drilling these wells and the operator's ability to effectively produce the new field. In addition to the cost of the required remedial work, which is easy to quantify, additional costs can be accrued from delays in production delivery, which are harder to quantify.
This paper presents case histories from Tierra del Fuego illustrating preplanning methodology, design strategies, and new technology implementations required to make this exploration project a success.
Background
A six-well development package was planned for the near offshore waters of Tierra del Fuego. Tierra del Fuego is a remote area of Argentina at the very southern tip of South America, 2,000 miles further south than Buenos Aries. The closest permanent oilfield base is located in Comodoro Rivadavia, an 8-hour drive from the shore base for this project located in Punta Quilla. As if the physical distances were not enough of a complication, the weather certainly finishes the deal. The average temperature is only 46 ºF with wind of 70 knots. These winds often cause the tides to reach 40 feet. Because of the currents associated with these tides, boat loading and offloading can only be done for a limited time each day.
Because of this difficult environment, an ERD drilling plan was selected to minimize the number of wells while maximizing the drainage area. In ERD wells, the horizontal extension is significantly longer than the depth. This is illustrated in Fig. 1. A common concern with ERD wells is whether cement can be effectively circulated across the entire openhole section, and if placed, whether it can form a successful annular seal. Previous exploratory work showed the fracture gradients to be in the 13.7 lb/gal range. Bottomhole circulating pressures are a combination of frictional and hydrostatic forces. With the long columns and low fracture gradients, slurry design needed to focus on creating a stable slurry with low rheologies to minimize frictional forces. To further decrease the overall circulating pressure, slurry density needed to be minimized as well.
Current state-of-the-art provides for three different methodologies for reducing the density of cement slurries.1–4 The earliest methods used extra water and corresponding bulk gelling materials to prevent the extra water from separating out of the slurry. This is the most economical way to build a cement slurry. All the extra water increases slurry yield requiring less total sacks to be purchased. Unfortunately all of this extra water degrades the set slurry properties and there is a lower density limit below which any extra water will over-dilute the slurry and initial set time is delayed beyond what is useable for the offshore market.
The second way to decrease slurry density is to foam the slurry with nitrogen. Foamed cements also have benefits and limitations. Despite the downhole engineering property benefits of delivery with a foamed cement, this method was not selected for this work because of the combination of difficulties associated with getting the required liquid nitrogen and associated equipment combined offshore and the increased job complexity.
