TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractTraditionally, Oil Base Mud (OBM) has been used by a major operator to drill horizontal wells in the Magellan Strait, Argentina. The operator was faced with additional challenges when drilling an exploratory well due to environmental concerns in a highly sensitive area and evaluation problems related to the use of OBM. Significant advances in water based drilling fluid design in the recent years have allowed water-based drilling fluid performance to approach that of OBM. This presented the operator and drilling fluids supplier with the opportunity of evaluating the application of water base drilling fluid on this well. The planning stage included laboratory testing, review of historical data and an evaluation of experience with similar shales in the area. A high performance water base drilling fluid containing both clay and shale stabilizers, an ROP enhancer and sealing agents was selected to drill the well. This paper presents the laboratory and field data generated during this project. The well was drilled through notoriously troublesome shales to total depth without the wellbore stability problems associated with more conventional water based muds. Gas kicks were controlled with no fluid solubility problems and the fluid exhibited excellent properties even when pressure parameters escalated higher than planned, requiring a higher mud density and high degree of temperature stability. The operator's expectations were met in this very difficult well including minimization of bit balling, near gauge hole and improved ROP in conjunction with optimum hydraulics.The evidence gathered on this project shows that a properly designed water base mud is a viable alternative to OBM in areas where environmental restrictions and formation evaluation problems are a concern.

TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractTraditionally, Oil Base Mud (OBM) has been used by a major operator to drill horizontal wells in the Magellan Strait, Argentina. The operator was faced with additional challenges when drilling an exploratory well due to environmental concerns in a highly sensitive area and evaluation problems related to the use of OBM. Significant advances in water based drilling fluid design in the recent years have allowed water-based drilling fluid performance to approach that of OBM. This presented the operator and drilling fluids supplier with the opportunity of evaluating the application of water base drilling fluid on this well. The planning stage included laboratory testing, review of historical data and an evaluation of experience with similar shales in the area. A high performance water base drilling fluid containing both clay and shale stabilizers, an ROP enhancer and sealing agents was selected to drill the well. This paper presents the laboratory and field data generated during this project. The well was drilled through notoriously troublesome shales to total depth without the wellbore stability problems associated with more conventional water based muds. Gas kicks were controlled with no fluid solubility problems and the fluid exhibited excellent properties even when pressure parameters escalated higher than planned, requiring a higher mud density and high degree of temperature stability. The operator's expectations were met in this very difficult well including minimization of bit balling, near gauge hole and improved ROP in conjunction with optimum hydraulics.The evidence gathered on this project shows that a properly designed water base mud is a viable alternative to OBM in areas where environmental restrictions and formation evaluation problems are a concern.

To help satisfy the demands for petroleum products, operators are being forced to drill in increasingly challenging environments. Combining 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. Many best practices were either established or re-enforced during the process. Attempting to circulate long columns of cement slurry during construction of 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 great 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 best practices learned from wells drilled offshore Tierra del Fuego using the latest cementing technologies as required to make this exploration project a success. Background A six-well development package has been drilled in the offshore waters of Tierra del Fuego. Tierra del Fuego is at the very southern tip of South America. The base for this project was located in Punta Quilla, an 8-hour drive from the closest permanent oilfield base, Comodoro Rivadavia, which is 2,000 miles south of Buenos Aries. As if the remoteness of this location was not complication enough, the weather is even more severe. The wind blows at 70 knots and the yearly average temperature is only 46° F. The high winds generate an equally troublesome tide, which often reaches 40 ft. The currents associated with these tides limit boat loading and offloading to a short period each day. To minimize the impact of this harsh environment, an ERD drilling plan was selected to develop this reservoir. With ERD, fewer wells could generate the required production. ERD wells typically have true vertical depths much less than measured depth or drilled length. This is illustrated by the well path from Aries-PH2 (Fig. 1). Effective cementation is always a concern with ERD wells. The primary concerns are:whether cement can be effectively circulated across the entire openhole section without losses, andif placed, whether it can efficiently displace the mud to form a successful annular seal. To further complicate the cementing of these long horizontal sections was the fracture gradient, which was expected to be in the 13.7 lb/gal range, requiring lightweight cement. Current state-of-the-art provides for three different methodologies for reducing the density of cement slurries.1-4 The original method called for the addition of bentonite to the slurry. With a specific gravity (SG) of 2.65, bentonite by itself will actually increase the density of most cement slurries.

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.

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