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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.

Traditionally, Non-Aqueous Fluids (NAF) have been used by a major operator to drill challenging wells in the Campos Basin, Brazil. Significant advances in water based drilling fluid design in the recent years have allowed water-based drilling fluid performance to approach that of NAF. Exploration in the new frontiers and optimized development well projects in deepwater Brazil have required a different approach regarding drilling fluid design due long step outs, difficult well trajectory, and the possibility of drilling horizontal wells in one step, thus avoiding intermediate casing strings. Although NAF are an ideal candidate for those applications, environmental concerns and logistic demands are still an issue and alternatives should be considered. HPWBM has been applied to replace NAF in some applications in deepwater and ultra deepwater (UDW) in the Campos Basin. This novel technology has been successfully applied to drill in UDW scenarios, reactive clays, dispersive shale, naturally micro fractured formation and horizontal wells. HPWBM characteristics are developed with:A new generation of encapsulation polymers;The use of amine chemistry to provide clay stability;The application of novel sealing polymer for shale stability; andExcellent mud lubricity characteristics. The lessons learned, as supported by case histories and lab data have contributed to system modifications which have improved performance. This work has also identified attributes needed to complete a drilling fluid design for the difficult wells to be drilled in the new exploration and development areas. The evolution of HPWBM drilling fluid design will be discussed along with how decisions were made. Introduction Today the industry is drilling more technically challenging wells difficult wells. Exploration and development operations have expanded globally as the economics of exploration and production for oil and gas have improved with advancements in drilling technology. Advanced drilling operations such as deep shelf, extended reach, horizontal and deepwater are technically challenging, inherently risky and expensive. With consideration to reducing drilling problems such as torque and drag, stuck pipe, low rate of penetration and well bore stability; these wells are generally drilled with emulsion-based muds. Nearly three quarters of the earth is ocean and a high prospect of hydrocarbon resources in addition to the other marine resources. That's why the industry is shifting from onshore drilling to offshore drilling. Published information indicates the presence of more than 20% of world's proven reserve in offshore geological structures. According to future production forecast of production reserves about 40–50% of future hydrocarbon recovery will be from offshore reserves. This is reflected by the increasing activity in the offshore environment with a gradual shift from shallow water drilling to deepwater drilling operations. This scenario is particularly critical in the drilling exploration of offshore Brazil where the country faces the challenge of increasing oil production and reaching energy self-sufficiency within the next few years. Petrobras is well known for extended deepwater experience, however exploration in the new frontiers of ultra deep water face new challenges.

Wells drilled in the Andean Mountain region of South America present significant challenges as a result of both operational and environmental factors. Wells located in the foothills along the basin are particularly difficult due tectonic stresses and unstable, probably, micro-fractured shales. Operators have experienced difficulties drilling wells using both water-based and oil-based muds (OBM). Environmental regulations hinder the use of OBM in many of those areas due to the potential environmental impact and costs associated with waste disposal. In many cases OBM has not prevented wellbore instability problems. This paper explains how a lack of understanding of regional geology and the practice of using successful drilling fluid design and drilling practices from other areas has led to wellbore problems. The water phase salinity of OBM and the use of the appropriate inhibitors in the drilling fluid play a key role in the minimization of wellbore problems. Although reactive clays are present in all the shales along the basin, they represent only 30 to 40 percent of the clay fraction, while non-expandable kaolinite clays are the major clay components. This paper explains how physical/mechanical effects are more important than inhibition in controlling these shales. Moreover, in some cases "excessive inhibition" due the presence of shale inhibitors such us potassium and high water phase salinity in OBM exacerbate the problems. Pore pressure transmission caused by fluid invasion is a major contributor to the observed problems. A combination of operational practices and improved fluid design minimizes mud and filtrate invasion. Troublesome shales in the Andean basin include, from north to south, the La Rosa and Icotetea in Venezuela, the Carbonera, Leon and Villeta in Colombia, the Napo in Ecuador, the Chonta in Peru and the Los Monos in Bolivia and Argentina. Case histories involving these shales are presented. Contrary to experiences in many other parts of the world, high water phase salinity OBM and potassium based water-based mud (WBM) are not the answer to shale stability problems. Rather, mud sealing properties, correct chemical composition and appropriate drilling practices are the key factors in maintaining wellbore stability. Introduction The challenges of drilling in the Andean Mountain region of South America are well documented. The presence of tectonic stresses combined with over pressures1 makes this a particularly challenging region. The stresses in this region were generated by the Andes Mountain orogeny. The geology is typified by steeply dipping sand/shale sequences. Many faults have been documented in this area. Claystones and shales dominate the lithology in this region. These clays and shales can be "sticky"2 at times, requiring the use of inhibitive drilling fluids to minimize the associated problems. The factors discussed above lead to wellbore stability being a major challenge when drilling in the Andean mountain region. These problems present the greatest challenge when drilling directional wells3. The drilling problems experienced in Colombia are well documented and include stuck pipe, high torque and drag, tortuous wellbores, twist-offs, poor cementing, and unplanned back-offs. Many of the difficulties encountered have been attributed to poor hole cleaning in enlarged hole resulting from wellbore instability. The cavings generated during the hole enlargement process have also presented hole cleaning challenges. Success in drilling wells in this region has been attributed variously to simplifying well design, understanding the tectonic stresses and their orientations, drilling fluid design4 and sound drilling practices.

The Karachaganak Field, which was discovered in 1979, is located in north-west Kazakhstan on the northern margin of the Pre-Caspian Basin. Production from this field contains 4 -4..5% H 2 S. The Kashagan field, located offshore in the North West part of the Caspian Sea, averages 10 -12% H 2 S concentration. An environmentally friendly H 2 S scavenger has been used in the Caspian area since 2006 in the invert emulsion drilling fluid systems. The operators had a standard practice of maintaining an excess of lime (2.5 kg/m 3 ) as a supplementary sulphide removal additive and also raised concerns regarding the lack of a testing procedure to monitor concentration. This issue was regarded as being very important on the grounds of the significant risk that existed in the absence of more accurate method of monitoring scavenger concentration other than just mass balance calculations. The lack of ability to measure scavenger concentration more precisely in the invert emulsion fluid returned to the liquid mud plant (LMP), led to increased cost for the operator to maintain a safe concentration. This paper discusses the design of a new method to be used in both OBM and WBM during field applications of the scavenger. The method is now used to monitor the excess scavenger in all the wells drilled in the Karachaganak field and to design subsequent treatments. The method has also contributed to improving the level of safety in the operation. The use of the proper scavenger concentration in combination with excess lime provides a primary and secondary barrier for protection against the influx of H 2 S