The influence of different factors on the asphaltene stability in crude oils was evaluated. Compositional studies and structural characterization of resins and asphaltenes were carried out to study a possible relationship between these properties and asphaltene deposition behavior. Asphaltenes were obtained by precipitation with n-heptane and characterized by elemental analysis and spectroscopic techniques. Low hydrogen to carbon ratios, high aromaticities, and high condensation of aromatic rings were the main characteristics of the asphaltenes from unstable crude oils. According to these results, the stability behavior of the asphaltenes was influenced strongly by their structural characteristics. However, it was also found that stabilization by means of a commercial inhibitor was affected by the composition of the crude oil. In particular, a high content of basic functionalities in the crude oil can be related to the decrease of the inhibitor's effectiveness (dodecylbenzene sulphonic acid). Based on these findings, some useful suggestions applicable to the oilfield operations are made.
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Accurate knowledge of drilling fluid behavior under actual conditions is required to maximize operational efficiency and to minimize cost and drilling fluid related risks on extreme high-pressure / high-temperature (HP/HT) wells. This paper identifies and discusses the major HP/HT drilling fluid challenges, recent innovations in fluid viscosity measurements under HP/HT conditions, drilling fluid designs stable to extreme HP/HT conditions, and other considerations in HP/HT drilling. Introduction Worldwide demand for energy continues to increase and is projected to average 2.0% per year out to 2030. Demand is widespread geographically but the most rapid growth is projected for nations outside the Organization for Economic Cooperation and Development (non-OECD nations) averaging 3.7% per year for non-OECD Asia.1 Providing adequate supply is driving the industry to explore areas previously unexplored, or minimally explored. A subset of this activity is HP/HT drilling. HP/HT drilling is not rigorously pursued during times of price uncertainty or low commodity pricing due to the relatively high lifting cost. The resurgence in HP/HT drilling stretches globally and encompasses areas such as the deep Gulf of Mexico Continental Shelf, northern India, Saudi Arabia, and Brunei. Historical HP/HT basins such as Indonesia, Thailand and northern Malaysia have also seen a selective increase in HP/HT activity. Several factors have combined to make deep gas increasingly attractive worldwide:Abundant infrastructure in the way of platforms, producing facilities, and pipelines that would allow new production to flow quickly to market.New technology such as 3D seismic and faster computers to locate potential formations. Drilling and Drilling Fluid Challenges Developing HP/HT prospects can require overcoming some formidable drilling challenges. Rigs capable of HP/HT drilling are larger due to requirements such as hook load, mud pumps, drill pipe and surface mud capacity to name a few. Due to these requirements, these rigs are more expensive. HP/HT wells, by definition, require a higher density fluid which typically requires high solids loading. High solids loading, the resulting higher pressures, combined with the competency of rock at depth, lead to low penetration rates, extending time on location and added drilling costs. In extreme cases, pressure, temperature, and acid gas levels can limit the selection and function of down-hole tools and fluid selection. These limitations can be so severe that MWD/LWD tools become unusable, rendering down-hole annular pressure measurements used for pressure management, unavailable. This places additional demands on the drilling fluid and temperature/hydraulic models as they become our best, if not our only source for down-hole pressure information. These models are based on surface inputs and laboratory measured fluid properties under down-hole conditions. During the planning stage for several potential record depth deep gas wells currently drilling or recently TD'd, not only did this information not exist, laboratory equipment capable of operation at the required temperatures and pressures didn't exist. Pressure/Volume/Temperature (PVT) Down-hole pressures are commonly calculated using TVD (true vertical depth) and surface measured mud weight reported from the rig. While this approach is adequate for less demanding wells, critical applications such as HP/HT and deepwater wells require adjustments for the pressure and temperature driven compression and expansion characteristics of the whole drilling fluid. These compression and expansion effects are quantified in fluid PVT measurement under expected down-hole conditions which, until recently, ranged from 15 psi/75°F to about 20,000 psi/350°F which covered industry needs. HP/HT drilling pressures and temperatures, however, can far exceed this envelope. Figure 1 illustrates isobaric PVT results on a commonly used base-fluid.
Efficient removal of the drilling fluid from tubulars, screens, and near-wellbore mineral surfaces is essential to the successful completion of wells. Cleaning and removal of the non-aqueous drilling fluid (NADF) from all of the well surfaces is not an easy task. Microemulsion fluids provide a highly-successful solution to this problem. Carefully designed and customized microemulsion fluids have been used to remediate and increase well productivity and injectivity in wells located in the Gulf of Guinea. Due to their uniform wellbore cleanup, optimum results were achieved in open-hole horizontal and highly-deviated wells. The robust microemulsion fluid systems are capable of accommodating changes in temperature, density, and salinity. They restore the water-wettability of the rock and increase injectivity or productivity of the wells. Some water injectors and oil producers from the Gulf of Guinea exhibited low injection or production rate. The purpose of pumping the treatment downhole was to improve injectivity by removing skin damage and screen blockage by using in-situ generated microemulsions, thus allowing efficient oil production from the formation or water injection at sustainable high-injection rates into the formation. Field data shows that injectivity and productivity increased after treatment. In some cases, the downhole pressure decreased for the injectors. Cleanup and removal of the NADF from the Gulf of Guinea wells was beyond the capabilities of conventional treatments. Laboratory tests confirmed that the in-situ microemulsion treatment removes the damage and helps to achieve the desired and predicted injection and production rates. This paper presents field data, before and after the successful microemulsion fluid treatment, and describes various laboratory tests performed prior to the Gulf of Guinea field applications.
Proper drilling fluids planning and execution play a major role in minimizing the likelihood of drilling difficulties such as those encountered in ultra-deep waters. Deepwater drilling is inherently expensive. Because of this, the proper selection of drilling fluids and other support technologies requires careful evaluation to diminish commonly observed deepwater challenges. These challenges include hydrate suppression, shallow water flows, logistics, lost circulation mitigation, viscosity profiles. Another critical deepwater challenge is properly managing ECD to maintain optimal well site safety and environmental compliance. This paper discusses the planning and execution of a drilling fluids program for a world record ultra-deepwater well. The well was located at a water depth of 10,011 ft. (3,051 m) in the Gulf of Mexico's Alaminos Canyon (Block 951). The well was drilled to TD (22,695 ft. [6,917 m]) using an advanced synthetic-based mud that met all Gulf of Mexico compliance requirements and advanced engineering software and tools. Introduction Since the mid-1990's, operators have been exploring and developing fields in ever-increasing water depths. As a result, the industry has identified techniques and developed new technology allowing them to safely and successfully drill wells at depths greater than 10,000 ft. The learning curve on these wells has been relatively steep and new records are being established at a rapid pace. Due to the presence of young sedimentary deposits many of these wells possess an extremely narrow margin between the pore pressure and reservoir's fracture pressure. Because of the greater the likelihood of drilling success. The use of a riserless drilling technique, known as Dynamic Kill Drilling (DKDSM), has proven instrumental in successfully pushing casing depths in deepwater and ultra-deepwater environments1. This technique played an important role in the success of this challenging deepwater project as the initial upper section was extended to obtain required LOT's at the 22" and 17–7/8" casing shoes. The unmatched combination of technical performance and environmental compliance presented by synthetic-based drilling fluids has made them the fluids of choice for the deepwater drilling in the Gulf of Mexico. Based on an overall analysis of this project the operator selected a proprietary synthetic compliant drilling fluid for the drilling of the well from the 18" × 22" section to TD. This decision was driven primarily by the presence of highly reactive shales in the deepwater formations and the system's prior performance in offset wells. In the deepwater environment several important factors must be considered. Chief among these considerations is the effect of temperature and pressure on the fluid system. As the water depth increases, temperature decreases and pressures at seabed rise. A typical temperature curve (Figure 1) for the Gulf of Mexico indicates an average seabed temperature lower than 40°F at depths greater than 4,000 ft. This cooling, combined with increased pressure, impacts the design of the drilling fluid for use in the well. Due to the chemical composition of the GOM-compliant synthetic-base fluids, the cooling will increase a base fluid's viscosity resulting in an increase in the overall drilling fluid viscosity. Because of this, particular importance must be paid to the fluid's viscosity profile in order to minimize potential problems such as formation fracturing which can result in costly lost circulation and non productive time. A proper viscosity is also critical in ensuring that adequate hole cleaning is maintained throughout the entire wellbore. Although planning for deepwater operations is an extremely complex process involving many technical disciplines, this paper will focus primarily on the planning and execution of this world-record deepwater well's drilling fluid program.
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