This paper presents the technical problems faced and the solutions found in the construction of a suitable static model of Bu Attifel field. The re-interpretation of the seismic volume, the construction of a detailed sedimentological model and the accurate correlations within the reservoir allowed the identification of several tectonic phases and, particularly, the intra-Nubian tectonics affecting only the lower levels of the reservoir. The conclusions of the geological study led to the identification of a very complex structural model due to the number, slope and, often, synsedimentary characteristics of the faults crossing the reservoir. During simulation, it was not possible to apply the usual simplifications to the structural model since a dynamic model able to optimise future infill well locations and enhanced oil recovery processes was required. It was then decided to build a simulation grid reconstructing both fault trend on the horizontal plane and their slope as accurately as possible. To this purpose, a specific methodology was followed. It consisted of two main phases: the generation of a conventional model with vertical faults and the conversion into a model containing sloping faults. The final result was the construction of a geological model and a simulation grid representing the structural complexity of the reservoir. Introduction Bu Attifel oil field, located in the Libyan desert about 400 km SE of Benghazi, has been producing since 1972. The field, which is one of the best Libyan oil field, has produced 30% of the original oil in place. In 1992, it was decided to build a new reservoir model able to guide the final development phase. The problem was the optimisation of location and number of the new wells (water injectors, oil producers and, if necessary, gas injectors). As a consequence, a very detailed geological, and particularly, structural model proved to be necessary to support the simulation study. The previous reservoir model had been made in 1987. Considerable difficulties were encountered in predicting the well performance along the southern flank of the field due to the high geological uncertainty in that area. Thus, a detailed reinterpretation of the 3D seismic volume, integrated with the analysis of all the geological data available, was performed. The resulting picture of the field showed that the reservoir is structurally complex for the presence of many sloping faults related to different tectonic events. Particularly, these faults control the areal distribution of the volcanic body that caused the geological failure of the wells located near the southern flank of the field. P. 129
This paper describes the successful drilling of a complex 3D High Dog Leg Severity (DLS) well trajectory with extended horizontal section, achieved thanks to the combined use of an innovative Rotary Steerable System (RSS) and a new type of High Performance Water-Based Mud (HPWBM).The well under discussion is a 6 in. side-track of an existing wellbore, drilled to optimize production in a field characterized by a carbonate reservoir. The challenge has been to reach the target and land the well in the shallowest part of the reservoir, far away from the Oil Water Contact (OWC). Furthermore, high Rate Of Penetration (ROP), smooth borehole, real time logs and good hole cleaning had to be provided to ensure optimum drilling performance while reducing the risk of wellbore stability issues.In the past, in this field, development wells have been generally drilled with a 2D trajectory design. In addition, standard drilling fluid practices have been using low weight calcium carbonate polymer muds to drill through the reservoir. Although generally successful, this resulted in high torque and low ROP in wells with extended horizontal sections.In the case history described in this paper, the side-track well trajectory has been planned with a DLS developed in three dimensional planes. An innovative hybrid RSS, specifically developed for high DLS applications, has been selected to drill the curve section. Moreover, a novel HPWBM has been introduced to drill the curve and the horizontal section. The use of the New Generation HPWBM has further optimized drilling performance, showing excellent results in reducing torque and increasing ROP, allowing to extend the horizontal section and maintain wellbore stability.The combined application of the innovative RSS and New Generation HPWBM has led to an improved control of the trajectory, reducing the well path tortuosity, and has greatly enhanced drilling performance compared to offset wells in the field. The curve section has been drilled with a DLS up to 10°/30 m and landed with a 90°inclination. Then, the side-track has reached the target with an extended lateral drain of 2010 m. In previous similar experiences, high torque values determined the maximum length of lateral drains.An average ROP of 3.8 m/h has been achieved while drilling the curve while an average 7.5 m/h ROP has been registered in the horizontal section.The outstanding results have opened up the way to further applications with similar challenging targets and high DLS requirements.
This paper highlights how the implementation of an extreme lean casing profile for the well construction, can really make the difference to achieve operational excellence. This is the case of the successful application in a mature field, where, for the first time, the reservoir target has been reached with a horizontal 8½Љ hole section within naturally fractured carbonate formations.In such occasion, the extreme lean profile has made it possible to reach the reservoir with a bigger production hole diameter, compared to the previous wells drilled in the same area; this has allowed the accomplishment of an outstanding production performance, also thanks to the lower pressure losses inside the 8 ½Љ open hole section.The reduced clearance between casing and borehole represents a distinctive characteristic of the extreme lean profile; this enables using two additional casing strings, as a contingency for well construction or as extra casings to reach deeper targets with the same production hole diameter or, as in this case, start with a smaller surface hole while simultaneously reaching the reservoir with a production hole diameter larger than those of conventional wells.Furthermore, the specific well design has allowed to reduce the drilling time, improve borehole stability thanks to the smaller hole sections, and decrease the environmental impact as a result of the reduced volume of cuttings produced.This paper provides a comprehensive description of the well requirements, the associated operational issues, the chosen solutions and the actual challenges encountered during drilling operations. The achieved results are also described.
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