A New MWD Concept for Geological Positioning of Horizontal Wells

Abstract

Abstract A key requirement in extended reach and horizontal drilling is to maintain the borehole trajectory within the productive interval. This is important for maximizing hydrocarbon recovery particularly when drilling thin beds and complexly folded and faulted reservoirs. The task, however. is often frustrated by uncertainty attached to the reservoir geology. This paper presents the conceptual ideas of an acoustic reference system for geological positioning of deviated and horizontal boreholes performed while drilling. The primary objective has been to extend the radial detection range from a few meters as of current geosteering tools up to at least 20 meters. By using acoustic reflection techniques, the borehole position is referenced to acoustic reflectors identifying critical stratigraphic bed boundaries or contacts. Theoretical analysis and experimental results, including cross well measurements with a full-scale prototype, demonstrate the feasibility of applying acoustic referencing for geosteering purposes. Introduction Horizontal production wells are increasingly being applied throughout the world. Starting in 1989, the number of horizontal wells in the Norwegian sector of the North Sea have increased rapidly. Data published by the Norwegian Petroleum Directorate (Fig. 1) shows that a total number of 102 horizontal wells were drilled between 1989 and 1994, which of 44 wells were drilled last year. Every third production well was drilled horizontally with a maximum inclination exceeding 800 in the reservoir section. Advances in steerable system technology and directional drilling performance have made operators increasingly confident that horizontal wells can be used with great success. Extended reach wells have been drilled with more than 7000 m horizontal reach to access reserves at reservoir flanks, and to develop marginal satellite fields from existing offshore platforms. Progressively more complex well profiles are being implemented. The designer well profile (Fig. 2), recently introduced in the North Sea, is a fully three-dimensional well design with several builds and turns. This type of profile has found its applications for penetrating multiplanar targets and optimizing wellbore placement in geologically intricate fields. Exploration and production well properties can be merged into a single well design. Geologically complex fields lend themselves to development using designer type well profiles. However, this also involves taking larger drilling and geological risks. Crucial to the success of these wells is to establish reliable stratigraphic position control while landing the well into the reservoir targets and during the reservoir navigation phase, insuring that the well stays in contact with the reservoir pay. High dogleg severities resulting from hole corrections to find or to re-enter the reservoir pay, may severely limit the horizontal reach and the final wellbore length exposed to the reservoir. Optimization of borehole elevation within the pay zone can also have a substantial impact maximizing production rates and minimizing gas and water coning problems (Fig. 3). Steering efficiency and geological position uncertainty are perhaps the greatest limitations in horizontal drilling today. The relative position uncertainty between a well and a critical near-wellbore bed boundary or contact is defined by the accuracy of MWD directional survey tools and the formation dip uncertainty. MWD tools measure the earth's gravity and magnetic field to determine inclination and azimuth. Knowledge of the course and position of the wellbore depends entirely on these two angles. Under normal operating conditions and with good quality control, errors in the inclination measurement, which is of most concern staying in the pay zone, is in the order of 0.20 (l confidence level). This error alone translates into a vertical depth uncertainty as large as 3.5 m per 1000 m along hole depth. P. 851