Data processing: structural design – modeling – simulation – and em – Modeling by mathematical expression
Reexamination Certificate
2000-02-15
2004-03-23
Broda, Esq., Samuel (Department: 2123)
Data processing: structural design, modeling, simulation, and em
Modeling by mathematical expression
C703S010000, C702S006000
Reexamination Certificate
active
06711529
ABSTRACT:
The present invention relates to a method for determining at least one optimal trajectory for reaching a fuzzy target in a three-dimensional medium starting from a point remote from the target.
In oil exploration, it is standard practice to seek to reach by drilling a geological structure situated in the subsurface and which might contain hydrocarbons. Very often, the start point for the drilling is positioned so as to take various constraints into account, in particular on-shore environmental constraints or off-shore optimization constraints.
Along with the growth in off-shore drilling, on account of the very high costs of marine production structures, it has been necessary to develop sophisticated techniques of directional drilling such that a single drilling start point can give access to several geological structures lying greater or lesser distances away in the subsurface. As a consequence of this, it is necessary to optimize the positioning of the production platform so as to obtain best access to all the sought-after geological structures. Therefore, the boreholes emanating from marine platforms sprout like upturned blooms in the subsurface. Ideally, the trajectory of a borehole lies in a vertical plane of chosen azimuth. It consists of a succession of rectilinear segments and arcs of circles strung together. The deviation of the borehole may be such that the trajectory of the borehole switches from a vertical direction at the origin to a horizontal direction when penetrating the geological structure.
A first problem has been knowing where the drilling tool was located at each instant during the progress of the borehole. This difficulty has been overcome firstly off-line through the use of single-shot, and then multi-shot photographic techniques which have made it possible to ascertain the azimuth and the inclination of the trajectory at regular intervals. These two measurements, allied with the drilled length, deduced from the length of the drill pipes, have made it possible to calculate the actual trajectory and hence the coordinates x, y, z of each measurement point in a three-dimensional orthonormal reference frame with the drilling kick-off as origin.
A second problem was knowing how to direct the drilling, in real time, towards a target point of the subsurface in as accurate a manner as possible. This technique did not experience a real upsurge until the development of directional turbodrilling associating the use of a downhole motor and a bent coupling. Initially, the measurements were carried out off-line by the single-shot photographic technique with each addition of drill pipes. Thereafter, the development of a special tool fitted with magnetometers and accelerometers and incorporated into the drill string (MWD standing for “Measurement While Drilling”) has made it possible to transmit azimuth and inclination of the trajectory in real time without interrupting drilling operations. More recently, the designing of flexible drill pipes has made it possible to reduce considerably the radii of curvature of trajectories, thus making it possible to switch from the vertical to the horizontal within a few tens of meters.
The duration of drilling is a critical parameter on account of the costs which it entails. This duration should therefore be reduced to a minimum. An effective way of reducing the duration of drilling is to reduce the trajectory of the drilling to an optimal length, for example a minimum. The problem of calculating the trajectory of the borehole from the start point to the location of the structure in the subsurface has thus been proposed. Posed in these terms, the problem has been solved for a number of years now. Thus, the Wellbore Planner® software, marketed by Landmark, makes it possible, among other things, to calculate various well trajectories deviated towards target structures chosen in the subsurface. Likewise, the article by M. Wael Helmi, F. Khalef, T. Darwish, entitled “Well design using computer model” and published by the Society of Petroleum Engineers SPE 37709 of 15-18/03/97, pages 299-307, proposes a method for calculating the trajectory of a borehole of minimum length between a start point and a target situated in the subsurface.
However, there is still a major obstacle to the complete effectiveness of the trajectory calculation, namely the uncertainty regarding the knowledge of the subsurface. Indeed, the subsurface is generally explored with the aid of seismic campaigns whose results are nowadays 3D seismic blocks. Depth migrations, which provide depthwise images of the subsurface, can be carried out using these seismic blocks. Chiefly distinguishable in these images are wave reflectors, called seismic horizons, which often correspond to boundaries of geological structures, which structures are the major objectives of oil exploration.
Whereas the shape of the geological structure may be relatively clearly apparent, its position in the subsurface is not accurately guaranteed since the depth migrations rely on approximate models of wave propagation velocities in the subsurface. It is therefore appreciated that the result of an uncertainty with regard to the velocity models causes, on concluding the calculations required for the migration, an uncertainty as to the position of the structures in the subsurface. In particular, this uncertainty becomes crucial when, for example, the structure is crossed by a fault dividing it into a compartment regarded as a hydrocarbon trap and a compartment containing no hydrocarbons, and when one wishes specifically to reach the hydrocarbon trap compartment and definitely not the other. Under these conditions, an error of a few meters in the position of the structure then becomes a source of major difficulties which translate into so many additional expenses to remedy same.
The consequences of imprecise knowledge regarding the position of structures were pointed out long ago and various studies have attempted to take them into account. In particular, in an article published in the JPT journal of February 1994, pages 143-149, published by the Society of Petroleum Engineers and entitled “A multidisciplined approach to designing targets for horizontal wells”, the authors S. T. Solomon et al focused on defining a target structure in three dimensions together with a certain tolerance. Their approach is based on a multidisciplinary consensus. The various petroleum professionals having defined their own view of the target are led to quantify reasonable uncertainties with regard to their specific parameters. For example, geologists and geophysicists advise of the uncertainty due to depth migration and reservoir specialists the uncertainties regarding the planes of contact of the fluids contained in the trap. These various uncertainties are subsequently weighted one against another. The final result is then a global volume, referred to as a target diagram, defined by the sum of the various volumes associated with each of the uncertain parameters. Specifically, with any contact point X known with uncertainty dX, there is associated the volume V(X) lying between X−dX and X+dX. The objective of the drilling, horizontal in this instance, is then as far as possible to reach the centre of the global volume.
As interesting as it may be, this approach does not offer the driller a trajectory. The driller will be allocated a target volume to be reached and it falls to him to choose the best trajectory to get there.
The subject of the present invention is a method of determining an optimal trajectory for reaching a target situated in a medium, for example the subsurface, and whose position is known in an uncertain manner, the said method making it possible to remedy the drawbacks and inadequacies of the prior techniques.
The method according to the invention for determining at least one optimal trajectory starting from a predetermined point of a medium so as to reach a target situated in this medium and comprising a centre and a peripheral boundary, the said target forming a centred volume, the position of each
Broda, Esq. Samuel
Elf Exploration Production
Ostrolenk Faber Gerb & Soffen, LLP
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