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Modern Geosteering Techniques

Authors: Stishenko Sergey

Starostin Ivan

One of the major current challenges facing the oil and gas industry is to increase the oil recovery factor. Since the 1970s, the average percentage of oil recovery in the country fell from 50 percent to 27 percent. This stock utilization rate is one of the lowest in the world. There are several methods of enhancing oil recovery, but this article is dedicated only to the practices of maximizing oil recovery factor by optimizing the horizontal wellbore position. This kind of optimization is carried out by geo-steering (geological drilling support). We shall briefly consider the different ways of geo-steering, ranging from simple, based on the use of standard logging devices to the most modern, using the integrated model, which includes structural data logging, directional survey and azimuthal logging obtained in on-line mode.

The most important geo-steering aspect is a timely based on the incoming data decision to correct the well path. At the moment, there are three possible while-drilling responses to change of lithology and formation properties:

1. Monitoring/survey

This option does not involve active steps and a change of the well path. Drilling is carried out according to a pre-arranged plan and eliminates the possibility of influencing the wellbore position. This method is passive, i.e. it involves only observation and no response. Consequently, in this case it is reasonable to talk about the absence of geo-steering. An example is the as-designed drilling, where the well path is not changed because of the rock properties change. The incoming log data is observed, but no decisions basing on these observations are made.

2. Log matching (non-azimuthal devices)

This is a well -proven method of geo-steering. It does not require expensive images for work and may operate well even if you have only one gamma-ray logging curve. The reactive method, when after drilling for a certain period the log data is processed, interpreted and the decision is made. In short, this method can be described as four successive steps:

  1. The construction of a 2-dimensional planar-cross formation model
  2. The calculation of synthetic logs along the drilled well path
  3. The actual and synthetic logs matching while drilling
  4. The model upgrading, the formation structure angle prognosis

The result is the definition of the bore stratigraphic position in the formation, as well the formation structure prognosis. The method graphic representation is shown in Fig. 1.

Figure 1. The method of matching operation concept.

The synthetic logs are based on the offset (reference) wells data. The offset (reference) well may be vertical or directional. Before drilling horizontal wells a reservoir model is created, which takes into account the regional structure dip angle obtained by the field geological model data. The synthetic logs along a horizontal well path are calculated basing on the reference well logs.

The actual horizontal well log is matched with the model synthetic log; then the model is upgraded for the maximum matching of the synthetic and actual logs. Thus, the wellbore stratigraphic position in the formation is defined and the formation structure performance is predicted basing on its regional dip angle and actual and synthetic logs matching. If necessary, the horizontal well path is corrected while drilling.

3. Determining the formation structure dip angle by the azimuthal log

Azimuthal logging provides more information than the average value or logging of one sector, but this entails cost increase. The azimuthal logging is the logging, which records the data from a particular sector (azimuth) of the borehole.

The curves of the upper and lower sectors are used for determining the formation dip angle along the well path. When the wellbore crosses the rock down the bedding, the resolution about the layer is provided first by the down sensor, then by the right and left side sensors and at last by the upper sensor.

When crossing the layer up the bedding, the order will be reverse. Using the spacing between the up and down sensor logs, we can calculate the layer structural dip angle in the path profile that makes it possible to determine the borehole current position.

The advantages of this method include:

  • An independent determination of the structure dip angle basing on the lateral homogeneity and the homogeneity of the bed thickness.
  • The direct determination of the borehole motion direction in the formation that prevents dual interpretation (for example, with the log matching method, two different sets of formation dip angles can sometimes give the same degree of correlation between the actual and synthetic logs). These problems never occur when using the image geo-steering.

But this method has several disadvantages, namely:

  • The lack of lithological contrast when drilling in a lithological homogeneous layer – the logging cannot fix changes even while drilling to the top or bottom of lithological layers.
  • The difficulty to determine the same layers correspondences of the up and down sensors loggings, in case the dip angle between the borehole and the bedding is small.
  • The high cost of logging

To achieve the geo-steering maximum effectiveness it is necessary to use combinations of the second and third methods described. With this approach, the formation structure dip angle will be evaluated by two independent methods and the evaluation accuracy will increase significantly. Besides, while drilling, it is imperative to get new log data (normal and azimuthal) as quickly as possible. Therefore the technique of transmitting the logging data in on-line mode becomes more widespread. In this case, geo-steering engineers may exclude routine procedures of data loading from their duties and give full attention to the support for well drilling.

In this regard the software packages of geological support for wells drilling have been developed and are actively being developed. Modern software solutions support the ability to combine the static geological models data with the results of different geo-steering methods. This helps geologists greatly improve the efficiency of optimizing the wellbore position.

The company Geosteering Technologies has developed the Software Package “Geonaft” (SP), intended for geological support for directional and horizontal wells drilling aimed to the most successful bore positioning in a target horizon.

The "Geonaft" allows using the methods of logging matching and the formation structure dip angle determination by azimuthal logging in a simple anduser-accessible form. The SP can also load data automatically in on-line mode. The SP general view is shown in Fig. 2.

Figure 2. The SP “Geonaft” general view.

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