Jaroslav Porada, Head of Horizontal Directional Drilling Division

Horizonty Group of Companies owns a large fund of old wells with high reuse potential. When designing the side track profile, experts faced the lack of inclinometry data of the old wells or the availability of data that is not confirmed (type of inclinometric device and its characteristics are unknown). This situation increases the risk of falling within a nearby well.

The next and the very important problem: once the well coordinates were restricted information. Thus, it is either lost or has a restricted access. It is often the case that the well coordinates do not reflect the real state of things.

Desiring to do everything well from the first time, our specialists began to take on the experience: how does it happen in international companies as they progress design stages .
It should be noted that all the rules, regulations, mathematical models for measuring the well profile are created by the ISCWSA (TheIndustrySteeringCommitteeonWellboreSurveyAccuracy) international organization. The organization acts solely on a volunteer basis and includes the representatives of all service companies and operators such as BP, Shell, Schlumberger, Weatherford, etc. Documents published by the ISCWSA are optional, but on their basis these companies create rules for sidetracking, high anti-collision risk drilling and measuring the well profile.

The organization issued two documents (SPE 67616 AccuracyPredictionforDirectionalMeasurementWhileDrilling and 90408 PredictionofWellborePositionAccuracyWhenSurveyedWithGyroscopicTools), describing the mathematical model of error while aligning with telemetry systems and gyroscopic inclinometers. Another published document ("Definition of the ISCWSA ErrorModel") introduces the concept of the "error code" and gives its definition. Why is it important to us? Each measuring device has its own accuracy according to its individual characteristics, it is assigned the error code, which is determined empirically. There is a file for each inclinometer containing all tolerances, errors and formulas used to determine the inaccuracy of well profile positioning.

What is the practical application? In our meaning, the well is a cylindrical object. It has a larger diameter on top, while a smaller diameter on the bottom. But when it comes to design, the shape of the well conditionally acquires the shape of a cut cone with a smaller size near the mouth of the well and its increase to the deep. It is called the "uncertainty ellipsoid". This is the amount of space where a well can be. Depending on how we measure, by what means we measure, whether we follow procedures or whether we meet all standards, the size of the uncertainty ellipsoid may vary.

What is the practical application? When designing, it is necessary to choose a profile so that the uncertainty ellipsoids do not intersect and do not touch. In doing so, we reduce any risk of intersecting with adjacent wells. That is, by having the appropriate error code and applying it correctly, we remove the risk of falling within a nearby well.

Let us look at the above with a simple example. Practically, there are four cases.
1. BLIND – no inclinometry data.
2. Inclinationonly (INC-ONLY) – should the inclinometry includes only zenith angle and depth readings.
3. MWD – measurements made through the telemetry system.
4. SPT-GTD – measuring the gleam of the well using gyroscopic inclinometer.

If we apply 4 different types of error for the same profile, then at a depth of 5,000 meters the size of the uncertainty ellipse will be 5.5 kilometers. It is the worst case. This approach to risk management was accepted by global service companies Schlumberger, Weatherford, BakerHughes, Halliburton. The second case provides for only zenith angle and depth (no azimuth). The situation is better, but the size of the uncertainty ellipse is 2,400 meters. The last two options are the availability of telemetry or gyroscope measurements. In that case the size of the uncertainty ellipse at a depth of 5,000 meters will be 42 and 29 meters, respectively.

So, if positioning accuracy of the wellbore is 42 m, then the geological target cannot be less than 42 m in order to guarantee the falling of the wellbore within the geological target with a probability of 95%. It is very important to use more accurate tools and choose the geological target according to the measurement program.

As the table shows, the most accurate is the gyroscopic inclinometer. It is not affected by metal objects.

If we want to comply with international standards, follow the example of the Big Four, we need to apply the same approaches that big companies use.

Having studied the market for the provision of gyroscopic inclinometer services, we realized that there are inclinometers in Ukraine, but they do not have ISCWSA certification. Accordingly, the measurement results will not be valid.

Our company decided to purchase a complete set of equipment that is certified and awarded by international organizations. It can be used to determine the profile of existing wells without stopping theie operation. The company’s geodetic service has high-precision tools to determine the coordinates of the wellhead.

The use of gyroscopic inclinometer reduces the risk of intersection with a nearby well, increases the probability of falling within geological target and reduces the operational drilling costs by waiving measurements using telemetry system.