Cost-effective vertical and fast drilling technology has been of great interest to drilling contractors of most oilfields, but it's difficult to implement. Conventional anti-deviation drilling methods are of limited use when confronted with high-tilt angle geologic structure. Systems such as VDS, Power V and VertiTrak can be used to control the deviation angle of a vertical hole, but the rental costs associated with them are high, and sometimes, prohibitive. Therefore, there is great potential for developing a new anti-deviation technology that is cost-effective.
Slide steering BHA with two stabilizers and a bent PDM shows good anti-deviation effect when it is in rotary driven on the surface of low-dip wells. This working mode can be termed as coupled mode because the bit is driven by both PDM and rotary table/top drive equipment. The whirl feature of this kind of BHA can helps to eliminate the deviation force caused by the asymmetry point direction of bit axis. At the same time the lateral vibration of BHA introduces a large amount of anti-deviation force on the bit. Simulation results show that this kind of anti-deviation force 1)is much larger than that of the pendulum BHA in the same tilt angle hole, and 2) decreases with tilt angle of hole. The calculation model involved is illustrated later in the paper.
The method above has been validated through experiments and applications in Tuha oilfield. 12 vertical wells in easily deviated geological structures have been successfully drilled applying this technology. As a result, not only the deviation angle is controlled effectively, but also penetration rate improves significantly. In particular, drilling period is reduced by 9.26 days, a 25% reduction as compared to the whole working time needed when conventional method was used.
In theory, how to drill a vertical well fast at low cost is a well-known difficult problem in the domain of well path control technology. A new anti-deviation technology with low cost would be used world widely, especial in land drilling of China.
There are two cases to drill a vertical hole. The first case is a whole vertical well, which is still a very important way to explore a new oil and gas source area. The other is the vertical hole section in the top of directional well. Almost all of the directional wells were designed with a vertical top hole section. Someone may think that a vertical well should be easier to drill and be more cost effective contrary to a deviated well. However, under certain conditions drilling the vertical bore hole course could be difficult, especially if the formation tends to deviate the bit from the planned well path, causing unacceptable well tortouosity and friction1.
It is validated that stiff or pendulum bottom hole assemblies could not maintain a vertical bore hole course especially in steep dip formation. The bias-shaft method2 and large WOB method3 were tested in China, but neither of them could solve this problem. High well tortuosity and doglegs were no peculiarity in certain areas when these methods were used. Moreover, "Repeated trips will increase non-drilling operation time in deep wells, thus frequently results in down hole problems"1. Effective deviation control could not be reached even if both WOB and ROP are very low.
Steerable motor directional technology may be used in this case and can drill a well vertically, but its significant disadvantages made this method cannot drill a perfect straight hole. A high tortouosity well path can be drilled by alternating between sliding and rotary mode frequently. This is also a time consuming method. Not only the required orientation of the tool face may take a lot of time, but also the maintaining orientation may slow down drilling progress. Moreover a modern surveying equipment such as MWD is needed to meet the need of these operations. Extra investment may go beyond the budget as a vertical well.