Horizontal wells

Figure 3.1 Planned well trajectory for a horizontal well...

Carbonate rocks are more frequently fractured than sandstones. In many cases open fractures in carbonate reservoirs provide high porosity / high permeability path ways for hydrocarbon production. The fractures will be continuously re-charged from the tight (low permeable) rock matrix. During field development, wells need to be planned to intersect as many natural fractures as possible, e.g. by drilling horizontal wells. 

It is worth noting that if field development using horizontal wells is under consideration, then horizontal appraisal wells will help to gather representative data and determine the benefits of this technique, which is further discussed in Section 9.3. 

Keywords compressibility, primary-, secondary- and enhanced oil-recovery, drive mechanisms (solution gas-, gas cap-, water-drive), secondary gas cap, first production date, build-up period, plateau period, production decline, water cut, Darcy s law, recovery factor, sweep efficiency, by-passing of oil, residual oil, relative permeability, production forecasts, offtake rate, coning, cusping, horizontal wells, reservoir simulation, material balance, rate dependent processes, pre-drilling. 

One category of remaining oil shown in the above diagram is unproducible oil in thin oil rims, which cannot be produced without coning in unwanted oil and/or gas. Horizontal wells are an ideal form of infill well in this situation, and will be discussed in Section 9.3. 

Horizontal wells were drilled as far back as the 1950s, but have gained great popularity in the last decade, as lower oil prices have forced companies to strive for technologies which reduce the cost of oil and gas recovery. Horizontal wells have potential advantage over vertical or deviated wells for three main reasons ... 

The increased exposure to the reservo/r results from the long horizontal sections which can be attained (between 500m and 1000m horizontal section is now common). Because the productivity index is a function of the length of reservoir drained by a well, horizontal wells can give higher productivities in laterally extensive reservoirs. As an initial estimate of the potential benefit of horizontal wells, one can use a rough rule of thumb, the... 

The geometry and reservoir quality have a very important influence on whether horizontal wells will realise a benefit compared to a vertical well, as demonstrated by the following example. 

Figure 9.5 Productivity improvement factor (PIF) for horizontal wells...

In the case of the very low vertical permeability, the horizontal well actually produces at a lower rate than the vertical well. Each of these examples assumes that the reservoir is a block, with uniform properties. The ultimate recovery from the horizontal well in the above examples Is unlikely to be different to that of the vertical well, and the major benefit is in the accelerated production achieved by the horizontal well. 

The PIF estimate is only a qualitative check on the potential benefit of a horizontal well. There is actually a diminishing return of production rate on the length of well drilled, due to increasing friction pressure drops with increasing well length, shown schematically in Figure 9.6. 

Figure 9.6 Production rate vs horizontal well length...

Horizontal wells have a large potential to connect laterally discontinuous features in heterogeneous or discontinuous reservoirs. If the reservoir quality is locally poor, the subsequent section of the reservoir may be of better quality, providing a healthy productivity for the well. If the reservoir is faulted or fractured a horizontal well may connect a series of fault blocks or natural fractures In a manner which would require many vertical wells. The ultimate recovery of a horizontal well is likely to be significantly greater than for a single vertical well. 

The third main application of horizontal wells is to reduce the effects of coning and cusping by changing the geometry of drainage c ose to the well. For example, a horizontal... 

Figure 9.8 Gas cresting in oil rim development with horizontal wells...

Completions in horizontal wells are also tailored to the individual reservoir. Figure 9.16 shows some options tor completing horizontal wells. 

Joshi, Sada D., (1991), Horizontal Well Technology, Pennwell... 

Sohd salt, ground and packaged in several particle size grades, can be used in saturated salt brines to increase the fluid density (28). However, sized salt is most often used as a water-soluble material for bridging or sealing porous formations. At one time the sized salt systems were used primarily for completion or workover operations, but use has increased as ddU-in fluids for horizontal wells (29). 

In Situ Air Stripping. An innovation to conventional pump and treat air stripping is in situ air stripping. Two horizontal wells are installed, one below the water table and one in the vadose zone. Air is injected in the lower well while contaminated soil vapor is extracted by vacuum through the upper well. 

Direction-Focused Gamma Rays. It is important to keep the trajectory of horizontal or nearly horizontal wells in the pay zone. By focusing the provenance of the gamma rays it is possible to determine if a shale boundary is approached from above or from below. 

An example of three horizontal wells drilled in a 2 m (6 ft) in the North Sea is shown in Figure 4-346. 

Suggestions have been made recently to drill horizontal branches in the horizontal portion of the horizontal wells. If this technique is developed, the drainage capacity of horizontal w ells will be even better. 

Figure 4-348 shows the proposed trajectory of a horizontal well. The planned mud weight is 12 Ib/gal water base. The drillstring will be as follows ... 

Jan, Y. H., and Harrel, J. W., MWD directional-focused Gamma Ray—A new tool for formation evaluation and drilling control in horizontal wells, SPWLA 28th Annual Logging Symp., p. A-1-17, 1987. 

Joshi, S. D., Horizontal Well Technology, PennWell Publishing Co., Tulsa, Oklahoma, 1991. 

Mixed metal hydroxide drilling muds have been successfully used in horizontal wells in tunneling under rivers, roads, and bays for drilling in fluids for drilling large-diameter holes with coiled tubing and to ream out cemented pipe. 

H. Dakhlia. A simulation study of polymer flooding and surfactant flooding using horizontal wells. PhD thesis, Texas Univ, Austin, 1995. 

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