Horizontal wells are gaining increased prominence in primary recovery and steaminjection - it is said that in ten years' time, 85% of all wells drilled willbe horizontal. Yet experimental and theoretical modeling of horizontal wellsare still inadequate. This paper explores both of these areas.
Equations were developed for three-phase and non-isothermal flow in thevicinity of a horizontal well, as well as flow inside the well. These were usedto obtain scaling criteria for designing laboratory experiments. The questionof skin factors for various types of perforations is also considered. Aninteresting idea offered is the use of variable diameter horizontalwells.
Two types of model experiments were carried out. In one case, isotherma1experiments were conducted in visual models, using gas and water, and varioustypes of horizontal wells. The results showed the flow patterns and the effectof gas segregation on well performance. The second type of experiments utilizeda steam model, with a concentric well. Temperature distributions weredetermined and used to interpret oil recoveries and pressure behavior.Limitations of horizontal well modeling methods are pointed out and solutionssuggested.
It has been reported extensively1,2 that horizontal wells offerprospects of better performance over conventional vertical wells. This improvedperformance is due primarily to larger contact area between the formation and ahorizontal well. This is especially true for thin oil formations where thereservoir contact area for a horizontal well can be hundreds of times that fora fully penetrating vertical well. Furthermore, horizontal wells whenstrategically placed can reduce water/gas coning and result in improvedcumulative recovery in view of the small drawdowns3. Horizontalwells are also believed to have a much better chance than vertical wells ofintersecting systems of vertical and horizontal fractures in an oil-bearingformation, and thus lead to higher production rates4. In steaminjection recovery of heavy oils and oil sands horizontal wells - especiallywhen combined with steam-assisted gravity drainage - could lead to highproduction rates, more uniform distribution of steam front leading to improvedsweep efficiency and cumulative oil recovery5,6.
One aspect that has been so far largely neglected in discussions abouthorizontal well performance concerns with the flow in the vicinity of andinside a horizontal well. Often, it has been assumed that a horizontal well hasinfinite conductivity and that the flow in the well is predominately laminar -meaning the pressure drop across the well is not significant.Dikken7 showed that this assumption is not necessarily true for mostpractical situations, and that pressure drops across horizontal wells cansignificantly affect their performance.
This paper attempts to contribute to an understanding of this aspect orhorizontal wells by presenting mathematical models or flow in the vicinity ofand inside a horizontal wellbore. These models were used to obtain scalingcriteria for a series of scaled model steam injection experiments. Data, including well pressure drop and fluid production rates in the presence of aperforated casing are also reported.
