Optimization of Well Length for Horizontal Drilling (2000)

Abstract Increase in the horizontal well length increases drainage area and well construction cost for drilling and related completion as well as other factors that decrease economics of the horizontal well. However, no reliable standards are currently available that take into account for the quantitative factors necessary for the determination of optimum well length for horizontal drilling. This paper presents the methodology developed to obtain the optimum horizontal well length based on the total economics as well as productivity index (PI) change associated with frictional loss effects in the long section of horizontal wellbore. The effects of various parameters affecting both productivity and economics of horizontal well drilling are analyzed quantitatively. The parameters involved in the analysis are: the change in productivity index due to frictional losses, reservoir characteristics (heterogeneity), well construction cost incorporated with learning curve, cash flow of the project Net Present Value (NPV), and drainage area in vertical/horizontal well. Integrated approach is proposed for the analysis of these parameters in order to achieve the overall optimum performance of a horizontal well drilling. This paper also presents how the optimum horizontal well length can provide economic benefits for the specific reservoir conditions. Step by step calculation procedure with examples is described with sets of graphical charts developed for illustrating the relationships between various parameters. The results can be used directly to determine the optimum horizontal well length by performing the sensitivity analysis of parameters involved. Introduction One of the advantages of the horizontal well is to achieve large reservoir contact area. As the well length for horizontal drilling is increased, its drainage area is also increased. Initially, it was believed that a horizontal well should be as long as possible. Current drilling technology allows wells to be drilled several thousands of feet long. However, a factor exists that can possibly limit the useful length of a horizontal well, i.e. frictional losses in the wellbore.1 Long horizontal wells or high flow rates result in increase in the frictional losses. This may be comparable with the drawdown at the producing end of the well. A portion of the downhole well would then be unproductive because of frictional losses. Recent experience with horizontal wells has revealed that in many circumstances the inflow performance of horizontal wells do not match with the expected productivity and that their deliverability may be affected by frictional losses along the wellbore. 2 This effect has serious implications where horizontal well section is very long because the productivity index is no longer directly proportional to the well length. A survey3 shows that horizontal drilling can be used on almost all reservoir setting, and its success rate reaches up to 95%. It provides convincing evidence that the implementation of horizontal drilling techniques has become almost routine. However, economic success of horizontal projects has not been widespread because only 54% were reported as economically successful. 3 Another point in horizontal drilling is the incremental expense over conventional drilling.