A Comparative Analysis of Pressure Transient Behaviour of Undulating and Horizontal Wells

Since the early 90's, the use of horizontal wells has become increasingly more popular in the development of oil and gas fields. Advances in drilling technology such as Measurement While Drilling (MWD) and Logging While Drilling (LWD) with accumulated knowledge/experience enable cost effective and more sophisticated horizontal wells to be drilled. Nevertheless, drilling a horizontal well that is perfectly parallel to the bedding plane is still distant from reality. All horizontal wells, to a certain extent, follow a sinusoidal trajectory merely because of steering problems, natural undulation of layers as well as presence of non-uniform layer thickness. In horizontal well applications ten to fifteen feet amplitude undulations are often considered as precise placement. Although MWD and LWD procedures minimize the possibility of exiting pay zones, while drilling horizontal wells in relatively thin formations there is always a real possibility to leave the pay zone even for a short period of time. A number of studies have appeared in the literature for pressure transient analysis of horizontal wells for rapid determination of permeability anisotropy, skin factors and average pressures. However, none of these forward and their accompanying inverse solutions discusses the validity and applicability of the offered protocols if the horizontal well in actuality is undulated. In this study, we analyse pressure transient response of undulating wells using a previously developed numerical model that can accommodate various types of well architectures including undulating wells. Our objective in this paper is to demonstrate when analytical solutions for pressure transient analysis of horizontal wells are still usable and when they become unusable for various undulation configurations studied, and show how numerical well test analysis procedures can be utilized when analytical models are inadequate. Example scenarios investigated in this paper show that the presence of intensive undulations in well trajectories may significantly shorten and even may overshadow the transient flow period that is already endangered by the presence of wellbore storage affects. Under these conditions the use of a numerical model, with options for undulating well architectures, will be necessary for accurate analysis of pressure transient data.