Abstract
The amount of oil producible from a horizontal well can easily be reduced if the reservoir has external boundaries which are capable of attenuating further pressure transient propagation through the reservoir. Although several operational measures, such as well stimulation, well re-entry, can be implemented to further boost well production, wellbore length modification by tapering is investigated in this paper. The investigation involves development of a mathematical model for evaluating well performance up to first linear flow period for a tapered horizontal well in an anisotropic reservoir of finite reservoir thickness. Dimensionless radius in the flow model for dimensionless pressure of a horizontal well in a laterally infinite reservoir during linear flow is tapered. The effects of the tapered radius on unsteady state dimensionless wellbore pressure were studied for a hypothetical anisotropic reservoir. Both infinite conductivity and uniform flux cases are considered. The effects of near wellbore problems such as wellbore skin and storage are not considered. Results obtained shows that dimensionless pressure increases as dimensionless time increases and the time at which initial radial flow starts, persists further than completions that are not tapered. Furthermore, tapering delays time at which linear flow starts. Two scenarios were observed for well performance with respect to well length. In the first case, well productivity decreases as the length of the tapered string increases for a particular dimensionless time, while second case shows that performance increases when both dimensionless length and time increases. Reservoir, wellbore, and fluid properties are also found to affect well productivity no matter the number of tapered strings used during well completion. For example, as reservoir pay thickness increases, well productivity also slightly increases. Also, the influence of infinite conductivity and uniform completion is not felt strongly on well productivity whether the lengths are tapered or not. Study shows (1) prolonged infinite acting flow period compared to single tubing completion. This means that time to reach reservoir external boundary is increased with tapered completion than single tubing completion. (2) there is improved production rate compared to single tubing completion. This means that more oil is recovered per unit time for tapered completion. (3) tapered completion requires less frequent stimulation since damage is spread over a larger area instead of concentrating on only a single tubing with same diameter across the pay zone. (4) Mechanical damage is mitigated since larger tubing penetrates the pay zone. Reservoir and well completion engineers are now encouraged to employ tapered completions in horizontal wells for improved well production especially if the reservoir has external boundaries that are inimical to well productivity.
