As the length of a horizontal well is increased, its contact with the reservoir increases. But at the same time, the resistance to flow in the well also increases, which has a direct negative effect on the productivity of the well. The overall performance of a horizontal well depends on the balance of these two opposing factors. No reliable tools are currently available that account for both these factors in the evaluation of horizontal well performance. A semi-analytical well-model is developed which can quantify the effects of both single phase oil and two-phase oil/gas flow pressure loss in the well on the overall well performance. The model is quite flexible and can incorporate any friction factor correlation. A methodology is developed to show the effects of various reservoir, fluid and well parameters on well productivity. We demonstrate that ignoring frictional effects could lead to unrealistically higher production estimates and low breakthrough times for water or gas. As a result of pressure drop in the well, breakthroughs occur first at the heel of the well. A methodology is also developed to calculate the optimum horizontal well length.
Most of the analytical work done in the past on horizontal well productivity either assumed that the well is infinitely conductive or the flow is uniform along the entire well length. References 1–6 are based on the assumption of steady state flow in the reservoir, 7–9 are for pseudo steady-state flow conditions and 10–16 are for transient flow. The assumption of uniform flow was made purely for mathematical convenience. The infinite conductivity assumption is a good assumption only in certain cases when the pressure drop in the wellbore is very small compared to the drawdown in the reservoir, otherwise, the pressure drop in the wellbore should also be taken into account.
Previous work can be categorized into three types:
Models for wells that are infinitely conductive and thus are not influenced by pressure drop in the well,
models where the reservoir is represented by an analytical model for single phase flow, and
general models that couple multiphase flow simulators with wells.
A horizontal wellbore can be treated to be infinitely conductive if the pressure drop in the wellbore is so small that ignoring it would not result in significant errors. Analytical equations with such an assumption are available in the literature. Based on the nature of the flow, any of the available equations can be used for a quick performance evaluation of a horizontal well in a single phase, homogeneous reservoir. A numerical simulator, on the other hand, can be used for a thorough analysis of a complex reservoir. Unless specified otherwise, most of the commercial simulators currently assume infinite conductivity in the wellbore. Using an analytical equation or a numerical simulator with the infinite conductivity well assumption for cases with significant pressure drop in the wellbore can lead to errors in the estimates.
Realizing the importance of wellbore pressure drop, a few groups have developed techniques to evaluate the effects of wellbore friction. This section outlines the available methods that couple the wellbore to an analytically approximated reservoir. The behavior of many homogenous, single phase reservoirs can be approximated analytically. Such analyses, though not too general in their applicability to complex reservoirs, give a good understanding of the nature of the problem. None of the approaches presented in this section take into account multiphase flow, or radial inflow effects in the wellbore.