Abstract

Production enhancement and ultimate recovery improvement have given horizontal wells the edge over vertical wells in many marginal reservoirs. However, it is more expensive to drill and complete a horizontal well than a vertical one. Therefore, to determine the economical feasibility of drilling a horizontal well, engineers need reliable methods to estimate its productivity.

After a broad literature review, a simple semi-analytical model has been developed in this study for predicting the productivity of horizontal oil wells. This model couples flow from a box-shaped drainage volume to flow in the wellbore. Along with friction, acceleration and fluid inflow effect, change in flow regime from laminar to turbulent is also taken into account to describe flow in the wellbore. The reservoir inflow model used in this productivity model represents flow in the reservoir using a combination of one-dimensional and two-dimensional models and also considers varying skin along the wellbore to account for the heterogeneity of near wellbore region due to drilling fluid invasion into formation. In addition, reservoir permeability anisotropy and convergence of flow to the wellbore have been taken into account in this inflow model. Comparison of this model with three existing models using field data reveals that the proposed model is more accurate due to more realistic modeling of reservoir inflow and wellbore flow.

Semi-analytical nature of this coupling model makes it comprehensive and applicable to reservoirs with varying conditions, especially heterogeneous reservoirs. Moreover, this productivity model can be easily extended to estimate the deliverability of multilateral wells by coupling the inflow performance of individual laterals with hydraulics in build-up sections and the main vertical section. A logical procedure for calculating the deliverability of multilateral wells by using this productivity model is described in this paper.

Introduction

A worldwide interest exists today in drilling horizontal wells to increase productivity. Productivity of a horizontal well can be greater than that of vertical wells for several reasons. First, horizontal wells can be open to a larger portion of the reservoir than vertical wells. A larger contact area allows lower drawdown to recover more oil and gas. Horizontal wells can be drilled perpendicular to oriented natural fractures and therefore intersect with more fractures. Also, it may be possible to induce multiple hydraulic fractures in a horizontal well. Increased productivity is not the only benefit of horizontal wells; improved sweep efficiency, reduced coning of water or gas and increased drainage area are other advantages of horizontal wells over vertical wells. Therefore, horizontal wells are believed to perform better than their vertical counterparts in thin reservoirs, naturally fractured reservoirs (double-porosity and discretely fractured), reservoirs with gas and water coning problems, reservoirs with favorable vertical permeability anisotropy, offshore environments where various wells are drilled from a central platform, and in various enhanced oil recovery projects.

Recent interest in horizontal wells has been accelerating because of improved drilling and completion technology. This has led to increased efficiency and economics in oil recovery. Increases in oil production rate and improvement in ultimate recovery has given horizontal wells the edge over vertical wells in many marginal reservoirs. However, it is more expensive to drill and complete a horizontal well than a vertical one. Therefore, to determine the economical feasibility of drilling a horizontal well, engineers need reliable methods to estimate its expected productivity.

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