Abstract

With the recent advancements in drilling technology, drilling undulating wells, which specifically target various zones in a reservoir, has become a rather routine application. In accordance, drilling engineers are able to steer the well to coincide with pre-established sinusoidal trajectories. This advancement in drilling practices calls for the enhancement of our understanding of the production characteristics of undulating wells. This paper provides an in-depth comparative analysis of production performances of open-hole completed and cased horizontal/sinusoidal wells.

Horizontal wells offer great advantages in thin-layered, tight sand gas reservoirs as they increase the productivity through a larger contact area with formations. In this study, a comprehensive performance comparison of open-hole vs. perforated horizontal wells in thin, tight sand gas reservoirs is presented. This is accomplished using a 3-D, single-phase flow model, which is capable of accommodating vertical, horizontal, undulating, perforated, and fractured wells. Effects of drilling damage and crushed zones are considered in the simulations. The parametric study presented in this paper includes various phase angles, shot densities, perforation lengths, and number of fractures. Additionally, the study considers effects of uneven perforation lengths on the topology of the flow fields generated around the perforations. The flow performance comparison is further extended to undulating wells in thin-layered, tight sand gas reservoirs with shale streaks. In the presence of shale streaks, it is observed that undulating wells, which penetrate individual isolated layers, perform more effectively than horizontal wells.

Introduction

Horizontal wells have been instrumental in development of thin-bedded, tight sand gas reservoirs. In this paper, flow performances of open-hole and cased horizontal/undulating wells are compared in a tight sand gas reservoir with and without the presence of shale streaks. A 3-D, single-phase flow model1 is used in the comparisons. The model approximates physical boundaries of wells as a collection of fine grid cells and utilizes the residual equation to determine the amount of flow. Local grid refinement is implemented to avoid the generation of redundant fine cells. Details pertaining to grid generation, local grid refinement, and model verifications are reported earlier.1,2

First, a homogenous, anisotropic reservoir is considered, and flow performances of open-hole horizontal wells are investigated with/without drilling damage. Later, flow performances of cased horizontal wells (perforated/fractured) are examined for various perforation lengths, shot angles, shot densities and number of fractures. Finally, performances of undulating wells in the presence of shale streaks are studied.

Open-Hole Horizontal Wells

Goode and Thambynayagam3 presented an analytical solution for pressure drawdown and buildup analysis of horizontal wells located in semi-infinite homogeneous and anisotropic systems. Fig. 1 compares the pressure response derived from the model used in this study against Goode and Thambynayagam's analytical solution for pressure drawdown, and displays excellent agreement between the analytical and numerical pressure solutions.

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