An Insight Into Development of Bottom Water Reservoirs
Authors: Zhao G. , Zhou J. , Liu X. 
Published: March 2010

While water production is an inevitable consequence in bottom water reservoirs, it is usually desirable to defer the onset or the rise of water coning as long as possible. Numerous mechanical and chemical methods have been applied to achieve this goal over recent decades. This paper presents new insights into improving oil production and reducing water production by considering flow barriers below horizontal well trajectories in formation regions with low permeabilities, especially natural ones such as shale bodies.

A 3D numerical simulation that applies the Computer Modelling Group's (CMG) STARS Simulator as a cost-effective way to investigate the effects of barriers on horizontal well performance in a bottom water reservoir has been conducted. More specifically, the effects of permeability, dimension, and position of barriers have been comprehensively analyzed when a horizontal well is implemented as a producer. The simulation results have shown that if barriers exist below a horizontal producer, water cut can be postponed and reduced greatly, and cumulative oil production can be increased. Cumulative water production can be decreased dramatically as well. To broaden the applicability of this new insight, some of the possible field implementing technologies, including fractured horizontal wells, small-scale CO2 injection, and solvent injection, are qualitatively simulated to determine their applicability for developing heavy oil in bottom water reservoirs using horizontal wells with the presence of barriers. The simulation results have shown that much better well performance can be reached with the help of barriers when these technologies are integrated systematically.

This new strategy shows a rather promising and economic way to develop bottom water reservoirs where the natural driving energy of the aquifer could benefit the oil production process. The results and understanding acquired from this study offer insights into the development of bottom water reservoirs.

Introduction

Water coning is a critical issue for conventional vertical well production in bottom water reservoirs. Generally speaking, the fluid production process creates a low-pressure region around the wellbore in the reservoir. This differential pressure causes the oilwater interface to deform into a cone shape, at which time the less viscous water phase is produced in preference to the more viscous oil phase. Consequently, the producing Water-Oil Ratio (WOR) increases quickly and readily reaches an uneconomic level. Darcy's law is still the fundamental principle behind this phenomenon. In the past, many researchers have conducted experimental, analytical, and numerical studies on water coning behaviour in vertical wells. Muskat and Wyckoff(1) published one of the early studies related to water coning in 1935. They observed that water coning is a rate-sensitive process and determined the critical oil rate, which is the maximum water-free production rate, by neglecting the shape of the cone. Later on, researchers(2, 3) directed their studies toward the calculation of the critical oil rate. Guo and Lee(4) indicated that the critical rate does not occur at zero wellbore penetration, as may intuitively be expected, but at a wellbore penetration of about one-third of the total oil-zone thickness for an isotropic reservoir.