The problem of water/gas coning into Iranian oil wells became one of the major concern in terms of productivity, increased water disposal and environmental effects.
Water/gas coning is a phenomena caused by an imbalance between the gravitational and viscous forces around the completion interval. This results in lower revenues and increased operating costs. Most of the works have been done by many investigations are associated with conventional oil and gas reservoirs. Very few studies are considered the occurrence of water/gas coning in naturally fractured reservoirs. Most of the productive oil wells in south of Iran are located in naturally fractured reservoirs. This work shows the actual water and gas coning data from Iranian natural fracture reservoirs. From these results, the phenomena of water/gas coning in natural fractured reservoir are explained.
Production from oil reservoirs overlying gas or underlying water or from a gas reservoir underlying water is always associated with coning. Water is in general produced from oil wells at a water cut that depends on the well and reservoir characteristics.
Coning can be avoided if the well is produced below its critical rate which is the maximum water-free or gas - free production rate. Critical rates have been studied extensively. The earliest documentation of critical rate studies was presented by Muskat and Wyckoff 1. Since then many researchers have considered flow parameters such as: unsteady state, pseudo - steady state, permeability heterogeneities, multi phase flow, oil zone thickness, ratio of gravity and viscous forces, well penetration, mobility ratios and horizontal wells into the critical rate calculations. The primary differences in the calculation methods are the assumptions made to implement simplifications. Muskat and Wyckoff assumed linear flow, whereas Meyer and Garder 2 invoked radial flow. Probably the earliest documentation of calculating the critical rate for water/gas coning for natural fracture reservoirs was presented by J. Birks 3. One of the studies about water coning in fractured reservoirs is done by Van Golf-Racht and Sonier4. For further review of coning see Chierici et.al. 5, Wheatly 6, yang and Wattenbarger 7, Chaperone 8, Guo and Lee 9, Al-Afaleg and Ershaghi10, Azar-Nejad and Tortike11 and Beattie and Roberts12.
Water coning studies can be subdivided into two chronologically distinct groups. The first group of studies published before 1970's deals mostly with the vertical wells and the second group of more recent studies deals mainly with horizontal wells. Most of the studies on vertical wells are either analytical or experimental whereas most of the studies on horizontal wells are either analytical or numerical. Recent advances in computer technology has caused to shift from scaled models to simulate models for complexity of actual reservoir problems.
In this work actual field data from the natural fractured reservoir wells are used to study the true mechanisms of coning.
Various investigators have attempted to provide coning models to determine; Critical rate, Breakthrough time and Water cut performance after breakthrough.