Water And Gas Coning/ Cresting A Technology Overview
Authors: Singhal A.K. 
Published: March 2010

Ever since Muskat presented his approximate theory on water coning in 1935, the primary research focus has been on design and performance optimization. Subjects such as critical production rates, time for the cone to break through, and production after break-through have been studied with physical, analog, analytical and numerical simulation models. Guidelines have been developed for optimal placement of wells in a stratified system and for situations involving simultaneous coning of water and gas.

For most practical situations, ultimate oil recovery was determined to be virtually independent of producing rates. This gave rise to the concepts of infilling wells in reduced spacing, producing wells at higher fluid rates, and periodic re-completion of wells to minimize coning thus mlpdmizing reserves produced. Associated problems such as haridrrng of large amounts of high water-cut effluent and treatment of effluent for the re-injection of water into the reservoir also received technological attention.

The advent of horizontal wells during the 1980's resulted in a significant increase in ‘clean’ oil rates and reserves (without coning). As a result, many otherwise marginal or uneconomic pools became viable. Consequently, the research efforts were, doubled not only to optimize operations, but also to find sup: porting technology to mitigate coning.

Two fundamental problems remain with conventional (vertical) and horizontal wells:Sustaining oil rates at economic levels andHandling and disposal of the effluent.

Several ideas have been developed and some have been tried with varying degrees of success. These include:Placing barriers to movement of water and gas around the well. These involve introducing water, oil, gas, gels, foams, polymers, or cementing agents to preferentially reduce permeability or relative permeability to water or gas; injecting oil into the water or gas zone; injecting gas into the water zone; and injecting foam into the gas zone to reduce relative permeability to gas.Modifying flowing pressure distribution around wells by i) producing water or gas via separate completion intervals' in the same well, or ii) reversing coning of oil into water or gas zone;Minimizing operating costs via sub-surface water separation and disposal of the separated water in the same well.

This paper presents a brief review of the current state-of-the-art.

Introduction

In many situations, recovery of oil is limited by excessive production of water or gas. After a period of low (or no) water and gas production, water-oil ratios and/or gas-oil ratios begin to increase substantially. Under these conditions, production rates may have to be restricted due to equipment, operating, regulatory, or environmental constraints. This results in not only reduced oil rates, but also in increased cost of handling (separation, processing, and disposal) of effluents and ultimately, in a rather premature shutting-in of the wells. Due to significant economic implications, the phenomenon of water and gas coning has received some attention in the literature.

What is Coning?

Over geological times, the reservoir fluids (oil, water, and gas) exist under static pressure (gravity-capillary) equilibrium.