Abstract

The water coning caused by the imbalance between gravity and viscous forces is the most important reason for water production in different fractured reservoirs. There are various controllable and uncontrollable parameters affecting this phenomenon. In this study different dynamic models were constructed to search for the key parameters affecting the coning process in both single-well and Cartesian multi-well models.

It has been determined that oil layer thickness, perforation thickness, fracture permeability and its orientation, especially horizontal not vertical fracture permeability, production rate, mobility ratio, and fracture storativity have the major role in water coning phenomenon. Also it has been determined that fracture spacing, aquifer strength and skin factor have insignificant effect on water coning in fractured reservoirs. The variation of water breakthrough time respect to each effective parameter has also been studied. We concluded that for any production program or adjusting the wells location, the parameter study is very important.

Multi-well studies using an Iranian fractured reservoir data show that the trend of dependency of water coning on each parameter is similar to the single-well model. However, in field scale, it is necessary to have all reservoir data including well location, and production history for a successful water coning simulation because a small pressure drawdown exerted by a far well will affect the cone shape and its breakthrough time.

Introduction

The production of water from oil producing wells is a common occurrence in oilfields. It may be attributed to one or more reasons such as normal rise of oil water contact, water coning, and/or water fingering. The water production increases the operating cost and it may also reduce both reserve and recovery.1 Among these mechanisms, water coning is a serious problem in many oilfields especially in some large Middle East oil reservoirs, where the oil zone has an aquifer underneath whether or not it serves as an active drive.2

Water coning is caused by an imbalance between the gravitational and viscous forces around the completion interval 1. In other words the flow of oil from the reservoir to the well introduces an upward dynamic force upon the reservoir fluids. This dynamic force due to wellbore drawdown causes the water at the bottom of the oil layer to rise to a certain point at which the dynamic force is balanced by the height of water beneath that point (Figure 1). Now, as the lateral distance from the wellbore increases, the pressure drawdown and the upward dynamic force decrease. Thus, the height of the balance point decreases as the distance from the wellbore increases. Therefore, the locus of the balance point is a stable cone shaped water-oil interface. At this stable situation oil flows above the interface while water remains stationary below the interface. 3

The extent of cone growth and/or its stabilization in conventional reservoirs depend on different factors such as mobility ratio, oil zone thickness, the extent of the well penetration, and vertical permeability; but the most important parameter is total production rate. 2 In case of fractured reservoir this problem is more complicated because a dual porosity system results in formation of two cones.

Depending on the rates, it may be developed a fast moving cone in the fracture and a slow moving one in the matrix. The relative position of the two cones is rate sensitive and is a function of reservoir properties.4 While there are many theoretical works in the case of conventional reservoirs 5, only limited analytical works are available for the ideal cases of fractured reservoirs such as Birks theory. 6, 7

In fractured reservoirs, critical rate are influenced by extra factors such as fracture storativity w, fracture transmissivity l, fractures pattern and their interaction to matrixes especially around the wellbore. Shorter breakthrough times and lower critical rates are predicted for fractured systems.4 In this study some simulation models have been constructed to analyze the effect of different parameters on water coning in single-well models (Figure 2). Furthermore some models have been constructed to analyze the coning phenomenon in multi-well configuration (Figure 3).

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