CHAPTER ONE

1.0          INTRODUCTION

1.1            Background of Study

An oil rim reservoir is a saturated reservoir with an oil column of limited thickness in the order of tens of feet, overlain by a gas cap and underlain by an aquifer. These reservoirs are common throughout the world, and despite their low pay thickness they can still contain substantial volumes of hydrocarbon-in-place (Fajhan and David, 2007). Ezzam et al. (2010) opined that maximizing oil recovery in this type of reservoirs is done by keeping the oil rim in contact with the producing wells at all times which is achievable by balancing the water-oil-contact (WOC) and gas-oil-contact (GOC) movement. For a thin oil rim reservoir with a large gas cap and strong aquifer, achieving the said goal is very challenging (Ezzam et al., 2010; Vijay et al., 1998). Rahim et al. (2013) mentioned water/gas coning and breakthrough, spread out resources, complicated production mechanism, presence of transition and invasion zones, oil smearing, and a low recovery factor of less than 18% as technical challenges of developing oil rim reservoirs.

Among the various challenges of producing oil rim reservoirs, water/gas coning and breakthrough is of utmost consequence. Coning is the mechanism whereby gas or water moves toward the production interval of an oil well in a cone or crestlike manner created by fluid production. It is caused by the pressure drawdown within the oil column close to the wellbore being sufficiently large to overcome viscous and gravity forces and draw the water

or gas into the well. The specific problems of water and gas coning are:

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·                     Costly added water and gas handling

·                     Gas production from the original or secondary gas cap reduces pressure without obtaining the displacement effects associated with gas drive

·                     Reduced efficiency of the depletion mechanism

·                     The water is often corrosive and its disposal costly

·                     The afflicted well may be abandoned early

·                     Loss of the total field overall recovery

Delaying the encroachment and production of gas and water are essentially the controlling factors in maximizing the field's ultimate oil recovery.

This problem of water and gas coning is so severe that exploitation of oil rims by means of vertical wells becomes technically and economically infeasible. When a vertical well is drilled through an oil rim reservoir, the length of the reservoir contact in the oil column is small. This low reservoir contact area and the large pressure drop that is associated with flow into a vertical well means that such wells are highly susceptible to coning. Since early 1980's horizontal wells have been used in such situations to defer the water and gas ingress and thus prolong the life of the producer (Vijay and James, 1998). Rahim et al (2013) stated that the use horizontal wells can significantly increase the well contact to the reservoir and can improve the well productivity even up to five times of the vertical wells in the oil rim reservoirs. Thus, the use of horizontal wells has made development of oil rim reservoirs economically viable.

Despite the fact that there are substantial advantages of using a horizontal well over a conventional vertical well in developing an oil rim reservoir, Kabir et al. (2004) stated that placement of horizontal wells in a thin-oil column (< 40 ft) is a challenge and depends on

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relative drive indices of the gas cap and the aquifer. Also, -toehe “heeffect”l in h wells, characteristics of the fluids involved, and variations in permeability can result in

unbalanced inflow along the horizontal section and accelerate early water breakthrough and uneven inflow downhole. Hence, horizontal wells are still subject to dual water and gas cresting. In the backdrop of this, research efforts were significantly increased not only to optimize the operations, but also to find supporting technologies to further mitigate the problem of coning in oil rim reservoirs.

To better control the water and gas movement within the limited oil column, horizontal wells with smart well completion (e.g., ICD, ICV, etc.) serves the purpose. Fajhan and David (2007) stated that the employment of intelligent completions in horizontal wells for developing thin oil column reservoirs has already produced many benefits. The application of Inflow Control Device (ICD) is to enhance sweep efficiency and hydrocarbon recovery by suppressing water and gas cones from high rate zones/area created either by heterogeneity or “heel-to-toe effect”in the horizontal wells. This will be achieved by evenly distributing the drawdown pressure along the entire well length and hence both the WOC and GOC move toward the horizontal leg in a more uniform pattern.

This study will investigate the opportunity that intelligent completions (inflow control devices, ICD) provide for efficient oil recovery from thin oil column reservoirs. A dynamic simulation study of an oil rim reservoir in the Niger Delta will be done using a dynamic simulation tool, in an attempt to highlight the benefits of developing an oil rim reservoir using a horizontal well with ICD over a conventional horizontal well without ICD.

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