Abstract

A study has been initiated to determine the best development scheme to recover oil and gas from two oil rim reservoirs off shore Abu Dhabi utilizing horizontal wells. One pilot horizontal well has already been drilled in the lower reservoir and few more are planned.

Due to uncertainties in the fluid contacts and reservoir characteristics, a vertical pilot hole had to be drilled and cored to decide on the optimum location of the horizontal hole.

A detailed core study was conducted to study the main rock types and lithofacies of this reservoir and a sector model was constructed to study the effect of different reservoir parameters on the performance of the horizontal well.

PVT data from the gas cap and the oil rim will be used to construct an EOS that will be used in the full field compositional model.

The results from this model and the performance of the pilot horizontal wells along with ongoing detailed geological study will help in determining the optimum development plan for these reservoirs.

Introduction

Two oil rims containing several hundred millions of barrels off shore Abu Dhabi are currently being studied for full field development. The conceptual plan calls for drilling several horizontal wells to drain the oil in place. Doing so the total number of wells can be reduced and less pressure draw down will be created and hence delay the gas and water break through.

The two oil rims are overlain by huge rich gas caps containing several trillion cubic feet of gas. Auto gas injection from underlain deep lean gas reservoir into the lower gas cap has been initiated several years ago in order to maintain reservoir pressure so that to minimize liquid drop out and oil migration toward the crest.

Historically two vertical producers were completed in the upper oil rim and two other ones from the lower oil rim. The two from the upper oil rim were re-completed later on in other horizons after short production, while the two from the lower oil rim were closed in due to water break through. No significant oil has been produced from the two oil rims while around 25% of OGIP from the lower reservoir has been produced from several gas producers and less than 1% from the upper one. The total gas injected so far in the lower zone constitute less than 5% of the free OGIP. This necessitates a rapid action to start a development plan to recover the remaining oil before it is displaced by the aquifer into the gas cap.

Reservoir Description

The Upper reservoir is a grain supported limestone sediments deposited in a shoal environment. It is composed of three distinct porous units separated by massive dense intervals. The upper unit is of the highest net thickness with an average of 30 ft, and the best reservoir characteristics.

The main porosity types are intergranular, intragranular and moldic types with log porosity ranges from 10% to 15%. The core horizontal permeability ranges from 280 to 600 mD.

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