Abstract

Presented in this paper are recent experimental and theoretical advancements in the coning reversal technique using an innovative completion method with downhole water sink (DWS). In this technique, a well is dual-completed in oil and water columns with a packer separating the two completions. Then, aninverse oil cone is created by draining the water from the water sink completion below oil water contact (OWC) while producing a water-free oil from the oil completion. The experiments were performed with a transparent Hele-Shaw physical analog that visualized all stages of water cone development, reversal, and creation of the inverse oil cone. Results presented in this paper show the effects of DWS design parameters on the reverse coning performance. The theoretical part of this study employed mathematical modeling of pressure distribution in Hele-Shaw analog. A combination of this mathematical model with the recently-published Moving Spherical Sink Method (MSSM) allows conversion of the results from the Hele-Shaw analog to the real reservoir conditions. The study shows how productivity of a "watered out" well can be recovered to give significant production of oil. Also, the oil produced from the oil completion was water-free. The results indicate that oil production from wells with DWS completions under condition of coning reversal may have high economic merit and is technically feasible.

Introduction

In the presence of bottom water, hydrocarbon production from a well is limited by the maximum, critical flow rate. If oil production rate is above this critical value, water breakthrough occurs. After the breakthrough, the water phase may dominate the total production rate to the extent that further operation of the well becomes uneconomical and the well must be shut-in. In the oil industry this phenomenon is also referred to as water coning.

Until recently, several technologies have been used by industry to fight water breakthrough to oil perforations due to coning. These methods include: keeping production rates below the critical value, perforating as far from the initial water-oil contact (OWC) as possible, or creating a water-blocking zone around the well by injecting crosslinking polymers or gels (polymer-gels watershut-off technology). Unfortunately, all these conventional methods do not solve the water breakthrough problem.

It is usually uneconomical to keep production rate in a well below the critical rate. Perforating far above the OWC reduces the length of the perforations and, thus, increases pressure drawdown around the well. The increased pressure drawdown may enhance water coning thus eliminating positive effect of staying away from OWC. Performance of the polymer-gel water shut-off technology is still a controversial issue. Field data shows that this technology works better for the water channeling/fingering (2-D) problem rather than bottom (3-D) water coning.

Downhole Water Sink (DWS) Technology. Since 1991 when the DWS technology was proposed DWS systems were pilot-tested, used in field operations, and investigated theoretically. DWS technology requires that an oil well be drilled through the oil-bearing zone to the underlying aquifer. Then, the well is dually-completed both in the oil and water zones. The oil and water perforations are separated by a packer. During production, oil flows into the conventional completion while water drains from below the initial OWC. Since water cones upward due to the pressure drop caused by oil production, an equal pressure drop in the water zone will keep the water from rising. As a result, the produced oil is water free, because the water drainage keeps the water-oil interface (OWI) below the oil perforations and prevents water breakthrough. P.425

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