Thin oil-rim reservoir poses challenges

Horizontal wells generally produce more, and more efficiently, than do vertical wells because they have more contact with the reservoir. However, these wells are subject to early water and gas coning toward the heel because of the flow’s frictional pressure drops along the horizontal section. Moreover, variations in permeability can result in unbalanced inflow along the horizontal section, accelerated early water and gas breakthrough, and uneven inflow downhole. These conditions can limit sweep efficiency and reduce hydrocarbon recovery from horizontal wells, leaving bypassed oil.

An operator in the Baram field offshore east Malaysia planned to develop the thin oil-rim reservoir by controlling the inflow of two horizontal wells to maximize recovery and retard the usually fast-approaching water coning without producing sand. To understand the effect of the ICDs, the operator completed two other horizontal wells, one with a stand-alone wire-wrapped screen and the other with an internal gravel pack; these wells were drilled 4.92-ft [1.5-m] updip in the same sand zone as the two wells with ICDs.

Balanced flow and effective draining

ResFlow ICDs are self-regulating choking devices that have two or more nozzles exposed to fluid inflow. They operate through nozzle restriction based on the Bernoulli pressure-drop effect. Because of the proven track record of the ResFlow nozzle-flow ICD, especially in the Middle East, the operator was aware of the advantages of the nozzle mechanism, which is less sensitive to downhole fluid viscosity changes.

The key advantage of using ResFlow ICDs was that they would balance the flow across the thin oil-rim layer for optimum sweep and effective draining without premature gassing out or watering out of the well. Two additional ICD features assured the operator that the proposed reservoir-centric approach would work: the capability to fine-tune nozzle configuration on the basis of the real-time LWD log from the PeriScope bed boundary mapper and the ability to swap different nozzle sizes and numbers at the rig site before the completion was run in the hole. After the job, the results of the ECLIPSE reservoir simulation of the ICD wells, including data from the geosteered trajectory and final ICD configuration, were compared with the projected results of the initial, proposed trajectory of that well. The results showed that the ResFlow ICD well with the precise well placement had a total cumulative oil production that was double that of the initial well without ICDs or precise well placement.

When production started in July 2007, the ResFlow ICD well was producing 100% oil at 2,200 bbl/d. Field production results since then have clearly shown the benefits of installing the ICDs. Water cut has been maintained at about 30%, compared with the water cut of an adjacent well with stand-alone wire-wrapped screens. In that well, the production water cut rose aggressively to 60%, jeopardizing the natural lift in the well.

The sustained lower water-cut production confirmed the effectiveness of the Bernoulli principle on the nozzles’ inflow: the higher the velocity of the high-mobile-phase water approaching the nozzles for inflow, the more backpressure was imposed by the nozzles to restrict water influx. As a result, the inflow of oil from low-productivity and low-permeability zones was eased in other sections of the horizontal sections to ensure that designed surface rates were achieved.