Drilling horizontal wells was a significant step for the oil industry. Even though the complexity and cost of drilling rose, economic returns to the operator rose as well.

Horizontal drilling meant increased production by maximizing the reservoir contact, and it meant drilling fewer wells than with vertical drilling.

Horizontal wells also enabled producing reserves from zones previously thought too difficult to reach, such as thin oil zones. And horizontal wells helped avoid drawdown-related problems associated with vertical wells, such as water and gas coning.

Although horizontal completion technology and techniques improved over the years, horizontal wells continue to face challenges related to uneven influx of reservoir fluid to the well bore, which results in early water and gas breakthrough. While some operators and regions can hold these problems to a manageable level, for others it substantially lowers overall productivity.

Defining the problem

Most oil companies are familiar with the problems associated with early water and gas breakthrough leading to reduced oil production. In the worst cases, the well has to be abandoned and valuable reserves left in the ground. Water and gas coning in the heel is often blamed for this. While this may be true for high-rate, long horizontal wells with low drawdown, coning in the heel may not be the only reason for early breakthrough.

For example, in homogenous formations with minor differences in permeability along the well bore, pressure drop from heel to toe inside the tubing can lead to uneven drawdown across the well bore. The drawdown can be higher in the heel and eventually lead to a breakthrough of water and gas.

Another reason for water and gas breakthrough is related to uneven permeability and fractures or differences in fluid mobility, such as in wells with high-viscosity oil. Since it becomes easier for the reservoir fluid to be produced through one section compared to the other, having an even drawdown under conditions of uneven permeability or uneven fluid mobility can lead to premature breakthrough of water or gas.

Finding the answers

One common way to handle these conditions is to wait until they occur. But by then it may be too late, and costly intervention operations may not give the desired results. The most effective way to solve the problem is to manage it at its origin — in the reservoir before the fluid enters the well bore.

In relatively homogenous reservoirs with higher drawdown in the heel, one solution is to balance the drawdown from heel to toe. This can be done by applying a controlled pressure drop from the annulus to the production pipe in the heel using inflow control devices (ICD), which in turn reduces the drawdown and the fluid rate from a particular section.

In a heterogeneous reservoir, where the drawdown is more equally distributed along the well bore, the drawdown should be reduced in high-permeability sections to allow low-productivity sections to flow more oil. Today, it is becoming common practice to achieve this equal distribution through the use of ICDs.

While using ICD technology may have a positive impact on initial overall production, the main benefit is to delay potential water or gas breakthroughs and allow more oil to be produced throughout the life of the well. Imagine, for example, the improvement in field economics that would follow if water and gas breakthroughs could be delayed two or three years.

The EquiFlow ICD creates a controlled pressure drop across the production tubing and is an integral part of the completion string, so it does not require additional installation time. The pressure drop is achieved by directing flow through a set of tubes inside the ICD. This device can be combined with Halliburton’s screen technology if sand control is needed.

Determining placement, number of units, and configuration of the ICDs has traditionally been done using numerical, steady-state near-wellbore software. This is a quick and effective method to run multiple scenarios and completion configurations to quantify the benefit of the ICD.

Since steady-state software does not offer the capability to investigate the long-term effects of the ICD on the reservoir, Halliburton has linked its QuikLook reservoir simulator with the AGR NETool nodal analysis package. This software package makes it possible to investigate the long-term effects of an EquiFlow ICD completion on the reservoir and its production performance, including recovery factor, cumulative production, and time to water/gas breakthrough.

If the formation is competent and stable, openhole completions can often increase reservoir contact and reduce completion cost. An important factor for a successful openhole completion is reliable zonal isolation. With the introduction of Halliburton’s Swellpacker isolation system, this is now available.

The Swellpacker system is based on elastomers that swell and expand in contact with oil or water to seal off the annulus. Nearly 14,000 Swellpacker systems have been installed by more than 175 operators and are considered standard completion equipment.

Zonal isolation enables breaking down the reservoir into fewer sections, and when combined with ICDs, provides an incremental flow control down to every basepipe joint. When using zonal isolation to its full potential, operators achieve more than just isolation; they have an effective way to manage the reservoir flow and increase recovery.

Putting ICDs to work

An operator that had completed several wells in a block experienced high water cut with some of the early openhole completions. Experience had shown that several large fractured zones connected to an aquifer. While the completion had recently been improved by isolating unwanted fractures, it was believed oil production could be further improved.

The planned well was modeled to design the configuration and placement of the EquiFlow completion system. Data from offset wells were used for the modeling exercise.

After the well was drilled, logging-while-drilling (LWD) data provided updated well information and helped identify highly fractured zones. The LWD data also helped verify the pre-modeled assumptions and allowed for optimum placement of the EquiFlow ICD and openhole packers. The highly fractured zones were blanked off as they were prone to produce high water cut.

The ICD/packer system was installed, and acid stimulation was performed through the EquiFlow ICD modules. Stimulating in this way improved the fluid injection profile to help ensure even placement of the acid and helped avoid pumping most of the fluid into high-permeability zones or fractures. Injectivity improved while acidizing, as indicated by a decrease in injection pressure, suggesting that the well was successfully stimulated.

A significant drop in water cut was observed a few days after putting the well on production, indicating the fractures were successfully isolated. Two more wells were completed with the EquiFlow ICD and Swellpacker system and are producing with water cuts well below the field average.

While horizontal drilling helps mitigate some of the challenges associated with water and gas coning in vertical wells, operators continue to seek ways to improve reserves recovery. Inflow control technology and zonal isolation systems are tools that are allowing better reservoir flow management and maximizing the value of horizontal wells.

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