Long horizontal wells are a double-edged sword for today's operators. On the one hand, they maximize reservoir contact and improve recovery efficiency and total well productivity. On the other hand, non-uniform flow profiles can result in premature water or gas breakthrough, screen plugging and erosion, all of which can reduce production, reserves, profitability and well life, and thus defeat the original purpose of the horizontal well.

Creating a uniform flow profile along the lateral section can optimize production while delaying water and gas coning and preventing erosion. Baker Oil Tools' Equalizer technology is the basis of a uniform flow system that has been proven to prevent water and/or gas coning in sandstone formations and prevent excessive water breakthrough in naturally fractured carbonate formations for long horizontal and highly deviated wells throughout the world. Lateral lengths have ranged from 1,100 ft to 12,000 ft (335 m to 3,660 m). In addition to single laterals, the system offers the benefit of uniform flow to wells with two or more laterals.

Why the need for uniform inflow?

Long horizontal and highly deviated well bores are being used in both new and brownfield developments to increase reservoir contact and improve field economics. However, along with the increase in reservoir contact come non-uniform flux profiles and frictional effects in the well bore that can limit the useful length of a horizontal section. The frictional effects associated with producing from the entire lateral in long horizontal wells can be significant relative to the typically low drawdown pressures required to achieve target production rates. If these frictional effects are neglected and inflow is not balanced along the lateral length of the well bore, the well faces the risk of premature breakthrough of water and/or gas as well as lower productivity and reduced recovery. In heterogeneous reservoirs, the point of breakthrough varies with permeability. In homogen-eous reservoirs, breakthrough commonly occurs at the heel of the lateral section (Figure 1).

A uniform inflow control system

The Equalizer technology-based uniform inflow control system consists of an inflow control device (ICD) integrated with a standalone premium sand screen or debris filter, depending on reservoir requirements. The ICD is a low-velocity flow regulator designed to equalize longitudinal inflow along the entire production profile, regardless of permeability variation, formation damage or location in the well bore (Figure 2). Highly productive zones cannot "overproduce" because they have less reservoir segment drawdown; at the same time, lower pressure in the reservoir-to-ICD region pulls harder on less productive zones to make them produce more. If the well path is close to either water or gas, the uniform flow profile prevents undesired coning effects that lead to premature water and/or gas breakthrough.

The ICD incorporates helical flow channels to impose pressure distribution along the entire length of the wellbore and control production rate through each screen element as a function of both the average drawdown pressure and the average productivity of the well (Figure 3). The number, length and cross-sectional area of the helical flow channels are based on reservoir and production requirements, with their configuration determined by numerical modeling and reservoir simulation. The design objective is to normalize permeability variation and wellbore effects to provide optimal pressure drop versus flow rate for inflow balancing.

Sand control benefits

Additional benefits of the uniform inflow control system accrue to horizontal wells completed in formations that require sand control at some point during the life of the reservoir. In these wells, the ICD reduces annular fluid flow velocity and optimizes the inflow velocity into each screen joint, thereby preventing erosion and screen failure typically associated with standalone screen completions. With the total production rate evenly distributed into the total number of screen joints in the horizontal section, fluid velocity into each screen joint is so low that screen erosion and screen plugging normally associated with standalone screen applications does not occur. Therefore, if the filtration media in the sand screen can be properly designed for solids control, integrating the uniform inflow control system with sand screens can provide sand control without the need for gravel packing. To date, no Equalizer ICD applications in sandstone reservoirs have required gravel packing for sand control.

Helping North Sea oil flow

In the 1980s, Norway's Troll field was originally considered to be a gas field with a thin oil layer that was uneconomical to produce because it was trapped in a high-permeability sandstone formation with both water and gas contacts. Today Troll is one of the largest oil producing fields in Norway, thanks to the development of extended-reach drilling (ERD) and uniform inflow control technology.

Making Troll oil recovery economically feasible required maximizing reservoir contact via horizontals while minimizing the potential for water or gas coning. It was this requirement that led to the development of uniform inflow control technology, jointly, by Norsk Hydro and Baker Oil Tools.

Prior to the development of the uniform inflow control system, the majority of production from long horizontals came from the first one-third of the well, closest to the heel (casing shoe), with the remaining interval contributing very little. The uniform inflow control system alleviated that situation and has enabled balanced production from more than 120 horizontal wells in the North Sea thus far. Among the longest completions to date was an 11,894-ft (3,627-m) openhole lateral at 18,113 ft (5,524 m) total depth and 5,177 ft (1,579 m) true vertical depth. The open hole was completed with 279 joints of 250 micron Excluder2000 standalone sand screens and Equalizer inflow control devices. The ICD screen assembly was hung off from the bottom of a 7- by 103⁄4-in. non-rotating, hydraulic set Flex-Lock Liner Hanger with a 7- by 103⁄4-in. Model ZXP liner top packer inside 103⁄4-in. casing. The ICDs were positioned along the horizontal openhole section to balance and effectively control the production inflow along its entirety. To achieve optimum well production, the inflow control devices were numerically modeled to determine appropriate placement and drawdown (Figure 4 and Table 1).

Savings in Saudi Arabia

To confirm the benefits of a uniform inflow control system, a horizontal well was drilled overbalanced to a total depth of 9,657 ft (2,945 m) with a total of 2,200 ft (671 m) across the reservoir sand and completed with Equalizer ICDs and four mechanical external casing packers. The packers were run to enhance the effect of the regulator by creating an annular barrier between sections of variable reservoir permeability. A 3-D simulation model enabled optimum placement of the inflow control system in the main sand, with gas above and water below.

To determine the completion efficiencies of each well type and help optimize future horizontal well completions, a spinner log was run in the well to validate the effective producing length. The preliminary analysis of the qualification well log indicated uniform flow contribution across the horizontal section with total rates of 8.0 MRBD and 10.0 MRBD at choke settings of 72⁄64 in. and 90⁄64 in., respectively. The estimated productivity index was about 160 stdb/psi for the completion.

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