During a year as SPE distinguished lecturer, Sada Joshi has travelled the world talking about the application of horizontal wells in reservoir management and the specific reservoir parameters that can help determine a commercially successful project. Tricia Young reports on the highlights from his presentation.

Horizontal wells are one of the most recent and promising reservoir management tools, according to Sada Joshi, president of Joshi Technologies International.

'This places a high priority on successful horizontal well operations since effective reservoir management assists production companies to maximise economic reserves ñ surely one of their top objectives', he said.

But to be successful, the reservoir management objectives need to be absolutely clear. They might include:

• Increasing production rates - seasonal or long term
• Accelerating reserves - reducing project life with related economic benefit
• Reducing gas and water production and so reducing surface facility and production costs
• Producing 'lost' reserves - thin oil rim application
• Reducing the number of wells required for a new field development or infill programme
• Bringing onstream currently unused wells or side-tracking current problem wells

Horizontal wells can be used to meet any one of these objectives and, at the same time, can improve the economics (rate of return, payout) of a field and reduce the production and development costs per barrel (oil) or per mcf (gas).

Using the US as an example, Sada illustrated that horizontal wells accounted for a nine per cent enhance-ment of reserves over the ten year period from 1985, and up to a two per cent increase in recovery rates. These improvements would be expected to be higher in the North Sea, he said.

With a combination of extended reach technology and horizontal drilling, the industry has already achieved departures of up to eight kilometres from the platform. This advance will be an important factor in accessing reserves that were previously inaccessible or uneconomic to produce.

Likewise multi-lateral wells. 'Multi-laterals are where horizontal wells were ten years ago,' Sada commented. He briefly described how they assist in meeting reservoir management and operational objectives. Multiple horizontal drainholes can drain multiple reservoirs at different elevations, or drain multiple zones simultaneously. Of the 300 plus multi-lateral wells drilled in the US, over 95 per cent are located in the Austin Chalk in Texas ñ typically with open hole completions.

The success of a number of newer North Sea fields has been dependent on horizontal wells. For the Haven field with reserves of 3.9 million barrels, a vertical well development was considered too risky especially given the marginal economics. More than two vertical wells would have been required, necessitating a platform with six slots. Instead, a development concept calling for two horizontal wells with a two-slot wellhead facility was adopted. Not only did this reduce costs but the horizontal wells have performed even better than predicted.

Horizontal wells are also being used to develop some of the new generation of heavy oil fields in the North Sea, such as Alba and Captain. Sada pointed out that around ten per cent of the worldís horizontal wells have been drilled in viscous or heavy oil zones. Texacoís Captain field lies in unconsolidated Cretaceous sands at a depth of 3000 ft and has estimated reserves of 300 million barrels of oil and 53 bcf of gas. The 19.2° API viscous crude will be produced via long-reach horizontal wells ñ up to 6000 ft.

Sada went on to discuss the reservoir applications for horizontal wells ñ of which a total of over 12,000 have been drilled in most of the major oil producing areas of the world. He looked in turn at the following areas:

• thin pay zones
• naturally fractured reservoirs
• gas and water coning in oil reservoirs
• heavy oil producing gas reservoirs
• gas storage reservoirs
• waterflood applications
• EOR (miscible, polymer, thermal)

Horizontal wells have made an important contribution to a number of naturally fractured reservoirs in Europe. Sada quoted the examples of the Lacq field in France which is 90 m thick and the Rospo Mare field in Italy which is 70 m thick.

As an application to reduce gas and/or water coning, horizontal wells have proved very effective, and Sada estimated that up to 25 per cent of all horizontal wells drilled around the world to date have had this prime objective. However, he warned that the pay zone needs to be at least 20 to 25 ft thick if the oil zone has a large gas cap ñ a minimum of 35 ft in limestone reservoirs. This is based upon the current drilling technology.

In gas reservoirs, horizontal wells have different applications depending on the permeability. In a low permeability envir-onment they can enhance the drainage area per well and also enhance the rate of production. For high permeability gas reservoirs, they may reduce near wellbore turbulence and near wellbore velocity, and may also help in sand control.

Horizontal wells can bring a number of advantages to waterflood applications. They may enhance injectivity and productivity and improve sweep efficiency, and Sada explained that they can reduce the waterflood response time in low permeability formations. ìHow-ever, a successful water injection project should be undertaken early in the field development and the well(s) drilled in a zone with low water saturation. Otherwise, the result might be extra water production,î he added.

Finally, horizontal wells have assisted in some successful EOR applications around the world ñ miscible flood in the Pembina Nisku field in Canada; polymer flood in the Chateaurenard field in France; and thermal oil recovery in Canada.

Having reviewed the application of horizontal wells for reservoir management, Sada went on to highlight the important reservoir parameters for economic success, as follows:

• fracture intensity and direction
• net pay
• vertical permeability
• areal anisotropy
• mud damage and clean up
• multi-well prospect
• well spacing
• geological control
• reservoir pressure
• completion and drilling costs
• wellbore size and length
• co-operation in various disciplines.

For example, fracture intensity and direction determine the number of fractures a horizontal well will intersect which, in turn, determines any productivity improvement. Sada pointed out that for primary recovery, the well should be perpendicular to natural fractures, and for secondary recovery, it should run parallel.

Just as any of the factors listed above can lead to a successful horizontal well operation, they can also lead to failure. Formation damage has resulted in many failures, particularly since its effects tend to be more pronounced in horizontal wells than in vertical ones. A good understanding of the geology is also essential. Unless recognised and taken into account, formation pinchout, variations in formation thickness, formation dip or faults with large vertical displacement can all spell disaster.

If production turns out to be less than expected in a horizontal well, Sada suggested that the most likely causes are reservoir heterogeneity, formation damage or pressure drop inside the well. 'Pressure drop inside the well is a particular danger in viscous oil reservoirs. If the well is too long at the well toe, flowing pressure may get too close to the reservoir pressure', he said.

But ending on a more positive note, Sada concluded that horizontal wells have been a major advance in oil field technology and have made a significant impact on existing fields as well as on new developments. Without them, many smaller or complex fields may never have been viable projects.

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