Multi-lateral wells will be key to future success

The industry must use multi-lateral wells if E&P companies are to move from being just highly productive to being the best-in-class.

So says the DTI, which estimates that multi-lateral wells in the UKCS could produce 600 million barrels of otherwise untapped oil reserves.

'Multi-lateral wells are one of a basket of improved oil recovery (IOR) techniques,' said Adrian Gregory of the DTI who was the first speaker at a recent SPE seminar on multi-lateral wells held in London. 'The imple-mentation of IOR strategy is the key to the competitiveness of E&P companies.'

Adrian defined a multi-lateral as a well with one or more branches from the main conduit, which offers obvious advantages over single vertical wells in terms of improved reservoir sweep coverage. He discussed the relationship between recovery factor, macroscopic sweep efficiency and microscopic displacement efficiency and suggested that multi-lateral technology could be the key to the solution of under performing wells.

Completions

The next speaker, Professor Jim Peden from the Horizontal Well Technology Unit, Heriot-Watt University, described the various completions available for a variety of multi-lateral geometries. These included the stacked trilateral, the dual opposed lateral and the planar trilateral, as shown in Figure 2. He reviewed the application of each multi-lateral well type to reservoirs with different characteristics, showing that opposed laterals are best suited to thin laterally extensive reservoirs and stacked laterals are most applicable to thick and layered reservoirs.

Jim described the work of the Horizontal Well Technology Unit in modelling the effect of the different multi-lateral wells, in a homogeneous base case reservoir model and a more complex heterogeneous model with vertical and lateral variations. In both models the multi-laterals produced slightly less when compared with two horizontal wells but total costs were significantly reduced in the multi-lateral case.

The application of multi-lateral wells in the UKCS will lead to major improvements in the mechanics of recovery, said Jim who added that a horizontal well converted to a multi-lateral costs a third of the price of two horizontal wells. However, more research is needed to compare their performance with horizontal wells. Another important area for future research will be the safety issues surrounding drilling multi-laterals under-balance.

Paul Lurie from BP Exploration gave an overview of multi-lateral well completions and developments in the technology. He stated that industry requirements are determined by the reservoir. In general this means multi-laterals need to have a hydraulic seal at the lateral junction which is installable with the minimum number of trips. It must be applicable to both new and old wells while, at the same time, making use of 'minimum risk' existing technology.

He gave an introduction to developments in the lateral joint technology outlining the Sperry-Sun LTBS composite wrap window system, the Halliburton composite joint system, the Baker Hughes 'Root' system, and the prototype IDS/Anadrill hinged joint system - all of which were discussed in-depth later in the day.

And multi-laterals can have their downsides, Paul warned. 'If a major problem occurs you could lose both of your bores.'

Majid Shafiq from Mobil described the practicalities of drilling multi-lateral wells in a case study presentation on the Galahad development within the Lancelot core area in the Southern North Sea. The reservoir in question is within the Rotliegendes Leman sandstone, and the anticlinal structure consists of a aeolian/fluvial dominated sequence with a worsening upward profile in terms of poroperm quality. The reservoir has a gas/water contact at 9585ft tvdss with approxi-mately 300ft gas above water.

A dual lateral well was proposed during the second appraisal stage of the field in 1994, with the objective of drilling a lower lateral into a high productivity lower zone, and an upper U-shaped lateral to cut across all other reservoir layers and flow baffles. A major requirement was that the lower lateral should be isolatable if excessive water was produced.

The lower lateral was drilled with a maximum deviation of 60 degrees to allow for wireline operations - the horizontal section drilled to a 6" hole and lined with a 4.5" slotted liner. It was suspended using a suspension packer with a wireline plug installed in the tailpipe and a trash cap on top of the packer. The upper lateral kicked off using a retrievable whipstock, milling the window and drilling a 6" hole with a final angle of 97 degrees through the main reservoir. A 4.5" slotted liner was run in and the final 500ft left open hole. Debris from the milling operation subsequently caused problems retrieving the suspension wireline plug and resulted in extended coiled tubing fishing operations.

Increased deliverability

During testing each isolated lateral produced at a rate of 36-37 mmscfd and the combined laterals had a rate of 64 mmscfd. Concluding, Majid said that although the multi-laterals had not produced conclusive benefits in terms of ultimate recovery they did allow increased deliverability and, in addition, had helped provide a better understanding of the reservoir structure.

The final talk of the morning session was given by Vigen Ghazarian of Phillips. Another case history from a gas well, his talk centred on the Alison field development in the southern gas basin of the North Sea. Alison is a lower Permian Rotliegendes reservoir and has been developed by a single subsea well. With relatively small closure and the complex faulted nature of its structure, the development well objectives were twofold. Firstly, to provide an opportunity to maximise the DCQ levels and secondly to optimise field reserves. Following interpretation of 3D seismic data and consideration of various options, the pre-drilling decision was based on drilling a dual lateral well.

The first lateral (B1) was drilled as a 6.5" hole into the Rotliegendes sandstone using gamma ray (LWD) for geosteering. A pseudo oil based mud was used to minimise formation damage. During drilling of this well, however, a non-sealing fault was encountered after 1,250ft which resulted in lost circulation. With this fault acting as a potential conduit for water production, the first lateral was therefore abandoned.

The second lateral (B1Z) kicked off at 9,964ft RT, and having drilled 2,250ft into the formation, Carboniferous was encountered unexpectedly. As a result, drilling was terminated and the lateral was completed with 5.5" production tubing and a 4.5" slotted liner across the reservoir. Poor performance during the production testing phase led to the decision to drill two additional laterals (B1Y and B1X).

Both new laterals were drilled using a retrievable whipstock with the final lateral (B1X) kicking off in the third lateral (B1Y). Production testing of the tri-lateral gave an initial clean up flow of 41 mmcfd and a main drawdown of 40 mmcfd. 'The excellent production rates were achieved at a cost of only 1.5 times that of a horizontal well in the southern North Sea,' Vigen noted, but he stressed the need to tailor multi-lateral wells to meet project objectives.

Bill Hay of Baker Hughes focused on the integrity of the lateral well junction - a topic some had voiced concern about during earlier presentations. He described a system which had both mechanical and hydraulic integrity at the junction.

Once the lateral branch has been created using an oriented retrievable whipstock system, tied back into a fixed datum packer bore, the lateral branch liner is run and cemented into place. This provides the mechanical integrity, he said. The liner stub is then washed over and removed from the mainbore casing leaving full ID.

The Sealed Root system is then run in (one trip), anchored and aligned into the same fixed datum packer bore as previously utilised for the creation of the lateral branch. The system is then set to provide the hydraulic integrity at the junction.

Proven technology

Bill explained that the system was currently under development with preliminary testing planned by the end of this year. The pressure integrity at the junction is expected to be in the region of 2,500 psi, and would be confirmed during testing. 'The advantages of this system are its use of proven oilfield technology and the ability to install the sealed junction completion in one trip,' Bill noted.

David Hill from Schlumberger-Anadrill described an innovative solution to the lateral joint integrity problem which has been developed by the Re-entry and Production Improvement Drilling (RAPID) team.

The RapidSeal Multidrain hinged joint system consists of a retractable steel outlet port within a mother casing which swings into an underreamed section of the original hole. A seal and latch system maintains positional permanence and hydraulic integrity. The joint does not have to be cemented and will hold 1000 psi burst pressure and 600 psi collapse.

David pointed out that the system has the advantage of allowing multiple installation without any adverse effects on production. In reply to a concern expressed by some delegates about a joint not opening properly, he said that it could be forced, milled, or pulled out of the hole if necessary.

Jim Longbottom described the progress in the development of Halliburton's Multi-Lateral System 3000. With the aid of a retrievable whipstock a window is milled in the existing casing downhole to allow the lateral wellbore to be drilled. While the lower parent wellbore is isolated, a liner is cemented into the lateral with a composite joint in the lateral junction. Cement around the junction area has elastomeric properties to provide a high impact strength impermeable seal. This junction design provides a mechanical connection of the lateral liner to parent wellbore casing, hydraulic isolation at the junction, and selective re-entry access to the lateral.

The lateral is the first to have one size drop from the parent casing (ie, 9-5/8" parent with 7" lateral) and full bore access to the lateral casing. In addition, selective through tubing access is provided to the laterals.

The technique was tried first in a full-scale surface test within 36" conduit pipe. Design modifications were made based on this testing, and a second abbreviated test was performed to verify them. A field trial in a Prudhoe Bay well showed that the components and contingency planning were viable, but unfortunately the system could not be fully installed due to open hole shale sloughing problems. Jim said that additional installations are planned early next year and will build on the lessons learned in the Alaska well.

The final presentation of the day was given by Doug Hamer of Sperry-Sun Drilling Services, part of Dresser Industries, who described the development of the Lateral Tie Back System (LTBS).

This system utilises a main bore liner with pre-milled windows. A pressure sleeve is pinned over the windows to give integrity up to 4000 psi burst pressure, thus allowing for conventional cementation of the primary casing. A whipstock landing profile is set in the casing collar immediately below the window joint. 'This unrestricted landing profile has been very successful and no problems have been encountered in positioning or retrieval of the whipstock,' Doug said.

Sperry-Sun has also developed the Retrievable Multi-Lateral System (RMLS) which offers another way to provide laterals in new wells, using a different design of pre-milled window. It uses the same casing collar landing profile and whipstock latch as the LTBS, but involves hanging the lateral liner through the cementing and then milling over to clear the main wellbore.

The Re-entry Drilling System (RDS) is a variant of the RMLS that uses the LTBS/RMLS-style landing profile, but the landing collar is carried on a casing packer. The window is provided by a novel milling system which is subject to a patent application.

'These systems allow the safe, accurate placement and orientation of whipstocks and an unrestricted connection between the lateral liner and main bore,' Doug added.

Chairing an informal wrap-up session, Jim Peden commented that the industry has some way to go before the use of multi-lateral wells is widespread. 'Not knowing the real economics of multi-lateral wells coupled with the mechanical risks involved means that the industry is not yet really comfortable with the concept. Although we are all aware that multi-laterals can lead to increased profits - if not by increased production then by a reduction in costs - more knowledge is needed on their long-term effects before we can reach the ultimate vision of one well per reservoir.'