The 6 levels of multilateral junctions are based on the degree of mechanical complexity and hydraulic isolation. (Image courtesy Schlumberger—The Oilfield Review)
When Alexander Grigoryan drilled the world’s first multilateral well in Bashkortostan, Russia, in 1953, who knew what a revolution he had started? Actually, few people in the west did know about this incredible breakthrough because not much information flowed out through the Iron Curtain in those days.

Grigoryan’s nine-branch multilateral produced 17 times more oil than the field average well, but only cost 1.5 times as much. This fact provides all of the incentive anyone needs to look into multilateral solutions for field development.

Today, multilateral wells are much less imposing than Grigoryan’s nine-branch beauty. To put things in perspective, it should be noted that despite all its branches, the pioneer well only produced a little more than 700 b/d of oil. But the idea took hold, largely driven by the development of offshore. As soon as offshore development ventured into deeper water, the cost of individual well jackets became prohibitive, and the era of the multi-well production platform dawned.

But even the largest of these behemoths could only support about 30 wells. Offshore drilling and production costs were high, and commodity prices were low, which necessitated making each well drilled from a platform count. Accordingly, the wells targeted only the largest, most prolific reservoirs.

When production from those wells declined, companies considered tapping some of the many satellite reservoirs bypassed originally because they were deemed less profitable.

Fortunately, commodity prices had firmed up in the interim, so reasonable arguments could be made to do a slot recovery project, mill a window, and drill a lateral branch (or branches) to target undeveloped blocks.

Exploration depends on skill, luck, but mostly on luck

Historically, the oil and gas industry has benefited from a level of serendipity, and in the multilateral saga, this came in the form of logging-while-drilling (LWD) capability timed to coincide with the lateral well debut. Getting down into highly deviated or horizontal wells with wireline tools was problematic, even when drillpipe-conveyed, but LWD systems gave directional drillers the “eyes” they needed to place laterals precisely where they needed to be. One famous North Sea lateral pierced no fewer than nine lens-like reservoirs.

The step from laterals to multilaterals was a challenging one and required a coordinated effort from the industry. The toughest problem was the junction where the various branches diverged. With significant engineering work and regulatory approval, the Technical Advancement of Multi Laterals (TAML) standard was launched and accepted (Figure 1). TAML defined each class of lateral junction, separating them into six levels with Level 6 being the most sophisticated. (Grigoryan’s branches would have been Level 1). With the standards set, the race was on
to develop hardware, software, and operational procedures to implement multilateral completions.

Today’s issues

The recent focus of multilateral technology has been on design and installation of lateral junctions, development and deployment of multilateral intervention tools, multilateral field development, and software programs to build economic models for cost-value analyses of multilateral options. Several case studies illustrate successful operations in multilateral wells.

Those engaged in projects involving multilaterals contend that every multilateral well could be treated as a “custom” design. Even with the TAML standards, no two wells seem to be alike. It may always be thus.

An ancient truism of this industry maintains, “Every well is different.” Perhaps this is the case, but surely at some point multilateral well construction and subsequent workover techniques will become somewhat standardized, and best practices will be developed.

Multilateral wells create the market for intelligent well systems (IWS) because, unless a company is extremely fortunate, someday the need to control production from either or both laterals arises. Accordingly a variety of solutions has evolved, from on/off valves to adjustable flow control devices. In the Middle East, several studies and simulations performed by Saudi Aramco have proven conclusively that enhanced productivity results from horizontal and multilateral wells. Not only are well and reservoir productivity enhanced, but rigless re-entry techniques are particularly effective in these wells at a greatly reduced cost.

Key to most multilateral interventions is a “multilateral tool” that has the ability to “find” the correct branch and guide the workover string or coiled tubing into it. Usually these tools have a positive identification feature that makes it virtually impossible for them to enter the wrong branch. Once inside the branch, all sorts of activities can take place, from reservoir stimulation services to jet-cleaning of the completion tubulars. Typically, for hydraulic fracturing, multilateral junctions must be able to withstand high differential pressures, calling for a TAML Level 4 or higher junction.

In the prolific Haradh field, near the southern end of the Ghawar complex, “smart multilateral systems” have been implemented with significant success. The combination of the multilateral well design and IWS is expected to enhance field recovery by delaying water coning and improving injector sweep efficiency. TAML Level 2 junctions are used and up to four laterals are drilled from each mother bore. Downhole sensors monitor pressure and temperature, and surface-operated hydraulic downhole chokes control flow from each lateral. Starting in 2005, 28 water injection wells were drilled and completed along with 13 observation wells intended to monitor the progression of the fluid front as it moves through the reservoir.

Chevron Energy Technology Co. has developed a semianalytical modeling program for predicting production performance under constant rate or pressure conditions of wells with any number of laterals. According to the company, laterals can be oriented in any direction, be of any length, and contain varying amounts of formation damage. Robust in design and application, the modeling program is effective in reservoirs with widely varying parameters such as porosity, permeability, fluid viscosity, or drainage volumes. Because there are many uncertainties in such a system, results can be enhanced by integration of production logging data or transient pressure testing and analysis. Fortunately, production logging strings can be configured to enter and take measurements in individual branches of a multilateral well using a version of the multilateral tool. A comprehensive model allows engineers to design well plans for lateral branches that optimize reservoir drainage with maximum economic advantage.

Recently, multilateral technology has been applied in unconventional gas development. Cimarex Energy has performed two multilateral completions by installing TAML Level 4 high-pressure junctions to allow hydraulic fracture stimulation of the reservoir. One of the primary objectives of the project was to perform it in such a way that the productivity of the multilateral completions exceeded that predicted for two separate horizontal drainholes had they been drilled and completed.

In Abu Dhabi, ZADCO designed and installed a multilateral completion system for two separate reservoirs using a multilateral tie-back. Well T-2 in the upper Zakum field was recompleted as a dual lateral dual producer using a re-entry drilling system and a dual bore deflector oriented and set in the 7-in. liner hanger. According to the company, the multilateral liner hanger system is the first successful application of this kind in the world. The system provides fullbore access to both laterals, offers a provision for selective water shutoff should either lateral water-out, and allows the possibility of drilling a secondary lateral through a pre-installed window joint in the upper lateral.

A final example is India’s Oil and Natural Gas Corp. (ONGC), whose company management has challenged engineers to increase overall production. With its principle asset, the mature Mumbai High field, the company planned an aggressive redevelopment program. Since the program’s initiation in 2000, 140 new wells have been drilled – an increase of 20% over the previous well population – and most of them have been horizontal or multilateral completions.

An additional 70 suboptimal wells were converted to extended-reach horizontal sidetracks to improve reservoir contact. According to ONGC, the program has been a success. Production declines at Mumbai High have been arrested, and production rates are on the rebound.