Multilateral completions on the verge of mainstream
New technology following path of high angle drilling
Marshall DeLuca
Survey Editor
- Advanced ML configuration includes selectivity, isolation, and conectivity.
- Original multilaterals were "barefoot" completions.
- Initial ML improvements include lateral isolation.
Early horizontal wells were "barefoot" completions, simple open hole wellbores to drain the well fluids to the main wellbore. Because most offshore reserves are found in less consolidated sands, it was only with the advent of sophisticated completions, including liners and sand control, that the offshore industry began to look closely at the new technology. Still widespread acceptance of the practice was hindered by operators fearful of exposing immense offshore investments to less than certain drilling practices. But as the intrepid mounted successes, the rest of the industry followed and horizontal and high angle drilling became the mainstay of the offshore industry.
So it is with multilateral (ML) well technology, the technological inheritor of high-angle and horizontal completion technology. By the end of 1996, several hundred ML completions had been done onshore. At the same time, their number offshore was far less than 50. Like horizontal completions years ago, most MLs onshore are multiple barefoot branches off a main wellbore with production commingled.
Also like high angle completion technology, multilaterals are becoming increasingly sophisticated offering re-enterability, lateral isolation, and mechanical integrity at the junctions of lateral and main wellbores. It is their increasing level of technology that is bringing them, if slowly and just in the past 18 months, into the repertoire of offshore oil industry practices.
Multilateral benefits
The choice of MLs is an economic decision that can at once lower surface and subsurface costs while enhancing production rates. In marginal reservoirs, they allow access to additional reserves through an existing main well bore. By circumventing the need to drill another main well bore, small reserves pockets can be reached through existing platforms without adding drilling slots.
This ability to tap and efficiently extract reserves through re-entry and by exposing more of the reservoir face to the drainage area, rather than through new single wells can easily make otherwise uneconomic projects profitable.
"Theoretically, you get two wells for the price of one-and-a-half wells," said BP engineer Tony Emerson, who believes the US Gulf of Mexico is ripe for ML technology.
When drilled in conjunction with high-accuracy geosteering tools, MLs can be precisely placed so as to strike pockets of reserves at their most advantageous spots and offer access to lenticular reservoirs and reservoirs that are strongly directional, vertically segregated, or contain natural fractures. Accurate placement also makes laterals good injection conduits to maximize reservoir pressure maintenance or sweep efficiency.
The most recent technological advances in ML promise to bring them the acceptance offshore that they have found onshore. The ability to re-enter laterals and set liners within them, to isolate laterals from each other, and to separate zones within individual laterals make the whole concept more viable than mere open hole laterals that until recently defined the science.
The latest technology takes production management downhole and promises to be an integral part of "smart wells" or "intelligent completions" that invoke the integration of computer processing and downhole hardware to optimize production strategies. The technology uses isolation techniques such as external casing packers and sliding sleeves to produce zones of varying pressures to maximize the value of the reserves. In its ultimate form, ML and smart well technology would be a highly efficient subterranean factory whose finished product was oil and gas.
One other ingredient, competitiveness, will help bring ML technology to the forefront of offshore completion strategies. Operating companies are keen to enlist early any new technologies that have passed the realm of experimentation, particularly since the post-depression days of the 1980s, which taught that survival went to the first and best at adopting new technologies. Their instinct to exploit the efficiencies of technology will soon override undue caution about its immaturity.
"As the future moves to more and more subsea completions, the future prospects of ML completions will become tied to the technical viability of intelligent systems," said Sperry-Sun Norway's Mike Cunnington. "And we are seeing new alliances formed between ML system vendors and specialist suppliers that give evidence to that fact."
Current applications
The earliest MLs were drilled more than 20 years ago. Their resemblance to the current completions are minimal. In fact, it would be a stretch to call them completions as little was done to affect production beyond drilling parallel sidetracks into the reservoir.
Today most ML technology development is being pursued by just three major service companies. Each has developed unique systems to accomplish the key goals of true ML completions: junction integrity, isolation, and lateral re-entry. And all three offer systems that range from simple barefoot holes of original vintage to complex completions that encourage wide flexibility in production strategy.
The most advanced systems have in fact been installed offshore and those companies are planning more. Norsk Hydro has used them in the North Sea and has identified more than 50 potential future ML applications. In the Gulf of Mexico and the Middle East, Shell and BP have used them and expressed interest in using more as has Phillips, in the North Sea's Eldfisk Field.
But true ML completions, that is those with re-entry and isolation technology, are relatively rare. Informal polls indicate less than 10% of all ML installations have been done offshore and though operators are hesitant to make available exact numbers there are certainly less than 40 such installations offshore worldwide. But the low numbers are a result of its relative immaturity. At the opening of 1995, there were no true ML completions offshore.
Technical obstacles to proliferation of MLs center on the ability to isolate producing laterals of differing pressures. If accelerated production is desired, often the most efficient answer to marginal field economics, simultaneous production of all zones is optimal. But that can be costly in an ML environment since it requires hydraulic isolation between laterals that at present vendors are not prepared to categorize (though they have reported pressure tests indicating such isolation has been obtained at the juncture, they do not make pressure differential rating claims for the junctions in the literature). However, true mechanical connectivity is now a reality and full pressure integrity is not far behind.
Original thought on MLs' place offshore centered on economically marginal fields, suggesting shallow water as their prime target since deepwater production has proven to be high enough to easily offset the extreme cost of doing business there. Besides it is those very same high costs that are associated with deepwater exploration and development that discourages managers from using anything but the most proven technology.
The technology is indeed quite complex and, therefore, more risk-laden than other completion strategies. Added to the natural complexity of offshore reservoir structures, the challenge to ML marketers is daunting.
"The industry is having trouble predicting the impact on reservoir drainage architecture which leads to a shortage of near-term prospects (for ML) that the reservoir engineer can feel comfortable with," said Cunnington, who added that he believed industry attitudes would reverse themselves with time. "Initial reluctance to commit to the increased investment of a ML well will give way to confidence in the benefits to be gained."
The future
Most observers believe it a foregone conclusion that ML technology will soon enter the mainstream of completion alternatives across all economic spectrums and water depths offshore in much the same way as did high angle drilling and completion technology just a decade earlier.
Real ML completion technology is only in its first generation. With time, its most glaring fault, is an inability to obtain a reliable hydraulic isolation at the junction. Other technologies will be combined with the current state of the art in the future.
Isolation within each lateral may be accomplished with inflatable packers and slotted liners, a feat accomplished in horizontal completions in a single run that included setting a hanger in the reach section of the well.
But for the present the industry is hesitant. Unproven technology takes time to graduate to the status of accepted practices in the oil and gas industry, particularly in the high-stake arena of deep waters.
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