Sand control complexity grows with water depth

Longer reach wellbores challenge industry's ability to control target formations

Rick Von Flatern
Technology Editor

Sintered fiber and metal powder or wire mesh screens replace gravel in some prepack screens (photo courtesy Pall Well Technology Co.). [6,949 bytes]
Long screen sections are installed in horizontal wells in unconsolidated offshore formations (photo courtesy Halliburton). [5,367 bytes]
Offshore, multilateral wells are further complicated by the need for sand control efforts (photo courtesy Baker Inteq). [6,200 bytes]

The petroleum industry is moving inexorably into greater water depths. To do so, it has spent millions of dollars and manhours to use technology developed on land to reduce the extreme costs of doing business there.

No small part of that effort has been to adopt high-angle, long-reach well technology to the unique environment of offshore. Early on, the attempt met resistance from an industry wary of using what it regarded as unproved technology on high-risk projects. But the resultant savings and high production rates of wells drilled directionally from a single seabed template have made the practice nearly common place, even in the most advanced, deepwater projects.

As with all technology born onshore, horizontal drilling required certain adaptations to make it suitable offshore. For instance, underbalanced drilling, made a common onshore practice in the early 1990s by the introduction of the rotating blowout presenter, has been done only experimentally offshore.

But, the most fundamental difference between onshore and offshore is basic rock characteristics. Whereas most onshore formations are hardened, older sands, the majority of target formations offshore are much younger unconsolidated sands and shales. As such, the need for sand control practices is a dominant concern when adapting or developing any new technology to subsea use.

The wellbore turns

Sand control has been achieved effectively in vertical wells by placing gravel, specifically sized to create a filter with pore spaces small enough to trap moving formation fines but large enough to accommodate commercial flow rates, between the production tubular and the producing formation. The process is effective because gravity helps create a thorough and efficient pack that concentrically surrounds the pipe.

But the introduction of high-angle and horizontal wells make efficient sand placement problematic. A major oil company study of just one of its high-angle gravel pack completions found that in one 1,500-ft horizontal section the well was producing only through a few randomly developed gaps in the gravel sleeve.

In response to the problem of uneven gravel distribution, many companies turned to prepacked screens. These screens, originally designed to hold gravel in place in a gravel pack completion, were modified and pressed into service as stand-alone filters. But stand-alone screens have problems of their own and others in the industry have continued to perfect long-reach gravel pack completions. As the number of their successes grow, they believe, gravel packing will be increasingly the completion technique of choice.

Stand-alone screens

The simplest way to avoid gravel packing problems is to not to do it. To accomplish that and still control sand movement, stand-alone sand control screens are manufactured with a filtering medium trapped between a protective shroud and a perforated base pipe and the entire assembly is put in the hole as a unit.

Stand-alone screen mediums are of three general types: gravel, sintered metal powder, and mesh screens. Those using gravel replace the need for a gravel pack job, that is placing gravel after the screen and base pipe are in place, with gravel between the screen and base pipe. They rely on the filter to replace gravel directly.

By placing it on a mesh screen and heating it to about 2,500 degrees F, sintered metal powder or fine fibers are melted together at their points of contact. This sintered medium forms pore spaces related to the size of the original material. Powder and pre-packed gravel screens act very much like gravel packing but their construction, according to manufacturers, makes them more durable and better suited to high-angle installations.

Mesh screens use layers of cross-hatched wires to form spaces measurable in microns to create a filtering medium of uniform porosity. By the nature of their construction, screen pore size is more uniform than either gravel packing or other prepacked materials.

Plugged filters

Building a screen is not as easy as it may sound at first blush. "Anyone can buy a piece of mesh that has 20% pore volume or change the mesh and get any volume you want," said Pall Well Technology senior vice president Razi Hashemi. "But that is not what you want downhole. You want as much pore volume as possible without sacrificing sand control."

And therein lies the rub. It is the task of both stand-alone screens and gravel packs to perform two apparently diametrically opposed tasks at once, as Hashemi puts it, to perform "a balancing act of sand retention without screen plugging."

The crux of the problem is that a medium whose pore space is small enough to catch minute particles risks becoming a plugged, impermeable blockage. In response to 15 years dealing with that paradox, Hashemi's company manufactures two basic prepacked filter mediums. One is of sintered metal powder for use in fine sand formations. The other uses sintered fibers more suited for coarse sand formations.

When screen plugging occurs, as it will inevitably to at least some degree, a situation similar to an uneven gravel pack is created. That is, flow is directed to clear areas of the screen where increasing flow velocity across a decreasing screen area leads to erosion.

Staving off plugged screens requires planning for the specifics of the job - most often a consideration of geography as well as geology. For example, while North Sea formations require sand control, they are composed of larger sand grains than in the Gulf of Mexico. Larger mesh screens may be used because the sands that form on them leave sufficient space between the grains to allow flow. In the US Gulf, on the other hand, fines flowing from the formation are easily small enough to form a filter cake on the screen of such a low permeability to severely restrict, and even halt, production.

Even drill-in fluids specifically designed for clarity to minimize formation damage in horizontal sections contain solids necessary for mud weight and fluid loss prevention. And unless the hole is well-conditioned to remove solids before completion operations begin, screens will begin plugging even as they are run in the hole.

The plugging problem is then exacerbated during production in open hole completions as the filter cake, left behind even after acid washes, comes off the wall. The damage depends on the nature of the filter cake. In a worst case scenario, plugging is early and complete. "In open hole, some [filter cakes] come off the wall in sheets and won't flow out," Montagna said. "Some come off in bits and flow out."

Screens face one more identity crisis. They must be at once sufficiently delicate to trap fines only thousandths of an inch in diameter, and rugged enough to survive being run through the tight turns of a short-radius horizontal well and uneven dips and climbs of extended reach well bores. Screens destroyed before reaching their destination are hardly a novelty, though manufacturers are working to lessen the chances of it happening.

Screen Solutions

Quantifying screen plugging problems has been a problem of its own, according to Halliburton Global Manager, Colby Ross. "One of the problems we have had and one of the reasons we are getting totally different answers to the same question is because the particle size being filtered in the different tests are different and some screens work better in some particle-size distributions than others," he said. "So there is, in the end, not one right answer and all screens have their place. It is a matter of identifying which is right for the proper application. We must establish parameters for each of the screen products and make sure to apply it correctly."

"What I have found in our tests of plugging with drilling muds or fines is that prepacked and powdered metal screens plugged faster than wire wrapped because they filter out everything," Montagna said. "We are being more relaxed in our approach. We are allowing some of the fines to be produced and therefore getting longer life out of these wells."

Hashemi said his company also saw the problem of drilling muds with powdered metal filter media. "Powdered media was not designed to have drilling fluids pass through it," he said. "That is why we initially face some plugging problems. But the new one is designed with every commercial drilling fluid considered."

Screen plugging solutions must be fashioned after the cause. That is, if plugging is a function of dirty completion fluids or filter cake, the answer lies in leaving the well to the completion team in better condition. But as any experienced manager knows, that is not always as easily done as said. Drilling engineers tend to understand drilling. Many of them consider their job over with a loggable wellbore. Completions engineers may need more and to get it requires communication.

"Not everyone wants to clean that hole because they are afraid of losing filter cake and losing the hole by fracturing the filter cake," said Montagna. "There is a mindset that wants (the well bore) full of mud."

What Montagna tells operators is if they wish to keep mud in the hole it must be conditioned to filter out particles larger than some size smaller than the screen is meant to filter so mud can be circulated out once the completion is done. In other words, screens must be part of the well plan from as early as the mud system design phase.

By choosing the mud and sand control system together, it is understood the well is going to produce some fines, at least at the onset of production. In doing so a new filter cake will be deposited on the screen exterior, what Montagna has called an in-situ gravel pack, whose permeability will be dictated by the screen pore size since it will be formed of all particles too large to pass through the screen.

"(Operators) can live with some initial fines production and we are getting longer life out of these wells and productivities are staying a little higher," Montagna said. "And, I believe it is because that initial filter cake is more permeable as opposed to a prepack or powdered metal with the tortuous path that begins to plug."

Screen mechanics

Getting the screens into place with minimal damage has also attracted technicians' attention. Pall has taken to welding their screens to high alloy base pipe as opposed to the old method of wrapping. And Purolator, another US-based company that has lately entered the screen market at the instigation of the industry, TIG welds their protective shrouds the length of the screen and then to the base pipe.

"With wire wrap screens you can apply a lot of torque, tension, and bending," said Ross, whose company he says has taken steps to make their screens stand up to the rigors of installation. "They can take a lot of abuse and a lot of damage and continue to work."

Indeed, Purolator has published test results that indicate their screens maintain their filtering ability despite 40% crushing-induced diameter reduction, 5% torsion deflection, and 50 degrees/100 ft bending. Even when the perforated base pipe cracked in tension the screen maintained its filtering ability. Similar results have been reported by other screen manufacturers.

The screen itself is the weak link in the assembly made up of a number of components attached to a longer, stronger base pipe. More specifically, according to some tests, the weldment between the wire wrap or filter medium cartridge and the base pipe were found to be especially vulnerable. That is because while installation exposes the assembly to bending, compression, and tensile stresses (all of which are heightened in high-angle wells), it is in tension that the weld and the base pipe is stretched far beyond the yield of the screens.

Montagna's company has approached the problem by adapting sliding sleeve technology to sleeve-based pipe mounting. In the sliding-sleeve scenario, the base pipe slides beneath the sleeve along O-ring seals especially developed for the purpose. In that way, the screen is spared many of the stresses felt by the stronger base pipe.

Gravel packing

When all is said and done, many sand-control experts say gravel packing is superior to stand-alone screens. Of course, no technology exists outside well economics. That is why Ross believes each technology has its place. "I think there is still going to be a tremendous number of wells that are going to benefit from just an open hole screen completion," Ross said.

"It would just make one well uneconomic while others wouldn't get enough out of a well if they didn't (gravel pack). It is just applying the proper technique at the proper time and not trying to do a one size fits all. Economics is the driver on most of this stuff."

The difficulty of obtaining efficient, concentric gravel packs has been the bane of adapting the technology to horizontal applications. Many believed it was impossible to lay the sand down evenly at high angles, as witnessed by the experience of the aforementioned operator. But progress, according to US/Johnson Filters' George Gillespie, has been made using a process he called "alternative path technology."

Gillespie reports that Mobil has developed a technology that uses a series of shunts fixed to the outside of the screen to allow fluid and sand slurry to bypass bridges formed between the screen and formation that are the beginning of gravel pack voids and backfill gravel behind them. "There have been no voids evident in Mobil's work after several hundred jobs over the past years," Gillespie said. "There have been no problems due to erosion or anything like that."

Montagna is championing gravel pack placement through accurate modeling. Their program was designed to predict pump rate and sand and gravel concentrations necessary to pack horizontal wells of different lengths and configurations. "It was tested in Trinidad where they did a 3,000-ft horizontal and sure enough things came out according to test."

The key to his company's approach, said Montagna, is identifying leak off to the formation and to pump at a rate sufficient to overcome it and carry the gravel the distance but not to fracture the formation. "To do that, you need to have about 80% returns," he said. "And there is a lot of work that goes into predicting that."

Montagna reports that when the proper rates are established the gravel lays down in the horizontal in what he calls and alpha-beta wave. He says he has observed the gravel laying down in a wave beginning at the top of the screen to the screen's toe in the alpha half of the process, and then work back towards the head of the assembly - the beta portion - filling in gaps as it goes.

Planning

As offshore petroleum industry problems have become more demanding with each additional mile operations move from shore, their solutions become more complex. Sand control in long-reach wells is no exception. But like all technical challenges of late, those involved with them voice certain familiar themes (no less appropriate for their familiarity) to accompany new technology - communication, planning, teamwork, integrated services.

"When we take on a horizontal well, we want to sell the complete process to the customer," said Montagna. "Drilling, hole cleaning, running the gravel pack."

The promoters of gravel pack over stand-alone screens in long-reach wells are being heard. More of them are being done in high-risk, high-profile projects including, it has been reported, Shell's latest deepwater success, Ram-Powell.