Single-trip showdown
  from: Offshore Engineer
  by: Rick von Flatern
  Monday, March 01, 2004

In completing a well from a deepwater rig, saving a little time means saving a lot of money. So when service companies talk of eliminating trips during sand control operations, operators and OE's US editor Rick von Flatern pay attention.

Traditionally, sand control operations include separate trips for perforating and sand placement and open hole completions often require yet another for acidizing to remove filter cake. On land and in shallower waters where the added cost incurred by numerous trips is small relative to the price tag of the overall well, multiple-trip operations are more easily tolerated.

But in deepwater, not only is the spread cost of the rig considerably higher, the time to trip in and out of the hole is longer. That combination of circumstances is further exacerbated by the fact these high-risk projects usually rely on numerous producing intervals to supply sufficient economic return. As a result, the added value attached to the ability to perform multiple tasks or to treat multiple zones in a single trip is significantly increased by deep water.

In recent years, service companies keenly aware of this situation have been delivering tools aimed at doing just that. Building on recently developed techniques that constitute modern sand control capabilities, they are spending time, money and intellectual capital to upgrade existing tools and to bring new systems to an arena that seems to constantly raise the stakes.

Longer and more numerous intervals to be treated per well, for instance, have resulted in demand for tools able to handle ever higher proppant loading and increased pump rates in higher downhole pressure and temperature environments. Adding to the sense of urgency to upgrade tool versatility and dependability is a spiraling cost of failure.

'When you are talking about frac packing in high-risk, high-reward environments everything is magnified,' explains Baker cased hole fracturing engineer Mike Lorenz. 'Pressure is higher, rates are higher and wear and tear on tools is higher.'

In response, service companies are increasingly turning to ways to do more in less time through what might accurately be termed multitasking completion systems. Towards that end they have developed and are continuing to develop systems able to perforate and place sand in a single run or to perform work on numerous zones in a single trip. Many companies already possess or are working on some variation of each.

Schlumberger's Perfpac system, for instance, is the product of evolution and is designed to perforate and gravel pack a single zone in one run. Baker Oil Tools' MZ Fraq is designed to allow cased hole frac packing in numerous zones once they have all been perforated. And BJ Services, as heir to ground breaking work done by its 2002 acquisition, OSCA, is soon to bring forward its Multizone Single Trip (MST) system that allows cased hole completions frac packing and acidizing of numerous zones in a single trip.

Shoot and pack
Traditionally, in cased holes, a gravel pack or frac operation meant one trip in the hole to perforate and another to run sand control equipment and service tools and to place the sand. Today these two jobs, particularly in the high-cost, high-reward wells that characterize the deepwater arena, can be done in a single trip and the benefits go beyond those attached to saving time.

'You are not only saving rig time but fluid loss also,' says Schlumberger's Chris Hines. 'Especially when you get in higher mud weights and consider zinc bromides and those kinds of fluids those savings can be considerable. The other thing is when you start losing fluids you damage the formation.'

Schlumberger has been running its Perfpac single trip system since 1996 but, says Hines, 'the entire process has been re-established, with modifications to the system being made as the times require'.

Changes over time, driven by the harsher environments of increasing well and water depth include raising temperature and pressure ratings for the system's critical IRIS valve from 8000psi and 300°F to 15,000psi and 350°F as well as modifications to the screens aimed at achieving more efficient packs across the sand face.

The upgraded IRIS valve is integral to the system and is run in the hole above the perforating guns, long-stroke sump packer and screens. It is a pressure pulse activated system that is actually two valves - a ball valve and a circulating valve.

'If we have unacceptable fluid losses we can circulate a light fluid in the hole and when we get that under control we can move down,' says Hines. 'The total operation of releasing the long stroke packer, moving down across the interval to get the screen across the sand face and setting the packer again, depending on how long the interval is, takes from 15 to 30 minutes. And when you are losing fluid that is a short time.'

Once the VRX plug has been dropped, the formation is ready for sand placement. The ball valve of the IRIS is opened and the circulating valve closed which creates a live annulus as well as a straight tube through which the job is pumped.

Fluid loss control while coming out of the hole once sand has been placed is accomplished via an isolation valve placed above the blank pipe. As the assembly is being retrieved, a shifting tool on the bottom of the wash pipe closes this formation isolation valve (FIV), itself designed with time savings in mind.

'Once the well is ready to go on production, you pulse down and open the FIV,' Hines explains. 'With the standard valves you would need to go in with a slick line tool and manually open that valve.'

The more zones the merrier
In a cased hole in deep water, savings are often realized more through treating multiple zones than in multitasking in a single run.

And while Baker Oil Tools' multizone frac packing system is on the sharp edge of such work, its roots, according to Mike Lorenz, go back decades.

'This is not a new system for Baker,' he observes. 'This is really approaching a fifth generation of our multiple zone, one-trip gravel pack system.The latest improvement incorporated things we have learned about design features and best practices in order to be able to frac pack wells with sand control tools.'

Baker's multizone system is based on its CK Fraq systems that Lorenz describes as 'a robust platform of hardware and software designed to make certain the requirements of any particular job do not exceed the limits of the system's equipment'.

Essentially the software program, dubbed XOT-Live, is used to determine if the service tools will survive what are essentially numerous gravel packing operations without the opportunity to come out of the hole for inspection or service. Of particular concern to the program is whether the crossover ports through which sand slurries must pass do not fail due to erosion, a catastrophic failure that could send proppant throughout the service tool, effectively ending the job and requiring considerable remedial work to correct.

'We figure out by testing and extrapolation and correlation to different schedules and proppant loadings to predict what sort of wear we can expect on a tool,' notes Baker engineer Max Trujillo. 'We have optimized the design to avoid the port being worn.We use the software with every job to get a feel as to the amount of wear we are going to see. We want to know if the port is going to survive this job and if not, or if it is close, we can modify the job, change the ramp schedule or maximum loading or maybe decrease the rate if that is possible.'

Multizone operations are preceded by perforation of all zones as well as a gage ring and junk basket run before setting the sump packer. The gravel packing tools and screens are run and snapped into the sump packer and the top packer and the lower isolation packer set and tested. Each zone is then treated, the excess sand reversed out and the procedure repeated up hole in series beginning with the lowermost.

'Basically we have a standard crossover tool we extend so that the port is at the zone to be treated,' adds Trujillo. 'We isolate the first zone at very bottom, pull the service string tools so the port is sealed off between two seal bores, set isolation packers, test the packers to make sure we are not getting communication and then move the tools back down.'

Because the communication path between the zone of interest and the surface is isolated, rising pressures that indicate a screen out is imminent are easily observed at the surface. At that point, the system is pulled up so as to expose reverse ports and positioned so that sand can be reversed out via flow down the casing annulus.

Equipment requirements dictate a 28ft minimum separation between zones but given that such small separations do not auger well for fracture height containment anyway, it is essentially a non-issue. Theoretically the system can do any number of zones up to the failure point of the tools as dictated by the software.

Another point of vulnerability likely in a single work string that is used multiple times is damage to seals used to establish fluid circulation. But the seals, according to Baker engineers, have been created for harsh treatment and placed on the service tool in a way that makes it unlikely any one seal will be over-used.

'At the most we would use any seal in two zones and we can make the call whether to use the seals again,' says Trujillo. 'Most times the intervals are different lengths and so different seals are being used. If a leak off test indicates some seal leakage, the team on site can decide whether to proceed and the seals are designed so that they can take considerable abuse before becoming damaged.'

Locating the tools in their proper place is, of course, critical to proper sand placement. The Baker system relies on what Lorenz called a 'hydraulically actuated smart collet' to align the inner string through which the sand is pumped with the outer string of sand screens and packers. The collet also accommodates weight on the system to keep the tool positioned through movement caused by rig heave or the hydraulic forces created downhole during pumping operations.

'It works with the rig's motion compensator,' says Baker engineer Buzz Speyrer of the system's ability to stay on bottom during rig movement. 'We set down as much weight as we want and then the sea height that can be tolerated depends on the rig and the compensator stroke length. If the compensator exceeds its stroke length that is when you get movement. But the real limit is capability of the frac vessel to operate safely.'

Pack and wash
MST, the latest offering from BJ Services, is now in its last stages of development and is expected to be in field trials in March. The MST service tool serves first as a traditional crossover tool for sand placement and then as a means to isolate the zone for spot acid washing and finally as a shifting tool.

'We first perforate all the zones,' explains BJ's Ed Smith. 'Then we run the equipment for all zones at zone, pump sand into the bottom zone and then come up and pump the middle zone and then the top zone - all through the same work string in one trip in the hole.'

Once the sand is in place at each interval, the service tool shifts an isolation valve to the closed position, accomplishing in a single stroke both fluid loss control and a measure of well control when it is time to run the production string.

The new system is the direct inheritor of the technology developed by BJ for its horizontal single trip system (HST) as a way to treat open hole horizontal sections in a single run. HST, in turn, traces its lineage to lessons learned on standard gravel packing systems such as the ISO or fixed systems and other early gravel pack systems that have become industry standards.

'In the standard or ISO system you have a crossover tool with a sliding sleeve with wash pipe attached to take returns and if you are in a fracture mode, you close off the formation and monitor the pressure on the formation through the tool,' explains BJ's tool development manager, Dewayne Turner, a pioneer of zone isolation used in conjunction with sand control completion tools. 'When you open the collet tool you create a communication path so fluid goes through the ports up the casing side. It is pretty standard.

'Once we did this we built an HST (horizontal single trip) system,' adds Turner. 'One of our main objectives in this was to maintain hydrostatic on the formation at all times until we had proppant or a filter media in place in order to keep the hole from caving in. We have accomplished that.'

As an added feature, the HST was designed to provide a method to clean up the interval just gravel packed by seating a ball on the top of the tool that at once seals the return ports and opens a channel back to the surface. The result is to convert the crossover tool to a simple piece of through pipe. 'Then, as you start out of the hole, you can seal across an interval and inject acid into the formation,' notes Turner.

The acid is delivered through a stinger with a polished outside diameter that stabs into inverted seals on the inside diameter of the packer, in effect providing a new set of seals for each zone being treated. The configuration forestalls concerns over repeated exposure of the seals to acid or to wear and tear as the tool is repeatedly pulled and re-set.

'We are finding it is more effective in resolving filter cake than what everyone else is doing, primarily on injectors since you can't push the filter cake out into the formation very well,' says BJ's Bryce Traweek. 'They say you can produce it but you can't get rid of it in the injector wells.'

From the horizontal to the multizone single trip (MST) system was a short leap. Leaning over a 6ft long engineering sequence drawing of the system, BJ's Gregg Stout walks through its application across three zones of varying size.

At the bottom of the lowest zone of interest is a sump packer with isolation packers set between the individual zones. 'The service tool is designed to locate in each zone as you move up the hole,' he says. 'On top of that is a work string that is smaller than normal to allow return flow. Attached above is a larger, typical work string. Once the system is run into the well and properly positioned, the top packer is set first and the all of the isolation packers are set simultaneously and tested.

'At each zone the tool is placed in the step down mode and the job is pumped. After fracturing each zone, the tool is picked up to the reverse position and excess slurry is reversed out.'

As with the horizontal system, when each successive zone is completed it is isolated by a sliding sleeve closed by the tool on the end of the wash pipe. 'When you complete the uppermost zone, you remove the service tool and run production tubing and seals into the top packer and seal bore receptacle,' says Stout. 'Then you can run slick line or coiled tubing to open or close sliding sleeves in the production screens to commingle or produce different zones.'

Dewayne Turner notes: 'We think this system could be the number one change in the industry. We have developed a special screen to use only one sleeve every one or two joints to minimize the amount of sleeves in the completion. And they have kick-off shoulders and profiles in the sleeves that allow you to open and close as many sleeves as you wish.'

Getting the sand in place
The type of long producing intervals in which single trip systems are most economically advantageous can create sand placement problems. This is particularly true in offshore operations where high perm reservoirs combined with heavy drilling and completion fluids maximize the threat of high fluid losses.

'Sometimes when doing a sand control job, the upper part of formation takes a lot of fluid and creates bridges at the top of the screen,' says Schlumberger's Chris Hines. 'If you bridge out at the very top, just above the blank, you might have a 10ft long void space at the very bottom of your screen.'

To address that eventuality, Schlumberger has employed what Hines calls alternative path technology, or shunts, that create pathways around bridges. Essentially tubes running alongside sand screens, shunts must at once provide an easy path for the sand slurry and, says Hines, be sized such that they are not so tight as to not be retrievable in the event of a problem.

'You have to look at friction pressures that you are going to be pumping down,' he adds. 'So you have to make sure the carrier fluid is right and the sand concentrations are right. There is actually a design to be done in using the shunts.'

Turner says BJ has taken another tack in handling long sections by making certain the slurry need never travel far along the screen. 'While packing, pressure set differential valves open in sequence as it feels pressure caused by the valve below it being covered with sand,' he says.

As a consequence, as the sand level rises the circulating port is essentially moved up the hole with the sand level and so circulation is relatively close to the bottom of the remaining open screen at all times.

Getting placed in the sand
An issue that has long dogged sand control treatments is the prospect of sticking tools with recently pumped proppant. When multiple jobs or multiple tasks are done with a single service tool string, it seems only reasonable to expect the concern is compounded.

But Baker engineers say such issues have been addressed in its design. 'It is intuitive that you should have a fair amount of space between the metal shoulder of the service tool and the seal bore to keep from getting stuck,' says Baker's Lorenz. 'But in using the proppants we have with the hardness they have, we have found that minimizing that space decreases your chances of getting stuck. Rather than using flow-through to eliminate sand from the void, we use very close tolerances to keep the sand out.'

Besides a square shoulder at the top of the sealing sub that is designed to keep sand ahead of the seal as it is pulled from the well, the seal bores are made harder than the sand itself and would shear the individual grains rather than allow them to become imbedded in the seal bore.

BJ and Schlumberger also say sticking is a non-issue as the tools involved are simply reconfigured ones with a long history dealing with sanding issues. 'We are taking tools we have on the shelf and screwing them together in a different configuration and using technology we have used in other places in a different way,' points out BJ's Ed Smith, explaining his company's confidence in the new MST system.

Schlumberger's Hines echoes the sentiment. 'Really we have no concern about sanding up. It is the same tool we use all the time and the bottom of the packer is designed to help clean debris. There is no more debris than you would have in a standard job.'

To work
Single trip systems, with their emphasis on time reduction have obvious allure in deepwater operations where that commodity is at a premium. They also all pay strict attention to fluid loss control and while the economic gain realized from not losing barrels of high-cost, heavy drilling fluids is obvious and easily quantifiable, it also results in less formation damage. And though such a gain is difficult to quantify, in the high-volume wells that often characterize deepwater prospects such an advantage likely translates to significant increases in production rates and ultimate reserves recovery.

In any case, single trip systems offer more than enough reasons to expect operators to embrace them for the kind of innovative tools they appear to be. OE