In laminated formations, limited-entry multizone fracturing completion practice has issues in yielding maximum well performance. Causes include reduced control of slurry/proppant placement; bypassing marginal zones; potentially breaching into water zones; and variability of the reservoir fracture parameters, including near-wellbore pressure differentials. Service

Figure 1. A very efficient multizone fracturing process allowed BJ crews to stimulate 253 zones in five wells in just 17 days. (Photos courtesy of BJ Services)
companies and operators today continue to seek new ways to provide economical pinpoint control of fracture placement and fast, simple operations for stimulating multiple-pay wells.
Even with diversion techniques, traditional limited-entry, staged fracturing cannot ensure all perforated intervals will be treated or that each zone will receive the treatment desired. This often results in a combination of over- and under-stimulated zones, including some that receive no stimulation at all. Thus, although traditional limited-entry treatments are inexpensive and carry little operational risk, they also are not certain to produce optimum stimulation results and hydrocarbon production.

One new completion option is the just-in-time-perforating (JITP) multizone completion technology licensed from ExxonMobil Upstream Research Company, which enables fast, accurate multipay perforating and fracturing. The technology has been field-proven, with more than 65 wells and 2,200 individual zones completed in the last few years.

The JITP process is a coordinated technical performance of pumping and perforating services companies. The wireline crew runs an assembly of select-fire perforating guns, typically prepared for six to 11 perforation sets. The first set of perforations is made, and then the perforating guns are pulled above the next zone to be perforated and stimulated. The pumping crew delivers the desired fracture stimulation treatment for the just-perforated zone, while the perforating guns are slowly lowered to the second zone. At the end of the first zone treatment, the pumping crew drops ball sealers in the flush fluid. The flush fluid behind the ball sealers becomes the pad fluid for the next zone’s fracture treatment. When the ball sealers arrive on the open perforations, the wellhead pressure spikes, and the wireline crew perforates the subsequent zone for stimulation. The result is overbalanced perforating and immediate fluid break-down of the newly perforated zone, and fracture treatment is initiated.

Coordination between the wireline and pumping crews is important to job execution. When the ball sealers arrive on the current zone being treated, the wireline company must fire the next gun immediately and follow quickly with pulling the perforating guns up-hole above the newly created perforations. For deeper completions, the pumping company may briefly reduce the fluid rate just prior to the ball sealer arrival. The rate slowdown is to stay within the limits of the wireline tension loading limits while pulling against the slurry flow friction and differential pressures at the perforating gun. The perforating guns are depth-correlated to casing collars before each perforation set is made to ensure the variation of wireline stretch is minimized.

During the JITP process, pumping reliability is critical, as are pressure gear and perforating gun performance. If the pumping stops, the ball sealers may float off the perforations, opening all the lower zones. Depending on the time lost and the number of zones affected, fracturing operations may either end (requiring a composite plug), or it may be possible to re-seat the balls by re-starting the fracture treatment. If a gun fails to fire, it may be possible to use the next select-fire portion of the gun to continue operations. If a screen-out occurs or seems likely, perforating of the next zone can be done, potentially avoiding disruption of the planned up-hole stimulation treatments for the perforating/fracturing event. This is a very flexible and forgiving completion methodology.

With proper execution and subject to license restrictions, the process can be used to fracture up to 22 zones per day (two perforating gun run events). On a single-well pad operation, a
Figure 2. The wellhead configurations allows for concurrent wireline and pumping operations as well as ball sealer deployment.
very quick turnaround of 2 to 3 hours is achievable because the perforating guns are being retrieved while the top zone fracture treatment is being performed. The second gun assembly is made up and run with a fracture composite plug to setting depth above the previously treated zones. With the plug set and the next zone perforated, the well is ready for fracture treatments without pulling the perforating assembly. On a multiple well pad, two wireline units working with one fracture pumping fleet can quickly move to a second well (less than 1 hour), achieving two perforating/fracturing events per day.

The 11-zone limitation on the gun string is mostly due to handling perforating gun assembly lengths at the surface and finishing treatments before there is excessive ball sealer fluid cutting over time. Ideally, the JITP process minimizes non-pumping time and crew working hours while improving job reliability with crew efficiency and safety.

Benefits to the operator include:
• Increased control of proppant placement. Pinpoint perforating and ball sealers provide positive isolation, ensuring each perforated zone receives its planned treatment, maximizing economic and technical gains.
• Fracture treatments designed per zone vs. per group of zones, allowing for optimizing pad volume percent, proppant concentration staging, fluid type, etc.
• Efficiency gains from rapid advancement from one zone to the next with limited non-pumping time.
• Immediate flowback, which minimizes potential damage from fluids sitting in the formation. Flowback can be initiated while perforating guns are retrieved from the well.
• Efficiencies that allow operators to avoid bypassing potentially productive zones that might be deemed marginal and add risk to the stimulation treatment when combined with pumping operations of other zones.
• Flexibility in decision-making: The decision to perforate or not perforate a particular zone can be made right up until the last moment if desired, to gauge how a lower zone stimulation treatment is received.
• Prompt hydrocarbon to sales. The well is online in hours compared with days for multizone plug-and-perforate operations. This can lead to increased reserves, higher initial production rates and flatter decline curves.
• Operational efficiencies and safety gained via fewer mobilizations to the well site and less non-pumping time during the completion phase.
• An ideal completion technique for many wells with multiple pay. For instance, in the Piceance Basin (Western Colorado) a typical vertical well has 20 to 60 productive intervals.

Five-well pad completion
In a recent extremely efficient Piceance fracture stimulation series in October and November 2006, a BJ Services stimulation crew from Grand Junction, Colo., used the JITP process to stimulate 253 zones in five wells, pumping 9.8 million lb of 40/70- and 100-mesh sand in just 17 days. Working in tandem with perforation crews from Oil Well Perforators Inc. of Casper, Wyo., the crew selectively fractured an average of 15 specific zones per day (130 hrs of total pumping time) in daylight-only operations.

A typical day on the five-well pad involved a perforating gun run and fracturing pumping event for 6 to 11 zones in one well in the morning and allowing it to start flow-back. The crews performed a second perforating/fracture event for another 6 to 11 zones in another well in the afternoon. After a brief clean-up flow period, both wells would be flowing to sales several hours later in the evening. This also allowed for a brief well test of each section of the well.

As an example, on the second day of the series, eight zones of Well B were treated in the morning and allowed to flow back overnight into the morning of the third day, while the first eight zones of Well C were stimulated. That afternoon, Well B was shut-in, a composite plug was set and eight more zones were stimulated. (To isolate the sections already stimulated, the gun assembly included a composite plug, which was set above the top Day 1
perforations).

At the end of the treatment, coiled tubing crews drilled out the plugs and cleaned out the wells. Performing more stimulation treatments per perforating/fracturing event has also let the operator reduce the number of composite plugs to drill out from each well. This further reduces well completion costs and risks of the clean-out operations.

JITP Operations
Rig-up involves some equipment particular to JITP. Most of the operation is based on running concurrent wireline and pumping operations, but it does not require specialized equipment. The wellhead connection is a “Y” block for the fluid injection with a wireline flange located on top. Pumping into the two sides of the wellhead reduces wireline erosion, as does continuous movement of the perforating assembly during pumping operations.

Several redundant systems provide over-pressure safety of the well and surface equipment. The pumping equipment pressure trips are set up in staged increments so that pumping is not completely stopped if perforating is slightly delayed or the perforating gun fails. For additional safety, two full-flow pressure relief valves are set at maximum operating pressure on the treating line. At 20 to 30 bpm, the well (as a closed chamber) builds up pressure very quickly (< 4 sec).

Clear communications between the pumping and wireline crews is essential to smooth and safe operations of the fracture treatments. This is achieved with dedicated radios and common displays between the pumping treatment control van and the wireline unit.

Summary
The JITP completion process has pinpoint placement control, efficiency and safety advantages compared with traditional multizone fracturing options. It may offer more optimal production results derived from the many features and benefits listed above. However, the technology is subject to license restrictions, depending on type, depth and location of the well and the operator’s partnership arrangements with the licensing company.

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