Getting the reservoir to give up its treasure involves careful design, technical innovation and no small amount of luck.

In his foreword to Reservoir Stimulation, the epic text published by John Wiley & Sons, Ahmed Abou-Sayed refers tongue-in-cheek to a popular 1970s era prescription for reservoir stimulation: "When everything else fails, frac it." Twenty chapters later, the reader should know everything there is to know about stimulation - both hydraulic fracturing and matrix treatments. However, we quickly found out that knowledge falls into three categories: what you know, what you don't know and what you don't know you don't know.

To find out what we didn't know, we cut right to the chase, "Tell us what's new in stimulation?" we asked some industry experts. For convenience, the answers have been divided into five categories: surface equipment, downhole tools and techniques, fluid technology, environmental concerns and enterprise solutions.

Surface equipment

Increasingly versatile coiled tubing units are playing a growing role in stimulation operations. Schlumberger is presently testing their new CT SEAS (Coiled Tubing Safer, Efficient Automated Solutions) offshore Norway for BP. This unit is an offshore version of their CT-EXPRESS land unit, and promises increased efficiency with lower CAPEX. According to the company, the new unit is smaller in size and weight, and its modular design makes for fewer lifts. Controls and operational monitoring are highly automated, reducing the number of personnel required onboard. Like all the company's units, CT SEAS is linked to a Web-based, real-time, ultra-secure data network, called InterACT, that allows clients to log on to the job from anywhere in the world using no special equipment to observe and monitor field operations. CT SEAS is predicted to reduced crew size by 30%, trucks on location by 20%, rig-up time by 75% and rig-down time by 40% over conventional CT units.

Halliburton has introduced hydrajet fracturing on CT that allows multiple fractures to be sited along the completion zone. Used in conjunction with acid treatment, the hydrajet process has showed excellent results in multilateral horizontal completions. The high-pressure jet initiates a crack in the borehole wall, which is then expanded by ramping up the annulus pressure. This has a multiplier effect on the fracturing pressure in the cavity, causing the fracture to propagate quickly.

An innovative real-time downhole mixing process whereby the fluid and proppant were infused with liquid CO2 downhole to generate a composite fracturing fluid produced several advantages, including reduced friction pressures and lower treatment pressures.

Downhole blending using either CO2 or N2 should provide comparable efficiency to surface-mixed foam systems, but has the added advantage of instant modification on the fly to optimize the treatment process. It provides the operator with a real-time method to control proppant concentration and placement.
Downhole tools and techniques

New bottomhole technology provides added reach for treatment in deeper wells with higher temperatures and pressures. Developments in inflatable packers operated singly or in straddle configuration are available that allow accurate placement of treatments. Baker Oil Tools' Inflatable Straddle Acidizing Packer (ISAP) can be set multiple times and is conveyed through tubing on coiled tubing.

Schlumberger reports its new CoilFLATE high pressure/high temperature through-tubing inflatable packer is rated at 375?F (190?C). In a Middle East application, the packer was emplaced holding 2,000 psi differential in September 2002, and is still in operation.

Fluid technology

Perhaps the most prolific area for new technology is in fluid design and chemistry. Halliburton recently unveiled its new MicroPolymer frac fluid, said to be the first polymer-based frac fluid that is reusable. Viscosity of the fluid is controlled by varying the PH, thus the polymer is unharmed and after filtering can be used again. Importantly, the new fluid reduces the demand for fresh water and spent fluid disposal, giving the product excellent marks for environmental impact.

Viscoelastic surfactant (VES) technology is paying big dividends by reducing friction pressures and increasing production. This gives access to deeper horizons, and will allow higher pumping rates, which keep a rapidly-developing fracture propagating and permits deep emplacement of the proppant. In a recent Schlumberger operation in North Africa, standard crosslinked polymers could only be pumped at 6-8 bbl/min, whereas VES fluids could be pumped at 12-14 bbl/min. VES products have evolved since the early 90s. Currently, they offer enhanced breaker properties, enhanced cleanup and higher fracture conductivity, all ultimately leading to higher production.

Viscoelastic Diverting Acid (VDA) is a new Schlumberger technology that is a polymer-free self-diverting matrix stimulation system. The key to VDA is its ability to generate in-situ fluid viscosity. The VDA becomes viscous when, as the acid spends, it comes in contact with water, but thins in the presence of hydrocarbon. This property offers several advantages including enhanced production by increasing zonal coverage in multi-zone reservoirs and extended intervals. When used in frac-pack operations, The VDA keeps the carrier from leaking off into a water zone, thus depositing the gravel in the wrong spot. The fluid also helps sweep residual drilling mud out of the hole. VDA helps keep fractures from aggregating around the heel of a horizontal drain hole. By thickening on contact with water, the VDA prevents the treatment from permeating the heel area and carries it deeper into the horizontal section. In vertical wells it allows all zones of varying permeability to be effectively stimulated. In horizontal wells, post-job production logs have shown stimulation of the entire openhole or cased hole section, from heel to toe.

Schlumberger offers a unique scale prevention technology, called ScalePROP, that utilizes porous proppant technology impregnated with a concentrated scale inhibition system. The treated proppant can be mixed at any fraction of total proppant volume. It can be impregnated with appropriate chemicals such as inhibitors designed to leech out over time to effectively protect the completion. Proppant impregnation can be customized, allowing for instantaneous tailor-made decisions on the type and concentration of chemicals to be deployed. For deepwater, Schlumberger has developed YF100EC encapsulated crosslinker that enables stimulation of deep wells at a high pump rate from a single vessel. The encapsulation allows delay times of up to 20 minutes to be achieved before the fluid crosslinks, giving time needed to carry the proppant laden fluid through the perforations or downhole restrictions. This allows the treatment to reach deeper frontiers while ensuring the desired system performance to and through the perforations, and well into the fracture.

BJ Services has an array of acid treatments than can be customized to fit specific applications. Their Sandstone Acid is a hydrogen fluoride (HF) based system that enhances damage removal and stimulation response. For non-HF applications such as carbonates or acid-sensitive sandstones, the company uses BJ S3 Acid. The treatment offers no danger of forming precipitates, and can be blended with scale inhibitors. Also recommended for carbonates is the company's XL Acid III that offers several benefits, principally deeper penetration of live acid into the formation.

Environmental concern

Particularly applicable in offshore operations new, "green" base fluids are making their appearance. Schlumberger's GreenSlurry system meets both of the stringent United Kingdom and Gulf of Mexico environmental guidelines, and is applicable in all types of fracturing and gravel packing operations. It features a unique "greener" carrier fluid that has a faster hydrating polymer time, and is accompanied by breaker technology that ensures efficient break and fracture cleanup.

BJ Services' Emerald FRAQ is a low residue guar borate-crosslinked polymer that boasts a very low oil and grease content. It is designed not to produce a sheen when discharged into the sea (where permitted). At the same time, the product demonstrates very good shear stability as well as excellent proppant transport and support in PH ranges above 9.5, and is rated for use in reservoirs above 300?F (150?C). Emerald FRAQ has been used in the Gulf of Mexico, and is now the standard offshore fracturing fluid for the company.

Enterprise solutions

One Houston operator said, "It's not about seeing how much sand you can pump anymore. We need optimum stimulations, not just big ones." More and more operators are working with their service providers to make use of all the data and information available - seismic, well logs, tests, drilling and production data - to design each fracture treatment specifically for the formation being treated. The operator reckoned that using an enterprise solution to design optimum treatments had allowed his company to save money on the treatments while improving the producibility of the reservoir and stretching out the workover cycle.

A technique whereby stimulations are planned on a macro scale comparing total cost of operations with incremental net present value of added production should result in maximum profitability for each reservoir stimulation campaign.

To do this efficiently requires a working knowledge management (KM) program. Oil companies are in various stages of adopting KM, which in its simplest definition must provide instantaneous global access to the company's knowledge base. Seamless integration of well and reservoir data allows its use in design and process control algorithms to create and implement optimal treatment programs.

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