Fractured Well Performance: Key to Fracture Treatment Success
- Hemanta Mukherjee (Schlumberger Oilfield Services)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- March 1999
- Document Type
- Journal Paper
- 54 - 59
- 1999. Society of Petroleum Engineers
- 2.4.1 Fracture design and containment, 3 Production and Well Operations, 5.6.4 Drillstem/Well Testing, 1.8 Formation Damage, 2.1.6 Frac and Pack, 5.4.6 Thermal Methods, 5.3.2 Multiphase Flow, 5.2.1 Phase Behavior and PVT Measurements, 1.13 Casing and Cementing, , 1.11 Drilling Fluids and Materials, 2.1.1 Perforating, 5.6.1 Open hole/cased hole log analysis, 2.5.2 Fracturing Materials (Fluids, Proppant), 4.1.2 Separation and Treating, 4.3.4 Scale, 5.5 Reservoir Simulation, 3.2.3 Hydraulic Fracturing Design, Implementation and Optimisation
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Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area,these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering.
Best production practice requires the optimum production rate from a wellwith maximum bottomhole flowing pressure (BHFP). This can be engineered only byreducing pressure losses in the reservoir-flow conduit that comprises thereservoir rock and the near-wellbore completion at or near the perforation.Under most producing conditions, an induced fracture with appropriate geometryminimizes near-wellbore pressure losses very efficiently. This paper exploresthis role of the hydraulic fracture, which results in many applications underdifferent reservoir conditions. Problems concerning placement of an optimallydesigned hydraulic fracture and common solutions also are discussed.
The basic objectives of hydraulic fracturing are to increase productivity orinjectivity and to improve the efficiency of steam injection in thermal floods.A more fundamental and alternative view to explain the role of inducedfracturing comes from Prats'1 contention that hydraulic fracturesextend the wellbore radius. There are a few ways to explain the profoundimplications of extended wellbores.2 The most practical explanationderives from understanding the pressure losses in the area of drainage. Darcy'slaw states that the pressure gradient in the direction of flow is directlyproportional to the velocity. This is stated mathematically in consistent unitsby
where v=q/A. This relationship also implies that the lower thevelocity, the lower the pressure gradient in the path of flow. In radialdrainage, with constant volumetric rate, the flow velocity in the radial-flowpath is maximum at the wellbore. Fig. 1 explains this point with realdimensions. The velocity at a wellbore of 6-in. radius is 2,000 times that atthe entry into the drainage (1,000 ft from the wellbore), assuming negligiblefluid entry within this drainage. From Darcy's law, this implies that, at thewellbore perimeter, the pressure gradient is 2,000 times greater than at thedrainage surface 1,000 ft from the wellbore. This also suggests that, if thewellbore diameter is increased to 100 ft from 6 in., the entry velocity intothis wellbore increases 10 times that at drainage. This translates to asubstantial net increase in BHFP by effectively increasing the wellbore radiusfrom 6 in. to 100 ft. Such an increase in the BHFP can be used to produce thewell at higher rates; at lower drawdown; or at a combination that considerssand-, water-, or gas-control problems.
For a fixed velocity, Darcy's law also implies that the pressure gradient isinversely proportional to the reservoir permeability: the lower the effectivepermeability of the flowing phase, the higher the pressure gradient. Near thewellbore, permeability is reduced through different radial-damage mechanisms,such as drilling-fluid invasion and production-induced mechanisms (e.g.,condensate dropout from gas, solids/fines deposit, sublimation of sulfur,paraffin deposit, and other scale deposits).
Consequently, the pressure gradient at the wellbore increases as a result ofboth increased velocity and the reduced permeability caused by damage. Inducedhydraulic fractures not only reduce pressure gradients near the wellbore byincreasing the surface area of fluid entry but also inhibit someproduction-induced-damage mechanisms by reducing drawdown and physicallybypassing these damaged areas. In many such production-induced-damagemitigations, fracturing can postpone the need for frequent matrix acidtreatments.
A fracture can be idealized as a slot induced in the rock, possibly openbetween acid-etched surfaces or filled with proppant to resist closure. It canbe shown analytically that the permeability of an open slot is proportional tothe square of slot width; this can be presented as
k=54.4×10 6 w2, (2)
where permeability, k, is in darcies and slot width, w, is ininches.
The permeability of a 0.01-in.-wide open slot is 5,440 darcies.3Because permeability is inversely proportional to the pressure loss throughporous media, the pressure losses at or near the wellbore can be minimized ifthe fluid flow can be directed through a fracture or a high-permeability slotfrom the reservoir to the wellbore. Consequently, the extent of thispressure-loss control determines the major fracture properties, such as itsphysical dimensions and the permeability with or without proppant.
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